Female Southern Elephant Seals Research Articles

Flexible preference of southern elephant seals for distinct mesoscale features within the Antarctic Circumpolar Current

The open ocean is a highly variable environment where marine top predators are thought to require optimized foraging strategies to locate and capture prey. Mesoscale and sub-mesoscale features are known to effect planktonic organisms but the response of top predators to these features results from behavioural choices and is poorly understood. Here, we investigated a multi-year database of at-sea distribution and behaviour of female Southern elephant seals (Mirounga leonina) to identify their preference for specific structures within the intense eddy field of the dynamic Antarctic Circumpolar Current (ACC). We distinguished two behavioural modes, i.e. travelling and intensive foraging, using state-space modelling. We employed multisatellite Lagrangian diagnostics to describe properties of (sub-)mesoscale oceanic circulation. Statistical analyses (GAMMs and Student’s t-tests) revealed relationships between elephant seal behaviour and (sub-)mesoscale features during the post-moulting period (January–August): travelling along thermal fronts and intensive foraging in cold and long-lived mesoscale water patches. A Lagrangian analysis suggests that these water patches – where the prey field likely developed and concentrated – corresponded to waters which have supported the bloom during spring. In contrast, no clear preference emerged at the (sub-)mesoscale during the post-breeding period (October–December), although seals were distributed within the Chlorophyll-rich water plume detaching from the plateau. We interpret this difference in terms of a seasonal change in the prey field. Our interdisciplinary approach contributes to elucidate the foraging strategies of top predators in a complex and dynamic environment. It also brings top down insights on prey distribution in remote areas where information on mid-trophic levels are strongly lacking and it identifies important physical-biological interactions relevant for ecosystem modelling and management.

Age-specific cost of first reproduction in female southern elephant seals.

When to commence breeding is a crucial life-history decision that may be the most important determinant of an individual's lifetime reproductive output and can have major consequences on population dynamics. The age at which individuals first reproduce is an important factor influencing the intensity of potential costs (e.g. reduced survival) involved in the first breeding event. However, quantifying age-related variation in the cost of first reproduction in wild animals remains challenging because of the difficulty in reliably recording the first breeding event. Here, using a multi-event capture-recapture model that accounts for both imperfect detection and uncertainty in the breeding status on an 18-year dataset involving 6637 individuals, we estimated age and state-specific survival of female elephant seals (Mirounga leonina) in the declining Macquarie Island population. We detected a clear cost of first reproduction on survival. This cost was higher for both younger first-time breeders and older first-time breeders compared with females recruiting at age four, the overall mean age at first reproduction. Neither earlier primiparity nor delaying primiparity appear to confer any evolutionary advantage, rather the optimal strategy seems to be to start breeding at a single age, 4 years.

Variation in body condition during the post-moult foraging trip of southern elephant seals and its consequences on diving behaviour.

Mature female southern elephant seals (Mirounga leonina) come ashore only in October to breed and in January to moult, spending the rest of the year foraging at sea. Mature females may lose as much as 50% of their body mass, mostly in lipid stores, during the breeding season due to fasting and lactation. When departing to sea, post-breeding females are negatively buoyant, and the relative change in body condition (i.e. density) during the foraging trip has previously been assessed by monitoring the descent rate during drift dives. However, relatively few drift dives are performed, resulting in low resolution of the temporal reconstruction of body condition change. In this study, six post-breeding females were equipped with time-depth recorders and accelerometers to investigate whether changes in active swimming effort and speed could be used as an alternative method of monitoring density variations throughout the foraging trip. In addition, we assessed the consequences of density change on the swimming efforts of individuals while diving and investigated the effects on dive duration. Both descent swimming speed and ascent swimming effort were found to be strongly correlated to descent rate during drift dives, enabling the fine-scale monitoring of seal density change over the whole trip. Negatively buoyant seals minimized swimming effort during descents, gliding down at slower speeds, and reduced their ascent swimming effort to maintain a nearly constant swimming speed as their buoyancy increased. One per cent of seal density variation over time was found to induce a 20% variation in swimming effort during dives with direct consequences on dive duration.

