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Temporal variations of fish assemblage in the surf zone of the Nakdong River Estuary, southeastern Korea Temporal variations of fish assemblage in the surf zone of the Nakdong River Estuary,...
Park, Joo Myun; Huh, Sung-Hoi; Baeck, Gun Wook
2015-09-03 00:00:00
Animal Cells and Systems, 2015 Vol. 19, No. 5, 350–358, http://dx.doi.org/10.1080/19768354.2015.1082930 Temporal variations of fish assemblage in the surf zone of the Nakdong River Estuary, southeastern Korea a b c Joo Myun Park , Sung-Hoi Huh and Gun Wook Baeck * a b Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia; Department of Oceanography, Pukyong National University, Busan 608-737, Korea; Department of Marine Biology & Aquaculture, College of Marine Science, Institute of Marine Industry, Gyeongsang National University, Tongyeong 650-160, Korea (Received 17 April 2015; received in revised form 22 June 2015; accepted 26 July 2015) To assess temporal variations in fish assemblages, fish samples were collected on a monthly basis in the surf zone of the Nakdong River Estuary in 2004. Surveys were conducted at two sites during day and night neap tides using a 10-m beach seine. In total, 2397 specimens (8146.0 g) comprising 27 species and 16 families were collected during the study period. The dominant fish species were Mugil cephalus, Plecoglossus altivelis, Favonigobius gymnauchen, Opsariichthys uncirostris amurensis, and Hyporhamphus sajori, which accounted for 84.6% of the total number of individuals. The fish sampled were primarily small species or the early juveniles of larger species. The species composition and abundance varied greatly, with a peak in the number of individuals in July (summer) and January (winter), and the peak in biomass during July (summer). The catch rate was low in the spring and autumn months due to the low abundance of the dominant species recorded. A cluster analysis revealed that the fish assemblages were characterized by distinct seasonal groups reflecting the reproductive habits of adult fishes. Fish abundance did not significantly differ between the two sites, although a significant difference in their abundance was detected between day and night, with more fish specimens collected at night than during the day at both sites. The Nakdong River Estuary is an important ecosystem for small and/ or juvenile fishes, providing a habitat within the estuary suitable for their development and migration. Keywords: Fish assemblage; Mugil cephalus; Plecoglossus altivelis; Favonigobius gymnauchen; Opsariichthys uncirostris amurensis; Hyporhamphus sajori; juvenile; surf zone; Nakdong River Estuary Introduction modifications of the river have diminished the freshwater flows that transport nutrients and sediments to estuaries, Surf zones, physically dynamic and complex environments threatening the existence of estuarine habitats (Gillanders with little habitat complexity, are physically determined by & Kingsford 2002; Flemer & Champ 2006). Environ- the total water and sediment movements (Komer 1998; mental change has affected the estuary ecosystem, and in McLachlan & Brown 2006). Sediment transportation by particular the population structure. Several studies have river currents is known to influence the topography of reported that animal community structures have been nearby surf zones (Komer 1998), and the importance of altered, with species composition and abundance changes sandy beach surf zones as transit routes and/or habitats occurring after the construction of the estuary dyke (Jang for the larvae and juveniles of many fish species is & Kim 1992; Hong et al. 1994; Kwak & Huh 2003). For widely recognized in various parts of the world example, Kwak and Huh (2003) reported that demersal (McLachlan & Brown 2006). The high proportion of species have become dominant due to changes in the sedi- juvenile fishes in these environments indicates that sandy ment environment caused by the irregular discharge of beaches provide important alternative nursery grounds for freshwater and variations in the movement of seawater. many estuarine species (Brown & McLachlan 1990). The Although many studies have investigated the commu- utilization of surf zones by large numbers of juvenile nity structure of surf zone fishes worldwide, the seasonal fishes is almost certainly due to the