EVOS Alaska Predator Ecosystem Experiment
Meso-Scale Interactions Between Seabirds and Forage Fish
Northern Gulf of Alaska
Summary
At intermediate spatial scales of 10s to 1,000s of km, the distribution of seabirds at colonies and at sea in the northern Gulf of Alaska (GOA) reflects regional patterns of productivity and forage fish abundance. Prince William Sound (PWS) is less productive than gulf waters, especially lower Cook Inlet (LCI). Low productivity in PWS may be attributed to limited circulation of nutrient-rich GOA water into PWS, stable stratification of water during summer, and a relative scarcity of shallow shelf habitat. Nonetheless, coastal areas of PWS support patchy concentrations of herring and sand lance, which are exploited by the dominant breeding seabirds; Marbled Murrelets and Black-legged Kittiwakes. Upwelling of cold, nutrient-rich GOA waters at the entrance to the shallow LCI estuary supports a high biomass of juvenile pollock, sand lance, and capelin, which in turn are exploited by large numbers of Common Murres, Kittiwakes, and puffins. A persistent feature in LCI is the plume of mixed GOA water that flows north into Kachemak Bay, enhancing forage fish and seabird production on the shallow east side of LCI. Waters on the west side of LCI are oceanographically distinct (warmer, less saline, outflowing), and much less productive for forage fish and seabirds. Patterns of seabird productivity and population change reflect forage fish dynamics at the above spatial scales, and over temporal scales of years to decades. Seabird productivity varies with overall forage fish abundance in a non-linear fashion, and in some areas and years, productivity is clearly limited by food availability. Decadal changes in forage fish stocks have altered the diets of many seabirds in the GOA, and influenced trends in productivity and population dynamics. In summary, meso-scale studies of seabird populations in the northern GOA reveal a high degree of local, short-term variability in population parameters, but a pervasive influence of meso-scale phenomena on populations in discrete oceanographic domains.
Principal Investigators
John F. Piatt, U.S. Geological Survey, Biological Resources Division, Anchorage, AK
David B. Irons, U.S. Fish and Wildlife Service, Anchorage, AK