Thermohaline structure and water masses in the north of Antarctic Peninsula from data collected in situ by southern elephant seals (Mirounga leonina)

A Península Antártica Oeste é uma das regiões do planeta que está se aquecendo rapidamente e apresenta alta diversidade animal principalmente ao longo de sua plataforma continental. Esta região também é de grande importância devido à mistura causada pela interação de águas do Mar de Weddell (MW), do Estreito de Bransfield (EB) e da Corrente Circumpolar Antártica (CCA) que transmitem características termohalinas e nutrientes de diferentes locais de formação e mais tarde se conectam com todos os oceanos do planeta. Entretanto, estudos sobre a variabilidade temporal das condições oceanográficas desta região que consequentemente determinam a formação de massas d'água, são escassos em função das dificuldades logísticas envolvidas em levantamentos oceanográficos e monitoramentos tradicionais durante os meses de inverno. Para esse trabalho, variações da estrutura termohalina e das massas d'água nas proximidades e abaixo do gelo marinho na região norte da Península Antártica (PA) e Mar da Escócia (ME) foram registradas entre os meses de fevereiro e novembro de 2008 por duas fêmeas de elefante-marinho do sul (EMS, Mirounga leonina) instrumentadas com Conductivity - Temperature - Depth/Satellite-Relay Data Logger (CTD - SRDL). 1330 perfis verticais de temperatura e salinidade foram coletados por estes indivíduos que foram instrumentados pelo Projeto MEOP-BR na Ilha Elefante, Shetlands do Sul. Esses perfis, juntamente com diagramas de estado espalhado permitiram a identificação de massas d'água e suas alterações na estrutura vertical oceânica. Dentre as massas d'água identificadas citamos: Água de Superfície Antártica (AASW), Água de Inverno (WW), Água Cálida Profunda (WDW), Água Cálida Profunda Modificada (MWDW), Água Circumpolar Profunda (CDW), Água Profunda Circumpolar Superior (UCDW), Água Profunda Circumpolar Inferior (LCDW) e Água de Plataforma de Gelo (ISW). Nossos resultados mostram que a estrutura vertical oceânica sofre alterações que tradicionalmente não podem ser monitoradas, principalmente durante o inverno austral, e que os EMS são importantes e modernas plataformas para coleta de dados oceanográficos, permitindo um aumento em nosso conhecimento dos processos oceanográficos na região antártica.

Spatial variation in female southern elephant seal mass change assessed by an accurate non-invasive photogrammetry method

AbstractPhysically weighing large marine mammals sequentially over time has presented researchers with a logistical challenge and has severely limited sample sizes. Using a well-established photogrammetry method we developed a simple mathematical method to calculate accurate mass measurements at specific stages in the life cycle of a top marine predator. Female southern elephant seals (n= 23) at Marion Island were sampled sequentially using photogrammetry and three-dimensional models (based on each photogrammetry project) were built for estimation of body mass. Simple equations were applied to obtain mass at critical instances in their life cycle. Marion Island elephant seal mass data was compared to data obtained from physically weighed elephant seals from King George, South Georgia and Macquarie islands. Females from Marion Island are smaller, but their percentage lactation mass loss is similar to females from these other populations. The similarity of percentage mass loss during lactation between different female populations illustrates the accuracy and practicality of the photogrammetric method over a temporal scale. Photogrammetric mass estimation can be used alongside datasets of physically weighed animals and can greatly benefit ecology and life history studies.