presence of rich food variation in the species composition and the abundance resources in the form of zooplankton, as well as the protec- of fishes in surf zones along the southern coast of Korea tion from predation provided by the shallowness, turbidity, is unknown. Several previous studies have suggested that and turbulence of these areas (Lasiak 1986). surf zones are important nursery grounds for larval and The Nakdong River, the second largest river in Korea, juvenile fishes, which have a relatively lower abundance is located in the southeastern part of the country. The in the western coastal surf zone habitats of Korea (Shin Nakdong River Estuary is an important spawning and & Lee 1990; Lee et al. 1997). Seasonal variations in fish nursery ground for many aquatic animals, and is a very pro- abundance are primarily the result of species-specific ductive fishery (Jeon 1987). In 1987, an estuary dyke was recruitment (Mariani 2001) and/or the interaction of constructed to prevent the influx of salt water into the species with environmental factors, especially water shallow freshwater in the estuary. These anthropogenic *Corresponding author. Email: gwbaeck@gnu.ac.kr © 2015 Korean Society for Integrative Biology ECOLOGY, POPULATION BIOLOGY & ANIMAL BEHAVIOR Animal Cells and Systems 351 temperature (Shin & Lee 1990; Ayvazian & Hyndes 1995; Field sampling Potter et al. 2001). Species-specific seasonal occurrence in The surveys were conducted monthly throughout 2004. a surf zone is also caused by ontogenetic shifts between The fish samples were collected by pulling a modified inshore and onshore habitats (Gibson et al. 1993). beach seine (10 m × 1.5 m; 10 mm mesh size) across 20 However, the fish assemblage in the surf zone of the m of surf water parallel to the coast. Sampling was repli- Nakdong River Estuary has not been studied, and the cated three times. In total, a 60-m tow was sampled on importance of the nursery habitats for marine residents each occasion during both the day and night low tide in and/or marine juveniles, including migrated species, has the surf zone of the estuary. The surface water temperature remained unknown. (using a thermometer) and salinity (via a salinometer) were Although sallow estuarine systems are important for also monitored during each sampling period. Specimens juvenile and migratory species as their habitat and migration were preserved immediately in 10% formalin in seawater pathway, there is little information on faunal assemblage of after capture and later transferred to 70% ethanol. These these ecosystems in Korea. The objective of the present samples were identified according to Masuda et al. study was to examine the seasonal and diel variations in (1984) and Kim et al. (2005). For larval and early juvenile the species composition and abundance of fishes inhabiting fishes, the criteria of Leis and Carson-Ewart (2000) and the surf zone of the Nakdong River Estuary. Okiyama (1989) were used for taxonomic identification. The size and weight of each fish specimen were recorded to the nearest millimeter in standard length and the Materials and methods nearest gram in wet weight, respectively. The guts were Study area then separated for a later gut content analysis. The study area was a sandy beach ∼1100 m wide, located at the eastern tip of the Nakdong River Estuary (35°04′N, 128° Developmental stages, and the ecological and feeding 96′ E; Figure 1). The mean tidal range is 1.2 m for the spring guilds of fishes tide and 0.4 m for the neap tide. A sandbank with a length of The fishes collected were categorized as larvae, juveniles, or ∼800 m and a width of 200 m protects the study area from adults, according to Kendall et al. (1984) and Leis and high wave action. Two sampling sites were established at Carson-Ewart (2000), with larvae representing the develop- each end of the beach to determine the ecological traits of mental stages between hatching and the attainment of a full fish assemblages in the study area (Figure 1). The two set of external meristic characteristics and juveniles being sites were separated by ∼1 km, and environmental con- the stage immediately following until reaching sexual ditions, such as the degree of wave action, water tempera- maturity. The five ecological guilds containing estuarine ture, salinity, and substrate grain size, were similar. residents (ER), freshwater species (FW), marine adventi- tious (MA), marine juvenile (MJ), and diadromous (catadro- mous, anadromous, or amphidromous) migrants (DM) were categorized according to general descriptions of Korean fishes (Chyung 1977; Kim et al. 2005). In addition, by ana- lyzing the gut contents, they were classified as zooplankton feeders (preying mostly on calanoid copepods, Z), benthic invertebrate feeders (mainly gammarid amphipods, B), fish feeders (F), or insect feeders (I). Data analysis The fish data were analyzed to obtain community variables. The diversity index (H′) was calculated using Shannon and Weaver’s formula to compare monthly population diversity (Shannon & Weaver 1949). Logarithmic transformations (log10(x + 1)) of both environmental variables and fish abundance (number and weight) data were performed to meet the assumptions of normality and homocedasticity for statistical tests and to reduce the bias of abundant species. A one-way analysis of variance (ANOVA) was applied to determine the significance of seasonal water temperature and salinity. A three-way ANOVA with an Figure 1. Map of the study area and the location of the two sampling sites in the surf zone of the Nakdong River Estuary. orthogonal design was used to analyze the spatial (two 352 J.M. Park et al. sites), seasonal (four seasons), and diel (day and night) significantly between two sites (F = 0.069, P = 0.794) or effects on fish abundance. between day and night (F= 1.591, P = 0.213). Salinity A one-way analysis of similarity (ANOSIM) was used ranged from 2.6 to 30.2‰ during the day and from 3.1 to compare the significant differences in the structure of the to 32.6‰ during the night. Salinity also changed signifi- assemblages between the two sites, day and night, and cantly among seasons (F = 25.391, P < .001), but no sig- among months. Hierarchical cluster analyses using Bray– nificant variation was observed between two sites (F = Curtis similarity were used for the classification and ordi- 1.938, P = 0.172) or between day and night (F = 0.094, nation of each month based on the number of individuals P = 0.761). of each species. The similarity index was subjected to an average linkage cluster analysis. To determine the species responsible for any seasonal variations in the structure of Fish species composition the assemblages, a similarity percentage (SIMPER) analy- In total, 2397 individuals (8146.0 g) belonging to 27 sis was applied using the PRIMER software package species, 17 families, and 9 orders were collected during (Clarke & Warwick 1994). the study period (Table 1), with Gobidae (5 species), Engraulidae (3 species), and Clupeidae (3 species) being the most widely represented families. Of these species, Results 2,202 individuals (91.9%) were juveniles and 195 (8.1%) Water temperature and salinity were adults. The numerically dominant species were Water temperature at the study site ranged from 9.9 to Mugil cephalus (26.2%), Plecoglossus altivelis (22.6%), 31.3°C during the day and from 9.7 to 28.7°C at night. Favonigobius gymnauchen (13.2%), Opsariichthys uncir- ostris amurensis (11.7%), and Hyporhamphus sajori The peak water temperature was recorded in July for both day and night measurements, with a decline begin- (10.9%), which together accounted for 84.6% of the ning in October and a minimum reached during January catch and 69.3% of the biomass. Most of the fish collected were small species or young juveniles. Only ∼8% of the at night and February during daytime (Figure 2(a)). Water temperature varied significantly among the total number were adults and consisted of 136 seasons (F = 77.601, P < .001), but did not vary F. gymnauchen,34 Leiognathus nuchalis,6 Acanthogobius lactipes,5 Acanthogobius flavimanus,4 Coilia nasus,3 Sardinella zunasi,1 Engraulis japonicus, and 1 Takifugu niphobles. Fish assemblages differed significantly among months (R = 0.602, P = 0.010); however, no significant differences in the species composition was observed between the two sites (R = –0.023, P = 0786) or between day and night (R = 0.046, P = 0.072) according to the ANOSIM. Seasonal variation in species composition The number of fish species varied with season (1–11 species), with the highest number recorded in July during daytime and in September at night (Figure 3(a)). The number of individuals also varied by season, with