EVOS Alaska Predator Ecosystem Experiment
Seabird Reproductive Energetics and the Fat Content of Fish: Revisiting the Junk Food Hypothesis
Summary
Factors affecting food availability of seabirds occur at many spatial scales ranging from 10s of meters to 1000s of kilometers. While broad scale oceanic processes (e.g. circulation and temperature) may affect general food abundance, it is often small scale events (e.g. local currents and upwelling) that affect how and where individual seabirds choose to forage. By observing individual seabirds and schools of fishes the APEX study has found several factors that influence how seabirds forage. In summer, the waters of PWS are stratified with little mixing and the near-surface fish schools (herring, sand lance, and capelin) are small, occur in low density, and are located close to shore. Seabirds in PWS respond by foraging singly or in small flocks close to shore. This is in contrast to Lower Cook Inlet (LCI) where there is strong tidal mixing of the water column, the fish are larger and more dense than in PWS, and occur offshore (capelin and pollock) as well as near shore (capelin and sand lance). Seabirds in LCI feed inshore and offshore and typically form large foraging flocks. We have observed foraging areas where fish schools occurred throughout the nesting season, however, we have also noted dramatic changes in species composition, abundance, and occurrence of prey over a period of several days or within a 24 hr period. When prey are predictable, seabirds learn and remember where prey can be found and individual birds return to the same area repeatedly. In this case, they do not always forage on fish that are closest to the colony, rather they pass by fish schools to return to the area where they have successfully foraged in the past. When prey availability changes daily due to tidal cycles birds respond by adjusting their foraging activities. Seabirds also change their foraging strategy in respect to prey abundance, when prey are scarce seabirds generally forage in flocks, but when prey are abundant they often forage alone. This behavior change likely increases their efficiency, by using other birds to find food when it is scarce, and decreasing risk of kleptoparisitism by foraging alone when prey are abundant.
Principal Investigators
David B. Irons, Robert Suryan, William D. Ostrand, and Gregory H. Golet, U.S. Fish and
Wildlife Service, Anchorage, AK
John F. Piatt, U.S. Geological Survey, Biological Resources Division, Anchorage, AK