Natal site fidelity by breeding female southern elephant seals in relation to their history of participation in the winter haulout

Of the four types of terrestrial haulout periods undertaken by southern elephant seals Mirounga leonina, only the purpose of the winter haulout is unknown. Returning to a haulout site from distant pelagic foraging grounds bears significant costs in terms of increased energy expenditure, reduced foraging time and increased exposure to predation; therefore, each haulout must serve a purpose. We examined the hypothesis that the winter haulout serves to maintain familiarity with the natal site, thereby increasing site fidelity. To this end, we analysed a long-term mark-recapture dataset for female southern elephant seals at Marion Island, Southern Ocean. Results indicate that, whereas greater natal site fidelity as primiparous females was associated with recorded presence ashore at the study site during the winter haulout as immatures, this was not the case for multiparous females. Furthermore, recorded presence ashore during both the moult haulouts as immatures, and all haulouts as immatures, irrespective of haulout type, was also associated with increased site fidelity. This suggests that any haulout at the natal island as an immature seal, whether for the moult or winter haulout, assists in maintaining site fidelity. Therefore, while the winter haulout facilitates greater natal site fidelity, whether this is the sole reason for this terrestrial period remains uncertain.

Foraging in the Darkness of the Southern Ocean: Influence of Bioluminescence on a Deep Diving Predator

How non-echolocating deep diving marine predators locate their prey while foraging remains mostly unknown. Female southern elephant seals (SES) (Mirounga leonina) have vision adapted to low intensity light with a peak sensitivity at 485 nm. This matches the wavelength of bioluminescence produced by a large range of marine organisms including myctophid fish, SES’s main prey. In this study, we investigated whether bioluminescence provides an accurate estimate of prey occurrence for SES. To do so, four SES were satellite-tracked during their post-breeding foraging trip and were equipped with Time-Depth-Recorders that also recorded light levels every two seconds. A total of 3386 dives were processed through a light-treatment model that detected light events higher than ambient level, i.e. bioluminescence events. The number of bioluminescence events was related to an index of foraging intensity for SES dives deep enough to avoid the influence of natural ambient light. The occurrence of bioluminescence was found to be negatively related to depth both at night and day. Foraging intensity was also positively related to bioluminescence both during day and night. This result suggests that bioluminescence likely provides SES with valuable indications of prey occurrence and might be a key element in predator-prey interactions in deep-dark marine environments.

Stable isotope analyses reveal individual variability in the trophic ecology of a top marine predator, the southern elephant seal

Identifying individuals' foraging strategies is critical to understanding the ecology of a species, and can provide the means to predict possible ecological responses to environmental change. Our study combines stable isotope analysis and satellite telemetry to study the variability in individual foraging strategies of adult female southern elephant seals (Mirounga leonina). Our hypothesis is that female elephant seals from the Western Antarctica Peninsula (WAP) display individual specialization in their diets. We captured adult female elephant seals (n = 56, 2005-2009) at Livingston Island (Antarctica), and instrumented them with SMRU-CTD satellite tags. We collected blood, fur, and vibrissae samples for δ(13)C and δ(15)N analyses. The mean values for all vibrissae were -21.0 ± 0.7‰ for δ(13)C, and 10.4 ± 0.8‰, for δ(15)N. The individual variability of δ(13)C (60%) was more important than the within-individual variability (40%) in explaining the total variance observed in our data. For δ(15)N, the results showed the opposite trend, with the within-individual variability (64%) contributing more to the total variance than the individual variability (36%), likely associated with the effect that the fasting periods have on δ(15)N values. Most individuals were specialists, as inferred from the low intra-individual variability of δ(13)C values with respect to the population variability, with half the individuals utilizing 31% or less of their available niche. We found eight different foraging strategies for these animals. Female elephant seals from the WAP are a diverse group of predators with individuals utilizing only a small portion of the total available niche, with the consequent potential to expand their foraging habits to exploit other resources or environments in the Southern Ocean.

Water column use and forage strategies of female southern elephant seals from Marion Island

The at-sea behaviour of marine top predators provides valuable insights into the distribution of prey species and strategies used by predators to exploit patchily distributed resources. We describe the water column usage and dive strategies of female southern elephant seals from Marion Island tracked between 2004 and 2008. Dives representing increases in forage effort were identified using a method that combines dive type analyses and the calculation of relative amounts of time that animals spend in the bottom phases of dives. Results from this analysis indicate that female elephant seals from Marion Island tend to display lower levels of forage effort closer to the island and display intensive opportunistic forage bouts that occur at a minimum distance of approximately 215 km from the island. Females from Marion Island dived deeper and for longer periods of time, compared to females from other populations. Most animals displayed positive diel vertical migration, evidently foraging pelagically on vertically migrating prey. A few animals displayed periods of reverse (negative) diel vertical migration, however, diving to deeper depths at night, compared to daytime. This behaviour is difficult to explain and prey species targeted during such periods unknown. Our results illustrate plasticity in foraging behaviour of southern elephant seals, as well as inter-population differences in forage strategies.

