Minimum population size estimates demonstrate an increase in southern elephant seals (Mirounga leonina) on Livingston Island, maritime Antarctica J. A. Gil-Delgado, J. A. Villaescusa, M. E. Diazmacip, D. Velazquez, E. Rico, M. Toro, A. Quesada & A. Camacho Polar Biology ISSN 0722-4060 Volume 36 Number 4 Polar Biol (2013) 36:607-610 DOI 10.1007/s00300-012-1280-6
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Author's personal copy Polar Biol (2013) 36:607–610 DOI 10.1007/s00300-012-1280-6
SHORT NOTE
Minimum population size estimates demonstrate an increase in southern elephant seals (Mirounga leonina) on Livingston Island, maritime Antarctica J. A. Gil-Delgado • J. A. Villaescusa • M. E. Diazmacip • D. Velazquez • E. Rico M. Toro • A. Quesada • A. Camacho
•
Received: 5 July 2012 / Revised: 4 December 2012 / Accepted: 11 December 2012 / Published online: 29 December 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract Southern elephant seals (Mirounga leonina) are apex predators of marine Antarctic food webs, and variations in their populations have been linked to environmental changes. Consequently, measuring and reporting the status of elephant seal populations provide insights into the environmental status of Antarctica. Here, we present new information on the size of the elephant seal subpopulation on Byers Peninsula (Livingston Island, South Shetland Islands, maritime Antarctica). Based on a total count of 1,510 pups, we estimated a total subpopulation size of 5,530 individuals by using a conversion factor of 3.5. This represents an increase of 150 % since the subpopulation was first counted 30 years ago. Based on this finding, we hypothesize that the overall South Georgia stock, to which the subpopulation we estimated on Byers Peninsula belongs, could be increasing instead of remaining stable as previously thought.
J. A. Gil-Delgado (&) J. A. Villaescusa M. E. Diazmacip A. Camacho Instituto Cavanilles de Biodiversidad y Biologı´a Evolutiva, Universidad de Valencia, Polı´gono La Coma s/n, 46980 Paterna, Spain e-mail:
[email protected] D. Velazquez A. Quesada Departamento de Biologı´a, Universidad Auto´noma de Madrid, 28049 Madrid, Spain E. Rico Departamento de Ecologı´a, Universidad Auto´noma de Madrid, 28049 Madrid, Spain M. Toro Centro de Estudios Hidrogra´ficos, CEDEX, Paseo Bajo Virgen del Puerto 3, 28005 Madrid, Spain
Keywords Antarctica Breeding population Byers Peninsula Mirounga leonina Pup
Introduction Southern elephant seals (Mirounga leonina) are marine predators with a circumpolar distribution. It is generally accepted that there are four main populations of this species (Laws 1994; McMahon et al. 2005). The annual cycle of adult females presents two pelagic stages that are separated by two land reproduction and molt periods (Pistorius et al. 2008). Pup births occur between August and December (Carlini et al. 2002). Elephant seal populations on the South Shetland Islands were first monitored in the 1960 s (Aguayo and Torres 1967). Whereas the available information about breeding populations on Livingston and Nelson Islands is scarce (Torres et al. 1981a, b; Carlini et al. 2003), the breeding population on King George Island has been monitored regularly (Vergani and Stanganelli 1990; Carlini et al. 2006). We report herein estimates of southern elephant seal’s subpopulation sizes on two beach areas of Byers Peninsula (Southern Beaches and President Beaches), which are the main distribution areas of these animals on Livingston Island (Aguayo and Torres 1967).
Materials and methods Study area Byers Peninsula (62°340 3500 to 62°400 3500 S and 60°540 1400 to 61°130 0700 W) is located at the western end of Livingston
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Island (South Shetland Islands). It is free of ice in summer (Toro et al. 2007). There are three groups of beaches on this peninsula: South Beaches and President Beaches on the western coast, and Robbery Beaches on the northern coast. Our study area includes South Beaches and President Beaches. The first area is approximately 18 km long, whereas the latter is approximately 11 km in length. Assumptions We assumed that the breeding pattern does not vary among the different southern elephant seal populations (Galimberti and Sanvito 2001). November and December are considered as the period of the year when onshore elephant seal densities peak (Salwicka and Sierakowski 1998). After the females arrive at the land breeding areas, there is a 5–7-day period before they give birth to the pups (Lewis 1996), followed by a 21–23-day interval between parturition to the departure of the females. Thus, females haul out on land over a total of 26–30 days. Pups, however, leave the breeding areas 6 weeks later (Campagna et al. 1993; Carlini et al. 2001). Consequently, that we assume that our estimates are accounting for all newborn pups. In any case, the figures we report here are conservative estimates representing minimum numbers, as some of the weaned pups could be at sea at the time of counting. Transect counts In the summer of 2008–2009, southern elephant seal counts were carried on both South Beaches and President Beaches. On South Beaches, counts covered a length of coast from Devils Point (62°400 1000 S, 61°110 0000 W) to Cerro Negro (62°390 4000 S, 61°000 0000 W), while those done on the President Beaches were carried out between Punta Ocoa (62°370 0500 S, 61°080 3000 W) and the eastern edge of the penguin colony near Devils Point. Counts of individuals were performed by means of straight-line transects on foot (using binoculars and approaching to large groups when spotted) between 22 December 2008 and 1 January 2009 (Fig. 1). Transect lengths were 11.28 and 6 km for South and President Beaches, respectively, and completely covered each beach length. Routes along the beaches were located at a distance of 50 m from the shoreline (Fig. 1). The number of counted pups was used to estimate the total elephant seal population by means of a conversion factor of 3.5, which has often been used in inventories of southern elephant seal populations (Laws 1994; Lewis 1996).