a peak in summer and winter for both the day and night samples (Figure 3(b)). Large numbers of fishes were collected in winter and summer when M. cephalus, P. altivelis, F. gymnauchen, and O. uncirostris amurensis were abun- dant, while the number collected was lowest in March. The fish biomass differed substantially among the different seasons (Figure 3(c)). The highest biomass was recorded in July and August in both day and night samples when rela- tively large M. cephalus specimens were abundant. The range of the diversity index was 0.20–1.29 in the day and 0.27–1.76 at night, and higher values were recorded in Figure 2. Monthly changes in the mean water temperature (a) and spring and autumn (Figure 3(d)). The diversity indices salinity (b) between day (black circle) and night (open circle) observations in the surf zone of the Nakdong River Estuary. were relatively low (<2.0) throughout the year. Animal Cells and Systems 353 Table 1. Species composition of fish caught using a beach seine in the surf zone of the Nakdong River Estuary on a monthly basis from January to December 2004 (developmental stage (DS): L, larvae; J, juvenile; A, adult), prey items (Cl, Cladocera; Co, Copepoda; Eu, Euphausiacea; Ga, Gammeridae Amphipoda; Ma, Macrura; My, Mysidacea; Pi, Pisces; Po, Polychaeta), ecological guild (EG), and feeding guild (FG)). Abundance Biomass (g) Family Species NN% WW% Size range (cm, SL) DS Prey items EG FG Congridae Conger myriaster 2 0.1 1.6 0.0 9.1–10.3 (TL) L Co MJ Z Engraulidae Coilia nasus 7 0.3 166.0 2.0 6.6–23.9 J, A Co DM Z Engraulis japonicus 2 0.1 3.6 0.0 5.4–6.2 J, A Co MA Z Thryssa kammalensis 27 1.1 31.2 0.4 3.9–5.1 J Co MJ Z Clupeidae Clupea pallasii 15 0.6 198.1 2.4 9.8–12.5 J Co MJ Z Konosirus punctatus 43 1.8 172.8 2.1 3.8–10.6 J Co, Cl MJ Z Sardinella zunasi 5 0.2 50.1 0.6 3.2–11.4 J, A Co MA Z Cyprinidae Hemibarbus labeo 73 3.0 410.1 5.0 5.1–9.7 J Ga, Po, Pi FR B, F Opsariichthys uncirostris 281 11.7 648.3 8.0 2.6–9.1 J Ga, In, Po FR B, I amurensis Osmeridae Plecoglossus altivelis 541 22.6 311.9 3.8 3.2–5.1 J Co DM Z Hypomesus niponensis 8 0.3 21.1 0.3 5.1–7.8 J Co DM Z Hemiramphidae Hyporhamphus sajori 262 10.9 1635.4 20.1 2.8–16.7 J Co MJ Z Scorpaenidae Sebastes schlegeli 5 0.2 5.1 0.1 3.4–3.8 J Ma, Ga MJ B Platycephalidae Platycephalus indicus 11 0.5 28.4 0.3 4.7–8.1 J Ga, Po MJ B Moronidae Lateolabrax japonicus 5 0.2 284.0 3.5 6.5–20.4 J Ma, Pi MJ F Carangidae Trachurus japonicus 4 0.2 1.8 0.0 3.1–3.8 J Cl MJ Z Leiognathidae Leiognathus nuchalis 38 1.6 605.3 7.4 2.0–10.7 J, A My, Co, Ga MA Z Mugilidae Mugil cephalus 628 26.2 2251.8 27.6 2.5–18.5 J Ga MJ B Champsodontidae Champsodon snyderi 4 0.2 1.9 0.0 3.1–3.6 J Is MJ B Gobiidae Acanthogobius flavimanus 69 2.9 432.8 5.3 3.4–14.4 J, A Ga, Po, Ma ER B Acanthogobius lactipes 16 0.7 37.1 0.5 4.7–6.2 J, A Ga, Po ER B Favonigobius gymnauchen 316 13.2 796.4 9.8 2.6–6.9 J, A Ga, Ma ER B Gymnogobius heptacanthus 1 0.0 1.2 0.0 4.6 J Eu ER Z Tridentiger trigonocephalus 2 0.1 1.7 0.0 3.1 J Ga ER B Paralichthyidae Paralichthys olivaceus 1 0.0 2.3 0.0 5.2 J My, Ma MJ B Pleuronectidae Kareius bicoloratus 1 0.0 0.4 0.0 2.5 J My MJ B Tetraodontidae Takifugu niphobles 30 1.3 45.7 0.6 1.6–9.4 J, A Po, Ga, Ma ER B Total 2397 8146.0 Temporal and spatial variations in the abundance of Cluster analysis fishes The dendrogram showed five clusters with a 50% level of Temporal and spatial variations in fish abundance (deter- similarity, which were characterized by samples from mined from the average number of individuals collected different months (Figure 5). Group A comprised samples per towing) were compared between the day and night collected during 3 months in winter (January, February, samples, the two sites, and among four seasons. The mean and December) characterized by high abundance, mainly number of individuals per haul varied between day and juveniles of P. altivelis. Group B included March and night, between stations 1 and 2, and among seasons, and April samples and was characterized by low fish abun- was highest at station 2 during the night in summer and dance, although the samples had a high proportion of lowest at station 2 during the day in spring (Figure 4). The M. cephalus. Groups C (May and June samples) and D overall abundance was higher in summer than other (July and August samples), which represented late spring seasons and during nighttime than daytime. Three-way and summer samples, respectively, were dominated by ANOVA results revealed that the mean number of individ- F. gymnauchen in group C, O. uncirostris amurensis in uals varied significantly between day and night, and group D, and M. cephalus in both groups. Group E was among seasons (P < .05). However, no significant differ- composed of autumn samples (September–November). ences were observed between sites, and no first- and These months were characterized by juveniles of second-order interactions were detected (P > .05; Table 2). H. sajori and several other species. 