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Timing of growth zone formations in otoliths of the snapper, Chrysophrys auratus, in subtropical and temperate waters differ and growth follows a parabolic relationship with latitude

Timing of growth zone formations in otoliths of the snapper, Chrysophrys auratus, in subtropical... Chrysophrys auratus was collected from one sub-tropical and two temperate regions spanning >2400 km along the coast of Western Australia (∼23.5–35.5° S). Marginal increment analysis demonstrated that, while a single opaque zone is formed in the otoliths of C. auratus each year, the period of deposition varies among regions. An opaque zone was formed in May to early September in the sub-tropical upper west coast, and thus when water temperatures were declining to their minima. In contrast, opaque zone formation occurred 3 months later in August to December in the temperate lower west and south coasts, when water temperatures were rising from their minima. The length and age distributions differed markedly among populations of C. auratus, with the strongest year classes varying among the three regions. Thus, it is likely that year class strength of C. auratus throughout its distribution along the coast of Western Australian is mostly related to local environmental conditions. Chrysophrys auratus grew far less rapidly and attained a smaller size in the warmer upper west coast than in the cooler temperate regions of the lower west and south coasts. A collation of data on C. auratus from ten populations in Australia and three in New Zealand showed that growth is greatest towards the mid-latitudes of its geographic range, i.e. at ∼31° S. Estimates of mean lengths at specified ages thus exhibit a parabolic relationship with latitude, with reduced growth (i.e. edge-of-range effects) occurring towards the latitudinal margins of the distribution of this sparid. Introduction The growth, maximum size and life span of individuals in the populations of many fish species vary widely among geographical regions (e.g. Boehlert and Kappenman, 1980; Robertson et al., 2005). There are also many cases where such life history characteristics vary in relation to latitude and thus water temperature (Atkinson, 1994; Kozłowski et al., 2004). These trends typically correspond to those predicted by the Metabolic Theory of Ecology (MTE), which is based on the relationship between the metabolism of poikilotherms, such as fishes, and environmental temperatures (Angilletta et al., 2004; Brown et al., 2004; Charnov and Gillooly, 2004; Lek et al., 2012). For example, the growth of a species, i.e. the rate at which the asymptotic length is approached (von Bertalanffy growth coefficient k), decreases from lower (warmer) to higher (cooler) latitudes (Atkinson, 1994; Kozłowski et al., 2004; Lek et al., 2012). However, the converse relationship between growth (i.e. length-at-age) and latitude (i.e. temperature) has been recorded for some fish species. For example, it has been proposed that the more rapid early growth of the foxfish Bodianus frenchii in a cooler than warmer environment is related to its ancestors having lived in cooler deeper waters and thus becoming better adapted to lower than higher water temperatures (Cossington et al., 2010). There are also cases where the growth characteristics of a species do not change consistently along a latitudinal gradient (e.g. similar growth curves, Hesp et al., 2004; Wakefield et al., 2013). Furthermore, it has been argued that a species would be more likely to perform better within the central part than towards the edges of its distribution, where conditions would be expected to be less favourable for that species (Pörtner et al., 2001; Osovitz and Hofmann, 2007). However, the MTE fails to capture localized ecological processes (e.g. habitat, oceanography), which may help explain departures from this ecological theory (Brown et al., 2004). The snapper Chrysophrys auratus (Forster 1801) is abundant in latitudes ranging from 25 to 40° S in temperate and sub-tropical waters of Australia and New Zealand (Paulin, 1990). This species occupies a wide range of habitats from estuaries to hypersaline environments (i.e. salinities > 70) and nearshore embayments to the upper continental slope (300 m, Woo and Fung, 1981; Moran et al., 1998; Fowler et al., 2004; Wakefield et al., 2011, 2015). As this sparid is highly valued by commercial and recreational anglers, its growth has been studied at numerous locations throughout its distribution (Paul and Tarring, 1980; Francis and Winstanley, 1989; Francis et al., 1992b; Millar et al., 1999; McGlennon et al., 2000; Jackson et al., 2010). These studies show that, in certain regions, C. auratus reaches maximum total lengths (TLs) of ∼1300 mm (1130 mm fork length, FL) and maximum ages of ∼40 years (Gomon et al., 2008; Norriss and Crisafulli, 2010). As biological data are available for several populations over its wide distribution, C. auratus provides a particularly good model for exploring the ways in which the growth characteristics of a species vary throughout an extensive latitudinal range. Despite the commercial and recreational importance of C. auratus along the extensive coastline of Western Australia, with a distributions that spans a latitude of ∼12° (Figure 1), information on the growth of this species in these waters is restricted to a single study of assemblages in the large subtropical marine embayment of Shark Bay at ∼26° S (Jackson et al., 2010). Figure 1. Open in new tabDownload slide Locations of the upper west, lower west and south coasts in Western Australia from which C. auratus was sampled. Figure 1. Open in new tabDownload slide Locations of the upper west, lower west and south coasts in Western Australia from which C. auratus was sampled. The annual formation of a single opaque and translucent growth zone in the otoliths of C. auratus has been validated, based on recaptured individuals with chemically-marked otoliths and marginal increment analyses (Ferrell et al., 1992; Francis et al., 1992b; Fowler et al., 2004; Jackson, 2007). However, the timing of opaque zone formation in the otoliths of C. auratus varies among locations. For example, this zone is formed from May to December in the otoliths of C. auratus in the subtropical waters of Shark Bay, Western Australia (Jackson, 2007), whereas in the cooler temperate waters of South Australia, an opaque zone was not recorded on the outer margin of otoliths until September and was not delineated in most fish until January (Fowler et al., 2004). In sparids, the opaque zone is typically formed during the cooler winter period and is indicative of slower growth, and the translucent zone is formed in the warmer summer period and is indicative of faster growth (Buxton and Clarke, 1991; Sarre and Potter, 2000; Hall et al., 2004; Hesp et al., 2004). Although variations in the formation of the opaque zone among conspecific populations are not well understood, variations in the timing of this formation have been found, for some sparids, to also coincide with spawning (Mann-Lang and Buxton, 1996; Sumpton, 2002). Previous studies have shown that the growth of individuals of C. auratus varies substantially throughout its distribution in Australia and New Zealand. For example, the mean asymptotic length estimated for C. auratus in South Australia at 33–35° S is typically ∼900 mm FL (Fowler et al., 2004; McGlennon, 2004), which is thus greater than the estimates of ∼750 mm FL in Shark Bay at ∼26° S (Jackson et al., 2010) and 600–700 mm FL in New Zealand at ∼37° S (Francis et al., 1992b; Gilbert and Sullivan, 1994). The extent to which the growth of C. auratus along the extensive Western Australian coastline, with a distribution covering ∼12° of latitude from subtropical to temperate waters, has yet to be investigated. Thus, while there is some evidence to suggest that C. auratus may typically grow larger toward the middle of its latitudinal range, the manner in which growth varies with respect to latitude (and thus water temperature) throughout its entire distribution, has yet to be investigated. During the present study, samples covering a wide length and age range of C. auratus were collected from along stretches of the upper west, lower west and south coasts of Western Australia (Figure 1) to: (i) Quantify variations in the timing of the formation of growth zones in otoliths in the subtropical and temperate regions. (ii) Test the hypothesis that, on the extensive Western Australian coast, the lengths at age of C. auratus approach their maximum more rapidly and have a smaller maximum size in the subtropical than temperate southern regions. (iii) Test the hypothesis that, on the basis of data collated from throughout the distribution of C. auratus, the growth of this sparid will be greatest in the middle of that latitudinal range. Material and methods Sampling regime Chrysophrys auratus was collected between 2002 and 2006 from oceanic waters on the upper west (23°30′–26°30′ S), lower west (31°00′–33°00′ S) and south coast (34°00′–35°30′ S and 115°30′–125°00′ E) regions of Western Australia (Figure 1). The majority of C. auratus were caught by line fishing (92–98% per region) with most sampled from commercial and recreational catches (90%). In all regions, these commercial and recreational line fishers targeted the same suite of demersal fish species and, throughout the year, employed a similar wide range of hook sizes and types and a variety of rig configurations. Although there may be subtle differences in the selectivity of fish among hooks of different sizes, the hooks typically used to target demersal teleosts by commercial and recreational fishers during this study were capable of sampling a very wide length range of C. auratus (Otway and Craig, 1993). Fish were sampled from commercial caches at least monthly in each region and predominantly from markets. On each of these sampling occasions, the majority or all of the C. auratus at the market were sampled. In each region, recreational catches were sampled through regular donations from fishers (see Fairclough et al., 2014) and at angling competitions. Additional samples were obtained in the two temperate regions from onboard licensed recreational fishing charter vessels. As the minimum legal length for retention (MLL) of C. auratus in Western Australia was 410 mm TL during this study, additional sampling involving line, trawl and trap fishing was undertaken by researchers to catch smaller individuals in each region. The overall sampling regime yielded substantial numbers of fish for each calendar month and, as recommended by Ricker (1969), data were pooled for subsequent analyses. Mean monthly water temperatures in the three regions were derived from in situ and satellite data at a resolution of 1° of latitude and longitude from 1982 to 2005, a period thus encompassing the lifespans of most individuals sampled (Reynolds et al., 2007). Somatic measurements and treatment of otoliths To enable comparisons of growth with other studies (e.g. Francis et al., 1992b; Coutin et al., 2003; McGlennon, 2004; Jackson et al., 2010), both the TL and FL of each C. auratus were measured to the nearest 1 mm. The sex could be determined by macroscopic examination of gonads of all but the small juveniles (∼300 mm TL, see Wakefield et al., 2015). The lengths and ages of the unsexed small juveniles were thus randomly assigned, in equal numbers, to the data sets for females and males in their respective regions. The sagittal otoliths of each C. auratus were removed, cleaned and stored dry (see Wakefield et al., 2016). The right otolith of each fish was embedded in epoxy resin and cut into ∼250–300 µm transverse sections using a low speed Isomet saw (Buehler Ltd.) with a diamond-tipped blade. The sections were rinsed in 2% hydrochloric acid for ∼15 s (see Gauldie et al., 1990) and mounted on a glass slide with a cover slip using casting resin. The opaque zones in each otolith section were counted under reflected light at 20–40 times magnification, without knowledge of the size or time of capture of the fish from which the otolith had come. The first opaque zone was readily distinguishable as its formation coincided with the development of an inflection point in the subcupular meshwork fibre zone (Francis et al., 1992a). Terminology used to describe otolith structure and marginal increment analyses follows Kalish et al. (1995) and Tuset et al. (2008). The trends exhibited by mean monthly marginal increments were used to ascertain visually whether the opaque zones in sagittal otoliths of C. auratus are typically formed annually and to compare the timing of their formation among the upper west, lower west and south coasts. The marginal increment on each otolith is the distance between the outer edge of the outermost opaque zone and the periphery of the otolith. Marginal increments were measured along an axis from the primordium to the crista inferior (ventral rim of the sulcus acusticus) of the otolith, perpendicular to the opaque zones and as close as possible to the sulcus acusticus. These measurements were made to the nearest 0.001 mm on images of the sectioned otoliths taken using a video camera (Leica DC300) attached to a dissecting microscope and employing computer imaging software (Leica IM1000). The marginal increment was then expressed as a proportion of the distance between the primordium and the outermost edge of the sole