Northern fur seals and fish predators feeding on pollock in the eastern Bering Sea: a look at summer prey availability and spatial overlap over 35 years

Over a million tons of walleye pollock are commercially caught annually in the eastern Bering Sea (EBS), and three times as much is consumed by predators, including northern fur seals (NFS), Pacific cod, arrowtooth flounder, Pacific halibut, and Alaska skates. Some of these pollock consumers have undergone dramatic population changes, like arrowtooth flounder tripling in biomass since the late 1970s while NFS pup production on the Pribilof Islands, AK has declined by almost 60%. About half of the world’s NFS population breeds or haul-out in the Pribilof Islands where the majority forage on the EBS shelf habitat and eat pollock. To look at potential competition between NFS seals and fish predators, we first used updated maps of multi-year aggregated foraging ranges for NFS and overlapped them with year-specific maps of pollock availability by size and bottom temperature using data from annual summer bottom trawl surveys from 1982 to 2018. We then added the year specific summer distribution of pollock, cod, arrowtooth flounder, halibut and Alaska skates to evaluate the spatial overlap of NFS foraging ranges with these competitors. Prey size preference by fish predators was estimated from stomach samples collected as part of the bottom trawl surveys. Interannual variability of pollock abundance and distribution, changes the availability of pollock to NFS; competing fish predator abundance, distribution, and their spatial overlap with NFS foraging ranges, further modifies pollock’s availability. The annual extent of the cold pool (a layer of cold bottom water that is 2°C or less), shifts/extends spatial overlaps from the outer into the middle shelf as cold temperatures push predators south and/or towards the outer shelf while warmer temperatures allow predators to move onto the shelf and further inshore. Increasing fish predator abundance, such as that of arrowtooth flounder and Alaska skate, or peak years as those for cod, further exacerbate such overlap and potential competition. This study helps inform ecosystem based management of northern fur seals, pollock, and its fish predators by providing insights into the effects of environmental conditions on their feeding interactions and potential competition.