MESOSCALE ACOUSTIC PATTERNS OF JUVENILE WALLEYE POLLOCK (THERAGRA-CHALCOGRAMMA) IN THE WESTERN GULF OF ALASKA

The mesoscale distribution patterns of age-0 walleye pollock (Theragra chalcogramma) in the western Gulf of Alaska were assessed through a c ombined acoustic and midwater trawl survey in 1990. Using a 38-kHz spl it-beam acoustic system and directed midwater trawling, we were able t o isolate concentrations of scatterers that were mainly age-0 pollock based on the expected target strength (-45 to -53 dB) derived from an empirical relationship of target strength Versus length. The concentra tions of these scatterers formed diffuse layers that were centered at a depth of 33 m during the night. During the day, scatterers descended to a depth of ca 100 m (up to 30 m off the bottom) where they coalesc ed into tight aggregations. A sharp thermocline did not inhibit the di el migration of these juveniles. Higher concentrations were found in i nner shelf regions as compared with inshore bays or offshore waters, b ut the varied topography appeared to have little effect on the distrib ution of these juveniles. These results are discussed in relation to t he trophic position of these fish as both predators and prey in this s ystem.