opaque zone when only one delineated opaque zone was present, and as a proportion of the distance between the outer edges of the two most recently delineated opaque zones when two or more such opaque zones were present. Each otolith was recorded as having either a translucent or opaque outer edge. The periodicity of opaque zone formation was determined using an approach described by Okamura and Semba (2009), based on the occurrence of marginal increments falling within the lower 30 percentile of values among each of the opaque zone count categories in each region. This involved fitting binomial models linked with von Mises distributions for circular data with the assumptions of no cycle, an annual cycle or a biannual cycle. The cyclic model with the smallest value of Akaike Information Criterion (AIC) was accepted as the model that best represented the marginal increment data. Ageing and growth The age of each C. auratus on its date of capture in each region was estimated using a combination of: (i) the number of delineated opaque zones, (ii) an allocated birth date corresponding to the approximate midpoint the spawning season (see Wakefield et al., 2015), (iii) the time of year when the opaque zones on the otoliths of the majority of C. auratus become delineated in that region, (iv) the year of life (i.e. first or second) when the first opaque zone is formed, determined from the progression of the discrete modes of young fish in sequential length-frequency distributions (see Wakefield et al., 2007,, 2011), and (v) the width of the marginal increment (i.e. relatively narrow or wide) in the months around the time of opaque zone delineation. The birth date selected for C. auratus in each region (1 June for the upper west coast and 1 November for the lower west and south coasts) was determined from trends exhibited by the mean monthly gonadosomatic indices and monthly prevalences of fish with gonads at stages IV (developed) and V (spawning, Wakefield et al., 2015). The von Bertalanffy growth equation was fitted to the TLs and FLs at age of females and males separately using, Lt=L∞(1−e−k(t−t0)) ⁠, where Lt=  the predicted TL or FL (mm) at age t (years), L∞ = the asymptotic length (mm), k= the growth coefficient (year−1), t= estimated age (years) and t0 = the theoretical age (years) at which fish would have zero length. The growth curves of females and males in each region and of the corresponding sex in different regions were then compared using a likelihood-ratio test (Cerrato, 1990). Results Marginal increment analyses The trend in mean monthly marginal increments of otoliths of C. auratus for all opaque zone categories in the upper west region declined rapidly from their maxima in late winter (i.e. ∼August) to their minima in mid spring (i.e. ∼October), and then increased gradually until June to August of the following year (Figure 2). Although fish with otoliths containing one opaque zone were caught in only a few months, the relative distributions of their marginal increments were consistent with those of older fish (Figure 2). Opaque zones were formed (i.e. observed on the outer margin) in otoliths of C. auratus in this region from May to early September, i.e. during the Austral winter and early spring, when water temperatures were declining and at their minima (Figures 2 and 3). In the upper west region, the period when a new translucent zone started to form on the periphery of otoliths of some fish coincided with the period when, in other fish, an opaque zone was still being formed. This resulted in marginal increments being either thin (i.e.<∼0.25) or wide (i.e.≥∼0.45) during this period. Whereby, otoliths with thin marginal increment widths possessed outer margins with a new (thin) translucent zone having just started to form, and those with wide increment widths possessed outer margins with a wide translucent or opaque outer margin (Figure 2). Irrespective of the number of opaque zones, the best fitting cyclic model to the marginal increment values of C. auratus from the upper west coast was that which assumed a single opaque zone is formed annually in otoliths (Table 1). Figure 2. Open in new tabDownload slide Individual (translucent outer margins grey circles, opaque outer margins white triangles) and mean monthly (±1 SE, lines) marginal increments in sagittal otoliths of C. auratus for each of five opaque zone categories in the upper west (left), lower west (middle) and south coast (right) regions. Figure 2. Open in new tabDownload slide Individual (translucent outer margins grey circles, opaque outer margins white triangles) and mean monthly (±1 SE, lines) marginal increments in sagittal otoliths of C. auratus for each of five opaque zone categories in the upper west (left), lower west (middle) and south coast (right) regions. Figure 3. Open in new tabDownload slide Mean monthly sea surface temperatures (±1 SE) for the (i) upper west, (ii) lower west, and (iii) south coast regions off Western Australia. Data were derived from mean daily values recorded between 1982 and 2005 (Reynolds et al., 2007). On the x-axis, the closed rectangles represent winter and summer months and the open rectangles autumn and spring months. Figure 3. Open in new tabDownload slide Mean monthly sea surface temperatures (±1 SE) for the (i) upper west, (ii) lower west, and (iii) south coast regions off Western Australia. Data were derived from mean daily values recorded between 1982 and 2005 (Reynolds et al., 2007). On the x-axis, the closed rectangles represent winter and summer months and the open rectangles autumn and spring months. Table 1 AIC values for binomial models assuming three different cycles (i.e. no, annual, or biannual cycle) fitted to the monthly proportions of C. auratus otoliths with marginal increments within the lower 30 percentile for each opaque zone category in each region Region . Opaque zones . No cycle . Annual cycle . Biannual cycle . Upper west coast 1 zone 52.1 38.3 45.1 2–3 zones 297.2 207.1 296.7 4–5 zones 406.5 254.4 401.4 6–7 zones 194.2 154.2 186.3 ≥8 zones 231.6 196.9 229.9 Lower west coast 1 zone 72.0 66.1 71.9 2–3 zones 284.3 235.0 284.0 4–5 zones 427.1 350.4 428.8 6–7 zones 265.1 186.1 239.6 ≥8 zones 515.7 504.6 483.2 South Coast 1 zone 72.0 43.3 66.5 2–3 zones 101.7 67.0 88.4 4–5 zones 187.3 112.0 161.4 6–7 zones 467.5 276.7 451.0 ≥8 zones 398.1 199.0 343.9 Region . Opaque zones . No cycle . Annual cycle . Biannual cycle . Upper west coast 1 zone 52.1 38.3 45.1 2–3 zones 297.2 207.1 296.7 4–5 zones 406.5 254.4 401.4 6–7 zones 194.2 154.2 186.3 ≥8 zones 231.6 196.9 229.9 Lower west coast 1 zone 72.0 66.1 71.9 2–3 zones 284.3 235.0 284.0 4–5 zones 427.1 350.4 428.8 6–7 zones 265.1 186.1 239.6 ≥8 zones 515.7 504.6 483.2 South Coast 1 zone 72.0 43.3 66.5 2–3 zones 101.7 67.0 88.4 4–5 zones 187.3 112.0 161.4 6–7 zones 467.5 276.7 451.0 ≥8 zones 398.1 199.0 343.9 Bold font denotes best model fit based on smallest AIC value. Table 1 AIC values for binomial models assuming three different cycles (i.e. no, annual, or biannual cycle) fitted to the monthly proportions of C. auratus otoliths with marginal increments within the lower 30 percentile for each opaque zone category in each region Region . Opaque zones . No cycle . Annual cycle . Biannual cycle . Upper west coast 1 zone 52.1 38.3 45.1 2–3 zones 297.2 207.1 296.7 4–5 zones 406.5 254.4 401.4 6–7 zones 194.2 154.2 186.3 ≥8 zones 231.6 196.9 229.9 Lower west coast 1 zone 72.0 66.1 71.9 2–3 zones 284.3 235.0 284.0 4–5 zones 427.1 350.4 428.8 6–7 zones 265.1 186.1 239.6 ≥8 zones 515.7 504.6 483.2 South Coast 1 zone 72.0 43.3 66.5 2–3 zones 101.7 67.0 88.4 4–5 zones 187.3 112.0 161.4 6–7 zones 467.5 276.7 451.0 ≥8 zones 398.1 199.0 343.9 Region . Opaque zones . No cycle . Annual cycle . Biannual cycle . Upper west coast 1 zone 52.1 38.3 45.1 2–3 zones 297.2 207.1 296.7 4–5 zones 406.5 254.4 401.4 6–7 zones 194.2 154.2 186.3 ≥8 zones 231.6 196.9 229.9 Lower west coast 1 zone 72.0 66.1 71.9 2–3 zones 284.3 235.0 284.0 4–5 zones 427.1 350.4 428.8 6–7 zones 265.1 186.1 239.6 ≥8 zones 515.7 504.6 483.2 South Coast 1 zone 72.0 43.3 66.5 2–3 zones 101.7 67.0 88.4 4–5 zones 187.3 112.0 161.4 6–7 zones 467.5 276.7 451.0 ≥8 zones 398.1 199.0 343.9 Bold font denotes best model fit based on smallest AIC value. On the lower west coast, the mean monthly marginal increments on otoliths with ≥ two opaque zones declined from their maxima in late spring to early summer, i.e. November to December, to their minima in mid-summer, i.e. January, and then increased to >0.6 in the December of the following year (Figure 2). The trends exhibited by the marginal increments on the otoliths of C. auratus on the south coast were similar to those of fish on the lower west coast (Table 1, Figure 2). The marginal increments in the otoliths of fish caught on the lower west coast also exhibited predominantly an annual cycle (Table 1), with opaque zone deposition occurring later than those on the upper west coast (i.e. August to December rather than May to September), when water temperatures were at or increasing from their minima (Figures 2 and 3). Thus, irrespective of the number of opaque zones, the marginal increments on the otoliths of C. auratus from each region underwent a single pronounced decline and then rise each year. This was consistent with the best fitting model to the marginal increment data being a single annual cycle, which strongly indicates that one opaque and one translucent zone is formed annually and that opaque zones in otoliths can be considered annuli when determining ages of C. auratus. Length- and age-frequency compositions In a previous study of C. auratus in waters near those sampled on the upper west coast, small 0+ aged fish (45–95 mm TL) were first caught in November, i.e. 4–5 months after spawning peaked (Wakefield et al., 2007). This cohort could be clearly traced through length-frequency distributions from sequential monthly samples from November until December of the following year. During the present study, small fish (120–129 mm TL) were first caught in March, which were similar in size to those sampled by Wakefield et al. (2007) at the same time of year. Although an opaque zone was not present in the otoliths of small fish caught in March (and also June), it was present in those of this cohort in September. The first opaque zone in the otoliths of C. auratus from the upper west coast thus becomes delineated in the second spring of life, when individuals are ∼14-months old (Figure 2). Substantial numbers of small 0+ C. auratus (68–96 mm TL) were first caught on the lower west coast in February, i.e. only 3 months after spawning peaked. Although the otoliths of none of these fish contained an opaque zone, it was present in fish of this cohort collected after December. As the above trends were paralleled by those of small fish on the south coast, the first opaque zone in the otoliths of C. auratus on both the lower west and south coasts becomes delineated when individuals are about 1-year old (Figure 2). The length- and age-frequency distributions of female and male C. auratus caught within any given region from 2002 to 2006 were similar, but differed markedly between regions (Figure 4). The length-frequency distributions for females and males from the upper west coast each contained a very broad mode, with the lengths of the majority of fish lying just to the right of the MLL of 410 mm TL. The length-frequency distributions for females and males on the lower west coast each contained a well-defined mode just to the right of the MLL, and another at a length slightly >800 mm TL and thus exceeding that of virtually all fish caught on the upper west coast (Figure 4). The length-frequency distributions for females and males on the south coast contained a single well defined mode just above 600 mm TL, lying between the two modes for the lower west coast and far greater than the MLL (Figure 4). As most samples came from commercial and recreational fishers, the numbers of C. auratus < the MLL of 410 mm were relatively small. Figure 4. Open in new tabDownload slide Length- (left) and age-frequency (right) histograms for female (white bars) and male (grey bars) C. auratus caught in the upper west, lower west and south coast regions of Western Australia from 2002 to 2006. Distributions do not include the small fish that could not be sexed macroscopically. Dashed line represents the minimum legal length for retention of 410 mm TL. Figure 4. Open in new tabDownload slide Length- (left) and age-frequency (right) histograms for female (white bars) and male (grey bars) C. auratus caught in the upper west, lower west and south coast regions of Western Australia from 2002 to 2006. Distributions do not include the small fish that could not be sexed macroscopically. Dashed line represents the minimum legal length for retention of 410 mm TL. The age compositions for female and male C. auratus in each region between 2002 and 2006 broadly parallel the trends exhibited by the length-frequency distributions (Figure 4). Thus, the age distribution for the two sexes on the upper west coast had a broad single mode at 4–6 years, whereas that for the lower west coast was bimodal at 4 and 11 years, and the south coast was unimodal at 7–8 years (Figure 4). The annual age-frequency distributions for C. auratus were used to track the relative abundance of age cohorts within and among regions from 2003 to 2005 (Figure 5). On the upper west coast, the relatively