Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Satellites, the All-Seeing Eyes in the Sky: Counting Elephant Seals from Space

Satellites, the All-Seeing Eyes in the Sky: Counting Elephant Seals from Space Regular censuses are fundamental for the management of animal populations but, are logistically challenging for species living in remote regions. The advent of readily accessible, high resolution satellite images of earth mean that it is possible to resolve relatively small (0.6 m) objects, sufficient to discern large animals. To illustrate how these advances can be used to count animals in remote regions, individual elephant seals (Mirounga leonina) were counted using satellite imagery. We used an image taken on 10/10/2011 to count elephant seals (n = 17906306 (95%CL)) on the isthmus of Macquarie Island, an estimate which overlapped with concurrent ground counts (n = 1991). The number of individuals per harem estimated using the two approaches were highly correlated, with a slope close to one and the estimated intercept also encompassing zero. This proof of concept opens the way for satellites to be used as a standard censusing technique for inaccessible and cryptically coloured species. Quantifying the population trends of higher order predators provides an especially informative and tractable indicator of ecosystem health. Citation: McMahon CR, Howe H, van den Hoff J, Alderman R, Brolsma H, et al. (2014) Satellites, the All-Seeing Eyes in the Sky: Counting Elephant Seals from Space. PLoS ONE 9(3): e92613. doi:10.1371/journal.pone.0092613 Editor: Yan Ropert-Coudert, Institut Pluridisciplinaire Hubert Curien, France Received January 8, 2014; Accepted February 24, 2014; Published March 20, 2014 Copyright:  2014 McMahon et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no support or funding to report. Competing Interests: The authors have declared that no competing interests exist. * E-mail: clive.mcmahon@utas.edu.au nesting habitat [4,7–9], determining rookery size and species Introduction composition [10] and quantifying temporal changes in colony size The population trends of apex predators provide invaluable [11]. Most seabird studies are presently restricted to identifying information on the state of the environment in which animals live entire colonies [12], not individuals, and therefore need to make because the status of a population is an integrated signal of the several assumptions about animal density and intra-colony state of the lower trophic levels and the environments that sustain distribution to make population estimates. Species that are larger them. Determining the bio-physical drivers of such change, and than the current satellite imagery resolution avoid such require- identifying which elements are changing, can be difficult, because ments. long times series are needed to accurately describe population Several studies have used satellite images to count larger species trends and to determine the relationships between population at the individual level [5,13], including marine mammals, fluctuations and broader environmental changes [1]. These time although pinnipeds have proven to be difficult [13]. The sole series rely on regular and accurate censuses which are difficult to exception is the Weddell seal (Leptonochotyes weddellii) whose dark maintain, especially for animals that occur on remote oceanic bodies were highly contrasted against the snow covered Antarctic islands or in inaccessible locations such as Antarctica [2], because sea-ice, making these an ideal test for the efficacy of satellite of logistical constraints in accessing these locations. imagery to inform about seal abundance and change over time [5]. Recently launched satellites such as Geo-Eye-1 (panchromatic, Southern elephant seals (Mirounga leonina) are the largest pinniped 0.5 m resolution and multispectral imagery 1.65 m resolution), species whose terrestrial breeding sites occur on remote sub- WorldView-1 (panchromatic, 0.6 m resolution), WorldView2 Antarctic islands scattered throughout the Southern Ocean. (panchromatic, 0.46 m resolution and 8-band multispectral Elephant seals are a relatively well-studied species and are imagery 1.8 m resolution) and QuickBird-2 (2.4 m multispectral, especially tractable for detecting and recording significant changes 0.6 m panchromatic) provide easily accessible high resolution in the Antarctic marine ecosystem because they are wide-ranging images of the Earth’s surface. This imagery increasingly provides a and therefore integrate environmental signals across ocean basins vital tool for the collection of information that addresses local to [14]. While it is possible to quantify their population status global-scale research and policy priorities [3]. High resolution through regular breeding season censuses surprisingly little is satellite images have allowed ecologists and wildlife demographers known on the global population status and trends of this species to undertake desktop censuses of the distribution and abundances especially the larger populations that breed on South Georgia and of animal populations, especially those in remote regions of the islands on the Kerguelen Plateau (.60% of the global population) globe or those that are sensitive to disturbance [4,5,6]. Satellite [15]. Those islands are logistically extremely difficult to census images have been particularly useful in identifying new seabird regularly given their size, remoteness and the treacherous terrain PLOS ONE | www.plosone.org 1 March 2014 | Volume 9 | Issue 3 | e92613 Remote Satellite Census of Seals that separates the breeding beaches on these islands. Consequently population information at those locations is sparse, collected infrequently and total island abundance estimates and population trends are