The foraging strategies of the Antarctic fur seals, Arctocephalus gazella
*Tuesday, October 10, 2011
Redfield Auditorium - 12:00 Noon
Dr. Morgane Viviant
Centre d'Etudes Biologiques de Chize, France
Predicting how predators will respond to environmental changes requires understanding how individuals adjust their foraging behavior according to their physical and biological environment. The optimal foraging theory predicts that predators will adjust their foraging strategy in order to maximize their net energy intake per unit time by considering the trade off between their physiology and prey availability.
The aim of this PhD was to understand the foraging strategies of a marine predator the Antarctic fur seals, Arctocephalus gazella breeding in the southern ocean in regard of the optimal foraging theory. The main goal is to understand if Antarctic fur seals adjust their diving behavior according to the trade-off between the quality of the environment prospected (prey abundance and accessibility at depth) and their own physiological constraints (Aerobic Dive Limit: ADL). To address these goals, a novel methodology based on accelerometers was used to detect prey capture attempts (i.e. prey encounters) of these predators while they are at sea.
The study shows that most of the fur seals diving activity occurred above depths of maximum prey density. Beyond a depth threshold of 55-60m, fur seals were physiologically constrained (exceeded their ADL on average), suggesting that they may face a trade-off between prey availability and accessibility at depth. This physiological constraint affected fur seals foraging decisions. They used exploratory dives, without bottom phase, only below this depth threshold. Moreover, at the scale of the entire night, when fur seals were foraging in bad quality patches (i.e. with low prey encounter rate), they increased their foraging effort by increasing the time spent at the bottom of dives and increasing their diving intensity. This is contradicting the optimal foraging theory that diving predator increase their foraging time in good quality patches. Taking into account both the physiological constraints of the animal and the time scale appears crucial to accurately predict the foraging success of a predator based on its diving behavior.
In the context of global warming, scientist predict an increase of the thermocline’s depth, which may affect the prey distribution in the water column by moving them downward. Antarctic fur seals are rapidly constraint in their foraging behavior with increasing depths. A small decrease in the accessibility of their prey at depth may thus negatively affect their foraging success and in turn their reproductive success.