Publications (45)108.45 Total impact
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Article: How large is large: estimating ecologically meaningful isotopic differences in observational studies of wild animals.
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ABSTRACT: In ecological studies of wildlife movements and foraging, bio-logging and isotopic data are routinely collected and increasingly analyzed in tandem. Such analyses have two shortcomings: (1) small sample size linked with the number of telemetric tags that can be deployed, and (2) the observational nature of isotopic gradients. Wildlife ecologists are thus put in a statistical conundrum known as the small n, large p problem. Using shrinkage regression, which directly addresses the issue of accurately estimating effects from sparse data, we studied what counts as a biologically meaningful isotopic difference (a prerequisite to delineate isoscapes) in the southern elephant seal (Mirounga leonina), a large and elusive marine predator. Seals foraging in Antarctic waters had a lower carbon isotopic value (by ≈ 2‰) than seals foraging either in the interfrontal zone or on the Kerguelen Plateau. The latter two foraging strategies were indistinguishable on the sole basis of δ(13) C values with our data. Shrinkage regression is a conservative statistical technique that has wide applicability in isotopic ecology to help separate robust biological signals from noise. Copyright © 2012 John Wiley & Sons, Ltd.Rapid Communications in Mass Spectrometry 12/2012; 26(23):2657-64. · 2.79 Impact Factor -
Article: Integrative modelling of animal movement: incorporating in situ habitat and behavioural information for a migratory marine predator.
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ABSTRACT: A fundamental goal in animal ecology is to quantify how environmental (and other) factors influence individual movement, as this is key to understanding responsiveness of populations to future change. However, quantitative interpretation of individual-based telemetry data is hampered by the complexity of, and error within, these multi-dimensional data. Here, we present an integrative hierarchical Bayesian state-space modelling approach where, for the first time, the mechanistic process model for the movement state of animals directly incorporates both environmental and other behavioural information, and observation and process model parameters are estimated within a single model. When applied to a migratory marine predator, the southern elephant seal (Mirounga leonina), we find the switch from directed to resident movement state was associated with colder water temperatures, relatively short dive bottom time and rapid descent rates. The approach presented here can have widespread utility for quantifying movement-behaviour (diving or other)-environment relationships across species and systems.Proceedings of the Royal Society B: Biological Sciences 11/2012; · 5.41 Impact Factor -
Article: Stable isotopes document winter trophic ecology and maternal investment of adult female southern elephant seals (Mirounga leonina) breeding at the Kerguelen Islands
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ABSTRACT: 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.Marine Biology 04/2012; 155(4):413-420. · 2.28 Impact Factor -
Article: Population trends of female Elephant Seals breeding on the Courbet Peninsula, îles Kerguelen
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ABSTRACT: 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 îles Kerguelen. In a previous analysis, Guinet et al. in Antarct Sci 11:193–197, 1999) suggested that the decline on îles Kerguelen might be over, as observed elsewhere. Using 10years 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 îles Kerguelen has been stable over the past 20years. 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. Keywords Mirounga leonina –Long-term monitoring–Population trend–Bayesian–Southern OceanPolar Biology 04/2012; 34(3):319-328. · 1.66 Impact Factor -
Article: O' mother where wert thou? Maternal strategies in the southern elephant seal: a stable isotope investigation.
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ABSTRACT: Maternal effects are widespread in ecology and can alter the dynamics of a population. We investigated the impact of maternal foraging strategies on offspring weaning mass--a proxy of maternal foraging success and of offspring survival--in southern elephant seals on îles Kerguelen. Using 4 years of data, we modelled pup weaning mass as a two-component mixture and used blood stable isotope values to discriminate between maternal foraging strategies previously identified from bio-logging studies. Carbon isotope ratio was a strong predictor of weaning mass, but the relationship was non-monotonic in contrast to a priori expectations. Females foraging in the interfrontal zone weaned pups with a smaller mass compared with females foraging in Antarctic waters. Pup mass was positively correlated with a proxy of global primary production in the interfrontal zone for small weanlings. Maternal effects, via a poor foraging efficiency in the 1970s, may help explain the large population decrease observed at that time on îles Kerguelen because of an overall decrease in pup weaning mass, survival and subsequent recruitment.Proceedings of the Royal Society B: Biological Sciences 03/2012; 279(1738):2681-90. · 5.41 Impact Factor -
Article: Hormonal responses to extreme fasting in subantarctic fur seal (Arctocephalus tropicalis) pups.