often based on smaller sub-areas. Typically, the number of number of breeding female elephant seals present in mid-October is taken as a reliable index of population size and used to monitor trends [16]. This is conventionally done by through annual ground counts which require considerable logistical investment that is difficult to guarantee from year-to-year. Unlike Weddell seals that are highly contrasted with their breeding substrate, female elephant seals are grey/brown coloured and aggregate in harems established on dark volcanic-sand beaches that offer little contrast, which may make them difficult to distinguish in satellite images. One of the best-studied southern elephant seal populations breeds on Macquarie Island. About 15% of all females breeding at Macquarie Island congregate in spatially stable harems on the Isthmus study area. Elephant seals found within this study area have been censused by foot on a near-daily basis throughout each breeding season from 1988 onward [17]. Such a committed survey effort within an accessible and well defined area makes the Isthmus an ideal site against which to test the idea that southern elephant seals breeding at very remote, difficult to access locations could be accurately censused from satellite imagery. This study aimed to use satellite imagery of the Macquarie Island isthmus study area captured during the October breeding season to count the number of female harems, the number of seals per harem and the total number of seals in the area. These estimates are then ground-truthed against counts of seals made on the same day. From this study we assess the applicability of the method to census seals observed at terrestrial breeding locations where their detection rates might be poor compared to ice- breeding species. Methods Figure 1. Macquarie Island and the main isthmus study area This study was carried out at Macquarie Island under ethics (upper left panel) and the location of Macquarie Island in the approval from the Australian Antarctic Animal Ethics Committee southern Pacific Ocean (lower right panel). (ASAC 2265) and the Tasmanian Parks and Wildlife Service. doi:10.1371/journal.pone.0092613.g001 To count the number of elephant seals from space we acquired an image of the Macquarie Island isthmus study area Isolated males and juveniles were excluded from the satellite (254.504531u, 158.934330u, Fig. 1) taken on a relatively cloud counts because female counts are the standard way of quantifying free day (10/10/2011) by the Geo-Eye satellite. The image is a population size in elephant seals. New born and nursing pups are GeoEye-1 satellite image (DigitalGlobe Catalog ID for the image approximately 1 m long at this time and coloured black, therefore inumber: 10504100009C3600) captured 10 October 2011 and the it is unlikely they would have been counted along with other larger off nadir angle is 25.8u (Fig. 2). Image processing was courtesy of harem seals. No effort was made to distinguish between adult male the Australian Antarctic Division Data Centre by Angela Bender and female seals within the harem polygons. Including males to produce a pan-sharpened orthorectified image would not greatly affect counts within the harems because typically (GE_10Oct2011_ps_orc). The resolution (pixel size) of the pan- there is approximately one male per 50 female seals [18] i.e. the sharpened orthorectified image is 0.5 metres. For our study we total count for the isthmus would be biased by only 40 seals given a used the panchromatic Geoeye1 image given this cloud free image total isthmus population of approximately 2000 adult female seals. coincided with concurrent ground counts on the island. While this It is also important to note that there is no diel pattern to elephant was sufficient for our purposes, multispectral sensors such as seal haulout as the seals remain ashore with their pups for 3 weeks, WorldView2 may be useful for other applications. so time of day that the image was taken relative to the ground A niave observer (HH) searched this image for harems within count is not a consideration in this study. the study area. We deliberatly used a niave observer to avoid Only once the satellite counts had been completed did we biases introduced by prior knowledge of the island or the compare them to the ground counts. Consequently, there was no distribution and number of harems. Each harem was delineated prior knowledge of the ground counts at the time of the analyses of by a polygon encompassing all animals and assigned a random the satellite images. Ground counts of individual harems of female number. Three counts in random order were made of each harem seals on the Macquarie Island isthmus were made by two from a constant virtual height of 300 m and all seals in the harems observers. When individual ground counts differed, subsequent were counted, using simple image viewing software (Google counts were done until the final tallies were within 5% [17]. These Earth). The mean of these three counts was taken to represent the counts formed part of the regular annual monitoring programme number of seals in that harem. whereby female elephant seals are counted throughout the PLOS ONE | www.plosone.org 2 March 2014 | Volume 9 | Issue 3 | e92613 Remote Satellite Census of Seals Figure 2. Southern elephant seals - Mirounga leonina, (a) congregate to breed annually in large groups known as harems on remote sub-Antarctic islands. These harems can be seen from space and individual seals are large enough (2.5 m long) to be counted (b). doi:10.1371/journal.pone.0092613.g002 breeding season at daily to weekly intervals. Fortuitously the A linear regression relating the number of seals per harem from ground count made on the 10th October 2011 coincided with satellite imagery and ground counts had a highly significant relationship (F = 107.3, r = 0.9062, Ground Counts acquisition of a cloud-free satellite image. 