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 combined acoustic and midwater trawl survey in 1990. Using a 38-kHz split-beam acoustic system and directed midwater trawling, we were able to 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 concentrations 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 coalesced into tight aggregations. A sharp thermocline did not inhibit the diel migration of these juveniles. Higher concentrations were found in inner shelf regions as compared with inshore bays or offshore waters, but the varied topography appeared to have little effect on the distribution of these juveniles. These results are discussed in relation to the trophic position of these fish as both predators and prey in this system.