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ABSTRACT: Surviving prolonged fasting implies closely regulated alterations in fuel provisioning to meet metabolic requirements, while preserving homeostasis. Little is known, however, of the endocrine regulations governing such metabolic adaptations in naturally fasting free-ranging animals. The hormonal responses to natural prolonged fasting and how they correlate to the metabolic adaptations observed, were investigated in subantarctic fur seal (Arctocephalus tropicalis) pups, which, because of the intermittent pattern of maternal attendance, repeatedly endure exceptionally long fasting episodes throughout their development (1-3 mo). Phase I fasting was characterized by a dramatic decrease in plasma insulin, glucagon, leptin, and total l-thyroxine (T(4)) associated with reductions in mass-specific resting metabolic rate (RMR), plasma triglycerides, glycerol, and urea-to-creatine ratio, while nonesterified fatty acids (NEFA) and β-OHB increased. In contrast, the metabolic steady-state of phase II fasting reached within 6 days was associated with minimal concentrations of insulin, glucagon, and leptin; unchanged cortisol and triiodothyronine (T(3)); and moderately increased T(4). The early fall in insulin and leptin may mediate the shift to the strategy of energy conservation, protein sparing, and primary reliance on body lipids observed in response to the cessation of feeding. In contrast to the typical mammalian starvation response, nonelevated cortisol and minimal glucagon levels may contribute to body protein preservation and downregulation of catabolic pathways, in general. Furthermore, thyroid hormones may be involved in a process of energy conservation, independent of pups' nutritional state. These original hormonal settings might reflect an adaptation to the otariid repeated fasting pattern and emphasize the crucial importance of a tight physiological control over metabolism to survive extreme energetic constraints.AJP Regulatory Integrative and Comparative Physiology 02/2012; 302(8):R929-40. · 3.34 Impact Factor -
Article: Relationship between Chlorophyll a Concentration, Light Attenuation and Diving Depth of the Southern Elephant Seal Mirounga leonina.
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ABSTRACT: Recently, a number of Antarctic marine environmental studies have used oceanographic parameters collected from instrumented top predators for ecological and physical information. Phytoplankton concentration is generally quantified through active measurement of chlorophyll fluorescence. In this study, light absorption coefficient (K(0.75)) was used as an indicator of phytoplankton concentration. This measurement, easy to obtain and requiring low electric power, allows for assessing of the fine scale horizontal structuring of phytoplankton. As part of this study, Southern elephant seals (SES) were simultaneously equipped with a fluorometer and a light logger. Along the SES tracks, variations in K(0.75) were strongly correlated with chlorophyll, a concentration measured by the fluorometer within the euphotic layer. With regards to SES foraging behaviour, bottom depth of the seal's dive was highly dependent on light intensity at 150 m, indicating that the vertical distribution of SES's prey such as myctophids is tightly related to light level. Therefore, change in phytoplankton concentration may not only have a direct effect on SES's prey abundance but may also determine their vertical accessibility with likely consequences on SES foraging efficiency.PLoS ONE 01/2012; 7(10):e47444. · 4.09 Impact Factor -
Article: Long-term species, sexual and individual variations in foraging strategies of fur seals revealed by stable isotopes in whiskers.
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ABSTRACT: Individual variations in the use of the species niche are an important component of diversity in trophic interactions. A challenge in testing consistency of individual foraging strategy is the repeated collection of information on the same individuals. The foraging strategies of sympatric fur seals (Arctocephalus gazella and A. tropicalis) were examined using the stable isotope signature of serially sampled whiskers. Most whiskers exhibited synchronous δ(13)C and δ(15)N oscillations that correspond to the seal annual movements over the long term (up to 8 years). δ(13)C and δ(15)N values were spread over large ranges, with differences between species, sexes and individuals. The main segregating mechanism operates at the spatial scale. Most seals favored foraging in subantarctic waters (where the Crozet Islands are located) where they fed on myctophids. However, A. gazella dispersed in the Antarctic Zone and A. tropicalis more in the subtropics. Gender differences in annual time budget shape the seal movements. Males that do not perform any parental care exhibited large isotopic oscillations reflecting broad annual migrations, while isotopic values of females confined to a limited foraging range during lactation exhibited smaller changes. Limited inter-individual isotopic variations occurred in female seals and in male A. tropicalis. In contrast, male A. gazella showed large inter-individual variations, with some males migrating repeatedly to high-Antarctic waters where they fed on krill, thus meaning that individual specialization occurred over years. Whisker isotopic signature yields unique long-term information on individual behaviour that integrates the spatial, trophic and temporal dimensions of the ecological niche. The method allows depicting the entire realized niche of the species, including some of its less well-known components such as age-, sex-, individual- and migration-related changes. It highlights intrapopulation heterogeneity in foraging strategies that could have important implications for likely demographic responses to environmental variability.PLoS ONE 01/2012; 7(3):e32916. · 4.09 Impact Factor -
Article: Foraging in the darkness of the southern ocean: influence of bioluminescence on a deep diving predator.
