Patterns of change in diets of two piscivorous seabird species during 35 years in the Pribilof Islands

As upper level predators in the marine ecosystem, seabirds reflect fluctuations in the marine environment that influence their prey supply. Studies of seabird diets thus provide insight into the physical and biological mechanisms that potentially drive population changes in both predators and their prey. The eastern Bering Sea shelf, among the most productive marine ecosystems in the world, has undergone significant restructuring in recent decades that is likely to continue in light of anticipated climatic change. Using a dataset spanning 35 years at two of the Pribilof Islands in the Bering Sea, we examined temporal patterns in diet and their relationships with oceanographic variables for black-legged kittiwakes (Rissa tridactyla) and thick-billed murres (Uria lomvia), two piscivorous seabirds with differing foraging strategies. Diets varied significantly among years and between islands and species. Our substantially expanded dataset supported conclusions found in previous studies of this system, including the importance of pollock, particularly age-0 class, in kittiwake diets and the absence of capelin in diets of either species since the late 1970s. Diets of both species contained more gadids at St. Paul Island and more squid and euphausiids at St. George Island, likely reflecting differences in foraging location between islands. We found some relationships between kittiwake diet and broad-scale oceanographic variables (Arctic Oscillation Index and regional summer sea surface temperature) but not with local physical variables. Almost no time series data exist on availability and abundance of zooplankton or forage fish species such as age-0 pollock, myctophids, and sandlance in the eastern Bering Sea. Our measure of diet (number of individuals within each prey type) appears too coarse for detecting complex relationships between local oceanographic variables and seabird responses, but may provide invaluable information about changes in forage fish stocks, which are frequently expensive or difficult to otherwise measure. Future diet analyses should increase emphasis on evaluating caloric input (i.e., size and nutrient composition of each prey type) as well as attempts to measure the diet of murre chicks.