Diet of juvenile southern elephant seals reappraised by stable isotopes in whiskers

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 424:247-258 (2011) - DOI: https://doi.org/10.3354/meps08769 Diet of juvenile southern elephant seals reappraised by stable isotopes in whiskers China Newland1,*, Iain C. Field1,2, Yves Cherel3, Christophe Guinet3, Corey J. A. Bradshaw4,5, Clive R. McMahon1,6, Mark A. Hindell1 1Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, PO Box 252-05, Hobart, TAS 7001, Australia 2Marine Mammal Research Group, Graduate School of the Environment, Macquarie University, Sydney, NSW 2109, Australia 3Centre d’Etudes Biologiques de Chizé, UPR 1934 du CNRS, BP 14, 79360 Villiers-en-Bois, France 4The Environment Institute and School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia 5South Australian Research and Development Institute, PO Box 120, Henley Beach, South Australia 5022, Australia 6School for Environmental Research, Charles Darwin University, Casuarina Campus, Darwin, Northern Territory 0909, Australia *Email: cnewland@utas.edu.au ABSTRACT: Declines in marine predator populations have been attributed to anthropogenic activity and environmental change. Southern elephant seals Mirounga leonina are major consumers of biomass in the eastern region of the Southern Ocean and have been declining in numbers since the 1960s. Previous studies have identified evidence for habitat and diet partitioning over a range of spatial and temporal scales between juveniles and adults in the Macquarie Island population. We first analysed the stable isotopes (SI) of 6 entire vibrissae from a dead adult female southern elephant seal from Kerguelen Islands to determine moult and growth patterns. Secondly we analysed the SI from the vibrissae of 102 juvenile southern elephant seals to investigate diet. The results from the growth pattern analysis indicated that vibrissae do not grow or moult simultaneously. However, it is likely that at least part of the vibrissae will have been produced sometime during the most recent trip to sea and will give a broad indication of diet. The subsequent SI analysis confirmed that juveniles are consuming greater proportions of fish species, and identified myctophids as the primary component of juvenile diet. Myctophids are also consumed by king penguins Aptenodytes patagonicus which have greatly increased in numbers recently in the Macquarie Island area. This may have presented the juvenile southern elephant seals with increased competition and may influence survival. KEY WORDS: Stable isotopes · Vibrissae growth patterns · Diet · Fish · Squid · Resource partitioning · Inter-specific competition Full text in pdf format PreviousNextCite this article as: Newland C, Field IC, Cherel Y, Guinet C, Bradshaw CJA, McMahon CR, Hindell MA (2011) Diet of juvenile southern elephant seals reappraised by stable isotopes in whiskers. Mar Ecol Prog Ser 424:247-258. https://doi.org/10.3354/meps08769 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 424. Online publication date: March 01, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Population trends of female Elephant Seals breeding on the Courbet Peninsula, îles Kerguelen

Southern Elephant Seals are upper marine predators of the Southern Ocean. As such, their population dynamics and fluctuations reflect environment conditions. Their worldwide populations crashed during the second half of the twentieth century for reasons not yet completely elucidated. Here, we studied the largest population of Southern Elephant Seals within the South Indian Ocean that are breeding on iles Kerguelen. In a previous analysis, Guinet et al. in Antarct Sci 11:193–197, 1999) suggested that the decline on iles Kerguelen might be over, as observed elsewhere. Using 10 years of additional data, we updated this analysis using state-of-the-art statistical methods to account for most uncertainties associated with count data. We showed that the population of female Southern Elephant Seals breeding on iles Kerguelen has been stable over the past 20 years. Despite concomitant global changes within the Southern Ocean, we did not find any evidence of a phenological shift in peak haul-out date of breeding females between the 1970s and the 2000s.