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ABSTRACT: 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.PLoS ONE 01/2012; 7(8):e43565. · 4.09 Impact Factor -
Article: Foraging fidelity as a recipe for a long life: foraging strategy and longevity in male Southern Elephant Seals.
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ABSTRACT: Identifying individual factors affecting life-span has long been of interest for biologists and demographers: how do some individuals manage to dodge the forces of mortality when the vast majority does not? Answering this question is not straightforward, partly because of the arduous task of accurately estimating longevity in wild animals, and of the statistical difficulties in correlating time-varying ecological covariables with a single number (time-to-event). Here we investigated the relationship between foraging strategy and life-span in an elusive and large marine predator: the Southern Elephant Seal (Mirounga leonina). Using teeth recovered from dead males on îles Kerguelen, Southern Ocean, we first aged specimens. Then we used stable isotopic measurements of carbon (δ13C) in dentin to study the effect of foraging location on individual life-span. Using a joint change-point/survival modelling approach which enabled us to describe the ontogenetic trajectory of foraging, we unveiled how a stable foraging strategy developed early in life positively covaried with longevity in male Southern Elephant Seals. Coupled with an appropriate statistical analysis, stable isotopes have the potential to tackle ecological questions of long standing interest but whose answer has been hampered by logistic constraints.PLoS ONE 01/2012; 7(4):e32026. · 4.09 Impact Factor -
Article: Delayed-Mode Calibration of Hydrographic Data Obtained from Animal-Borne Satellite Relay Data Loggers
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ABSTRACT: A delayed-mode calibration procedure is presented to improve the quality of hydrographic data from CTD-Satellite Relay Data Loggers (CTD-SRDL) deployed on elephant seals. This procedure is applied on a dataset obtained with 10 CTD-SRDLs deployed at Kerguelen Islands in 2007. A comparison of CTD-SRDLs with a ship-based CTD system is first presented. A pressure-effect correction, linear with pressure, is deduced for both temperature and salinity measurements. An external field effect on the conductivity sensor is also detected, inducing an additional salinity offset. The salinity offset cannot be estimated directly from the ship-based CTD comparisons, because the attachment of the CTD-SRDL on the seal head modifies the magnitude of the external field effect. Two methods are proposed for estimating a posteriori the salinity offset. The first method uses the stable salinity maximum characterizing the Lower Circumpolar Deep Water (LCDW), sampled by seals foraging south of the Southern Antarctic Circumpolar Current Front. Where this approach is not possible, a statistical method of cross-comparison of CTD-SRDLs surface salinity measurements is used over the sluggish Northern Kerguelen Plateau. Accuracies are respectively estimated as +/- 0.02 degrees C for temperature and +/- 0.1 for derived salinity without corrections. The delayed-mode calibration significantly improves the CTD-SRDL data, improving accuracies to +/- 0.01 degrees C and +/- 0.03, respectively. A better salinity accuracy of +/- 0.02 is achieved when the LCDW method can be used. For CTD-SRDLs where ship-based CTD comparisons are not available, the expected accuracy would be +/- 0.02 degrees C for temperature and +/- 0.04 for the derived salinity.Journal of Atmospheric and Oceanic Technology 01/2011; 28(6):787-801. · 2.26 Impact Factor -
Conference Proceeding: Foraging habitats of top predators, and Areas of Ecological Significance, on the Kerguelen Plateau
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ABSTRACT: Avian and mammalian predators play a key role in the Kerguelen Plateau ecosystem, both with respect to structuring the marine community and its response to anthropogenic influences, such as climate change and commercial fisheries. A powerful way to determine regions of particular ecological importance is to identify Areas of Ecological Significance (AES): regions that are utilized by multiple predator species. Such concentrations of foraging activity are indicative of enhanced primary and/or secondary productivity. These are regions that require specialised management efforts, and which are of considerable importance in the development of ecological models and climate