354 J.M. Park et al. Table 2. Results of a three-way ANOVA testing the effects of day and night, sampling site, and season on the abundance of fish. Source d.f. MS Fp-Value Diel 1 1.825 9.536 0.004 Site 1 0.000 0.001 0.970 Season 3 0.661 3.453 0.028 diel × site 1 0.000 0.000 0.988 diel × season 3 0.113 0.590 0.626 site × season 3 0.085 0.444 0.724 diel × site × season 3 0.101 0.526 0.668 The SIMPER analysis revealed the species that contrib- uted most to the similarity in each season (Table 3). Nine species contributed more than 90% of the similarity between the groups, two species (M. cephalus and P. altivelis) being dominant in group A, and four (M. cephalus, P. altivelis, H. sajori, and K. punctatus)in group B. Group C consisted primarily of four species (M. cephalus, F. gymnauchen, H. sajori, and A. flavimanus). Six species (M. cephalus, F. gymnauchen, O. uncirostris amurensis, H. sajori, A. flavimanus, and L. nuchalis) were dominant in group D, and four (F. gymnauchen, H. sajori, A. flavimanus, and L. nuchalis) dominated group E. Ecological and feeding guild analysis From the cluster analysis in Figure 5, five ecological guilds were identified in each of the selected groups (Figure 6(a)). Figure 3. Monthly variations in the number of species (a), individ- uals (b), biomass (c), and diversity index (d) between day and night in the surf zone of the Nakdong River Estuary. Figure 5. A dendrogram of the hierarchical cluster analysis based on the number of individuals of each fish species in each month of 2004 (left figure). Fish assemblages were divided into five groups (A–E) at 50% similarity. The percentage abundance of dominant fish species and other minor species for each month is shown Figure 4. Seasonal variations in the mean number of individuals (right figure). Fish abbreviations: Fg, F. gymnauchen; Hl, H. per haul (± standard error) in stations 1 and 2 between day and labeo; Hs, H. sajori; Mc, M. cephalus; Ou, O. uncirostris amuren- night in the surf zone of the Nakdong River Estuary. sis; Pa, P. altivelis; Ot, other species. Animal Cells and Systems 355 Table 3. Average similarity and percentage contribution (%) from a SIMPER analysis of the most common species contributing to >90% of each group by season (clusters in Figure 5). Group Group Group Group Group A B C D E Average similarity 56.26 53.77 68.85 55.32 53.60 M. cephalus 30.50 53.60 33.66 17.85 H. niponensis 66.79 14.87 F. gymnauchen 47.11 9.64 15.24 O. uncirostris 40.41 amurensis H. sajori 10.51 4.81 8.15 56.22 H. labeo A. flavimanus 4.81 11.52 10.02 K. punctatus 21.02 L. nuchalis 5.15 12.87 By analyzing the ecological guilds by the number of species within each guild, of the 27 species identified in this study, six were estuarine residents (ER) (five goby species and T. niphobles), and they were abundant in groups C and D. Three diadromous migrant species were also identified, among which P. altivelis was abundant in winter (group A), and the other two species were occasion- ally present in spring and autumn. Thirteen marine juven- iles (MJ) were abundant year-round, with most occurring occasionally in small numbers and for a short period; Figure 6. The number of individuals from each ecological (a) and only M. cephalus (spring and summer) and H. sajori feeding (b) guild in each group resulting from the cluster analysis in Figure 5. Ecological guilds: estuarine residents (ER), fresh- (autumn) were abundant in particular seasons. High water species (FW), marine adventitious (MA), marine juvenile numbers of two freshwater species (Hemibarbus labeo (MJ), and diadromous (catadromous, anadromous, or amphidro- and O. uncirostris amurensis) occurred in summer (group mous) migrants (DM). Feeding guilds: zooplankton feeder (Z), D), while