strong 1991 year class declined in abundance from 2003 to 2004 and had essentially disappeared by 2005. This accounts for the age-frequency distributions changing between 2003 and 2005 from bimodal to unimodal and the percentage of fish > 8-years old declining from 41.2 to only 10.5% (Figure 5). Figure 5. Open in new tabDownload slide Percentage frequency of occurrence of sequential age cohorts of C. auratus (≥MLL of 410 mm TL) caught in the upper west, lower west (white bars, outside Cockburn Sound; grey bars, inside Cockburn Sound) and south coast regions of Western Australia in three sequential years. Figure 5. Open in new tabDownload slide Percentage frequency of occurrence of sequential age cohorts of C. auratus (≥MLL of 410 mm TL) caught in the upper west, lower west (white bars, outside Cockburn Sound; grey bars, inside Cockburn Sound) and south coast regions of Western Australia in three sequential years. The vast majority of C. auratus caught outside the embayment of Cockburn Sound on the lower west coast in 2002–2004 were < 6-years old, with the 2000 year class the most abundant in the latter 2 years (Figure 5). In contrast, virtually all fish caught by commercial and recreational fishing in Cockburn Sound in those 3 years were >6 years old (Figure 5). The 1991 and 1992 year classes were the most abundant in Cockburn Sound in 2002, but both of these, and particularly the 1992 year class, declined progressively over the next 2 years (Figure 5). The 1996 year class was by far the strongest in the catches of C. auratus on the south coast in 2003–2005 (Figure 5). The maximum lengths and ages of C. auratus were 864 mm and 30 years on the upper west coast, 1056 mm and 29 years on the lower west coast and 1083 mm and 38 years on the south coast (Table 2). Table 2 von Bertalanffy growth parameters (estimates and associated 95% upper and lower confidence limits) for curves fitted to the TLs at age of female and male C. auratus in three regions of Western Australia . . TL∞(mm) . k (y−1) . t0(y) . Amax . Lmax . n . r2 . Upper west coast  Female   Estimate 691 0.21 −0.33 30.0 864 639 0.90   Upper 709 0.23 −0.22   Lower 672 0.20 −0.45  Male   Estimate 648 0.25 −0.25 22.1 840 545 0.88   Upper 670 0.27 −0.13   Lower 626 0.22 −0.37 Lower west coast  Female   Estimate 1150 0.12 −0.41 24.8 1051 872 0.89   Upper 1199 0.13 −0.22   Lower 1101 0.11 −0.60  Male   Estimate 1127 0.12 −0.46 28.8 1056 802 0.88   Upper 1178 0.13 −0.24   Lower 1075 0.11 −0.67 South coast  Female   Estimate 1013 0.11 −0.94 29.4 1083 896 0.83   Upper 1065 0.13 −0.65   Lower 962 0.10 −1.23  Male   Estimate 950 0.13 −0.61 37.8 999 818 0.84   Upper 989 0.15 −0.36   Lower 911 0.12 −0.86 Sexes combined  Upper west coast   Estimate 681 0.22 −0.42 1152 0.87   Upper 695 0.23 −0.31   Lower 666 0.20 −0.53  Lower west coast   Estimate 1136 0.12 −0.42 1689 0.88   Upper 1171 0.13 −0.27   Lower 1101 0.11 −0.56  South coast   Estimate 986 0.11 −1.22 2094 0.84   Upper 1017 0.12 −1.05   Lower 956 0.11 −1.38 . . TL∞(mm) . k (y−1) . t0(y) . Amax . Lmax . n . r2 . Upper west coast  Female   Estimate 691 0.21 −0.33 30.0 864 639 0.90   Upper 709 0.23 −0.22   Lower 672 0.20 −0.45  Male   Estimate 648 0.25 −0.25 22.1 840 545 0.88   Upper 670 0.27 −0.13   Lower 626 0.22 −0.37 Lower west coast  Female   Estimate 1150 0.12 −0.41 24.8 1051 872 0.89   Upper 1199 0.13 −0.22   Lower 1101 0.11 −0.60  Male   Estimate 1127 0.12 −0.46 28.8 1056 802 0.88   Upper 1178 0.13 −0.24   Lower 1075 0.11 −0.67 South coast  Female   Estimate 1013 0.11 −0.94 29.4 1083 896 0.83   Upper 1065 0.13 −0.65   Lower 962 0.10 −1.23  Male   Estimate 950 0.13 −0.61 37.8 999 818 0.84   Upper 989 0.15 −0.36   Lower 911 0.12 −0.86 Sexes combined  Upper west coast   Estimate 681 0.22 −0.42 1152 0.87   Upper 695 0.23 −0.31   Lower 666 0.20 −0.53  Lower west coast   Estimate 1136 0.12 −0.42 1689 0.88   Upper 1171 0.13 −0.27   Lower 1101 0.11 −0.56  South coast   Estimate 986 0.11 −1.22 2094 0.84   Upper 1017 0.12 −1.05   Lower 956 0.11 −1.38 TL∞, hypothetical asymptotic length at an infinite age; k, growth coefficient; t0, hypothetical age at zero length; Amax, maximum age; Lmax, maximum length; n, sample size (juveniles and adults combined); r2, coefficient of determination. Table 2 von Bertalanffy growth parameters (estimates and associated 95% upper and lower confidence limits) for curves fitted to the TLs at age of female and male C. auratus in three regions of Western Australia . . TL∞(mm) . k (y−1) . t0(y) . Amax . Lmax . n . r2 . Upper west coast  Female   Estimate 691 0.21 −0.33 30.0 864 639 0.90   Upper 709 0.23 −0.22   Lower 672 0.20 −0.45  Male   Estimate 648 0.25 −0.25 22.1 840 545 0.88   Upper 670 0.27 −0.13   Lower 626 0.22 −0.37 Lower west coast  Female   Estimate 1150 0.12 −0.41 24.8 1051 872 0.89   Upper 1199 0.13 −0.22   Lower 1101 0.11 −0.60  Male   Estimate 1127 0.12 −0.46 28.8 1056 802 0.88   Upper 1178 0.13 −0.24   Lower 1075 0.11 −0.67 South coast  Female   Estimate 1013 0.11 −0.94 29.4 1083 896 0.83   Upper 1065 0.13 −0.65   Lower 962 0.10 −1.23  Male   Estimate 950 0.13 −0.61 37.8 999 818 0.84   Upper 989 0.15 −0.36   Lower 911 0.12 −0.86 Sexes combined  Upper west coast   Estimate 681 0.22 −0.42 1152 0.87   Upper 695 0.23 −0.31   Lower 666 0.20 −0.53  Lower west coast   Estimate 1136 0.12 −0.42 1689 0.88   Upper 1171 0.13 −0.27   Lower 1101 0.11 −0.56  South coast   Estimate 986 0.11 −1.22 2094 0.84   Upper 1017 0.12 −1.05   Lower 956 0.11 −1.38 . . TL∞(mm) . k (y−1) . t0(y) . Amax . Lmax . n . r2 . Upper west coast  Female   Estimate 691 0.21 −0.33 30.0 864 639 0.90   Upper 709 0.23 −0.22   Lower 672 0.20 −0.45  Male   Estimate 648 0.25 −0.25 22.1 840 545 0.88   Upper 670 0.27 −0.13   Lower 626 0.22 −0.37 Lower west coast  Female   Estimate 1150 0.12 −0.41 24.8 1051 872 0.89   Upper 1199 0.13 −0.22   Lower 1101 0.11 −0.60  Male   Estimate 1127 0.12 −0.46 28.8 1056 802 0.88   Upper 1178 0.13 −0.24   Lower 1075 0.11 −0.67 South coast  Female   Estimate 1013 0.11 −0.94 29.4 1083 896 0.83   Upper 1065 0.13 −0.65   Lower 962 0.10 −1.23  Male   Estimate 950 0.13 −0.61 37.8 999 818 0.84   Upper 989 0.15 −0.36   Lower 911 0.12 −0.86 Sexes combined  Upper west coast   Estimate 681 0.22 −0.42 1152 0.87   Upper 695 0.23 −0.31   Lower 666 0.20 −0.53  Lower west coast   Estimate 1136 0.12 −0.42 1689 0.88   Upper 1171 0.13 −0.27   Lower 1101 0.11 −0.56  South coast   Estimate 986 0.11 −1.22 2094 0.84   Upper 1017 0.12 −1.05   Lower 956 0.11 −1.38 TL∞, hypothetical asymptotic length at an infinite age; k, growth coefficient; t0, hypothetical age at zero length; Amax, maximum age; Lmax, maximum length; n, sample size (juveniles and adults combined); r2, coefficient of determination. Growth The von Bertalanffy growth curves provided good fits to the TLs at age for female and male C. auratus in each region, with each coefficient of determination ≥ 0.83 (Table 2). The parameters for the growth curves fitted to the FLs at age for these fish and for those from elsewhere throughout Australia and New Zealand are given in Table 3. Although growth of the two sexes in each region differed significantly (p < 0.05, Figures 6 and 7), the differences in the lengths estimated from the von Bertalanffy equation for the females and males of C. auratus at any age in each region were small. For example, the differences between the sexes at 15 years of age were only 4.5, 1.4, and 1.9% for the upper west, lower west, and south coasts, respectively, and sex differences are thus considered of limited biological significance. Figure 6. Open in new tabDownload slide von Bertalanffy growth curves fitted to the lengths-at-age of unsexed (white triangles), female (grey circles, left) and male (grey circles, right) C. auratus caught in the upper west, lower west, and south coast regions of Western Australia. Figure 6. Open in new tabDownload slide von Bertalanffy growth curves fitted to the lengths-at-age of unsexed (white triangles), female (grey circles, left) and male (grey circles, right) C. auratus caught in the upper west, lower west, and south coast regions of Western Australia. Figure 7. Open in new tabDownload slide von Bertalanffy growth curves (±95% CIs) for female and male C. auratus from the upper west, lower west and south coast regions of Western Australia. Figure 7. Open in new tabDownload slide von Bertalanffy growth curves (±95% CIs) for female and male C. auratus from the upper west, lower west and south coast regions of Western Australia. Table 3 von Bertalanffy growth parameters for curves fitted to the FLs at age of C. auratus in Australia and New Zealand and derived using fish that had been aged using the growth zones in sectioned otoliths Location . Latitude (ºS) . FL∞ (mm) . k (y−1) . t0 (y) . References . Western Australia  Eastern Gulf, Shark Bay 25.8 755 0.17 −0.02 Jackson et al. (2010)  Denham Sound, Shark Bay 25.3 728 0.15 −0.08  Freycinet Estuary, Shark Bay 26.4 770 0.17 0.1  Upper west coast 25.0 589 0.22 −0.23 This study  Lower west coast 32.0 1004 0.12 −0.23  South coast 35.0 876 0.11 −1.15  Lower mid-west coast 28.0 682 0.15 −1.26 Wise et al. (2007) and  South west coast 33.5 805 0.13 −1.05 Lenanton et al. (2009) South Australia  Various 902 0.19 −2.86 McGlennon (2004)  Northern Spencer Gulf 33.3 984 0.12 −0.35 Fowler et al. (2004)  Southern Spencer Gulf 34.6 632 0.21 −0.11  Northern Gulf St Vincent 34.6 907 0.16 −0.08  Southern Gulf St Vincent 35.4 895 0.13 −0.06 Victoria  Various 38.1 857 0.12 0 Coutin et al. (2003) New South Wales  Various 34.0 574 0.14 −2.29 Ferrell (2004) Queensland  Various 27.3 792 0.08 −2.45 Sumpton (2002) New Zealand  Various 720 0.11 −0.75 Francis et al. (1992b)  Hauraki Gulf 36.6 382 0.23 −0.61 Vooren and Coombs (1977)  Hauraki Gulf 36.6 588 0.10 −1.11 Gilbert and Sullivan (1994)  West coast 37.5 667 0.16 −0.11 Location . Latitude (ºS) . FL∞ (mm) . k (y−1) . t0 (y) . References . Western Australia  Eastern Gulf, Shark Bay 25.8 755 0.17 −0.02 Jackson et al. (2010)  Denham Sound, Shark Bay 25.3 728 0.15 −0.08  Freycinet Estuary, Shark Bay 26.4 770 0.17 0.1  Upper west coast 25.0 589 0.22 −0.23 This study  Lower west coast 32.0 1004 0.12 −0.23  South coast 35.0 876 0.11 −1.15  Lower mid-west coast 28.0 682 0.15 −1.26 Wise et al. (2007) and  South west coast 33.5 805 0.13 −1.05 Lenanton et al. (2009) South Australia  Various 902 0.19 −2.86 McGlennon (2004)  Northern Spencer Gulf 33.3 984 0.12 −0.35 Fowler et al. (2004)  Southern Spencer Gulf 34.6 632 0.21 −0.11  Northern Gulf St Vincent 34.6 907 0.16 −0.08  Southern Gulf St Vincent 35.4 895 0.13 −0.06 Victoria  Various 38.1 857 0.12 0 Coutin et al. (2003) New South Wales  Various 34.0 574 0.14 −2.29 Ferrell (2004) Queensland  Various 27.3 792 0.08 −2.45 Sumpton (2002) New Zealand  Various 720 0.11 −0.75 Francis et al. (1992b)  Hauraki Gulf 36.6 382 0.23 −0.61 Vooren and Coombs (1977)  Hauraki Gulf 36.6 588 0.10 −1.11 Gilbert and Sullivan (1994)  West coast 37.5 667 0.16 −0.11 FL∞, hypothetical asymptotic length at age infinity; k, growth coefficient; t0, hypothetical age at zero length. Table 3 von Bertalanffy growth parameters for curves fitted to the FLs at age of C. auratus in Australia and New Zealand and derived using fish that had been aged using the growth zones in sectioned otoliths Location . Latitude (ºS) . FL∞ (mm) . k (y−1) . t0 (y) . References . Western Australia  Eastern Gulf, Shark Bay 25.8 755 0.17 −0.02 Jackson et al. (2010)  Denham Sound, Shark Bay 25.3 728 0.15 −0.08  Freycinet Estuary, Shark Bay 26.4 770 0.17 0.1  Upper west coast 25.0 589 0.22 −0.23 This study  Lower west coast 32.0 1004 0.12 −0.23  South coast 35.0 876 0.11 −1.15  Lower mid-west coast 28.0 682 0.15 −1.26 Wise et al. (2007) and  South west coast 33.5 805 0.13 −1.05 Lenanton et al. (2009) South Australia  Various 902 0.19 −2.86 McGlennon (2004)  Northern Spencer Gulf 33.3 984 0.12 −0.35 Fowler et al. (2004)  Southern Spencer Gulf 34.6 632 0.21 −0.11  Northern Gulf St Vincent 34.6 907 0.16 −0.08  Southern Gulf St Vincent 35.4 895 0.13 −0.06 Victoria  Various 38.1 857 0.12 0 Coutin et al. (2003) New South Wales  Various 34.0 574 0.14 −2.29 Ferrell (2004) Queensland  Various 27.3 792 0.08 −2.45 Sumpton (2002) New Zealand  Various 720 0.11 −0.75 Francis et al. (1992b)  Hauraki Gulf 36.6 382 0.23 −0.61 Vooren and Coombs (1977)  Hauraki Gulf 36.6 588 0.10 −1.11 Gilbert and Sullivan (1994)  West coast 37.5 667 0.16 −0.11 Location . Latitude (ºS) . FL∞ (mm) . k (y−1) . t0 (y) . References . Western Australia  Eastern Gulf, Shark Bay 25.8 755 0.17 −0.02 Jackson et al. (2010)  Denham Sound, Shark Bay 25.3 728 0.15 −0.08  Freycinet Estuary, Shark Bay 26.4 770 0.17 0.1  Upper west coast 25.0 589 0.22 −0.23 This study  Lower west coast 32.0 1004 0.12 −0.23  South coast 35.0 876 0.11 −1.15  Lower mid-west coast 28.0 682 0.15 −1.26 Wise et al. (2007) and  South west coast 33.5 805 0.13 −1.05 Lenanton et al. (2009) South Australia  Various 902 0.19 −2.86 McGlennon (2004)  Northern Spencer Gulf 33.3 984 0.12 −0.35 Fowler et al. (2004)  Southern Spencer Gulf 34.6 632 0.21 −0.11  Northern Gulf St Vincent 34.6 907 0.16 −0.08  Southern Gulf St Vincent 35.4 895 0.13 −0.06 Victoria  Various 38.1 857 0.12 0 Coutin et al. (2003) New South Wales  Various 34.0 574 0.14 −2.29 Ferrell (2004) Queensland  Various 27.3 792 0.08 −2.45 Sumpton (2002) New Zealand  Various 720 0.11 −0.75 Francis et al. (1992b)  Hauraki Gulf 36.6 382 0.23 −0.61 Vooren and Coombs (1977)  Hauraki Gulf 36.6 588 0.10 −1.11 Gilbert and Sullivan (1994)  West coast 