monitoring systems. This study integrates tracking and diving data from a suite of predator species collected as part of both the French and Australian Antarctic programs at Kerguelen Islands and Heard Island respectively. Tracking and/or dive data for Macaroni and King penguins, southern Elephant seals, Antarctic Fur seals and Black-browed albatross were analysed. The estimated path for each animal was derived using state-space models, which also allocated each location to either "transit" or "search" behavioural modes. For diving species, dive depth data were temporally allocated along the path, providing information on three-dimensional habitat use. Areas of Ecological Significance for each species and for the combined suite of predators were identified using Kernel Density analysis. The role of bathymetry, ocean circulation and other environmental factors underlying the AES were established using deterministic models, which can be used to predict predator foraging habitats across the entire plateau.The Kerguelen Plateau: marine ecosystem and fisheries, Concarneau, France; 01/2011 -
Chapter: Geostatistical Modelling of Wildlife Populations: A Non-stationary Hierarchical Model for Count Data
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ABSTRACT: We propose a hierarchical model coupled to geostatistics to deal with a non-gaussian data distribution and take explicitly into account complex spatial structures (i.e. trends, patchiness and random fluctuations). A common characteristic of animal count data is a distribution that is both zero-inflated and heavy tailed. In such cases, empirical variograms are no more robust and most structural analyses result in poor and noisy estimated spatial variogram structures. Thus kriged maps feature a broad variance of prediction. Moreover, due to the heterogeneity of wildlife population habitats, a nonstationary model is often required. To avoid these difficulties, we propose a hierarchical model that assumes that the count data follow a Poisson distribution given a theoretical sighting density which is a latent variable to be estimate. This density is modelled as the product of a positive long range trend by a positive stationary random field, characterized by a unit mean and a variogram function. A first estimate of the drift is used to obtain an estimate of the variogram of residuals including a correction term for variance coming from the Poisson distribution and weights due to the non-constant spatial mean. Then a kriging procedure similar to a modified universal kriging is implemented to directly map the latent density from raw count data. An application on fin whale data illustrates the effectiveness of the method in mapping animal density in a context that is presumably non-stationary.07/2010: pages 1-12; -
Article: Mesoscale eddies as foraging area of a deep-diving predator, the southern elephant seal
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ABSTRACT: Physical processes, such as mesoscale eddies, play an important role in ocean mixing and concentrating biological productivity, which provides great feeding opportunities for many marine top predators. This study describes the presumed foraging behaviour of southern elephant seals Mirounga leonina within an intense eddy field along the Polar Front in relation to the physical features of the eddies targeted by foraging seals, especially in the vertical dimension. Combining satellite measurements and data from tags deployed on elephant seals, we showed that some seals (10 of 38) preferentially selected the edge of cyclonic structures, generally associated with a high eddy kinetic energy, during intensive foraging phases. Moreover, we observed that elephant seals dived deeper within cyclonic eddies compared with anti-cyclonic structures. The behaviour of seals well illustrates the function of the spin-up of cyclonic eddies, where the upwelling of nutrients occurs at the centre, and high productivity and high concentration of prey occur at the edge. Productivity, aggregation of resources and accessibility of prey associated with cyclonic eddies probably act together to determine the selection of these structures by foraging elephant seals. However, eddies are not the only favourable foraging habitat within the Polar Frontal Zone, and the identification of oceanographic conditions associated with these other foraging zones requires further work.Marine Ecology Progress Series 06/2010; 408:251-264. · 2.71 Impact Factor -
Article: Source level estimation of two blue whale subspecies in southwestern Indian Ocean.