three marine adventitious (MA) fishes were benthic invertebrate feeder (B), fish feeder (F), and insect minor species in all groups. feeder (I). By analyzing the gut contents, four feeding guilds were identified (Table 1), and all of the guilds were represented surf zone on the midwestern coast of Korea (Lee et al. only in group D (Figure 6(b)). Zooplankton feeders (Z) and 1997) and also other estuarine systems (Pombo et al. benthic invertebrate feeders (B) occurred in all groups, 2005; Veiga et al. 2006; Plavan et al. 2010). In general, a with zooplankton feeders being more common in groups shallow surf zone experiences a wide range of water temp- A and E, and benthic invertebrate feeders being more eratures throughout the year because these areas are readily common in groups C and D. Two fish feeders (large- influenced by the heating and cooling of the adjacent land. sized H. labeo and L. japonicus) accounted for a small pro- Extremely low salinity was found in the surf zone of the portion of the fish in groups D and E, and insect feeders Nakdong River Estuary, especially in summer due to the (∼40% of O. uncirostris amurensis) were represented heavy rainfall during the rainy season. These dramatic only in the summer (group D). environmental variations might contribute to the formation of distinct biological communities. Twenty-seven fish species were recorded in the surf Discussion zone of the Nakdong River Estuary, with M. cephalus, P. altivelis, F. gymnauchen, O. uncirostris amurensis, and A broad range of water temperatures (9.7–31.3°C) and H. sajori being numerically dominant. The fishes collected extremely low salinity (∼2.0‰) were observed in the in this study area were predominantly from small species study area compared to other coastal areas. The large and/or juveniles of marine fishes, indicating that the surf range in water temperature and low salinity in the study zone of the estuary functions as a nursery area. This area are similar to the environmental characteristic of the 356 J.M. Park et al. conclusion is in general agreement with other studies on winter, especially in January, because of the strong domi- estuaries elsewhere in the world (Lee et al. 1997; nance of juvenile P. altivelis, which migrated into the Strydom 2003; Barreiros et al. 2004; Nanami & Endo estuary during this period (Chyung 1977; Yamada et al. 2007; Inoue et al. 2008; Plavan et al. 2010). The signifi- 1995). These seasonal changes in fish assemblages might cantly greater abundance of juveniles than adults at our be due to a resource partitioning strategy that reduces com- study site indicated that these species were likely to be petition for food or may reflect species responses to subop- dependent on the surf zone for shelter, survival, and timal physical environmental conditions, or a combination refuge from predators during the early stages of their life of these two factors (Akin et al. 2003). cycle. Most of the marine species that use the estuary as In terms of the diel changes in fish assemblages, the a nursery ground have commercial and/or recreational mean abundances were higher at night than in the value. For example, among the dominant species, daytime (Figure 4). Nagelkerken et al. (2002) reported M. cephalus is harvested as a food fish, and P. altivelis that in general, the abundance of fishes living in shallow and H. sajori are valued in recreational fishing (NFRDI coastal areas, such as seagrass and intertidal habitats, is 2004; Kim et al. 2005). Several minor species, such as Clu- higher during the night. Gibson et al. (1996) recorded the peiformes fishes, also have commercial value in Korea. highest abundance of such fish at night, indicating an Seasonal variation in both species composition and onshore migration at dusk followed by an offshore abundance was considerable for fish communities utilizing migration at dawn, which may have been determined by the surf zone. The dominant species had a distinct pattern their feeding activity and predator avoidance. Quinn and of seasonal occurrence, and fish assemblages were conse- Kojis (1987) found more species, individuals, and quently categorized into five seasonal groups by a cluster biomass immediately after dusk, probably due to changes analysis. The seasonal abundance of many species can be in illumination, gear efficiency, and/or the behavior of