1,10 = 22.03+0.946*Satellite counts). The slope of this relationship was The satellite counts for each harem were compared to the 0.96460.21(95% confidence limit) which encompasses 1.0 and the corresponding ground counts for each harem using linear intercept of the relationship was 22.03637.79 (95% confidence regression. The satellite counts and the ground counts of seal limit) thereby also encompassing zero. The line of parity (where numbers were taken to be equivalent when the 95% CL of the ground counts equal satellite counts) is also contained within the mean satellite estimate overlapped the ground count. 95% confidence intervals of the relationship (the dotted line in Fig 3). Results Here we demonstrate that despite their cryptic colouration, vital Discussion demographic information in the form of population censuses for We show that individual elephant seals can be reliably and elephant seals can be collected remotely by satellites, and that the accurately counted from simple, single-spectrum satellite images abundance of seals estimated from satellite imagery were an using manual counting, thereby demonstrating the quick simple accurate representation of numbers of seals counted on the beach and relatively inexpensive utility of the technique. While it was on the same day. The Geo-Eye satellite image contained sufficient undoubtedly fortuitous that an image was available that was taken resolution to firstly locate all 12 harems on the isthmus study area on a cloud free day corresponding to a concurrent ground count, (Fig. 2). Further, the mean satellite count of elephant seals within this limitation can be resolved given that many of the satellites can the study area was 17906306 (n = 3 counts; 95% confidence limit) be directed to take specific images at specific times, although at seals, compares well with the ground count of the study area made some cost to the user. We found no difference between satellite on the same day: 1991 adult females within 12 harems, which lies and ground counts for: the total number of harems, the total within the confidence limits of the satellite estimate. number of seals on the isthmus, or the number of seals per harem, PLOS ONE | www.plosone.org 3 March 2014 | Volume 9 | Issue 3 | e92613 Remote Satellite Census of Seals Figure 3. The number of seals counted from space was highly correlated to the actual numbers on beach determined by simultaneous ground counts (Ground Counts = 20.11+1.0003*Satellite counts). The solid line represents the line of best fit, the dashed lines indicated the 95% Confidence limits of that line, and the dotted line is the line of parity (i.e. Ground Counts = Satellite counts). doi:10.1371/journal.pone.0092613.g003 demonstrating that remotely sensed images can reliably be used to There are several sources of variance within the estimates. The census robustly elephant seals on remote sub-Antarctic islands. first is discerning the individual seals. Given that the pixel size of Earlier work by La Rue [5] demonstrated that individual Weddell the images is 0.6 m, and that the average length of an adult female seals could be accurately counted from space, but our study is the seal is 2.4 m and the width is 1.4 m, the seals will be represented in first to successfully count animals with poor contrast against their 4–6 pixels, it is unlikely that individual seals were missed. background i.e. dark bodies on a dark background. This finding Consequently count variance is most likely due to our in ability greatly broadens the utility of the method, which can potentially to distinguish seals from similar sized rocks and from shading due be used in many terrestrial and coastal situations. to sun angle and from animals casting shadows onto adjacent seals Despite the congruence of the mean satellite estimates with the in the tightly packed breeding harems [18]. While the harems are ground counts there was nonetheless some disparity between the tightly packed, elephant seals do not lie on top of one another satellite derived estimates and ground counts. The degree of this during the breeding season, and missing seals because they are disparity i.e. the difference between the two types of counts, is stacked on top of each other is unlikely. important because the power to detect inter-annual variability and Resolving the inherent errors of detecting seals in natural abundance trends relies on the precision of annual census data. settings from satellite images can be relatively easily overcome by PLOS ONE | www.plosone.org 4 March 2014 | Volume 9 | Issue 3 | e92613 Remote Satellite Census of Seals (i) increasing the number of replicate counts of the images, thereby the potential to revolutionize how animals are censused at a time reducing the variance around the counts, (ii) by obtaining images when robust demographic information including longitudinal on other days during the breeding season when detection count series to quantify population trends and growth rates are parameters may have improved and (iii) by adjusting the image sorely needed, especially in the light of the current biodiversity to improve contrast and sharpen edges. This would improve our crisis [21–22]. capacity to distinguish males within the harems and therefore give more accurate counts of females. Alternative methods such as Acknowledgments aerial surveys could also improve the accuracy of counts but while We thank the 2011 ANARE expeditioners to Macquarie Island who aerial surveys give better resolution and hence higher accuracy, conducted the ground counts. The satellite imagery is provided courtesy of such surveys are expensive and often sub-Antarctic islands are not DigitalGlobe 