37.5 667 0.16 −0.11 FL∞, hypothetical asymptotic length at age infinity; k, growth coefficient; t0, hypothetical age at zero length. The von Bertalanffy growth curves for both females and males on the upper west coast were significantly different from those on the lower west coast (p < 0.001), and these differed from those of the corresponding sex on the south coast (p < 0.01). These differences reflected, in part, marked variations in the asymptotic lengths (L∞), with those for females, for example, being far lower on the upper west coast (674 mm) than on both the lower west (1150 mm) and south coasts (1013 mm, Table 2). In contrast, the growth coefficient (k) for females was greater on the upper west coast (0.23 year−1) than on both the lower west (0.12 year−1) and south coasts (0.11 year−1). These differences in growth parameters are reflected in an increasing divergence of the growth curves for those regions after fish have reached 3 years of age, and particularly with those from the upper west coast (Figure 7). Discussion Formation of opaque zones in otoliths Marginal increment analyses on otoliths of C. auratus from different climatic regions of Western Australia demonstrated that an opaque zone has typically become fully formed in otoliths from the upper west coast by October, and thus 3 months earlier than in those of populations on both the lower west and south coasts. In the sub-tropical upper west coast, opaque zones were deposited in otoliths from May to September and therefore when water temperatures were declining to their minima. In contrast, the opaque zones in the otoliths of C. auratus from the two more southern and temperate regions were deposited 3 months later, i.e. from August to December, when water temperatures were rising from their minima. The variations in the timing of opaque zone formation of C. auratus in Western Australia are consistent with results of previous studies of this species from locations throughout Australasia (Ferrell et al., 1992; Francis et al., 1992b; Fowler et al., 2004). For example, the outer edge of the opaque zone has typically become delineated in otoliths of C. auratus by November and December in New South Wales (Ferrell et al., 1992; Ferrell, 2004) and New Zealand (Francis et al., 1992b), respectively, but this often does not occur until February in South Australia (Fowler et al., 2004). By definition, the ability to detect delineation of a newly-formed opaque zone on the outer margin of otoliths relies on the deposition of a certain amount of translucent material. It is likely that any time lag associated with the detection of opaque zone delineation was very similar for C. auratus among the upper west, lower west and south coasts. Thus, the observed regional variation in the timing of opaque zone formation (and delineation) reflects responses to other factors. In particular, the timing of opaque zone formation in each region corresponds closely with both water temperature minima and spawning periods of this species (Wakefield et al., 2015). In the co-occurring Rhabdosargus sarba, which spawns at a similar time in coastal waters on the upper west and lower west coasts, there is little difference in the timing of opaque zone delineation, which is consistent with the view that reproduction may influence opaque zone formation in some sparids (Hesp and Potter, 2003; Hesp et al., 2004). Furthermore, Mann-Lang and Buxton (1996) provided evidence that the timing of opaque zone formation in otoliths of several South African sparids is closely associated with reproduction. Variations in year class strength This study showed that the length composition of populations of C. auratus differed markedly among the three regions. As would be expected for samples collected from commercial and recreational fishers in a heavily-exploited fishery, the numbers of C. auratus caught on the lower west and upper west coasts increased sharply as the TL reached the MLL of 410 mm for this species. However, this was not the case on the south coast, where the numbers of C. auratus did not start rising conspicuously until the lengths of fish were well above the MLL. These differences are due to the fact that, unlike the situation on the upper and lower west coasts, the younger age classes, i.e. 3+, 4+, and 5+, were not well represented on the south coast. Although C. auratus is considered to be genetically homogenous along the west coast of Australia, otolith microchemistry indicates that adults do not typically mix over scales of hundreds of kilometres (Gardner and Chaplin, 2011; Fairclough et al., 2013). This is supported by tag-recapture studies in the upper west and lower west coasts, where the vast majority of adults were recaptured within ∼35 km from their tagged location (Moran et al., 2003; Wakefield et al., 2011). The very pronounced bimodality in the length-frequency distributions for C. auratus on the lower west coast, contrasts markedly with the forms of the distributions in both the upper west and south coasts. The greater abundances of larger and older C. auratus on the lower west coast were due to the presence of large mature individuals that aggregate and spawn each year in Cockburn and Warnbro Sounds (Wakefield, 2010; Wakefield et al., 2011, 2015; Breheny et al., 2012). However, the migration ranges of the large majority of these spawning individuals include adjacent shelf waters within a few hundred kilometres, and thus well within the lower west coast region (Wakefield et al., 2011). The age-frequency data for the upper west coast showed that, in 2003–2005, C. auratus started to be caught in substantial numbers when they had reached an age approximating that at which the MLL is attained, i.e. 4 years of age, with their numbers peaking 1 year later. The very marked decline in the proportion of fish >8-years old in the samples collected between 2003 and both 2004 and 2005 suggests that fishing has had a pronounced influence on the abundance of the older age classes. This marked decline reflects the reduced abundances, of the initially very strong 1991 year class (and also 1992 year class) between 2003 and 2005. The age-frequency distributions for C. auratus in Cockburn Sound on the lower west coast demonstrated that the catches of fish >7-years old were dominated by the 1991 and 1992 year classes in 2002–2004, together with the 1996 year class in the last 2 of those years. Considering the 1996 year class did not become abundant in the catches until 2003, it appears that the individuals of C. auratus do not typically aggregate in Cockburn Sound until they are ∼7 years of age (see Wakefield et al., 2011). Although relatively few older fish were caught in Cockburn Sound in 2002, 2003, or 2004, the 1991 year class, which corresponded to the oldest cohort that was well represented in each year, remained relatively strong throughout those 3 years. The fact that, on the south coast, the catches were dominated to such a marked extent by the 1996 year class and those for some years were very low is consistent with the evidence that spawning and/or recruitment on this coast is very limited in some years (Wakefield et al., 2015). Fowler and Jennings (2003) found that recruitment success in South Australia was greatest in years when summer water temperatures were warmest. This parallels the situation in Port Phillip Bay in Victoria, which is a major nursery area for C. auratus in that state (Hamer and Jenkins, 2004; Hamer et al., 2005). Moreover, in New Zealand, the recruitment of C. auratus was positively correlated with water temperature in the 5 months following spawning (Francis, 1993; Francis et al., 1997). It is noteworthy that the 1991 year class was particularly strong on the upper west coast, Cockburn Sound and probably on the south coast, and also in South Australia (Fowler et al., 2005; Fowler and McGlennon, 2011). This suggests that, in that year, environmental conditions were especially favourable for recruitment throughout Western Australia and South Australia. Although, water temperatures on the upper west coast were above average in the winter of 1991, when spawning would have occurred, those for the subsequent spring and early summer months were the coldest recorded for those years (Reynolds et al., 2007). Water temperatures in spring and early summer on the lower west and south coasts in 1991, when spawning and settlement would have occurred, were the lowest since 1990, but were highest in those months in 1996, when, like 1991, recruitment was particularly strong (Reynolds et al., 2007). Moreover, years of strong recruitment do not consistently coincide with the annual strength of either the La Niña (stronger current) or El Niño (weaker current) events that influence the nutrient poor, southward-flowing Leeuwin current that dominates the oceanographic climate along the west coast of Australia (Feng et al., 2003). It thus appears that, on rare occasions, favourable environmental conditions coincide over large areas of this species distribution and result in consistent strong annual recruitment within discreet spawning/nursery areas (i.e. 1991, see Wakefield, 2010). However, it is more typical that localized environmental conditions within the discrete spawning and nursery areas for C. auratus influence annual recruitment abundance. Comparisons of growth with other snapper populations As hypothesized, C. auratus grew more rapidly in the temperate region of the lower west coast than in the sub-tropical region of the upper west coast of Australia. For example, by 10 years of age, females had reached 820 mm on the lower west coast compared with only 610 mm on the upper west coast. Furthermore, the maximum total length was far greater for the population on the lower west than upper west coast, i.e. 1056 vs. 864 mm. C.auratus also grows far more rapidly and reaches a larger size on the temperate south coast than subtropical upper west coast of Western Australia with females, on average, attaining a total length of 720 mm at 10 years of age and having a maximum length of 1083 mm. As C.auratus has a relatively large latitudinal range and its growth is well documented, it provided an excellent model for studying the relationship between the growth of a demersal teleost and water temperature. FLs at 5, 10, and 20 years were calculated from the growth curves for C. auratus in Australia and New Zealand, for which t0 did not differ from zero by more than 2 years. Quadratic curves were fitted to describe the relationship between the estimated lengths at each of the above ages and the latitude of the region from which samples had been collected (Figure 8). This plot demonstrates that, for each age, the lengths increased to reach a peak at ∼31° S and then declined. This parabolic relationship implies that the growth of C. auratus is greatest towards the mid-part of the latitudinal range of this species, with edge-of-range effects resulting in reduced growth towards the higher and lower latitudes of their distribution (Pörtner et al., 2001; Osovitz and Hofmann, 2007). This relationship is therefore inconsistent with the linear correlation between growth and temperature (and latitude) as predicted by the MTE. Although factors other than temperature affect growth (e.g. food availability, density and mortality), their combined influence in the case of C. auratus is clearly consistent with the overarching parabolic latitudinal relationship throughout Australasia. Thus, by implication, temperature appears to be a dominant factor influencing growth. The information provided by this study on growth variation over a wide latitudinal range will be useful for predicting the effects of climate change on this and similar teleost species occupying subtropical and temperate oceans elsewhere. Figure 8. Open in new tabDownload slide Quadratic curves fitted to estimated lengths at ages 5 (circles), 10 (squares), and 20 (triangles) years derived from von Bertalanffy growth parameters for C. auratus at various locations in Australia and New Zealand (see Table 3). Data have been restricted to those studies where t0 did not vary from zero by more than 2 years. Data for Fowler (2004) in South Australia represent the average of the four values provided. Figure 8. Open in new tabDownload slide Quadratic curves fitted to estimated lengths at ages 5 (circles), 10 (squares), and 20 (triangles) years derived from von Bertalanffy growth parameters for C. auratus at various locations in Australia and New Zealand (see Table 3). Data have been restricted to those studies where t0 did not vary from zero by more than 2 years. Data for Fowler (2004) in South Australia represent the average of the four values provided. Acknowledgements Our gratitude is extended to commercial and recreational fishers and fish processors in each region for providing fish and to staff and students from Murdoch University and staff from the Department of Fisheries, Western Australia for help with sampling. 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Comparative Biochemistry and Physiology , 69A : 237 – 242 . Google Scholar Crossref Search ADS WorldCat © International Council for the Exploration of the Sea 2016. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png ICES Journal of Marine Science Oxford University Press

Timing of growth zone formations in otoliths of the snapper, Chrysophrys auratus, in subtropical and temperate waters differ and growth follows a parabolic relationship with latitude