Stable isotopes reveal the trophic position and mesopelagic fish diet of female southern elephant seals breeding on the Kerguelen Islands

Trophic interactions between organisms are the main drivers of ecosystem dynamics, but scant dietary information is available for wide-ranging predators during migration. We investi- gated feeding habits of a key consumer of the Southern Ocean, the southern elephant seal Miroun- gia leonina, by comparing its blood δ 13 C and δ 15 N values with those of various marine organisms, including crustaceans, squid, fishes, seabirds and fur seals. At the end of winter, δ 13 C values (-23.1 to -20.1‰) indicate that female elephant seals forage mainly in the vicinity of the Polar Front and in the Polar Frontal Zone. Trophic levels derived from δ 15 N values (trophic level = 4.6) show that the southern elephant seal is a top consumer in the pelagic ecosystem that is dominated by colossal squid. The mean δ 15 N value of seals (10.1 ± 0.3‰) indicates that they are not crustacean eaters, but instead feed on crustacean-eating prey. Surprisingly, most of the previously identified prey species have isotope δ 13 C and δ 15 N values that do not fit with those of potential food items. The most singular pattern to emerge from δ 15 N values of predators and prey is that female seals are likely to feed on myctophid fishes, not squid or Patagonian toothfish. We therefore suggest that they occupy a unique trophic niche amongst the guild of air-breathing, diving vertebrates by feeding on mesopelagic fish at great depths. In turn, this finding emphasizes the role of myctophids in oceanic waters and will help to quantify and model fluxes of matter and energy within the pelagic ecosystem of the Southern Ocean.

Tracking changes in relative body composition of southern elephant seals using swim speed data

Changes in buoyancy during an animal’s time at sea are a powerful tool for inferring spatial and temporal foraging success. Buoyancy can be difficult to measure, but in some species of seal, drift components of dives can be used. We used swim speed data from adult female southern elephant seals Mirounga leonina using geo-locating velocity-time-depth recorders during 2004 postlactation (PL; n = 7) and 2002, 2004 and 2005 post-moult (PM; n = 18) foraging trips to detect periods of passive drifting during diving. In addition to the characteristic drift dives of elephant seals, drifting also occurred during putative foraging dives. We used generalised linear models (GLMs) to examine the relationship between body lipid content measured on land and several diving variables collected within a week of these measurements being taken. The strongest support (deviance explained = 90%) was for the model including drift rate (77%), seal length (12%) and descent rate (2%). Estimates of body lipid, based on the GLM, were predicted for each day of the foraging trips. Areas where seals increased their relative lipid content from one day to the next corresponded well with areas in which the seals spent the greatest amount of time. Inferring foraging success from positive changes in drift rate has so far been limited to elephant seals which perform characteristic drift dives, but the addition of swim speed data to detect short periods of stationary behaviour allows for this method to be expanded to a greater range of ocean predators.

Stable isotopes document winter trophic ecology and maternal investment of adult female southern elephant seals (Mirounga leonina) breeding at the Kerguelen Islands

Individual specialisation is widespread and can affect a population’s ecological and evolutionary dynamics. Whether intra-specific niche differences can influence reproductive investment was examined in a marine mammal, the southern elephant seal (Mirounga leonina), whose females were known to forage in two different areas during the austral winter. The study was conducted at Kerguelen Islands (49°21′S, 70°18′E), southern Indian Ocean, in late winter–early spring 2006. Pups were used as proxies of their mothers’ biology and combined information on their weaning mass (a proxy of females’ foraging success and short-term fitness) together with their blood δ13C value (a proxy of female foraging zone). First, the use of isotopic signature of pups was validated to study the female foraging ecology during their pre-breeding trip by demonstrating that δ13C and δ15N values of pups and their mothers were positively and linearly correlated. Then, blood samples were taken from a large number of newly-weaned pups, which were also weighed, to provide information at the population level. Estimated δ13C values of female seals encompassed a large range of values (from −23.7 to −19.1‰) with an unimodal frequency distribution, suggesting no contrasted foraging areas within the population. No significant relationship was found between pup weaning mass and their carbon signature, indicating no link between female foraging areas and maternal foraging success and investment. Finally, blood δ13C and δ15N values gave new insights into southern elephant seal ecology, suggesting that females mainly foraged north of the Polar Front where they preyed upon myctophid fish in late winter.