2013. DigitalGlobe confirm that publication of the satellite within range of survey aircraft. image under the agreement with the Australian Antarctic Division is Our findings illustrate the general utility of using satellite permitted. We thank the Australian Antarctic Division Data centre (David acquired census information to accurately enumerate the numbers Smith) for providing Figure 1 and Angela Bender for the pan-sharpened of individual large-bodied animals in the wild. While our findings orthorectified image. are a manifestly useful tool for counting elephant seals at remote and rarely visited islands such as Heard Island and the remote Author Contributions beaches of South Georgia which contain about 62% of the Conceived and designed the experiments: CRM JvdH MAH. Performed World’s elephant seals population for which there are no the experiments: CRM HH JvdH HB MAH. Analyzed the data: CRM contemporary counts. This techniques of counting seals using HH JvdH HB MAH. Contributed reagents/materials/analysis tools: JvdH satellites can be extended to other Sothern Ocean pinnipeds such RA CRM HB MAH. Wrote the paper: CRM HH JvdH RA HB MAH. as pack-ice seals, other marine mammals such as whales or large Naive observer: HH. Gave informed consent to participate in the study terrestrial animals such as zebra, camels, elephants, bison and verbally, this was not documented in writing: HH. Supervised HH’s study: savannah ungulates that occur in open terrain [19,20]. This has JvdH MAH. References 12. LaRue MA, Ainley DG, Swanson M, Dugger KM, Lyver POB, et al. (2013) 1. McMahon CR, Bester MN, Hindell MA, Brook BW, Bradshaw CJA (2009) Climate Change Winners: Receding Ice Fields Facilitate Colony Expansion and Shifting trends: detecting environmentally mediated regulation in long-lived Altered Dynamics in an Ade´lie Penguin Metapopulation. Plos One 8: e60568. marine vertebrates using time-series data Oecologia 159: 69–82. 13. Laliberte AS, Ripple WJ (2003) Automated wildlife counts from remotely sensed 2. Schofield O, Ducklow HW, Martinson DG, Meredith MP, Moline MA, et al. imagery. Wildl Soc Bull 31: 362–371. (2010) How do polar marine ecosystems respond to rapid climate change? 14. Hindell MA, Bradshaw CJA, Guinet C, Harcourt RG (2003) Ecosystem Science 328: 1520–1523. monitoring and modelling: can marine mammals signal or predict change? In: 3. Horning N, Robinson J, Sterling E, Turner W, Spector S (2010) Remote sensing Gales N, Hindell MA, Kirkwood R, editors. Marine mammals and humans: for ecology and conservation: Oxford University Press. 448 p. towards a sustainable balance. Melbourne: CSIRO Publishing. 4. Hughes BJ, Martin GR, Reynolds SJ (2011) The use of Google Earth (TM) 15. McMahon CR, Bester MN, Burton HR, Hindell MA, Bradshaw CJA (2005) satellite imagery to detect the nests of masked boobies Sula dactylatra. Wildl Biol Population status, trends and a re-examination of the hypotheses explaining the 17: 210–216. recent declines of the southern elephant seal Mirounga leonina. Mamm Rev 35: 5. LaRue MA, Rotella JJ, Garrott RA, Siniff DB, Ainley DG, et al. (2011) Satellite 82–100. imagery can be used to detect variation in abundance of Weddell seals 16. Hindell MA, Burton HR (1987) Past and present status of the southern elephant (Leptonychotes weddellii) in Erebus Bay, Antarctica. Polar Biol 34: 1727–1737. seal (Mirounga leonina) at Macquarie Island. J Zool (Lond) 231: 365–380. 6. Platonov NG, Mordvintsev IN, Rozhnov VV (2013) The possibility of using high 17. Van den Hoff J, Burton HR, Raymond B (2007) The population trend of resolution satellite images for detection of marine mammals. Biol Bull 40: 197– southern elephant seals (Mirounga leonina L.) at Macquarie Island (1952–2004). Polar Biol 30: 1275–1283. 7. Fretwell PT, Trathan PN (2009) Penguins from space: faecal stains reveal the 18. McMahon CR, Bradshaw CJA (2004) Harem choice and breeding experience of location of emperor penguin colonies. Glob Ecol Biogeogr 18: 543–552. female southern elephant seals influence offspring survival. Behav Ecol Sociobiol 8. Fretwell PT, LaRue MA, Morin P, Kooyman GL, Wienecke B, et al. (2012) An 55: 349–362. Emperor Penguin Population Estimate: The First Global, Synoptic Survey of a 19. Gutro R (2005) Satellite Data to Track Wildlife: Elephants in Space. Available: Species from Space. Plos One 7. http://www.nasa.gov/vision/earth/lookingatearth/elephants_space.html. Ac- 9. Trathan PN, Fretwell PT, Stonehouse B (2011) First Recorded Loss of an cessed 2013 October 1. Emperor Penguin Colony in the Recent Period of Antarctic Regional Warming: 20. Yang Z (2012) Evaluating high resolution GeoEye-1 satellite imagery for Implications for Other Colonies. Plos One 6. mapping wildlife in open savannahs. Enschede, The Netherlands University of 10. Lynch HJ, White R, Black AD, Naveen R (2012) Detection, differentiation, and Twente 1–61 p. abundance estimation of penguin species by high-resolution satellite imagery. 21. Reich PB, Tilman D, Isbell F, Mueller K, Hobbie SE, et al. (2012) Impacts of Polar Biol 35: 963–968. biodiversity loss escalate through time as redundancy fades. Science 336: 589– 11. Naveen R, Lynch HJ, Forrest S, Mueller T, Polito M (2012) First direct, site- wide penguin survey at Deception Island, Antarctica, suggests significant 22. Hooper DU, Adair EC, Cardinale BJ, Byrnes JEK, Hungate BA, et al. (2012) A global synthesis reveals biodiversity loss as a major driver of ecosystem change. declines in breeding chinstrap penguins. Polar Biol 35: 1879–1888. Nature 486: 105–108. PLOS ONE | www.plosone.org 5 March 2014 | Volume 9 | Issue 3 | e92613 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png PLoS ONE Pubmed Central

Satellites, the All-Seeing Eyes in the Sky: Counting Elephant Seals from Space

Loading next page...
