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

Foraging strategies and their resulting efficiency (energy gain to cost ratio) affect animals’ survival and reproductive success and can be linked to population dynamics. However, they have rarely been studied quantitatively in free-ranging animals. We investigated foraging strategies and efficiencies of wild northern fur seals Callorhinus ursinus during their breeding season to understand potential links to the observed population decline in the Bering Sea. We equipped 20 lactating females with biologgers to determine at-sea foraging behaviours. We measured energy expenditure while foraging using the doubly-labelled water method, and energy gained using (1) the types and energy densities of prey consumed, and (2) the number of prey capture attempts (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 efficiencies of females that foraged on-shelf in neritic waters. However, neritic seals made comparatively shorter trips, and likely fed their pups ~20 to 25% more frequently. The presence of these strategies which either favor foraging efficiency (energy) or frequency of nursing (time) might be maintained in the population because they have similar net fitness outcomes. However, neither strategy appears to simultaneously maximize time and energy allocated to nursing, with potential impacts on the survival of pups during their first year at sea.