Seasonal survival and the relative cost of first reproduction in adult female southern elephant seals

The annual cycle of adult female southern elephant seals (Mirounga leonina) can be divided into 2 pelagic phases, separated by relatively short terrestrial phases: breeding and molting. We used resighting data collected from tagged female southern elephant seals at Marion Island during 1986–1999 to investigate seasonal survival during the 2 pelagic phases in relation to reproductive experience. Mean postbreeding (pelagic phase between breeding and molting, about 62 days) survival of primiparous females was 0.830 compared to 0.912 for moreexperienced females. Postmolting (pelagic phase between molting and breeding, about 255 days) survival was 0.847 (0.960 when rescaled to 62 days for comparative purposes) and was not dependent on reproductive experience. Postbreeding survival of experienced females was higher than postmolting survival, but per unit time the opposite applied. A 2-stage survival model, in which survival was constrained to be constant before 1994 (when the population was declining) and from 1994 onward (during the stable phase), had overwhelming support from the data. Postbreeding survival of primiparous females increased from 0.799 before 1994 to 0.880 from 1994 onward. Postmolting survival of all females also increased from 0.817 to 0.872 over the same period. Postbreeding and postmolting mortality risk varied independently over time, demonstrating the importance of an intra-annual approach in population studies of southern elephant seals.

The effects of El Niño–La Niña on reproductive parameters of elephant seals feeding in the Bellingshausen Sea

AbstractAim To assess the impacts of El Niño–La Niña events on the pup weaning mass and diet of female southern elephant seals (Mirounga leonina) feeding in the Bellingshausen Sea, Antarctica, and to understand the ecological processes that drive these impacts.Location Atlantic southern elephant seal weaning mass and diet were measured at King George Island (62º14′ S, 58º30′ W). Feeding areas for pregnant female seals from King George Island are located west of Alexander Island in the Bellingshausen Sea.Methods Data on weaning mass were collected between 1985 and 1994 during the breeding season (September–November). Moulting females were anaesthetized and cephalopod beaks were isolated and identified from stomach contents obtained from stomach lavages. Sea‐surface temperature anomaly (SSTA) data for the ‘El Niño 3.4’ geographical region (5º N–5º S, 120º W–170º W) were used to define El Niño–Southern Oscillation (ENSO) event years (grouped as El Niño, La Niña and Neutral) as well as the strength of each ENSO event year. Using data from the US National Center for Environmental Prediction, temperature, sea ice concentration and atmospheric pressure anomalies in the Bellingshausen Sea were calculated from March to August, corresponding to the feeding period of pregnant female seals.Results Positive temperature anomalies and negative pressure anomalies in the Bellingshausen Sea were observed during La Niña years and negative temperature anomalies and positive pressure anomalies during El Niño years. These data correlate with sea ice concentration anomalies, which are highly negative during La Niña years and highly positive during El Niño years. Warm temperature conditions in the Bellingshausen Sea during La Niña years are strongly related to both higher weaning mass in elephant seals and to an increase in squid beaks in the stomach contents of females.Main conclusions It is possible that higher elephant seal weaning masses in La Niña years correlate with warmer waters in the Bellingshausen Sea leading to the rapid growth of squid and their more frequent descents to depths frequented by elephant seals. This results in increased predation by pregnant females, leading to a greater mass among weaned pups. This hypothesis may guide future research about interactions between climate and the marine biosphere.