 
/lp/pubmed-central/satellites-the-all-seeing-eyes-in-the-sky-counting-elephant-seals-from-JIELK4J4dX

References (25)

Publisher
Pubmed Central
Copyright
© 2014 McMahon et al
ISSN
1932-6203
eISSN
1932-6203
DOI
10.1371/journal.pone.0092613
Publisher site
See Article on Publisher Site

Abstract

Regular censuses are fundamental for the management of animal populations but, are logistically challenging for species living in remote regions. The advent of readily accessible, high resolution satellite images of earth mean that it is possible to resolve relatively small (0.6 m) objects, sufficient to discern large animals. To illustrate how these advances can be used to count animals in remote regions, individual elephant seals (Mirounga leonina) were counted using satellite imagery. We used an image taken on 10/10/2011 to count elephant seals (n = 17906306 (95%CL)) on the isthmus of Macquarie Island, an estimate which overlapped with concurrent ground counts (n = 1991). The number of individuals per harem estimated using the two approaches were highly correlated, with a slope close to one and the estimated intercept also encompassing zero. This proof of concept opens the way for satellites to be used as a standard censusing technique for inaccessible and cryptically coloured species. Quantifying the population trends of higher order predators provides an especially informative and tractable indicator of ecosystem health. Citation: McMahon CR, Howe H, van den Hoff J, Alderman R, Brolsma H, et al. (2014) Satellites, the All-Seeing Eyes in the Sky: Counting Elephant Seals from Space. PLoS ONE 9(3): e92613. doi:10.1371/journal.pone.0092613 Editor: Yan Ropert-Coudert, Institut Pluridisciplinaire Hubert Curien, France Received January 8, 2014; Accepted February 24, 2014; Published March 20, 2014 Copyright:  2014 McMahon et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no support or funding to report. Competing Interests: The authors have declared that no competing interests exist. * E-mail: clive.mcmahon@utas.edu.au nesting habitat [4,7–9], determining rookery size and species Introduction composition [10] and quantifying temporal changes in colony size The population trends of apex predators provide invaluable [11]. Most seabird studies are presently restricted to identifying information on the state of the environment in which animals live entire colonies [12], not individuals, and therefore need to make because the status of a population is an integrated signal of the several assumptions about animal density and intra-colony state of the lower trophic levels and the environments that sustain distribution to make population estimates. Species that are larger them. Determining the bio-physical drivers of such change, and than the current satellite imagery resolution avoid such require- identifying which elements are changing, can be difficult, because ments. long times series are needed to accurately describe population Several studies have used satellite images to count larger species trends and to determine the relationships between population at the individual level [5,13], including marine mammals, fluctuations and broader environmental changes [1]. These time although pinnipeds have proven to be difficult [13]. The sole series rely on regular and accurate censuses which are difficult to exception is the Weddell seal (Leptonochotyes weddellii) whose dark maintain, especially for animals that occur on remote oceanic bodies were highly contrasted against the snow covered Antarctic islands or in inaccessible locations such as Antarctica [2], because sea-ice, making these an ideal test for the efficacy of satellite of logistical constraints in accessing these locations. imagery to inform about seal abundance and change over time [5]. Recently launched satellites such as Geo-Eye-1 (panchromatic, Southern elephant seals (Mirounga leonina) are the largest pinniped 0.5 m resolution and multispectral imagery 1.65 m resolution), species whose terrestrial breeding sites occur on remote sub- WorldView-1 (panchromatic, 0.6 m resolution), WorldView2 Antarctic islands scattered throughout the Southern Ocean. (panchromatic, 0.46 m resolution and 8-band multispectral Elephant seals are a relatively well-studied species and are imagery 1.8 m resolution) and QuickBird-2 (2.4 m multispectral, especially tractable for detecting and recording significant changes 0.6 m panchromatic) provide easily accessible high resolution in the Antarctic marine ecosystem because they are wide-ranging images of the Earth’s surface. This imagery increasingly provides a and therefore integrate environmental signals across ocean basins vital tool for the collection of information that addresses local to [14]. While it is possible to quantify their population status global-scale research and policy priorities [3]. High resolution through regular breeding season censuses surprisingly little is satellite images have allowed ecologists and wildlife demographers known on the global population status and trends of this species to undertake desktop censuses of the distribution and abundances especially the larger populations that breed on South Georgia and of animal populations, especially those in remote regions of the islands on the Kerguelen Plateau (.60% of the global population) globe or those that are sensitive to disturbance [4,5,6]. Satellite [15]. Those islands are logistically extremely difficult to census images have been particularly useful in identifying new seabird regularly given their size, remoteness and the treacherous terrain PLOS ONE | www.plosone.org 1 March 2014 | Volume 9 | Issue 3 | e92613 Remote Satellite Census of Seals that separates the breeding beaches on these islands. Consequently population information at those locations is sparse, collected