23 research outputs found
Taking movement data to new depths : Inferring prey availability and patch profitability from seabird foraging behavior
Funded byNatural Environment Research Council. Grant Number: NE/K007440/1 and Marine Scotland Science and Seabird Tracking and Research (STAR) Project led by the Royal Society for the Protection of Birds (RSPB)Peer reviewedPublisher PD
Stratégies d'acquisition des ressources en proies et coût du transport chez l'éléphant de mer austral
The Southern Ocean is a fragile ecosystem whose dynamics are influenced by climate change that will structure the spatio-temporal distribution of resources. The objective of this PhD was to investigate the foraging strategies used by the southern elephant seal, under energetic (cost of transport and costs of access to the resource) and temporal (time at the bottom of a dive and aerobic dive limit) constraints. Using a set of animal-borne data loggers allowed us to reconstruct their three-dimensional path underwater, but also to calculate their swimming effort, the number of prey encountered and their energy expenditure. At the dive level, our study shows that elephant seals adapt their path, but also the time spent at the bottom, depending on the number of prey encountered. For high local prey density, they spent more time at the bottom, and concentrated their foraging effort in areas restricted search, characterized by a decrease in speed and an increase in horizontal sinuosity. Beyond 550 m, the cost of access to resources becomes greater than the cost of acquisition ; they must therefore deal with a trade-off between prey accessibility and availability. While feeding, these seals increase their buoyancy, reducing furthermore their energy expenditure. This study also shows a spatio-temporal structure of the energy expenditure that appears to be related to feeding success and therefore to prey resources distribution.LâocĂ©an austral est un Ă©cosystĂšme fragile dont la dynamique est influencĂ©e par des variations climatiques qui vont structurer la distribution spatio-temporelle des ressources. Lâobjectif de cette thĂšse Ă©tait dâĂ©tudier les stratĂ©gies dâacquisition des ressources en proies mises en place par lâĂ©lĂ©phant de mer austral face aux contraintes Ă©nergĂ©tiques (coĂ»ts du transport et coĂ»ts dâaccĂšs Ă la ressource) et temporelles (temps passĂ© au fond dâune plongĂ©e et limite de plongĂ©e aĂ©robie) auxquelles il est soumis. Lâutilisation dâun ensemble dâenregistreurs de donnĂ©es dĂ©ployĂ©s sur ces animaux a permis de reconstruire en trois dimensions leur plongĂ©e, mais Ă©galement de calculer leur effort de nage, le nombre de proies rencontrĂ©es ainsi que leur dĂ©pense Ă©nergĂ©tique. Notre Ă©tude montre quâĂ lâĂ©chelle dâune plongĂ©e, les Ă©lĂ©phants de mer adaptent leur trajectoire, mais Ă©galement le temps quâils passent au fond, en fonction du nombre de proies rencontrĂ©es. Pour des densitĂ©s locales de proie importantes, ils passent plus de temps au fond, et concentrent leur recherche en zone restreinte, caractĂ©risĂ©e par une diminution de la vitesse et une augmentation de la sinuositĂ© horizontale. Au-delĂ de 550 m, le coĂ»t dâaccĂšs aux ressources devient supĂ©rieur aux coĂ»ts dâacquisition ; ils doivent alors faire face Ă un compromis entre lâaccessibilitĂ© et la disponibilitĂ© en proies. Ă mesure quâils sâalimentent, ces phoques augmentent leur flottabilitĂ©, diminuant de surcroĂźt leur dĂ©pense Ă©nergĂ©tique. Cette Ă©tude dĂ©montre Ă©galement une structuration spatio-temporelle de cette dĂ©pense Ă©nergĂ©tique qui semble ĂȘtre liĂ©e au succĂšs dâalimentation et donc Ă la distribution des ressources en proies
Resource acquisition strategies and cost of transport in southern elephant seal