The population trend of southern elephant seals (Mirounga leonina L.) at Macquarie Island (1952–2004)

Total numbers of adult female southern elephant seals (cows) breeding at Macquarie Island were determined for 19 of the 52 year period between 1952 and 2004. Totals for 1952–1987 (exc. 1959 and 1985) were estimated from the relationship between censuses of the isthmus study area and concurrent censuses for the whole island. Totals for 1987–2004 were obtained by direct census of the entire island in mid-October. Cow numbers decreased from a maximum of about 40,000 in the 1950s to a minimum of 18,300 in 2000, but then increased slightly to 19,200 in 2004. Nonlinear and post-hoc linear analysis of the count data identified 1999 as the year when the exponential rate of change (r) slowed from −1.4% per annum to near zero. The rate of change was not uniform for each census sub-area counted (1987–2004), suggesting that certain terrestrially based density-dependent mechanisms were influencing the annual distribution of cows.

Sex-specific foraging strategies and resource partitioning in the southern elephant seal (Mirounga leonina)

The evolution of resource specializations is poorly understood, especially in marine systems. The southern elephant seal (Mirounga leonina) is the largest of the phocid seals, sexually dimorphic, and thought to prey predominantly on fish and squid. We collected vibrissae from male and female southern elephant seals, and assessed stable C and N isotope ratios along the length of the vibrissae. Given that whiskers grow slowly, this sampling strategy reflects any variation in feeding behaviour over a period of time. We found that isotopic variation among females was relatively small, and that the apparent prey choice and trophic level of females was different from that for males. Further, males showed a very broad range of trophic/prey choice positions, grouped into several clusters, and this included isotopic values too low to match a broad range of potential fish and cephalopod prey tested. One of these clusters overlapped with data for South American sea lions (Otaria flavescens), which were measured for comparison. Both male southern elephant seals and southern sea lions forage over the continental shelf, providing the potential for competition. We discuss the possibility that individual southern elephant seals are pursuing specialist foraging strategies to avoid competition, both with one another, and with the South American sea lions that breed nearby.

Development of individual recognition of female southern elephant seals, Mirounga leonina, from Punta Norte Península Valdés, applying principal components analysis

AbstractAim The development of software able to provide individual recognition of southern elephant seals, as a tool to study colonies. This analysis was performed within a framework of studies concerning environmental dispersion produced by the El Niño Southern Oscillation effect in the Southern Ocean Ecosystem.Location Digital photographs of reproductive female elephant seals were taken at Punta Norte (Península Valdés, Patagonia; 42°05′ S, 63°45′ W) during the 2002 breeding season (August to November). The data set under analysis is composed of 96 elephant seal images for a population of 56 individuals.Method Identification of specimens was carried out using digital pictures taken with a digital video camera, and processed through the ‘Eigenfaces’ method, which is based on principal components analysis. Special care was taken to control possible variations among images of the same individual, like distance, angle, light intensity, etc. To deal with these variations, an initial alignment procedure is proposed to have all images framed; in addition an initial histogram equalization was used which attenuates any potential variation in light intensity. The software was developed in IDL5.5 language.Results A complete set of empirical results is displayed showing the potential effectiveness of this technique. Individual recognition and pertinence to different population subsets (harems) tests have been carried out. A principal result of this work is that all 96 elephant seal images (representing 56 individuals) were correctly identified.Conclusion The Eigenfaces method can be used successfully for identification of elephant seals. With the appropriate preparatory treatment of images, high performance results can be expected.

Growth of female southern elephant seals Mirounga leonina at Macquarie Island

Body growth of 137 female southern elephant seals (Mirounga leonina) over 1 year of age was investigated at subantarctic Macquarie Island. An asymptotic straight line, snout–tail body length of 2.57±0.03 m was estimated to be attained at 9 years of age, using a three-parameter Gompertz equation. A significant increase of approximately 0.1 m (5%) in mean body length of females between 1 and 10 years of age was estimated to have occurred between the 1950–1960s and 1990s at Macquarie Island. This is consistent with a reduction in both the rate of population decline and the age of onset of sexual maturity. Age determination using dental cementum layers and the importance of standardised measurements in pinniped growth studies are discussed.