Diel patterns of behavior in juvenile walleye pollock, Theragra chalcogramma

In laboratory experiments, we contrasted the behavior of juvenile walleye pollock under dark and light conditions, comparing group cohesion, activity levels, vertical distribution, and movement into cold water when the water column was thermally stratified. At night the fish displayed a tendency for movement into the upper water column and were less active, with groups more dispersed than during the day. Time spent in cold water under stratified conditions did not differ between day and night. These results are interpreted relative to the potential ecological benefits and costs of particular behaviors, and their application in designing effective sampling surveys of juvenile abundances.