infrequently and total island abundance estimates and population trends are often based on smaller sub-areas. Typically, the number of number of breeding female elephant seals present in mid-October is taken as a reliable index of population size and used to monitor trends [16]. This is conventionally done by through annual ground counts which require considerable logistical investment that is difficult to guarantee from year-to-year. Unlike Weddell seals that are highly contrasted with their breeding substrate, female elephant seals are grey/brown coloured and aggregate in harems established on dark volcanic-sand beaches that offer little contrast, which may make them difficult to distinguish in satellite images. One of the best-studied southern elephant seal populations breeds on Macquarie Island. About 15% of all females breeding at Macquarie Island congregate in spatially stable harems on the Isthmus study area. Elephant seals found within this study area have been censused by foot on a near-daily basis throughout each breeding season from 1988 onward [17]. Such a committed survey effort within an accessible and well defined area makes the Isthmus an ideal site against which to test the idea that southern elephant seals breeding at very remote, difficult to access locations could be accurately censused from satellite imagery. This study aimed to use satellite imagery of the Macquarie Island isthmus study area captured during the October breeding season to count the number of female harems, the number of seals per harem and the total number of seals in the area. These estimates are then ground-truthed against counts of seals made on the same day. From this study we assess the applicability of the method to census seals observed at terrestrial breeding locations where their detection rates might be poor compared to ice- breeding species. Methods Figure 1. Macquarie Island and the main isthmus study area This study was carried out at Macquarie Island under ethics (upper left panel) and the location of Macquarie Island in the approval from the Australian Antarctic Animal Ethics Committee southern Pacific Ocean (lower right panel). (ASAC 2265) and the Tasmanian Parks and Wildlife Service. doi:10.1371/journal.pone.0092613.g001 To count the number of elephant seals from space we acquired an image of the Macquarie Island isthmus study area Isolated males and juveniles were excluded from the satellite (254.504531u, 158.934330u, Fig. 1) taken on a relatively cloud counts because female counts are the standard way of quantifying free day (10/10/2011) by the Geo-Eye satellite. The image is a population size in elephant seals. New born and nursing pups are GeoEye-1 satellite image (DigitalGlobe Catalog ID for the image approximately 1 m long at this time and coloured black, therefore inumber: 10504100009C3600) captured 10 October 2011 and the it is unlikely they would have been counted along with other larger off nadir angle is 25.8u (Fig. 2). Image processing was courtesy of harem seals. No effort was made to distinguish between adult male the Australian Antarctic Division Data Centre by Angela Bender and female seals within the harem polygons. Including males to produce a pan-sharpened orthorectified image would not greatly affect counts within the harems because typically (GE_10Oct2011_ps_orc). The resolution (pixel size) of the pan- there is approximately one male per 50 female seals [18] i.e. the sharpened orthorectified image is 0.5 metres. For our study we total count for the isthmus would be biased by only 40 seals given a used the panchromatic Geoeye1 image given this cloud free image total isthmus population of approximately 2000 adult female seals. coincided with concurrent ground counts on the island. While this It is also important to note that there is no diel pattern to elephant was sufficient for our purposes, multispectral sensors such as seal haulout as the seals remain ashore with their pups for 3 weeks, WorldView2 may be useful for other applications. so time of day that the image was taken relative to the ground A niave observer (HH) searched this image for harems within count is not a consideration in this study. the study area. We deliberatly used a niave observer to avoid Only once the satellite counts had been completed did we biases introduced by prior knowledge of the island or the compare them to the ground counts. Consequently, there was no distribution and number of harems. Each harem was delineated prior knowledge of the ground counts at the time of the analyses of by a polygon encompassing all animals and assigned a random the satellite images. Ground counts of individual harems of female number. Three counts in random order were made of each harem seals on the Macquarie Island isthmus were made by two from a constant virtual height of 300 m and all seals in the harems observers. When individual ground counts differed, subsequent were counted, using simple image viewing software (Google counts were done until the final tallies were within 5% [17]. These Earth). The mean of these three counts was taken to represent the counts formed part of the regular annual monitoring programme number of seals in that harem. whereby female elephant seals are counted throughout the PLOS ONE | www.plosone.org 2 March 2014 | Volume 9 | Issue 3 | e92613 Remote Satellite Census of Seals Figure 2. Southern elephant seals - Mirounga leonina, (a) congregate to breed annually in large groups known as harems on remote sub-Antarctic islands. These harems can be seen from space and individual seals are large enough (2.5 m long) to be counted (b). doi:10.1371/journal.pone.0092613.g002 breeding season at daily to weekly intervals. Fortuitously the A linear regression relating the number of seals per harem from ground count made on the 10th October 2011 coincided with satellite imagery and ground counts had a highly significant relationship (F = 107.3, r = 0.9062, Ground Counts acquisition of a cloud-free satellite image. 