LâocĂ©an austral est un Ă©cosystĂšme fragile dont la dynamique est influencĂ©e par des variations climatiques qui vont structurer la distribution spatio-temporelle des ressources. Lâobjectif de cette thĂšse Ă©tait dâĂ©tudier les stratĂ©gies dâacquisition des ressources en proies mises en place par lâĂ©lĂ©phant de mer austral face aux contraintes Ă©nergĂ©tiques (coĂ»ts du transport et coĂ»ts dâaccĂšs Ă la ressource) et temporelles (temps passĂ© au fond dâune plongĂ©e et limite de plongĂ©e aĂ©robie) auxquelles il est soumis. Lâutilisation dâun ensemble dâenregistreurs de donnĂ©es dĂ©ployĂ©s sur ces animaux a permis de reconstruire en trois dimensions leur plongĂ©e, mais Ă©galement de calculer leur effort de nage, le nombre de proies rencontrĂ©es ainsi que leur dĂ©pense Ă©nergĂ©tique. Notre Ă©tude montre quâĂ lâĂ©chelle dâune plongĂ©e, les Ă©lĂ©phants de mer adaptent leur trajectoire, mais Ă©galement le temps quâils passent au fond, en fonction du nombre de proies rencontrĂ©es. Pour des densitĂ©s locales de proie importantes, ils passent plus de temps au fond, et concentrent leur recherche en zone restreinte, caractĂ©risĂ©e par une diminution de la vitesse et une augmentation de la sinuositĂ© horizontale. Au-delĂ de 550 m, le coĂ»t dâaccĂšs aux ressources devient supĂ©rieur aux coĂ»ts dâacquisition ; ils doivent alors faire face Ă un compromis entre lâaccessibilitĂ© et la disponibilitĂ© en proies. Ă mesure quâils sâalimentent, ces phoques augmentent leur flottabilitĂ©, diminuant de surcroĂźt leur dĂ©pense Ă©nergĂ©tique. Cette Ă©tude dĂ©montre Ă©galement une structuration spatio-temporelle de cette dĂ©pense Ă©nergĂ©tique qui semble ĂȘtre liĂ©e au succĂšs dâalimentation et donc Ă la distribution des ressources en proies.The Southern Ocean is a fragile ecosystem whose dynamics are influenced by climate change that will structure the spatio-temporal distribution of resources. The objective of this PhD was to investigate the foraging strategies used by the southern elephant seal, under energetic (cost of transport and costs of access to the resource) and temporal (time at the bottom of a dive and aerobic dive limit) constraints. Using a set of animal-borne data loggers allowed us to reconstruct their three-dimensional path underwater, but also to calculate their swimming effort, the number of prey encountered and their energy expenditure. At the dive level, our study shows that elephant seals adapt their path, but also the time spent at the bottom, depending on the number of prey encountered. For high local prey density, they spent more time at the bottom, and concentrated their foraging effort in areas restricted search, characterized by a decrease in speed and an increase in horizontal sinuosity. Beyond 550 m, the cost of access to resources becomes greater than the cost of acquisition ; they must therefore deal with a trade-off between prey accessibility and availability. While feeding, these seals increase their buoyancy, reducing furthermore their energy expenditure. This study also shows a spatio-temporal structure of the energy expenditure that appears to be related to feeding success and therefore to prey resources distribution
Three-dimensional space use during the bottom phase of southern elephant seal dives
International audienceBackground: In marine pelagic ecosystems, the spatial distribution of biomass is heterogeneous and dynamic. At large scales, physical processes are the main driving forces of biomass distribution. At fine scales, both biotic and abiotic parameters are likely to be key determinants in the horizontal and vertical distribution of biomass, with direct consequences on the foraging behaviour of diving predators. However, fine scale three-dimensional (3D) spatial interactions between diving predators and their prey are still poorly known. Results: We reconstructed and examined the patterns of southern elephant seals 3D path during the bottom phase of their dives, and related them to estimated prey encounter density. We found that southern elephant seal tracks at bottom are strongly dominated by a single horizontal direction. In high prey density areas, seals travelled shorter distances but their track remained strongly orientated according to a main linear direction. Horizontal, and more importantly, vertical deviations from this main direction, were related negatively to the estimated prey density. We found that prey encounter density decreased with diving depth but tended to be more predictable. Conclusion: Southern elephant seal behaviour during the bottom phase of their dives suggest that the prey are dispersed and distributed into layers in which their density relates to the vertical spread of the layer. The linear trajectories performed by the elephant seals would allow to explore the largest volume of water, maximizing the opportunities of prey encounter, while travelling great horizontal distance