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Oxford University Press
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© International Council for the Exploration of the Sea 2016. All rights reserved. For Permissions, please email: journals.permissions@oup.com
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1054-3139
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1095-9289
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10.1093/icesjms/fsw137
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Abstract

Chrysophrys auratus was collected from one sub-tropical and two temperate regions spanning >2400 km along the coast of Western Australia (∼23.5–35.5° S). Marginal increment analysis demonstrated that, while a single opaque zone is formed in the otoliths of C. auratus each year, the period of deposition varies among regions. An opaque zone was formed in May to early September in the sub-tropical upper west coast, and thus when water temperatures were declining to their minima. In contrast, opaque zone formation occurred 3 months later in August to December in the temperate lower west and south coasts, when water temperatures were rising from their minima. The length and age distributions differed markedly among populations of C. auratus, with the strongest year classes varying among the three regions. Thus, it is likely that year class strength of C. auratus throughout its distribution along the coast of Western Australian is mostly related to local environmental conditions. Chrysophrys auratus grew far less rapidly and attained a smaller size in the warmer upper west coast than in the cooler temperate regions of the lower west and south coasts. A collation of data on C. auratus from ten populations in Australia and three in New Zealand showed that growth is greatest towards the mid-latitudes of its geographic range, i.e. at ∼31° S. Estimates of mean lengths at specified ages thus exhibit a parabolic relationship with latitude, with reduced growth (i.e. edge-of-range effects) occurring towards the latitudinal margins of the distribution of this sparid. Introduction The growth, maximum size and life span of individuals in the populations of many fish species vary widely among geographical regions (e.g. Boehlert and Kappenman, 1980; Robertson et al., 2005). There are also many cases where such life history characteristics vary in relation to latitude and thus water temperature (Atkinson, 1994; Kozłowski et al., 2004). These trends typically correspond to those predicted by the Metabolic Theory of Ecology (MTE), which is based on the relationship between the metabolism of poikilotherms, such as fishes, and environmental temperatures (Angilletta et al., 2004; Brown et al., 2004; Charnov and Gillooly, 2004; Lek et al., 2012). For example, the growth of a species, i.e. the rate at which the asymptotic length is approached (von Bertalanffy growth coefficient k), decreases from lower (warmer) to higher (cooler) latitudes (Atkinson, 1994; Kozłowski et al., 2004; Lek et al., 2012). However, the converse relationship between growth (i.e. length-at-age) and latitude (i.e. temperature) has been recorded for some fish species. For example, it has been proposed that the more rapid early growth of the foxfish Bodianus frenchii in a cooler than warmer environment is related to its ancestors having lived in cooler deeper waters and thus becoming better adapted to lower than higher water temperatures (Cossington et al., 2010). There are also cases where the growth characteristics of a species do not change consistently along a latitudinal gradient (e.g. similar growth curves, Hesp et al., 2004; Wakefield et al., 2013). Furthermore, it has been argued that a species would be more likely to perform better within the central part than towards the edges of its distribution, where conditions would be expected to be less favourable for that species (Pörtner et al., 2001; Osovitz and Hofmann, 2007). However, the MTE fails to capture localized ecological processes (e.g. habitat, oceanography), which may help explain departures from this ecological theory (Brown et al., 2004). The snapper Chrysophrys auratus (Forster 1801) is abundant in latitudes ranging from 25 to 40° S in temperate and sub-tropical waters of Australia and New Zealand (Paulin, 1990). This species occupies a wide range of habitats from estuaries to hypersaline environments (i.e. salinities > 70) and nearshore embayments to the upper continental slope (300 m, Woo and Fung, 1981; Moran et al., 1998; Fowler et al., 2004; Wakefield et al., 2011, 2015). As this sparid is highly valued by commercial and recreational anglers, its growth has been studied at numerous locations throughout its distribution (Paul and Tarring, 1980; Francis and Winstanley, 1989; Francis et al., 1992b; Millar et al., 1999; McGlennon et al., 2000; Jackson et al., 2010). These studies show that, in certain regions, C. auratus reaches maximum total lengths (TLs) of ∼1300 mm (1130 mm fork length, FL) and maximum ages of ∼40 years (Gomon et al., 2008; Norriss and Crisafulli, 2010). As biological data are available for several populations over its wide distribution, C. auratus provides a particularly good model for exploring the ways in which the growth characteristics of a species vary throughout an extensive latitudinal range. Despite the commercial and recreational importance of C. auratus along the extensive coastline of Western Australia, with a distributions that spans a latitude of ∼12° (Figure 1), information on the growth of this species in these waters is restricted to a single study of assemblages in the large subtropical marine embayment of Shark Bay at ∼26° S (Jackson et al., 2010). Figure 1. Open in new tabDownload slide Locations of the upper west, lower west and south coasts in Western Australia from which C. auratus was sampled. Figure 1. Open in new tabDownload slide Locations of the upper west, lower west and south coasts in Western Australia from which C. auratus was sampled. The annual formation of a single opaque and translucent growth zone in the otoliths of C. auratus has been validated, based on recaptured individuals with chemically-marked otoliths and marginal increment analyses (Ferrell et al., 1992; Francis et al., 1992b; Fowler et al., 2004; Jackson, 2007). However, the timing of opaque zone formation in the otoliths of C. auratus varies among locations. For example, this zone is formed from May to December in the otoliths of C. auratus in the subtropical waters of Shark Bay, Western Australia (Jackson, 2007), whereas in the cooler temperate waters of South Australia, an opaque zone was not recorded on the outer margin of otoliths until September and was not delineated in most fish until January (Fowler et al., 2004). In sparids, the opaque zone is typically formed during the cooler winter period and is indicative of slower growth, and the translucent zone is formed in the warmer summer period and is indicative of faster growth (Buxton and Clarke, 1991; Sarre and Potter, 2000; Hall et al., 2004; Hesp et al., 2004). Although variations in the formation of the opaque zone among conspecific populations are not well understood, variations in the timing of this formation have been found, for some sparids, to also coincide with spawning (Mann-Lang and Buxton, 1996; Sumpton, 2002). Previous studies have shown that the growth of individuals of C. auratus varies substantially throughout its distribution in Australia and New Zealand. For example, the mean asymptotic length estimated for C. auratus in South Australia at 33–35° S is typically ∼900 mm FL (Fowler et al., 2004; McGlennon, 2004), which is thus greater than the estimates of ∼750 mm FL in Shark Bay at ∼26° S (Jackson et al., 2010) and 600–700 mm FL in New Zealand at ∼37° S (Francis et al., 1992b; Gilbert and Sullivan, 1994). The extent to which the growth of C. auratus along the extensive Western Australian coastline, with a distribution covering ∼12° of latitude from subtropical to temperate waters, has yet to be investigated. Thus, while there is some evidence to suggest that C. auratus may typically grow larger toward the middle of its latitudinal range, the manner in which growth varies with respect to latitude (and thus water temperature) throughout its entire distribution, has yet to be investigated. During the present study, samples covering a wide length and age range of C. auratus were collected from along stretches of the upper west, lower west and south coasts of Western Australia (Figure 1) to: (i) Quantify variations in the timing of the formation of growth zones in otoliths in the subtropical and temperate regions. (ii) Test the hypothesis that, on the extensive Western Australian coast, the lengths at age of C. auratus approach their maximum more rapidly and have a smaller maximum size in the subtropical than temperate southern regions. (iii) Test the hypothesis that, on the basis of data collated from throughout the distribution of C. auratus, the growth of this sparid will be greatest in the middle of that latitudinal range. Material and methods Sampling regime Chrysophrys auratus was collected between 2002 and 2006 from oceanic waters on the upper west (23°30′–26°30′ S), lower west (31°00′–33°00′ S) and south coast (34°00′–35°30′ S and 115°30′–125°00′ E) regions of Western Australia (Figure 1). The majority of C. auratus were caught by line fishing (92–98% per region) with most sampled from commercial and recreational catches (90%). In all regions, these commercial and recreational line fishers targeted the same suite of demersal fish species and, throughout the year, employed a similar wide range of hook sizes and types and a variety of rig configurations. Although there may be subtle differences in the selectivity of fish among hooks of different sizes, the hooks typically used to target demersal teleosts by commercial and recreational fishers during this study were capable of sampling a very wide length range of C. auratus (Otway and Craig, 1993). Fish were sampled from commercial caches at least monthly in each region and predominantly from markets. On each of these sampling occasions, the majority or all of the C. auratus at the market were sampled. In each region, recreational catches were sampled through regular donations from fishers (see Fairclough et al., 2014) and at angling competitions. Additional samples were obtained in the two temperate regions from onboard licensed recreational fishing charter vessels. As the minimum legal length for retention (MLL) of C. auratus in Western Australia was 410 mm TL during this study, additional sampling involving line, trawl and trap fishing was undertaken by researchers to catch smaller individuals in each region. The overall sampling regime yielded substantial numbers of fish for each calendar month and, as recommended by Ricker (1969), data were pooled for subsequent analyses. Mean monthly water temperatures in the three regions were derived from in situ and satellite data at a resolution of 1° of latitude and longitude from 1982 to 2005, a period thus encompassing the lifespans of most individuals sampled (Reynolds et al., 2007). Somatic measurements and treatment of otoliths To enable comparisons of growth with other studies (e.g. Francis et al., 1992b; Coutin et al., 2003; McGlennon, 2004; Jackson et al., 2010), both the TL and FL of each C. auratus were measured to the nearest 1 mm. The sex could be determined by macroscopic examination of gonads of all but the small juveniles (∼300 mm TL, see Wakefield et al., 2015). The lengths and ages of the unsexed small juveniles were thus randomly assigned, in equal numbers, to the data sets for females and males in their respective regions. The sagittal otoliths of each C. auratus were removed, cleaned and stored dry (see Wakefield et al., 2016). The right otolith of each fish was embedded in epoxy resin and cut into ∼250–300 µm transverse sections using a low speed Isomet saw (Buehler Ltd.) with a diamond-tipped blade. The sections were rinsed in 2% hydrochloric acid for ∼15 s (see Gauldie et al., 1990) and mounted on a glass slide with a cover slip using casting resin. The opaque zones in each otolith section were counted under reflected light at 20–40 times magnification, without knowledge of the size or time of capture of the fish from which the otolith had come. The first opaque zone was readily distinguishable as its formation coincided with the development of an inflection point in the subcupular meshwork fibre zone (Francis et al., 1992a). Terminology used to describe otolith structure and marginal increment analyses follows Kalish et al. (1995) and Tuset et al. (2008). The trends exhibited by mean monthly marginal increments were used to ascertain visually whether the opaque zones in sagittal otoliths of C. auratus are typically formed annually and to compare the timing of their formation among the upper west, lower west and south coasts. The marginal increment on each otolith is the distance between the outer edge of the outermost opaque zone and the periphery of the otolith. Marginal increments were measured along an axis from the primordium to the crista inferior (ventral rim of the sulcus acusticus) of the otolith, perpendicular to the opaque zones and as close as possible to the sulcus acusticus. These measurements were made to the nearest 0.001 mm on images of the sectioned otoliths taken using a video camera (Leica DC300) attached to a dissecting microscope