Results and discussion A total number of 1,510 southern elephant seal pups were counted at both beach areas. Based on this figure, a
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minimum subpopulation size of 5,530 individuals was estimated (Table 1). Pup density (abundance per km) was greater on Southern Beaches (121.4 pups km-1) than on President Beaches (18.2 pups km-1). Our conservative estimate suggest an increase of about 150 % on Byers Peninsula (Livingston Island) in comparison with the estimate of 2,051 elephant seals made in the early 1980 s (Torres et al. 1981a, b). Globally, elephant seal populations have either stabilized or grown recently (McMahon et al. 2005; Ferrari et al. 2009; Authier et al. 2011), and our finding of increasing elephant seal numbers on Livingston Island is in accordance with these reports. The biggest southern elephant seal population is found on South Georgia Island (Boyd et al. 1996). The stable and relatively benign environmental conditions on Byers Peninsula, as well as its wide ice-free areas during summer, may explain why, even in an assumed stable population context (Boyd et al. 1996; McMahon et al. 2005), subpopulations can spread their spatial distribution toward areas with better breeding zones, such as those on Byers Peninsula. Based on our finding of an increased subpopulation on Byers Peninsula, we suggest that the entire South Georgia stock may also be increasing instead of being stable as currently assumed. Our results also emphasize the need for wider simultaneous censuses to cover the other areas that are possibly occupied by individuals of the South Georgia stock, such as the Magellan Strait, which is currently being colonized by southern elephant seals belonging to this population (Gibbons and Miranda 2001). The hypothetical increase in southern elephant seal populations may be related to the evident rise in temperatures in maritime Antarctica (Quayle et al. 2002), which may have a great effect on overall food availability and intrinsic population features of southern elephant seals (de Little et al. 2007; Thumbs et al. 2011). Females and males show a tendency to forage on the Antarctic continental shelf (Bornemann et al. 2000; Bradshaw et al. 2003; Tosh et al. 2009), although climatic events such as El Nin˜o/La Nin˜a oscillations may also relate to the southern elephant seal production based on resource availability (Vergani et al. 2001; McMahon and Burton 2005). However, the potential size of elephant seal stocks may be limited by the availability of space (Bradshaw et al. 2002; McMahon et al. 2009; Pistorius et al. 2011; Herrando-Perez et al. 2012). Our study suggests that access to breeding beaches on Livingston Island is not limited on Livingston Island and that the recent availability of these sites may partially explain the increased numbers of breeding elephant seals on the island. In addition, the occurrence of southern elephant seals breeding in recent times and in areas located at higher latitudes, such as Anvers Island (ASPA 113 2009), suggests that the breeding range of this species is
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Fig. 1 Study area on the Byers Peninsula. The broken line shows the transect counts performed in this study (dotted lines)
Table 1 Southern elephant seal counts on Byers Peninsula performed in Spring-Summer of 2008 and 2009 Beach/source
Date
Distance (km)
Pups
Total population
Southern Beaches
22–24/12/2008
11.28
1,470
5,145
President Beaches
1/1/2009
6
110
385
Total
17.28
1,580
5,530
Torres et al. (1981a)
January 1978
2,521
Torres et al. (1981b)
November 1981
2,051
Acknowledgments This paper is dedicated to the memory of Dr. Alejandro Carlini, whose contributions to polar sciences has illuminated our knowledge. This work has been supported by several grants from the Spanish Ministries of Education and Science and of Science and Innovation as follows: CGL2005-06549-C02-02, CGL2007-29841-E; CTM2008-05205-E; CGL2005-06549-C02-01; POL2006-06635, to A.C. and A.Q. We thank C.R. McMahon, C. Campagna, and M.N. Bester for their important contributions by reviewing the manuscript and to E. Cooper, J. Gonza´lez-Solis and C. Rochera for their assistance in the field transects. We are also very grateful to A. Aguayo and J. Acevedo for providing help to find important forgotten references. Finally, we thank the professional translator service of Helen Warburton, for checking the English language.
Pups (P) and P 9 3.5 were the estimate population. We also show previous estimates of population size (Torres et al. 1981a, b)
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