FY 1998

Distribution of juvenile pollock relative to frontal structure near the Pribilof Islands, Bering Sea

Brodeur, R.D., M.T. Wilson, J.M. Napp, P.J. Stabeno, and S. Salo

In Proceedings of Forage Fishes in Marine Ecosystems, AL-SG-97-01, Alaska Sea Grant College Program, 573–589 (1997)

A tidal front exists around the Pribilof Islands in the eastern Bering Sea which separates weakly stratified nearshore water from the strongly stratified (two-layer) middle shelf water farther offshore. Enhanced mixing and vertical flux of nutrients at the structural front results in high abundances of phytoplankton, zooplankton, and micronekton. Using midwater trawl observations from September 1994 and 1995, we examined abundance, distribution, size composition, and feeding habits of age-0 walleye pollock in relation to the physics and biology of the structural front north of St. Paul Island. The frontal region occurred 12-20 km offshore during both years, but the thermocline was much shallower in 1995 (~25 m) than in 1994 (45 m). The isothermal (<8°C) inner domain had low chlorophyll, and relatively low small (>153 µm) and large (>505 µm) zooplankton concentrations. The highest chlorophyll and small zooplankton concentrations occurred seaward of the front and were lower both inshore and offshore of the front. Large zooplankton (mainly euphausiids and cnidarians) were most abundant in the stratified offshore waters. By number, age-0 walleye pollock (Theragra chalcogramma) dominated the catches of midwater trawls (>99% and >95% in 1994 and 1995, respectively), although large medusae dominated the total biomass. Age-0 walleye pollock densities were the highest at the front or inshore of the front. Age-0 pollock densities averaged about three times higher in 1994 than in 1995. Age-0 pollock were significantly smaller in 1994 inshore and at the front (mean = 43.4 and 44.6 mm, respectively) than in 1995 (52.2 and 51.7 mm).