attributed to their reproductive habits. For example, the the animals. Despite the diel changes in the occurrence of increase in the abundance of M. cephalus (from late some species in the Nakdong River Estuary, no strong evi- spring to summer), H. sajori (in autumn), and P. altivelis dence for the existence of distinct day and/or night commu- (in winter) occurred mainly after their respective spawning nities was identified. The observed diel changes in seasons (Nishida 1978; Novikov et al. 2002; Hsu et al. abundance were likely to be caused by shifts in the abun- 2007). These species appear to use the surf zone as a dance of a particular species rather than by their presence nursery ground, whereas the consistent occurrence of and/or absence. F. gymnauchen and A. flavimanus in this area suggests In terms of ecological guilds, five ecological systems that these species undergo their entire life cycle in this were identified. Marine juvenile species and estuary resi- zone. However, O. uncirostris amurensis, a freshwater dents were the more diverse groups caught during our species, was predominant in the summer fish community. study. Similar results were reported in many other O. uncirostris amurensis and also H. labeo may be shallow estuarine systems (Veiga et al. 2006; Cardoso washed away to the tip of the Nakdong River Estuary by et al. 2011). In terms of abundance, marine juveniles domi- the increased freshwater discharge. Therefore, the species nated the community with 1008 individuals (42.4% of the composition in summer was dominated by freshwater total number of individuals), followed by diadromous species and euryhaline marine species such as M. cephalus, migrant species (23.2%) and estuarine residents (18.1%). F. gymnauchen, A. flavimanus,and T. niphobles. The guilds in the summer (group A) and winter (group In the Nakdong River Estuary, the number of individ- D) fish communities were dominated by freshwater and uals peaked in summer and winter. These peaks in abun- diadromous migrant species, respectively. We found differ- dance corresponded closely with the peak abundance of ent guild proportions in the fish community collected by M. cephalus in July and P. altivelis in January. Seasonal small trawls in the estuary (Kwak & Hun 2003; Lee et al. variations in fish abundance are a common feature of 2009). Marine stenohaline adult fishes were rare in this dynamic ecosystems such as estuaries. The shifts in the study, although they were the dominant guild in previous abundance of some species were mainly due to their repro- studies, which may be attributable to difficulties in ductive periods because most individuals sampled were approaching the surf zone for adult fishes due to the low young-of-the-year juveniles in the first year of their life salinity and shallow water depth. In addition, the low cycle. The higher fish abundance in summer found in habitat diversity of the surf zone may lead to a different this study was similar to observations reported in the surf fish assemblage (Mathieson et al. 2000). zone on the western coast of Korea (Shin & Lee 1990; Freshwater species are reportedly a minor group in the Lee et al. 1997). Many studies worldwide have reported surf zone occurring with variable intensity depending on low fish abundance in the surf zone during winter due to the estuary (Pombo et al. 2005; Veiga et al. 2006; Inoue the extremely low water temperature (Lasiak 1984; et al. 2008). However, two cyprinid species, O. uncirostris Strydom 2003; Veiga et al. 2006). However, in this study, amurensis and H. labeo, were caught in large numbers in a relatively high fish abundance was recorded during this study during summer when the salinity was low Animal Cells and Systems 357 enough for their occurrence and survival, as both species can dominance of marine juveniles, low diversity, and the pres- tolerate brackish waters (Kim et al. 2005). They will be ence of freshwater species. swept away to the estuary by heavy rainfall in summer, although they are originally freshwater species. Also, Acknowledgements three diadromous species were identified, which used the We are grateful to Dr Hyun Gi Choo, Korean Ocean & Fisheries estuary as a stopover during migration. One species, Institute and Dr Ha Won Kim, Pukyong National University for P. altivelis, was common during winter both in terms of assistance