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 similar to 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.