Acoustic measurements of post-dive cardiac responses in southern elephant seals ( Mirounga leonina ) during surfacing at sea
International audienceMeasuring physiological data in free-ranging marine mammals remains challenging, owing to their far-ranging foraging habitat. Yet, it is important to understand how these divers recover from effort expended underwater, as marine mammals can perform deep and recurrent dives. Among them, southern elephant seals (Mirounga leonina) are one of the most extreme divers, diving continuously at great depth and for long duration while travelling over large distances within the Southern Ocean. To determine how they manage post-dive recovery, we deployed hydrophones on four post-breeding female southern elephant seals. Cardiac data were extracted from sound recordings when the animal was at the surface breathing. Mean heart rate at the surface was 102.4±4.9 beats.min(-1) and seals spent on average 121±20 s breathing. During these surface intervals, the instantaneous heart rate is increasing with time. Elephant seals are supposed to drastically slow their heart rate (bradycardia) while they are deep underwater, and increase it (tachycardia) during the ascent towards the surface. Our finding suggests that tachycardia continues while the animal stays breathing at the surface. Also, the measured mean heart rate at the surface was unrelated to the duration and swimming effort of the dive prior to the surface interval. Recovery (at the surface) after physical effort (underwater) appears to be related to the overall number of heart beats performed at the surface, and therefore total surface duration. Southern elephant seals recover from dives by adjusting the time spent at the surface rather than their heart rat
How elephant seals (Mirounga leonina) adjust their fine scale horizontal movement and diving behaviour in relation to prey encounter rate
International audienceUnderstanding the diving behaviour of diving predators in relation to concomitant prey distributioncould have major practical applications in conservation biology by allowing theassessment of how changes in fine scale prey distribution impact foraging efficiency andultimately population dynamics. The southern elephant seal (Mirounga leonina, hereafterSES), the largest phocid, is a major predator of the southern ocean feeding on myctophidsand cephalopods. Because of its large size it can carry bio-loggers with minimal disturbance.Moreover, it has great diving abilities and a wide foraging habitat. Thus, the SES is a wellsuited model species to study predator diving behaviour and the distribution of ecologicallyimportant prey species in the Southern Ocean. In this study, we examined how SESs adjusttheir diving behaviour and horizontal movements in response to fine scale prey encounterdensities using high resolution accelerometers, magnetometers, pressure sensors andGPS loggers. When high prey encounter rates were encountered, animals responded by (1)diving and returning to the surface with steeper angles, reducing the duration of transit divephases (thus improving dive efficiency), and (2) exhibiting more horizontally and verticallysinuous bottom phases. In these cases, the distance travelled horizontally at the surfacewas reduced. This behaviour is likely to counteract horizontal displacement from water currents,as they try to remain within favourable prey patches. The prey encounter rate at thebottom of dives decreased with increasing diving depth, suggesting a combined effect ofdecreased accessibility and prey density with increasing depth. Prey encounter rate alsodecreased when the bottom phases of dives were spread across larger vertical extents ofthe water column. This result suggests that the vertical aggregation of prey can regulateprey density, and as a consequence impact the foraging success of SESs. To our knowledge,this is one of only a handful of studies showing how the vertical distributions and structureof prey fields influence the prey encounter rates of a diving predator
Measuring the marine soundscape of the indian ocean with southern elephant seals used as acoustic gliders of opportunity
International audienc