1,10 = 22.03+0.946*Satellite counts). The slope of this relationship was The satellite counts for each harem were compared to the 0.96460.21(95% confidence limit) which encompasses 1.0 and the corresponding ground counts for each harem using linear intercept of the relationship was 22.03637.79 (95% confidence regression. The satellite counts and the ground counts of seal limit) thereby also encompassing zero. The line of parity (where numbers were taken to be equivalent when the 95% CL of the ground counts equal satellite counts) is also contained within the mean satellite estimate overlapped the ground count. 95% confidence intervals of the relationship (the dotted line in Fig 3). Results Here we demonstrate that despite their cryptic colouration, vital Discussion demographic information in the form of population censuses for We show that individual elephant seals can be reliably and elephant seals can be collected remotely by satellites, and that the accurately counted from simple, single-spectrum satellite images abundance of seals estimated from satellite imagery were an using manual counting, thereby demonstrating the quick simple accurate representation of numbers of seals counted on the beach and relatively inexpensive utility of the technique. While it was on the same day. The Geo-Eye satellite image contained sufficient undoubtedly fortuitous that an image was available that was taken resolution to firstly locate all 12 harems on the isthmus study area on a cloud free day corresponding to a concurrent ground count, (Fig. 2). Further, the mean satellite count of elephant seals within this limitation can be resolved given that many of the satellites can the study area was 17906306 (n = 3 counts; 95% confidence limit) be directed to take specific images at specific times, although at seals, compares well with the ground count of the study area made some cost to the user. We found no difference between satellite on the same day: 1991 adult females within 12 harems, which lies and ground counts for: the total number of harems, the total within the confidence limits of the satellite estimate. number of seals on the isthmus, or the number of seals per harem, PLOS ONE | www.plosone.org 3 March 2014 | Volume 9 | Issue 3 | e92613 Remote Satellite Census of Seals Figure 3. The number of seals counted from space was highly correlated to the actual numbers on beach determined by simultaneous ground counts (Ground Counts = 20.11+1.0003*Satellite counts). The solid line represents the line of best fit, the dashed lines indicated the 95% Confidence limits of that line, and the dotted line is the line of parity (i.e. Ground Counts = Satellite counts). doi:10.1371/journal.pone.0092613.g003 demonstrating that remotely sensed images can reliably be used to There are several sources of variance within the estimates. The census robustly elephant seals on remote sub-Antarctic islands. first is discerning the individual seals. Given that the pixel size of Earlier work by La Rue [5] demonstrated that individual Weddell the images is 0.6 m, and that the average length of an adult female seals could be accurately counted from space, but our study is the seal is 2.4 m and the width is 1.4 m, the seals will be represented in first to successfully count animals with poor contrast against their 4–6 pixels, it is unlikely that individual seals were missed. background i.e. dark bodies on a dark background. This finding Consequently count variance is most likely due to our in ability greatly broadens the utility of the method, which can potentially to distinguish seals from similar sized rocks and from shading due be used in many terrestrial and coastal situations. to sun angle and from animals casting shadows onto adjacent seals Despite the congruence of the mean satellite estimates with the in the tightly packed breeding harems [18]. While the harems are ground counts there was nonetheless some disparity between the tightly packed, elephant seals do not lie on top of one another satellite derived estimates and ground counts. The degree of this during the breeding season, and missing seals because they are disparity i.e. the difference between the two types of counts, is stacked on top of each other is unlikely. important because the power to detect inter-annual variability and Resolving the inherent errors of detecting seals in natural abundance trends relies on the precision of annual census data. settings from satellite images can be relatively easily overcome by PLOS ONE | www.plosone.org 4 March 2014 | Volume 9 | Issue 3 | e92613 Remote Satellite Census of Seals (i) increasing the number of replicate counts of the images, thereby the potential to revolutionize how animals are censused at a time reducing the variance around the counts, (ii) by obtaining images when robust demographic information including longitudinal on other days during the breeding season when detection count series to quantify population trends and growth rates are parameters may have improved and (iii) by adjusting the image sorely needed, especially in the light of the current biodiversity to improve contrast and sharpen edges. This would improve our crisis [21–22]. capacity to distinguish males within the harems and therefore give more accurate counts of females. Alternative methods such as Acknowledgments aerial surveys could also improve the accuracy of counts but while We thank the 2011 ANARE expeditioners to Macquarie Island who aerial surveys give better resolution and hence higher accuracy, conducted the ground counts. The satellite imagery is provided courtesy