and employing computer imaging software (Leica IM1000). The marginal increment was then expressed as a proportion of the distance between the primordium and the outermost edge of the sole opaque zone when only one delineated opaque zone was present, and as a proportion of the distance between the outer edges of the two most recently delineated opaque zones when two or more such opaque zones were present. Each otolith was recorded as having either a translucent or opaque outer edge. The periodicity of opaque zone formation was determined using an approach described by Okamura and Semba (2009), based on the occurrence of marginal increments falling within the lower 30 percentile of values among each of the opaque zone count categories in each region. This involved fitting binomial models linked with von Mises distributions for circular data with the assumptions of no cycle, an annual cycle or a biannual cycle. The cyclic model with the smallest value of Akaike Information Criterion (AIC) was accepted as the model that best represented the marginal increment data. Ageing and growth The age of each C. auratus on its date of capture in each region was estimated using a combination of: (i) the number of delineated opaque zones, (ii) an allocated birth date corresponding to the approximate midpoint the spawning season (see Wakefield et al., 2015), (iii) the time of year when the opaque zones on the otoliths of the majority of C. auratus become delineated in that region, (iv) the year of life (i.e. first or second) when the first opaque zone is formed, determined from the progression of the discrete modes of young fish in sequential length-frequency distributions (see Wakefield et al., 2007,, 2011), and (v) the width of the marginal increment (i.e. relatively narrow or wide) in the months around the time of opaque zone delineation. The birth date selected for C. auratus in each region (1 June for the upper west coast and 1 November for the lower west and south coasts) was determined from trends exhibited by the mean monthly gonadosomatic indices and monthly prevalences of fish with gonads at stages IV (developed) and V (spawning, Wakefield et al., 2015). The von Bertalanffy growth equation was fitted to the TLs and FLs at age of females and males separately using, Lt=L∞(1−e−k(t−t0)) ⁠, where Lt=  the predicted TL or FL (mm) at age t (years), L∞ = the asymptotic length (mm), k= the growth coefficient (year−1), t= estimated age (years) and t0 = the theoretical age (years) at which fish would have zero length. The growth curves of females and males in each region and of the corresponding sex in different regions were then compared using a likelihood-ratio test (Cerrato, 1990). Results Marginal increment analyses The trend in mean monthly marginal increments of otoliths of C. auratus for all opaque zone categories in the upper west region declined rapidly from their maxima in late winter (i.e. ∼August) to their minima in mid spring (i.e. ∼October), and then increased gradually until June to August of the following year (Figure 2). Although fish with otoliths containing one opaque zone were caught in only a few months, the relative distributions of their marginal increments were consistent with those of older fish (Figure 2). Opaque zones were formed (i.e. observed on the outer margin) in otoliths of C. auratus in this region from May to early September, i.e. during the Austral winter and early spring, when water temperatures were declining and at their minima (Figures 2 and 3). In the upper west region, the period when a new translucent zone started to form on the periphery of otoliths of some fish coincided with the period when, in other fish, an opaque zone was still being formed. This resulted in marginal increments being either thin (i.e.<∼0.25) or wide (i.e.≥∼0.45) during this period. Whereby, otoliths with thin marginal increment widths possessed outer margins with a new (thin) translucent zone having just started to form, and those with wide increment widths possessed outer margins with a wide translucent or opaque outer margin (Figure 2). Irrespective of the number of opaque zones, the best fitting cyclic model to the marginal increment values of C. auratus from the upper west coast was that which assumed a single opaque zone is formed annually in otoliths (Table 1). Figure 2. Open in new tabDownload slide Individual (translucent outer margins grey circles, opaque outer margins white triangles) and mean monthly (±1 SE, lines) marginal increments in sagittal otoliths of C. auratus for each of five opaque zone categories in the upper west (left), lower west (middle) and south coast (right) regions. Figure 2. Open in new tabDownload slide Individual (translucent outer margins grey circles, opaque outer margins white triangles) and mean monthly (±1 SE, lines) marginal increments in sagittal otoliths of C. auratus for each of five opaque zone categories in the upper west (left), lower west (middle) and south coast (right) regions. Figure 3. Open in new tabDownload slide Mean monthly sea surface temperatures (±1 SE) for the (i) upper west, (ii) lower west, and (iii) south coast regions off Western Australia. Data were derived from mean daily values recorded between 1982 and 2005 (Reynolds et al., 2007). On the x-axis, the closed rectangles represent winter and summer months and the open rectangles autumn and spring months. Figure 3. Open in new tabDownload slide Mean monthly sea surface temperatures (±1 SE) for the (i) upper west, (ii) lower west, and (iii) south coast regions off Western Australia. Data were derived from mean daily values recorded between 1982 and 2005 (Reynolds et al., 2007). On the x-axis, the closed rectangles represent winter and summer months and the open rectangles autumn and spring months. Table 1 AIC values for binomial models assuming three different cycles (i.e. no, annual, or biannual cycle) fitted to the monthly proportions of C. auratus otoliths with marginal increments within the lower 30 percentile for each opaque zone category in each region Region . Opaque zones . No cycle . Annual cycle . Biannual cycle . Upper west coast 1 zone 52.1 38.3 45.1 2–3 zones 297.2 207.1 296.7 4–5 zones 406.5 254.4 401.4 6–7 zones 194.2 154.2 186.3 ≥8 zones 231.6 196.9 229.9 Lower west coast 1 zone 72.0 66.1 71.9 2–3 zones 284.3 235.0 284.0 4–5 zones 427.1 350.4 428.8 6–7 zones 265.1 186.1 239.6 ≥8 zones 515.7 504.6 483.2 South Coast 1 zone 72.0 43.3 66.5 2–3 zones 101.7 67.0 88.4 4–5 zones 187.3 112.0 161.4 6–7 zones 467.5 276.7 451.0 ≥8 zones 398.1 199.0 343.9 Region . Opaque zones . No cycle . Annual cycle . Biannual cycle . Upper west coast 1 zone 52.1 38.3 45.1 2–3 zones 297.2 207.1 296.7 4–5 zones 406.5 254.4 401.4 6–7 zones 194.2 154.2 186.3 ≥8 zones 231.6 196.9 229.9 Lower west coast 1 zone 72.0 66.1 71.9 2–3 zones 284.3 235.0 284.0 4–5 zones 427.1 350.4 428.8 6–7 zones 265.1 186.1 239.6 ≥8 zones 515.7 504.6 483.2 South Coast 1 zone 72.0 43.3 66.5 2–3 zones 101.7 67.0 88.4 4–5 zones 187.3 112.0 161.4 6–7 zones 467.5 276.7 451.0 ≥8 zones 398.1 199.0 343.9 Bold font denotes best model fit based on smallest AIC value. Table 1 AIC values for binomial models assuming three different cycles (i.e. no, annual, or biannual cycle) fitted to the monthly proportions of C. auratus otoliths with marginal increments within the lower 30 percentile for each opaque zone category in each region Region . Opaque zones . No cycle . Annual cycle . Biannual cycle . Upper west coast 1 zone 52.1 38.3 45.1 2–3 zones 297.2 207.1 296.7 4–5 zones 406.5 254.4 401.4 6–7 zones 194.2 154.2 186.3 ≥8 zones 231.6 196.9 229.9 Lower west coast 1 zone 72.0 66.1 71.9 2–3 zones 284.3 235.0 284.0 4–5 zones 427.1 350.4 428.8 6–7 zones 265.1 186.1 239.6 ≥8 zones 515.7 504.6 483.2 South Coast 1 zone 72.0 43.3 66.5 2–3 zones 101.7 67.0 88.4 4–5 zones 187.3 112.0 161.4 6–7 zones 467.5 276.7 451.0 ≥8 zones 398.1 199.0 343.9 Region . Opaque zones . No cycle . Annual cycle . Biannual cycle . Upper west coast 1 zone 52.1 38.3 45.1 2–3 zones 297.2 207.1 296.7 4–5 zones 406.5 254.4 401.4 6–7 zones 194.2 154.2 186.3 ≥8 zones 231.6 196.9 229.9 Lower west coast 1 zone 72.0 66.1 71.9 2–3 zones 284.3 235.0 284.0 4–5 zones 427.1 350.4 428.8 6–7 zones 265.1 186.1 239.6 ≥8 zones 515.7 504.6 483.2 South Coast 1 zone 72.0 43.3 66.5 2–3 zones 101.7 67.0 88.4 4–5 zones 187.3 112.0 161.4 6–7 zones 467.5 276.7 451.0 ≥8 zones 398.1 199.0 343.9 Bold font denotes best model fit based on smallest AIC value. On the lower west coast, the mean monthly marginal increments on otoliths with ≥ two opaque zones declined from their maxima in late spring to early summer, i.e. November to December, to their minima in mid-summer, i.e. January, and then increased to >0.6 in the December of the following year (Figure 2). The trends exhibited by the marginal increments on the otoliths of C. auratus on the south coast were similar to those of fish on the lower west coast (Table 1, Figure 2). The marginal increments in the otoliths of fish caught on the lower west coast also exhibited predominantly an annual cycle (Table 1), with opaque zone deposition occurring later than those on the upper west coast (i.e. August to December rather than May to September), when water temperatures were at or increasing from their minima (Figures 2 and 3). Thus, irrespective of the number of opaque zones, the marginal increments on the otoliths of C. auratus from each region underwent a single pronounced decline and then rise each year. This was consistent with the best fitting model to the marginal increment data being a single annual cycle, which strongly indicates that one opaque and one translucent zone is formed annually and that opaque zones in otoliths can be considered annuli when determining ages of C. auratus. Length- and age-frequency compositions In a previous study of C. auratus in waters near those sampled on the upper west coast, small 0+ aged fish (45–95 mm TL) were first caught in November, i.e. 4–5 months after spawning peaked (Wakefield et al., 2007). This cohort could be clearly traced through length-frequency distributions from sequential monthly samples from November until December of the following year. During the present study, small fish (120–129 mm TL) were first caught in March, which were similar in size to those sampled by Wakefield et al. (2007) at the same time of year. Although an opaque zone was not present in the otoliths of small fish caught in March (and also June), it was present in those of this cohort in September. The first opaque zone in the otoliths of C. auratus from the upper west coast thus becomes delineated in the second spring of life, when individuals are ∼14-months old (Figure 2). Substantial numbers of small 0+ C. auratus (68–96 mm TL) were first caught on the lower west coast in February, i.e. only 3 months after spawning peaked. Although the otoliths of none of these fish contained an opaque zone, it was present in fish of this cohort collected after December. As the above trends were paralleled by those of small fish on the south coast, the first opaque zone in the otoliths of C. auratus on both the lower west and south coasts becomes delineated when individuals are about 1-year old (Figure 2). The length- and age-frequency distributions of female and male C. auratus caught within any given region from 2002 to 2006 were similar, but differed markedly between regions (Figure 4). The length-frequency distributions for females and males from the upper west coast each contained a very broad mode, with the lengths of the majority of fish lying just to the right of the MLL of 410 mm TL. The length-frequency distributions for females and males on the lower west coast each contained a well-defined mode just to the right of the MLL, and another at a length slightly >800 mm TL and thus exceeding that of virtually all fish caught on the upper west coast (Figure 4). The length-frequency distributions for females and males on the south coast contained a single well defined mode just above 600 mm TL, lying between the two modes for the lower west coast and far greater than the MLL (Figure 4). As most samples came from commercial and recreational fishers, the numbers of C. auratus < the MLL of 410 mm were relatively small. Figure 4. Open in new tabDownload slide Length- (left) and age-frequency (right) histograms for female (white bars) and male (grey bars) C. auratus caught in the upper west, lower west and south coast regions of Western Australia from 2002 to 2006. Distributions do not include the small fish that could not be sexed macroscopically. Dashed line represents the minimum legal length for retention of 410 mm TL. Figure 4. Open in new tabDownload slide Length- (left) and age-frequency (right) histograms for female (white bars) and male (grey bars) C. auratus caught in the upper west, lower west and south coast regions of Western Australia from 2002 to 2006. Distributions do not include the small fish that could not be sexed macroscopically. Dashed line represents the minimum legal length for retention of 410 mm TL. The age compositions for female and male C. auratus in each region between 2002 and 2006 broadly parallel the trends exhibited by the length-frequency distributions (Figure 4). Thus, the age distribution for the two sexes on the upper west coast had a broad single mode at 4–6 years, whereas that for the lower west coast was bimodal at 4 and 11 years, and the south coast was unimodal at 7–8 years (Figure 4). The annual