Northern fur seal foraging behaviors, food webs, and interactions with oceanographic features in the eastern Bering Sea

Foraging patterns of juvenile male and adult female northern fur seals (Callorhinus ursinus) captured on two St. Paul Island, AK rookeries and haul-out sites were examined during contrasting years of eastern Bering Sea shelf and basin domain oceanography and walleye pollock ( Theragra chalcogramma) stock structure, distribution, and abundance. Satellite-dive recorders, time-depth recorders, and satellite transmitters were employed to monitor and record fur seal movements and their dive depths while on foraging trips. Juvenile male fur seals spent more time and had more dives outside those habitats most frequently used by adult females, but spent similar amounts of time and had similar numbers of dives as adult females within their foraging habitats. Diets were similar between the sexes suggesting that juvenile males were competing with adult females for prey, but due to differential constraints imposed on females to nourish their pups, males could travel farther and for longer while searching for prey. Fur seals spent significantly more time in some years foraging in the basin domain and mostly along anticyclonic eddy edges. However, autumnal storms in 2005 and 2006 disrupted basin foraging causing fur seals to spend more time at sea or shift their distribution from the basin domain to the middle and outer shelf domains. When foraging on the shelf, fur seals responded to the presence of the cold pool by spending more time in the outer domain and occupying similar thermal regimes to those preferred by walleye pollock. In addition, dive patterns were consistent with the dominant year class of walleye pollock and its age-related behavior in the vertical water column. These parallel behaviors between fur seals and walleye pollock prompted and expanded the analysis to revisit previous reports of fur seal diet, adult female trip durations, and pup weights and examine their relationship to the estimated abundance of walleye pollock. In years with above average numbers of age-1-5 walleye pollock, fur seals showed a higher frequency of occurrence of walleye pollock in their diet, adult females had shorter trips to sea, and pups weights were heavier. Thus, the usage of satellite telemetry and dive recorders on juvenile male and adult female northern fur seals revealed important biophysical relationships, supported historical observations, and generated new ideas to consider when examining the current northern fur seal population decline on the Pribilof Islands.