Trade-offs between foraging efficiency and pup feeding rate of lactating northern fur seals in a declining population

International audience; Foraging strategies and their resulting efficiency (energy gain to cost ratio) affectanimals’ survival and reproductive success and can be linked to population dynamics. However,they have rarely been studied quantitatively in free-ranging animals. We investigated foragingstrategies and efficiencies of wild northern fur seals Callorhinus ursinus during their breedingseason to understand potential links to the observed population decline in the Bering Sea. Weequipped 20 lactating females with biologgers to determine at-sea foraging behaviours. We measuredenergy expenditure while foraging using the doubly-labelled water method, and energygained using (1) the types and energy densities of prey consumed, and (2) the number of prey captureattempts (from acceleration data). Our results show that seals employed 2 foraging strategies:one group (40%) fed mostly in oceanic waters on small, high energy-density prey, while the other(60%) stayed over the shallow continental shelf feeding mostly on larger, lower quality fish.Females foraging in oceanic waters captured 3 times more prey, and had double the foraging efficienciesof females that foraged on-shelf in neritic waters. However, neritic seals made comparativelyshorter trips, and likely fed their pups ~20 to 25% more frequently. The presence of thesestrategies which either favor foraging efficiency (energy) or frequency of nursing (time) might bemaintained in the population because they have similar net fitness outcomes. However, neitherstrategy appears to simultaneously maximize time and energy allocated to nursing, with potentialimpacts on the survival of pups during their first year at sea.