age-frequency distributions for C. auratus were used to track the relative abundance of age cohorts within and among regions from 2003 to 2005 (Figure 5). On the upper west coast, the relatively strong 1991 year class declined in abundance from 2003 to 2004 and had essentially disappeared by 2005. This accounts for the age-frequency distributions changing between 2003 and 2005 from bimodal to unimodal and the percentage of fish > 8-years old declining from 41.2 to only 10.5% (Figure 5). Figure 5. Open in new tabDownload slide Percentage frequency of occurrence of sequential age cohorts of C. auratus (≥MLL of 410 mm TL) caught in the upper west, lower west (white bars, outside Cockburn Sound; grey bars, inside Cockburn Sound) and south coast regions of Western Australia in three sequential years. Figure 5. Open in new tabDownload slide Percentage frequency of occurrence of sequential age cohorts of C. auratus (≥MLL of 410 mm TL) caught in the upper west, lower west (white bars, outside Cockburn Sound; grey bars, inside Cockburn Sound) and south coast regions of Western Australia in three sequential years. The vast majority of C. auratus caught outside the embayment of Cockburn Sound on the lower west coast in 2002–2004 were < 6-years old, with the 2000 year class the most abundant in the latter 2 years (Figure 5). In contrast, virtually all fish caught by commercial and recreational fishing in Cockburn Sound in those 3 years were >6 years old (Figure 5). The 1991 and 1992 year classes were the most abundant in Cockburn Sound in 2002, but both of these, and particularly the 1992 year class, declined progressively over the next 2 years (Figure 5). The 1996 year class was by far the strongest in the catches of C. auratus on the south coast in 2003–2005 (Figure 5). The maximum lengths and ages of C. auratus were 864 mm and 30 years on the upper west coast, 1056 mm and 29 years on the lower west coast and 1083 mm and 38 years on the south coast (Table 2). Table 2 von Bertalanffy growth parameters (estimates and associated 95% upper and lower confidence limits) for curves fitted to the TLs at age of female and male C. auratus in three regions of Western Australia . . TL∞(mm) . k (y−1) . t0(y) . Amax . Lmax . n . r2 . Upper west coast  Female   Estimate 691 0.21 −0.33 30.0 864 639 0.90   Upper 709 0.23 −0.22   Lower 672 0.20 −0.45  Male   Estimate 648 0.25 −0.25 22.1 840 545 0.88   Upper 670 0.27 −0.13   Lower 626 0.22 −0.37 Lower west coast  Female   Estimate 1150 0.12 −0.41 24.8 1051 872 0.89   Upper 1199 0.13 −0.22   Lower 1101 0.11 −0.60  Male   Estimate 1127 0.12 −0.46 28.8 1056 802 0.88   Upper 1178 0.13 −0.24   Lower 1075 0.11 −0.67 South coast  Female   Estimate 1013 0.11 −0.94 29.4 1083 896 0.83   Upper 1065 0.13 −0.65   Lower 962 0.10 −1.23  Male   Estimate 950 0.13 −0.61 37.8 999 818 0.84   Upper 989 0.15 −0.36   Lower 911 0.12 −0.86 Sexes combined  Upper west coast   Estimate 681 0.22 −0.42 1152 0.87   Upper 695 0.23 −0.31   Lower 666 0.20 −0.53  Lower west coast   Estimate 1136 0.12 −0.42 1689 0.88   Upper 1171 0.13 −0.27   Lower 1101 0.11 −0.56  South coast   Estimate 986 0.11 −1.22 2094 0.84   Upper 1017 0.12 −1.05   Lower 956 0.11 −1.38 . . TL∞(mm) . k (y−1) . t0(y) . Amax . Lmax . n . r2 . Upper west coast  Female   Estimate 691 0.21 −0.33 30.0 864 639 0.90   Upper 709 0.23 −0.22   Lower 672 0.20 −0.45  Male   Estimate 648 0.25 −0.25 22.1 840 545 0.88   Upper 670 0.27 −0.13   Lower 626 0.22 −0.37 Lower west coast  Female   Estimate 1150 0.12 −0.41 24.8 1051 872 0.89   Upper 1199 0.13 −0.22   Lower 1101 0.11 −0.60  Male   Estimate 1127 0.12 −0.46 28.8 1056 802 0.88   Upper 1178 0.13 −0.24   Lower 1075 0.11 −0.67 South coast  Female   Estimate 1013 0.11 −0.94 29.4 1083 896 0.83   Upper 1065 0.13 −0.65   Lower 962 0.10 −1.23  Male   Estimate 950 0.13 −0.61 37.8 999 818 0.84   Upper 989 0.15 −0.36   Lower 911 0.12 −0.86 Sexes combined  Upper west coast   Estimate 681 0.22 −0.42 1152 0.87   Upper 695 0.23 −0.31   Lower 666 0.20 −0.53  Lower west coast   Estimate 1136 0.12 −0.42 1689 0.88   Upper 1171 0.13 −0.27   Lower 1101 0.11 −0.56  South coast   Estimate 986 0.11 −1.22 2094 0.84   Upper 1017 0.12 −1.05   Lower 956 0.11 −1.38 TL∞, hypothetical asymptotic length at an infinite age; k, growth coefficient; t0, hypothetical age at zero length; Amax, maximum age; Lmax, maximum length; n, sample size (juveniles and adults combined); r2, coefficient of determination. Table 2 von Bertalanffy growth parameters (estimates and associated 95% upper and lower confidence limits) for curves fitted to the TLs at age of female and male C. auratus in three regions of Western Australia . . TL∞(mm) . k (y−1) . t0(y) . Amax . Lmax . n . r2 . Upper west coast  Female   Estimate 691 0.21 −0.33 30.0 864 639 0.90   Upper 709 0.23 −0.22   Lower 672 0.20 −0.45  Male   Estimate 648 0.25 −0.25 22.1 840 545 0.88   Upper 670 0.27 −0.13   Lower 626 0.22 −0.37 Lower west coast  Female   Estimate 1150 0.12 −0.41 24.8 1051 872 0.89   Upper 1199 0.13 −0.22   Lower 1101 0.11 −0.60  Male   Estimate 1127 0.12 −0.46 28.8 1056 802 0.88   Upper 1178 0.13 −0.24   Lower 1075 0.11 −0.67 South coast  Female   Estimate 1013 0.11 −0.94 29.4 1083 896 0.83   Upper 1065 0.13 −0.65   Lower 962 0.10 −1.23  Male   Estimate 950 0.13 −0.61 37.8 999 818 0.84   Upper 989 0.15 −0.36   Lower 911 0.12 −0.86 Sexes combined  Upper west coast   Estimate 681 0.22 −0.42 1152 0.87   Upper 695 0.23 −0.31   Lower 666 0.20 −0.53  Lower west coast   Estimate 1136 0.12 −0.42 1689 0.88   Upper 1171 0.13 −0.27   Lower 1101 0.11 −0.56  South coast   Estimate 986 0.11 −1.22 2094 0.84   Upper 1017 0.12 −1.05   Lower 956 0.11 −1.38 . . TL∞(mm) . k (y−1) . t0(y) . Amax . Lmax . n . r2 . Upper west coast  Female   Estimate 691 0.21 −0.33 30.0 864 639 0.90   Upper 709 0.23 −0.22   Lower 672 0.20 −0.45  Male   Estimate 648 0.25 −0.25 22.1 840 545 0.88   Upper 670 0.27 −0.13   Lower 626 0.22 −0.37 Lower west coast  Female   Estimate 1150 0.12 −0.41 24.8 1051 872 0.89   Upper 1199 0.13 −0.22   Lower 1101 0.11 −0.60  Male   Estimate 1127 0.12 −0.46 28.8 1056 802 0.88   Upper 1178 0.13 −0.24   Lower 1075 0.11 −0.67 South coast  Female   Estimate 1013 0.11 −0.94 29.4 1083 896 0.83   Upper 1065 0.13 −0.65   Lower 962 0.10 −1.23  Male   Estimate 950 0.13 −0.61 37.8 999 818 0.84   Upper 989 0.15 −0.36   Lower 911 0.12 −0.86 Sexes combined  Upper west coast   Estimate 681 0.22 −0.42 1152 0.87   Upper 695 0.23 −0.31   Lower 666 0.20 −0.53  Lower west coast   Estimate 1136 0.12 −0.42 1689 0.88   Upper 1171 0.13 −0.27   Lower 1101 0.11 −0.56  South coast   Estimate 986 0.11 −1.22 2094 0.84   Upper 1017 0.12 −1.05   Lower 956 0.11 −1.38 TL∞, hypothetical asymptotic length at an infinite age; k, growth coefficient; t0, hypothetical age at zero length; Amax, maximum age; Lmax, maximum length; n, sample size (juveniles and adults combined); r2, coefficient of determination. Growth The von Bertalanffy growth curves provided good fits to the TLs at age for female and male C. auratus in each region, with each coefficient of determination ≥ 0.83 (Table 2). The parameters for the growth curves fitted to the FLs at age for these fish and for those from elsewhere throughout Australia and New Zealand are given in Table 3. Although growth of the two sexes in each region differed significantly (p < 0.05, Figures 6 and 7), the differences in the lengths estimated from the von Bertalanffy equation for the females and males of C. auratus at any age in each region were small. For example, the differences between the sexes at 15 years of age were only 4.5, 1.4, and 1.9% for the upper west, lower west, and south coasts, respectively, and sex differences are thus considered of limited biological significance. Figure 6. Open in new tabDownload slide von Bertalanffy growth curves fitted to the lengths-at-age of unsexed (white triangles), female (grey circles, left) and male (grey circles, right) C. auratus caught in the upper west, lower west, and south coast regions of Western Australia. Figure 6. Open in new tabDownload slide von Bertalanffy growth curves fitted to the lengths-at-age of unsexed (white triangles), female (grey circles, left) and male (grey circles, right) C. auratus caught in the upper west, lower west, and south coast regions of Western Australia. Figure 7. Open in new tabDownload slide von Bertalanffy growth curves (±95% CIs) for female and male C. auratus from the upper west, lower west and south coast regions of Western Australia. Figure 7. Open in new tabDownload slide von Bertalanffy growth curves (±95% CIs) for female and male C. auratus from the upper west, lower west and south coast regions of Western Australia. Table 3 von Bertalanffy growth parameters for curves fitted to the FLs at age of C. auratus in Australia and New Zealand and derived using fish that had been aged using the growth zones in sectioned otoliths Location . Latitude (ºS) . FL∞ (mm) . k (y−1) . t0 (y) . References . Western Australia  Eastern Gulf, Shark Bay 25.8 755 0.17 −0.02 Jackson et al. (2010)  Denham Sound, Shark Bay 25.3 728 0.15 −0.08  Freycinet Estuary, Shark Bay 26.4 770 0.17 0.1  Upper west coast 25.0 589 0.22 −0.23 This study  Lower west coast 32.0 1004 0.12 −0.23  South coast 35.0 876 0.11 −1.15  Lower mid-west coast 28.0 682 0.15 −1.26 Wise et al. (2007) and  South west coast 33.5 805 0.13 −1.05 Lenanton et al. (2009) South Australia  Various 902 0.19 −2.86 McGlennon (2004)  Northern Spencer Gulf 33.3 984 0.12 −0.35 Fowler et al. (2004)  Southern Spencer Gulf 34.6 632 0.21 −0.11  Northern Gulf St Vincent 34.6 907 0.16 −0.08  Southern Gulf St Vincent 35.4 895 0.13 −0.06 Victoria  Various 38.1 857 0.12 0 Coutin et al. (2003) New South Wales  Various 34.0 574 0.14 −2.29 Ferrell (2004) Queensland  Various 27.3 792 0.08 −2.45 Sumpton (2002) New Zealand  Various 720 0.11 −0.75 Francis et al. (1992b)  Hauraki Gulf 36.6 382 0.23 −0.61 Vooren and Coombs (1977)  Hauraki Gulf 36.6 588 0.10 −1.11 Gilbert and Sullivan (1994)  West coast 37.5 667 0.16 −0.11 Location . Latitude (ºS) . FL∞ (mm) . k (y−1) . t0 (y) . References . Western Australia  Eastern Gulf, Shark Bay 25.8 755 0.17 −0.02 Jackson et al. (2010)  Denham Sound, Shark Bay 25.3 728 0.15 −0.08  Freycinet Estuary, Shark Bay 26.4 770 0.17 0.1  Upper west coast 25.0 589 0.22 −0.23 This study  Lower west coast 32.0 1004 0.12 −0.23  South coast 35.0 876 0.11 −1.15  Lower mid-west coast 28.0 682 0.15 −1.26 Wise et al. (2007) and  South west coast 33.5 805 0.13 −1.05 Lenanton et al. (2009) South Australia  Various 902 0.19 −2.86 McGlennon (2004)  Northern Spencer Gulf 33.3 984 0.12 −0.35 Fowler et al. (2004)  Southern Spencer Gulf 34.6 632 0.21 −0.11  Northern Gulf St Vincent 34.6 907 0.16 −0.08  Southern Gulf St Vincent 35.4 895 0.13 −0.06 Victoria  Various 38.1 857 0.12 0 Coutin et al. (2003) New South Wales  Various 34.0 574 0.14 −2.29 Ferrell (2004) Queensland  Various 27.3 792 0.08 −2.45 Sumpton (2002) New Zealand  Various 720 0.11 −0.75 Francis et al. (1992b)  Hauraki Gulf 36.6 382 0.23 −0.61 Vooren and Coombs (1977)  Hauraki Gulf 36.6 588 0.10 −1.11 Gilbert and Sullivan (1994)  West coast 37.5 667 0.16 −0.11 FL∞, hypothetical asymptotic length at age infinity; k, growth coefficient; t0, hypothetical age at zero length. Table 3 von Bertalanffy growth parameters for curves fitted to the FLs at age of C. auratus in Australia and New Zealand and derived using fish that had been aged using the growth zones in sectioned otoliths Location . Latitude (ºS) . FL∞ (mm) . k (y−1) . t0 (y) . References . Western Australia  Eastern Gulf, Shark Bay 25.8 755 0.17 −0.02 Jackson et al. (2010)  Denham Sound, Shark Bay 25.3 728 0.15 −0.08  Freycinet Estuary, Shark Bay 26.4 770 0.17 0.1  Upper west coast 25.0 589 0.22 −0.23 This study  Lower west coast 32.0 1004 0.12 −0.23  South coast 35.0 876 0.11 −1.15  Lower mid-west coast 28.0 682 0.15 −1.26 Wise et al. (2007) and  South west coast 33.5 805 0.13 −1.05 Lenanton et al. (2009) South Australia  Various 902 0.19 −2.86 McGlennon (2004)  Northern Spencer Gulf 33.3 984 0.12 −0.35 Fowler et al. (2004)  Southern Spencer Gulf 34.6 632 0.21 −0.11  Northern Gulf St Vincent 34.6 907 0.16 −0.08  Southern Gulf St Vincent 35.4 895 0.13 −0.06 Victoria  Various 38.1 857 0.12 0 Coutin et al. (2003) New South Wales  Various 34.0 574 0.14 −2.29 Ferrell (2004) Queensland  Various 27.3 792 0.08 −2.45 Sumpton (2002) New Zealand  Various 720 0.11 −0.75 Francis et al. (1992b)  Hauraki Gulf 36.6 382 0.23 −0.61 Vooren and Coombs (1977)  Hauraki Gulf 36.6 588 0.10 −1.11 Gilbert and Sullivan (1994)  West coast 37.5 667 0.16 −0.11 Location . Latitude (ºS) . FL∞ (mm) . k (y−1) . t0 (y) . References . Western Australia  Eastern Gulf, Shark Bay 25.8 755 0.17 −0.02 Jackson et al. (2010)  Denham Sound, Shark Bay 25.3 728 0.15 −0.08  Freycinet Estuary, Shark Bay 26.4 770 0.17 0.1  Upper west coast 25.0 589 0.22 −0.23 This study  Lower west coast 32.0 1004 0.12 −0.23  South coast 35.0 876 0.11 −1.15  Lower mid-west coast 28.0 682 0.15 −1.26 Wise et al. (2007) and  South west coast 33.5 805 0.13 −1.05 Lenanton et al. (2009) South Australia  Various 902 0.19 −2.86 McGlennon (2004)  Northern Spencer Gulf 33.3 984 0.12 −0.35 Fowler et al. (2004)  Southern Spencer Gulf 34.6 632 0.21 −0.11  Northern Gulf St Vincent 34.6 907 0.16 −0.08  Southern Gulf St Vincent 35.4 895 0.13 −0.06 Victoria  Various 38.1 857 0.12 0 Coutin et al. (2003) New South Wales  Various 34.0 574 0.14 −2.29 Ferrell (2004) Queensland  Various 27.3 792 0.08 −2.45 Sumpton (2002) New