1. AT 25 DE MAYO / KING GEORGE ISLAND DISCUSSION and CONCLUSIONS
SEASONAL VARIATION IN THE NUMBER OF BREEDING FEMALES OF SOUTHERN ELEPHANT SEALS,
AT 25 DE MAYO / KING GEORGE ISLAND
Mennucci J. A1., Negrete J1,2., Juáres M. A.1,2, Santos M1., Coria N. R.1, Márquez M.E.1
1 Instituto Antártico Argentino, Depto. Biología de Predadores Tope, Buenos Aires, Argentina.
2 CONICET, Buenos Aires, Argentina.
Corresponding author:
BACKGROUND
The effects of global warming on pinniped populations are evident mainly during the breeding season or the preceding months, due to variations in the availability of breeding sites and/or related to changes in the abundance and distribution of their principal prey-items (Fig. 1).
The Southern Elephant Seal (SES) Mirounga leonina is an ice-tolerant species with a circumpolar distribution. Their annual phenology varies between populations.
At the South Shetland Islands:
- Pre-breeding foraging trips extend from March to September
- Reproduction occurs during the austral spring (October – December) in ice-free beaches
- The overall foraging habitat of SES females that breed at Potter Peninsula (PP) includes the southwest of the Antarctic Peninsula and the surroundings of the South Shetland Islands.
Objective:
To examine the SES breeding population fluctuation at PP and its relationship with sea ice extent at the Western Antarctic Peninsula (WAP).
Figure 1: Diagram of the effects of global warming
MATERIALS AND METHODS
Study Area:
ASPA No 132, Stranger Point, 25 de Mayo/King George Island (62°14´S, 58°40´W), South Shetland (Fig. 2).
Population size of breeding females:
Cows were counted during the austral spring and summer, from 1995 to 2010.
Following Caughley (1977) the intrinsic rate of natural change was calculated as:
r = ln (nt/n0)/t
where: n = number of breeding females
Survival and return rates:
From 2000 weaned SES pups were branded and tagged with flipper plastic tags. Data on animals recaptured over an 8-year period (2004 to 2011) were analyzed.
Survival: percentage of cows alive in year x of the total number of marked adult (over 4 years old) females resighted during the breeding or moulting periods.
Return rate: percentage of breeding females which returned to breed in at least one subsequent year.
The results represent minimal values since there is not available information for the whole subpopulation.
Sea ice extent:
For the study period, average monthly sea ice coverage for the WAP, was provided by the Palmer LTER program (
To evaluate putative effects of sea ice extent:
- July-September average: relation to breeding sites availability
- March-September average: relation to the pre-breeding foraging trip
Statistical analyses:
The strength of the association between pairs of variables was measured using Pearson product–moment correlations. We used simple linear regressions to test for trends in the abundance of females.
Mean ± standard deviation and coefficient of variation (CV) of survival and return rates were calculated to determine the interannual variability.
Figure 2: Study area. Antarctic Special Protected Area No 132, Potter Peninsula, 25 de Mayo Island.(Modified from KGIS Project)
Figure 3: SES breeding females seasonal variation and overall decline at Potter Peninsula from 1995 to 2010.
RESULTS
An overall cow population decline was recorded (F1,12 = 15.42, P = 0.002, r2 = 0.56) between 1995 and 2010 with an intrinsic rate of natural change of Maximum number of breeding females showed an inter-annual variation (Fig.3).
The return rate fluctuated seasonally (29.71 ± %, CV = 34.20%) while survival rate remained constant (80.62 ± %, CV = 12.79%) Fig. 4.
The number of breeding females and WAP sea ice extent showed a positive linear relationship both for July-September (r2 = 0.52, P = 0.004, n = 14) and March-September (r2 = 0.35, P = 0.024, n = 14).
Both survival and return rates were not related to sea ice extent (July-September; March-September; r2 < 0.4, P > 0.05, n = 6).
Figure 4: SES breeding females survival and return rates at Potter Peninsula from 2004 to 2010.
DISCUSSION and CONCLUSIONS
For the study period, breeding female population size showed a strong interannual fluctuation and an overall decline of 33% (Fig. 3). Considering that 825 cows bred during 1980 at PP (Vergani and Stanganelli 1990), the female SES number declined a 62% in the last 30 years ( ).
SES at Potter Peninsula could choose to skip breeding seasons in this area, as tagged females were resighted during moulting but not during the breeding haulout in the same season.
A reduction of the WAP sea ice extent was correlated with a lower number of breeding females at PP. Nevertheless, sea ice extent was not related to either return or survival rates, possibly due to the number of years used in the analysis.
Two main effects of global warming:
Changes in prey distribution and abundance can precede a shift in predators demography (Simmonds and Isaac 2007).
Reduction of sea ice creates additional areas for breeding and moulting for ice-tolerant seals (Siniff et al. 2008).
SES population fluctuation at Potter Peninsula
Since SES show a relative plasticity in their diet composition (Daneri et al. 2000, Daneri and Carlini 2002) and fidelity to specific foraging regions gives them an energetic advantage (Bradshaw et al. 2004), it could be convenient to choose new breeding areas (Authier et al. 2012), thus minimizing the mean distance between the breeding area and foraging locations. Movement between sub-populations as a response to an emerging habitat was proposed by Hall et al. (2006) and De Bruyn et al. (2009) in warm climate Holocene conditions. At Anvers Island, South WAP, breeding female populations are increasing while sea ice extent is decreasing (Siniff et al. 2008). On the contrary, at 25 de Mayo Island the opposite relation was recorded.
We propose that the fluctuation observed in the SES population, and part of the decline observed, could be related to the choice of new ice-free breeding sites, closer to the foraging areas.
Available data are part of the Marine Mammals Research Program of the IAA, led from 1995 to 2010 by Dr. Alejandro Carlini.
Acknowledgments
Sharon Stammerjohn. Palmer Station, Antarctica Long-Term Ecological Research Program. Data from the Palmer LTER data repository were supported by Office of Polar Programs, NSF Grants OPP , OPP and OPP
To the Dirección Nacional del Antártico, Instituto Antártico Argentino for the logistic support. We want to thank all the people who helped collect data associated with this study.