with field sampling. abundance and biomass, but two more diadromous species had low abundance. P. altivelis is a typical amphidromous Disclosure statement fish, which is present nearshore in the Northwest Pacific No potential conflict of interest was reported by the authors. from late autumn to spring during the early stages of its life cycle (Chyung 1977; Yamada et al. 1995). Only a few studies have reported the occurrence of P. altivelis in the References Nakdong River Estuary (Kang et al. 2012) due to the block- Akin S, Winemiller KO, Gelwick FP. 2003. Seasonal and spatial age of their migration route to the upper river by the estuary variations in fish and macrocrustacean assemblage structure in Mad Island Marsh estuary, Texas. Estuar Coast Shelf Sci. dyke. Our results confirmed that large numbers of juvenile 57:269–282. P. altivelis were present in the surf zone, which will contrib- Ayvazian SG, Hyndes GA. 1995. Surf zone fish assemblages in ute to the conservation and management of this species. southwestern Australia: do adjacent nearshore habitats and With respect to the feeding guilds, an overall domi- the warm Leeuwin Current influence the characteristics of nance of taxa feeding on both benthic invertebrates and the fish fauna? Mar Biol. 122:527–536. Baeck GW, Park JM, Huh SH, Kim HJ, Jeong JM. 2014. Feeding zooplanktons was observed. The dominant invertebrates, habits of Kammal thryssa Thryssa kammalensis (Bleeker, calanoid copepods, mysids, and amphipods, were impor- 1849) in the coastal waters of Gadeok-do, Korea. Anim tant food resources for many fishes in the surf zone of Cells Syst. 18:154–159. the Nakdong River Estuary. Zooplankton and benthic Barreiros JP, Figna V, Hostim-Silva M, Santos RS. 2004. invertebrate feeders, which were the most numerous fish Seasonal changes in a sandy beach fish assemblage at Canto Grande, Santa Catarina. South Brazil J Coast Res. in both species and individual numbers, primarily con- 203:862–870. sumed these prey items. These food resources were also Brown AC, McLachlan A. 1990. Ecology of sandy shores. abundant in the diet of small-sized pelagic and benthic Amsterdam: Elsevier; xii 328 p. fishes such as Thryssa kammalensis (Baeck et al., 2014) Cardoso I, França S, Pais MP, Henriques S, da Fonseca LC. 2011. and Repomucenus lunatus (Huh et al., 2013) which Fish assemblages of small estuaries of the Portuguese coast: a functional approach. Estuar Coast Shelf Sci. 93:40–46. occurred in large numbers around the study area. The indi- Chyung MK. 1977. The fishes of Korea. Seoul: Il Ji Sa Publishing vidual numbers of zooplankton and benthic invertebrate Co; 727 p. feeders displayed significant responses to seasonal differ- Clarke KR, Warwick RM. 1994. Changes in marine communities: ences in prey abundance, tending toward higher densities an approach to statistical analysis and interpretation. of calanoid copepods and mysids in groups A and E Plymouth: Plymouth Marine Laboratory; 144 p. Flemer DA and Champ MA. 2006. What is the future fate of estu- (early spring, autumn and winter), and gammarid amphi- arine given nutrient over-enrichment, freshwater diversion pods and polychaetes in groups C and D (late spring and and low flow? Mar Poll Bull. 52:247–258. summer). For example, small zooplanktivorous juveniles Gibson RN, Ansell AD, Robb L. 1993. Seasonal and annual vari- of P. altivelis and H. sajori tended to accumulate in the ations in abundance and species composition of fish and low-temperature seasons (autumn and winter), when zoo- macrocrustacean communities on a Scottish sandy beach. Mar Ecol Prog Ser. 98:89–105. plankton densities tended to be higher (Jeong et al. Gibson RN, Robb L, Burrows MT, Ansell DA. 1996. Tidal, diel 2004), while most of the benthic invertebrate feeders, and longer term changes in the distribution of fishes on a which were mainly estuarine residents, were abundant Scottish sandy beach. Mar Ecol Prog Ser. 130:1–17. during warm seasons (late spring and summer). Gillanders BM, Kingsford MJ. 2002. Impact of changes in flow of In conclusion, the Nakdong River Estuary is an impor- freshwater on estuarine and open coastal habitats and the associated organisms. Ocean Mar Biol Ann Rev. 40:233–309. tant ecosystem for many fishes. Of the 27 species caught in Hong SY, Ma CW, Lim HS. 1994. 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