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ABSTRACT: Blue whales produce intense, stereotypic low frequency calls that are particularly well suited for transmission over long distances. Because these calls vary geographically, they can be used to gain insight into subspecies distribution. In the Southwestern Indian Ocean, acoustic data from a triad of calibrated hydrophones maintained by the International Monitoring System provided data on blue whale calls from two subspecies: Antarctic and pygmy blue whales. Using time difference of arrival and least-squares hyperbolic methods, the range and location of calling whales were determined. By using received level of calls and propagation modeling, call source levels of both subspecies were estimated. The average call source level was estimated to 179+/-5 dB re 1 microPa(rms) at 1 m over the 17-30 Hz band for Antarctic blue whale and 174+/-1 dB re 1 microPa(rms) at 1 m over the 17-50 Hz band for pygmy blue whale. According to previous estimates, slight variations in the source level could be due to inter-individual differences, inter-subspecies variations and the calculation method. These are the first reported source level estimations for blue whales in the Indian Ocean. Such data are critical to estimate detection ranges of calling blue whales.The Journal of the Acoustical Society of America 06/2010; 127(6):3800-8. · 1.55 Impact Factor -
Article: Looking at the unseen: combining animal bio‐logging and stable isotopes to reveal a shift in the ecological niche of a deep diving predator
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ABSTRACT: Understanding how marine top predators exploit their environment is a central topic in marine ecology. Among all methodologies used to investigate this part of ecology, electronic devices are very useful to track animals' movements and foraging habitats, but they do not provide any dietary information. Stable isotopes provide information on trophic levels but remain imprecise to identify small spatial-scale habitats. In this study, we combined the two approaches to obtain a synoptic view of the foraging behaviour variability of southern elephant seals Mirounga leonina. Our results suggested marked differences in distribution, diving behaviour, foraging habitats, trophic levels, and dietary habits of elephant seals according to their sex and age. Thus, we characterized main foraging habitats over the Kerguelen-Heard Plateau and the Antarctic shelf for juvenile males, while females foraged mainly in oceanic waters of the Polar Frontal Zone and the Antarctic Zone. In addition, we highlighted the ontogeny of niche partitioning in this sexually dimorphic species. While females did not exhibit a major dietary shift in relation to their age and their breeding status, a different picture emerged for males. Young males had a trophic level identical to that of all females. However, at 3–4 yr of age, males showed a progressive increase in trophic level. The inter-annual combination of bio-logging and stable isotopes could provide a powerful tool to investigate possible shifts in ecological niche between years according to environmental changes.Ecography 03/2010; 33(4):709 - 719. · 4.19 Impact Factor -
Article: Physiological response to extreme fasting in subantarctic fur seal (Arctocephalus tropicalis) pups: metabolic rates, energy reserve utilization, and water fluxes.
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ABSTRACT: Surviving prolonged fasting requires various metabolic adaptations, such as energy and protein sparing, notably when animals are simultaneously engaged in energy-demanding processes such as growth. Due to the intermittent pattern of maternal attendance, subantarctic fur seal pups have to repeatedly endure exceptionally long fasting episodes throughout the 10-mo rearing period while preparing for nutritional independence. Their metabolic responses to natural prolonged fasting (33.4 +/- 3.3 days) were investigated at 7 mo of age. Within 4-6 fasting days, pups shifted into a stage of metabolic economy characterized by a minimal rate of body mass loss (0.7%/day) and decreased resting metabolic rate (5.9 +/- 0.1 ml O(2)xkg(-1)xday(-1)) that was only 10% above the level predicted for adult terrestrial mammals. Field metabolic rate (289 +/- 10 kJxkg(-1)xday(-1)) and water influx (7.9 +/- 0.9 mlxkg(-1)xday(-1)) were also among the lowest reported for any young otariid, suggesting minimized energy allocation to behavioral activity and thermoregulation. Furthermore, lean tissue degradation was dramatically reduced. High initial adiposity (>48%) and predominant reliance on lipid catabolism likely contributed to the exceptional degree of protein sparing attained. Blood chemistry supported these findings and suggested utilization of alternative fuels, such as beta-hydroxybutyrate and de novo synthesized glucose from fat-released glycerol. Regardless of sex and body condition, pups tended to adopt a convergent strategy of extreme energy and lean body mass conservation that appears highly adaptive for it allows some tissue growth during the repeated episodes of prolonged fasting they experience throughout their development.AJP Regulatory Integrative and Comparative Physiology 09/2009; 297(5):R1582-92. · 3.34 Impact Factor -
Article: Whisker isotopic signature depicts migration patterns and multi-year intra- and inter-individual foraging strategies in fur seals