Zealand  Various 720 0.11 −0.75 Francis et al. (1992b)  Hauraki Gulf 36.6 382 0.23 −0.61 Vooren and Coombs (1977)  Hauraki Gulf 36.6 588 0.10 −1.11 Gilbert and Sullivan (1994)  West coast 37.5 667 0.16 −0.11 FL∞, hypothetical asymptotic length at age infinity; k, growth coefficient; t0, hypothetical age at zero length. The von Bertalanffy growth curves for both females and males on the upper west coast were significantly different from those on the lower west coast (p < 0.001), and these differed from those of the corresponding sex on the south coast (p < 0.01). These differences reflected, in part, marked variations in the asymptotic lengths (L∞), with those for females, for example, being far lower on the upper west coast (674 mm) than on both the lower west (1150 mm) and south coasts (1013 mm, Table 2). In contrast, the growth coefficient (k) for females was greater on the upper west coast (0.23 year−1) than on both the lower west (0.12 year−1) and south coasts (0.11 year−1). These differences in growth parameters are reflected in an increasing divergence of the growth curves for those regions after fish have reached 3 years of age, and particularly with those from the upper west coast (Figure 7). Discussion Formation of opaque zones in otoliths Marginal increment analyses on otoliths of C. auratus from different climatic regions of Western Australia demonstrated that an opaque zone has typically become fully formed in otoliths from the upper west coast by October, and thus 3 months earlier than in those of populations on both the lower west and south coasts. In the sub-tropical upper west coast, opaque zones were deposited in otoliths from May to September and therefore when water temperatures were declining to their minima. In contrast, the opaque zones in the otoliths of C. auratus from the two more southern and temperate regions were deposited 3 months later, i.e. from August to December, when water temperatures were rising from their minima. The variations in the timing of opaque zone formation of C. auratus in Western Australia are consistent with results of previous studies of this species from locations throughout Australasia (Ferrell et al., 1992; Francis et al., 1992b; Fowler et al., 2004). For example, the outer edge of the opaque zone has typically become delineated in otoliths of C. auratus by November and December in New South Wales (Ferrell et al., 1992; Ferrell, 2004) and New Zealand (Francis et al., 1992b), respectively, but this often does not occur until February in South Australia (Fowler et al., 2004). By definition, the ability to detect delineation of a newly-formed opaque zone on the outer margin of otoliths relies on the deposition of a certain amount of translucent material. It is likely that any time lag associated with the detection of opaque zone delineation was very similar for C. auratus among the upper west, lower west and south coasts. Thus, the observed regional variation in the timing of opaque zone formation (and delineation) reflects responses to other factors. In particular, the timing of opaque zone formation in each region corresponds closely with both water temperature minima and spawning periods of this species (Wakefield et al., 2015). In the co-occurring Rhabdosargus sarba, which spawns at a similar time in coastal waters on the upper west and lower west coasts, there is little difference in the timing of opaque zone delineation, which is consistent with the view that reproduction may influence opaque zone formation in some sparids (Hesp and Potter, 2003; Hesp et al., 2004). Furthermore, Mann-Lang and Buxton (1996) provided evidence that the timing of opaque zone formation in otoliths of several South African sparids is closely associated with reproduction. Variations in year class strength This study showed that the length composition of populations of C. auratus differed markedly among the three regions. As would be expected for samples collected from commercial and recreational fishers in a heavily-exploited fishery, the numbers of C. auratus caught on the lower west and upper west coasts increased sharply as the TL reached the MLL of 410 mm for this species. However, this was not the case on the south coast, where the numbers of C. auratus did not start rising conspicuously until the lengths of fish were well above the MLL. These differences are due to the fact that, unlike the situation on the upper and lower west coasts, the younger age classes, i.e. 3+, 4+, and 5+, were not well represented on the south coast. Although C. auratus is considered to be genetically homogenous along the west coast of Australia, otolith microchemistry indicates that adults do not typically mix over scales of hundreds of kilometres (Gardner and Chaplin, 2011; Fairclough et al., 2013). This is supported by tag-recapture studies in the upper west and lower west coasts, where the vast majority of adults were recaptured within ∼35 km from their tagged location (Moran et al., 2003; Wakefield et al., 2011). The very pronounced bimodality in the length-frequency distributions for C. auratus on the lower west coast, contrasts markedly with the forms of the distributions in both the upper west and south coasts. The greater abundances of larger and older C. auratus on the lower west coast were due to the presence of large mature individuals that aggregate and spawn each year in Cockburn and Warnbro Sounds (Wakefield, 2010; Wakefield et al., 2011, 2015; Breheny et al., 2012). However, the migration ranges of the large majority of these spawning individuals include adjacent shelf waters within a few hundred kilometres, and thus well within the lower west coast region (Wakefield et al., 2011). The age-frequency data for the upper west coast showed that, in 2003–2005, C. auratus started to be caught in substantial numbers when they had reached an age approximating that at which the MLL is attained, i.e. 4 years of age, with their numbers peaking 1 year later. The very marked decline in the proportion of fish >8-years old in the samples collected between 2003 and both 2004 and 2005 suggests that fishing has had a pronounced influence on the abundance of the older age classes. This marked decline reflects the reduced abundances, of the initially very strong 1991 year class (and also 1992 year class) between 2003 and 2005. The age-frequency distributions for C. auratus in Cockburn Sound on the lower west coast demonstrated that the catches of fish >7-years old were dominated by the 1991 and 1992 year classes in 2002–2004, together with the 1996 year class in the last 2 of those years. Considering the 1996 year class did not become abundant in the catches until 2003, it appears that the individuals of C. auratus do not typically aggregate in Cockburn Sound until they are ∼7 years of age (see Wakefield et al., 2011). Although relatively few older fish were caught in Cockburn Sound in 2002, 2003, or 2004, the 1991 year class, which corresponded to the oldest cohort that was well represented in each year, remained relatively strong throughout those 3 years. The fact that, on the south coast, the catches were dominated to such a marked extent by the 1996 year class and those for some years were very low is consistent with the evidence that spawning and/or recruitment on this coast is very limited in some years (Wakefield et al., 2015). Fowler and Jennings (2003) found that recruitment success in South Australia was greatest in years when summer water temperatures were warmest. This parallels the situation in Port Phillip Bay in Victoria, which is a major nursery area for C. auratus in that state (Hamer and Jenkins, 2004; Hamer et al., 2005). Moreover, in New Zealand, the recruitment of C. auratus was positively correlated with water temperature in the 5 months following spawning (Francis, 1993; Francis et al., 1997). It is noteworthy that the 1991 year class was particularly strong on the upper west coast, Cockburn Sound and probably on the south coast, and also in South Australia (Fowler et al., 2005; Fowler and McGlennon, 2011). This suggests that, in that year, environmental conditions were especially favourable for recruitment throughout Western Australia and South Australia. Although, water temperatures on the upper west coast were above average in the winter of 1991, when spawning would have occurred, those for the subsequent spring and early summer months were the coldest recorded for those years (Reynolds et al., 2007). Water temperatures in spring and early summer on the lower west and south coasts in 1991, when spawning and settlement would have occurred, were the lowest since 1990, but were highest in those months in 1996, when, like 1991, recruitment was particularly strong (Reynolds et al., 2007). Moreover, years of strong recruitment do not consistently coincide with the annual strength of either the La Niña (stronger current) or El Niño (weaker current) events that influence the nutrient poor, southward-flowing Leeuwin current that dominates the oceanographic climate along the west coast of Australia (Feng et al., 2003). It thus appears that, on rare occasions, favourable environmental conditions coincide over large areas of this species distribution and result in consistent strong annual recruitment within discreet spawning/nursery areas (i.e. 1991, see Wakefield, 2010). However, it is more typical that localized environmental conditions within the discrete spawning and nursery areas for C. auratus influence annual recruitment abundance. Comparisons of growth with other snapper populations As hypothesized, C. auratus grew more rapidly in the temperate region of the lower west coast than in the sub-tropical region of the upper west coast of Australia. For example, by 10 years of age, females had reached 820 mm on the lower west coast compared with only 610 mm on the upper west coast. Furthermore, the maximum total length was far greater for the population on the lower west than upper west coast, i.e. 1056 vs. 864 mm. C.auratus also grows far more rapidly and reaches a larger size on the temperate south coast than subtropical upper west coast of Western Australia with females, on average, attaining a total length of 720 mm at 10 years of age and having a maximum length of 1083 mm. As C.auratus has a relatively large latitudinal range and its growth is well documented, it provided an excellent model for studying the relationship between the growth of a demersal teleost and water temperature. FLs at 5, 10, and 20 years were calculated from the growth curves for C. auratus in Australia and New Zealand, for which t0 did not differ from zero by more than 2 years. Quadratic curves were fitted to describe the relationship between the estimated lengths at each of the above ages and the latitude of the region from which samples had been collected (Figure 8). This plot demonstrates that, for each age, the lengths increased to reach a peak at ∼31° S and then declined. This parabolic relationship implies that the growth of C. auratus is greatest towards the mid-part of the latitudinal range of this species, with edge-of-range effects resulting in reduced growth towards the higher and lower latitudes of their distribution (Pörtner et al., 2001; Osovitz and Hofmann, 2007). This relationship is therefore inconsistent with the linear correlation between growth and temperature (and latitude) as predicted by the MTE. Although factors other than temperature affect growth (e.g. food availability, density and mortality), their combined influence in the case of C. auratus is clearly consistent with the overarching parabolic latitudinal relationship throughout Australasia. Thus, by implication, temperature appears to be a dominant factor influencing growth. The information provided by this study on growth variation over a wide latitudinal range will be useful for predicting the effects of climate change on this and similar teleost species occupying subtropical and temperate oceans elsewhere. Figure 8. Open in new tabDownload slide Quadratic curves fitted to estimated lengths at ages 5 (circles), 10 (squares), and 20 (triangles) years derived from von Bertalanffy growth parameters for C. auratus at various locations in Australia and New Zealand (see Table 3). Data have been restricted to those studies where t0 did not vary from zero by more than 2 years. Data for Fowler (2004) in South Australia represent the average of the four values provided. Figure 8. Open in new tabDownload slide Quadratic curves fitted to estimated lengths at ages 5 (circles), 10 (squares), and 20 (triangles) years derived from von Bertalanffy growth parameters for C. auratus at various locations in Australia and New Zealand (see Table 3). Data have been restricted to those studies where t0 did not vary from zero by more than 2 years. Data for Fowler (2004) in South Australia represent the average of the four values provided. Acknowledgements Our gratitude is extended to commercial and recreational fishers and fish processors in each region for providing fish and to staff and students from Murdoch University and staff from the Department of Fisheries, Western Australia for help with sampling. 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Comparative Biochemistry and Physiology , 69A : 237 – 242 . Google Scholar Crossref Search ADS WorldCat © International Council for the Exploration of the Sea 2016. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

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