MEPS 392:223-237 (2009)  -  DOI: https://doi.org/10.3354/meps08160

ABSTRACT: Physical heterogeneity affects the geographic structure of coastal ocean ecosystems by influencing the spatial distribution of zooplankton and their nektonic predators. This was examined in the western Gulf of Alaska (GOA) during late summer (2000, 2001, and 2003) when seasonal increases in the Alaska Coastal Current flow and additions of Age-0 fishes to the nektonic community of small neritic fishes coincided with declining zooplankton abundance. The 48-site sampling grid was divided into 5 meso-scale areas based on physical condition (temperature, salinity, net current velocity). Larval crabs and fishes, and krill were the only zooplankton taxa for which the population density consistently differed by area regardless of diel period (day, night) or year. Larval crabs and fishes concentrated near shore in warm, low-salinity water. Krill concentrated in high-flow areas, which, over the shelf, were associated with sea valleys. Dominant fishes were walleye pollock Theragra chalcogramma, capelin Mallotus villosus, and eulachon Thaleichthys pacificus. Few fish occurred beyond the shelf in the cold, swift-flowing Alaskan Stream boundary current. Over the shelf, Age-1+ walleye pollock and eulachon aggregated with krill in high-flow areas. Smaller fishes (Age-0 walleye pollock and capelin) were more variably distributed, and were not well associated with taxon-specific zooplankton densities. Thus, relatively high flow in proximity to sea valleys was associated with concentrations of krill and fishes that presumably were able to efficiently forage on krill in high-flow areas. The year-to-year consistency in patterns is a noteworthy indication that geographic structure in the coastal GOA ecosystem may resist climate forcing.

KEY WORDS: Walleye pollock · Capelin · Eulachon · Zooplankton · Population density · Body size