of such surveys are expensive and often sub-Antarctic islands are not DigitalGlobe 2013. DigitalGlobe confirm that publication of the satellite within range of survey aircraft. image under the agreement with the Australian Antarctic Division is Our findings illustrate the general utility of using satellite permitted. We thank the Australian Antarctic Division Data centre (David acquired census information to accurately enumerate the numbers Smith) for providing Figure 1 and Angela Bender for the pan-sharpened of individual large-bodied animals in the wild. While our findings orthorectified image. are a manifestly useful tool for counting elephant seals at remote and rarely visited islands such as Heard Island and the remote Author Contributions beaches of South Georgia which contain about 62% of the Conceived and designed the experiments: CRM JvdH MAH. Performed World’s elephant seals population for which there are no the experiments: CRM HH JvdH HB MAH. Analyzed the data: CRM contemporary counts. This techniques of counting seals using HH JvdH HB MAH. Contributed reagents/materials/analysis tools: JvdH satellites can be extended to other Sothern Ocean pinnipeds such RA CRM HB MAH. Wrote the paper: CRM HH JvdH RA HB MAH. as pack-ice seals, other marine mammals such as whales or large Naive observer: HH. Gave informed consent to participate in the study terrestrial animals such as zebra, camels, elephants, bison and verbally, this was not documented in writing: HH. Supervised HH’s study: savannah ungulates that occur in open terrain [19,20]. This has JvdH MAH. References 12. LaRue MA, Ainley DG, Swanson M, Dugger KM, Lyver POB, et al. (2013) 1. McMahon CR, Bester MN, Hindell MA, Brook BW, Bradshaw CJA (2009) Climate Change Winners: Receding Ice Fields Facilitate Colony Expansion and Shifting trends: detecting environmentally mediated regulation in long-lived Altered Dynamics in an Ade´lie Penguin Metapopulation. Plos One 8: e60568. marine vertebrates using time-series data Oecologia 159: 69–82. 13. Laliberte AS, Ripple WJ (2003) Automated wildlife counts from remotely sensed 2. Schofield O, Ducklow HW, Martinson DG, Meredith MP, Moline MA, et al. imagery. Wildl Soc Bull 31: 362–371. (2010) How do polar marine ecosystems respond to rapid climate change? 14. Hindell MA, Bradshaw CJA, Guinet C, Harcourt RG (2003) Ecosystem Science 328: 1520–1523. monitoring and modelling: can marine mammals signal or predict change? In: 3. Horning N, Robinson J, Sterling E, Turner W, Spector S (2010) Remote sensing Gales N, Hindell MA, Kirkwood R, editors. Marine mammals and humans: for ecology and conservation: Oxford University Press. 448 p. towards a sustainable balance. Melbourne: CSIRO Publishing. 4. Hughes BJ, Martin GR, Reynolds SJ (2011) The use of Google Earth (TM) 15. McMahon CR, Bester MN, Burton HR, Hindell MA, Bradshaw CJA (2005) satellite imagery to detect the nests of masked boobies Sula dactylatra. Wildl Biol Population status, trends and a re-examination of the hypotheses explaining the 17: 210–216. recent declines of the southern elephant seal Mirounga leonina. Mamm Rev 35: 5. LaRue MA, Rotella JJ, Garrott RA, Siniff DB, Ainley DG, et al. (2011) Satellite 82–100. imagery can be used to detect variation in abundance of Weddell seals 16. Hindell MA, Burton HR (1987) Past and present status of the southern elephant (Leptonychotes weddellii) in Erebus Bay, Antarctica. Polar Biol 34: 1727–1737. seal (Mirounga leonina) at Macquarie Island. J Zool (Lond) 231: 365–380. 6. Platonov NG, Mordvintsev IN, Rozhnov VV (2013) The possibility of using high 17. Van den Hoff J, Burton HR, Raymond B (2007) The population trend of resolution satellite images for detection of marine mammals. Biol Bull 40: 197– southern elephant seals (Mirounga leonina L.) at Macquarie Island (1952–2004). Polar Biol 30: 1275–1283. 7. Fretwell PT, Trathan PN (2009) Penguins from space: faecal stains reveal the 18. McMahon CR, Bradshaw CJA (2004) Harem choice and breeding experience of location of emperor penguin colonies. Glob Ecol Biogeogr 18: 543–552. female southern elephant seals influence offspring survival. Behav Ecol Sociobiol 8. Fretwell PT, LaRue MA, Morin P, Kooyman GL, Wienecke B, et al. (2012) An 55: 349–362. Emperor Penguin Population Estimate: The First Global, Synoptic Survey of a 19. Gutro R (2005) Satellite Data to Track Wildlife: Elephants in Space. Available: Species from Space. Plos One 7. http://www.nasa.gov/vision/earth/lookingatearth/elephants_space.html. Ac- 9. Trathan PN, Fretwell PT, Stonehouse B (2011) First Recorded Loss of an cessed 2013 October 1. Emperor Penguin Colony in the Recent Period of Antarctic Regional Warming: 20. Yang Z (2012) Evaluating high resolution GeoEye-1 satellite imagery for Implications for Other Colonies. Plos One 6. mapping wildlife in open savannahs. Enschede, The Netherlands University of 10. Lynch HJ, White R, Black AD, Naveen R (2012) Detection, differentiation, and Twente 1–61 p. abundance estimation of penguin species by high-resolution satellite imagery. 21. Reich PB, Tilman D, Isbell F, Mueller K, Hobbie SE, et al. (2012) Impacts of Polar Biol 35: 963–968. biodiversity loss escalate through time as redundancy fades. Science 336: 589– 11. Naveen R, Lynch HJ, Forrest S, Mueller T, Polito M (2012) First direct, site- wide penguin survey at Deception Island, Antarctica, suggests significant 22. Hooper DU, Adair EC, Cardinale BJ, Byrnes JEK, Hungate BA, et al. (2012) A global synthesis reveals biodiversity loss as a major driver of ecosystem change. declines in breeding chinstrap penguins. Polar Biol 35: 1879–1888. Nature 486: 105–108. PLOS ONE | www.plosone.org 5 March 2014 | Volume 9 | Issue 3 | e92613

Journal

PLoS ONEPubmed Central

Published: Mar 20, 2014

There are no references for this article.