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ABSTRACT: The movement and dietary history of individuals can be studied using stable isotope records in archival keratinous tissues. Here, we present a chronology of temporally fine-scale data on the trophic niche of otariid seals by measuring the isotopic signature of serially sampled whiskers. Whiskers of male Antarctic fur seals breeding at the Crozet Islands showed synchronous and regular oscillations in both their d13C and d15N values that are likely to represent their annual migrations over the long term (mean 4.8 years). At the population level, male Antarctic fur seals showed substantial variation in both d13C and d15N values, occupying nearly all the ‘isotopic space' created by the diversity of potential oceanic habitats (from high Antarctica to the subtropics) and prey (from Antarctic krill to subantarctic and subtropical mesopelagic fishes). At the individual level, whisker isotopic signatures depict a large diversity of foraging strategies. Some seals remained in either subantarctic or Antarctic waters, while the migratory cycle of most animals encompassed a wide latitudinal gradient where they fed on different prey. The isotopic signature ofwhiskers, therefore, revealed new multi-year foraging strategies of male Antarctic fur seals and is a powerful tool for investigating the ecological niche during cryptic stages of mammals' lifeBiology letters 09/2009; · 3.76 Impact Factor -
Article: Observations of the Fawn Trough Current over the Kerguelen Plateau from instrumented elephant seals
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ABSTRACT: Due to its great meridional extent and relatively shallow depths, the Kerguelen Plateau constitutes a major barrier to the eastward flowing Antarctic Circumpolar Current in the Indian sector of the Southern Ocean. While most of the Antarctic Circumpolar Current transport is deflected north of the Kerguelen Islands, the remainder (∼ 50 Sv, 1 Sv = 106 m3 s− 1) must pass south of the islands, most probably through the Fawn and Princess Elizabeth Troughs. However, the paucity of finely resolved quasi-synoptic hydrographic data in this remote and infrequently sampled area has limited the progress in our knowledge of the regional circulation. Since 2004, a new approach using elephant seals from the Kerguelen Islands as autonomous oceanographic profilers has provided new information on the hydrography over the Kerguelen Plateau, covering the entire Antarctic Zone between the Polar Front and Antarctica, with a mean along-track resolution of about 25 km. These finely resolved bio-logged data revealed details of a strong northeastward current found across the Fawn Trough (sill depth: 2600 m; 56°S, 78°E). This so-called Fawn Trough Current transports cold Antarctic waters found mostly south of the Elan Bank, between the Ice Limit (58°S) and the Antarctic Divergence (64°S) in the eastern Enderby Basin, toward the Australian–Antarctic Basin. Our analysis also demonstrates that the Deep Western Boundary Current, which carries cold Antarctic water along the eastern flank of the southern Kerguelen Plateau collides with Fawn Trough Current at the outlet of the Fawn Trough sill. In other words, the Fawn Trough constitutes a veritable bottleneck, channelling the quasi-totality of the Antarctic Circumpolar flow found south of the Polar Front. Thanks to the unprecedented fine resolution of seal-borne data, a branch of flow centered at the Winter Water isotherm of 1 °C is also revealed along the northern escarpment of the Elan Bank, and then along the southern edge of Heard Island. Further analysis of different supplementary data reveals a complex circulation pattern in the entire Enderby Basin, with several distinctive branches of flow being strongly controlled by prominent topographic features such as the Southwest Indian Ridge, Conrad Rise, Elan Bank, and Kerguelen Plateau. This newly emerged frontal structure refines considerably previous large-scale circulation schematics of the area.Journal of Marine Systems 04/2009; 78:377-393. · 2.13 Impact Factor -
Article: Acoustic localization of two distinct blue whale (Balaenoptera musculus) subspecies in the South-West Indian Ocean.
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ABSTRACT: Analysis of one year of acoustic signal recordings from the five permanent autonomous hydrophones of the International Monitoring System in the South-West Indian Ocean reveals low frequency with high intensity calls produced by two blue whale subspecies. The "Antarctic" or "true" blue whale (B. m. intermedia) calls and the "Madagascar-type" Pygmy blue whale calls (B. m. brevicauda) were automatically detected through the matched filtering method. The potential movements were investigated by using the time difference of arrival (TDOA) of calls to assess the bearing of the sound source. The fully range dependent parabolic equation code (RAM - range-dependent acoustic model) and the PMCC code (progressive multi-channel correlation) are applied to estimate the range between our system and the vocalising animals. Our results show that (1) the variation of call number revealed two distinct patterns of seasonal whale occurrences and (2) the distances from the hydrophones to the blue whales reached up to 50 km. Tracking whales is possible when whales are concentrated of the hydrophone array.The Journal of the Acoustical Society of America 06/2008; 123(5):3774. · 1.55 Impact Factor
Top co-authors
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Institutions
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2009–2012
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University of Melbourne
- Department of Zoology
Melbourne, Victoria, Australia
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2004–2008
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French National Centre for Scientific Research
Lyon, Rhone-Alpes, France
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