Characterizing fish schools in relation to the marine environment and their use by seabirds in lower Cook Inlet, Alaska

The primary purpose of this study has been to investigate the effects of environmental variability on seabird distributions and abundances at sea in lower Cook Inlet, Alaska. Sources of variability in seabird densities were determined by assessing spatial and temporal variability in the physical environment, and examining concurrent patterns of primary production, and zooplankton and forage fish community structure. Surveys were conducted during 1996--1999. Effort was focused around three seabird colonies, the Barren Islands (oceanic waters), Gull Island in Kachemak Bay (oceanic and estuarine waters), and Chisik Island (estuarine waters). Spatial heterogeneity in the physical oceanographic environment of lower Cook Inlet was modeled as three marine-estuarine gradients characterized by temperature, salinity, bottom depth, and turbidity. Both zooplankton and forage fish communities were structured by these gradients in the physical environment. Acoustic transects and mid-water trawls were used to measure variability in species-specific fish densities and school areas among oceanographically distinct areas. Trawl catches were dominated numerically by Pacific sand lance (Ammodytes hexapterus ), Pacific herring (Clupea pallasi), and walleye pollock (Theragra chalcogramma). Sand lance schools were significantly smaller and more dense than schools of other species, occurring in waters that were warm and shallow. Age-0 walleye pollock schools were the most dispersed and largest, occurring in waters that were cold, deep, and high salinity. Sand lance and capelin (Mallotus villosus) showed flexibility in school characters, displaying increased density under presumably good conditions and dispersing under poor conditions. The lowest fish densities were measured in the area with high turbidity, where schools were both small and diffuse, regardless of species. The lowest densities of kittiwakes (Rissa tridactyla) and murres (Uria aalge ) were also measured in this area. Kittiwakes and murres responded numerically to differences in prey abundance among colonies, resulting in statistically significant sigmoidal aggregative response curves. Abundance and distribution at all ecosystem levels were affected much more by spatial variability in physical oceanography than by interannual variability. Examination of physical-biological linkages in lower Cook Inlet supports the concept of "bottom-up control," i.e., that variability in the physical environment structures higher trophic-level communities by influencing their distribution and abundance across space and time.