Effects of physical factors on the vertical distribution of larval walleye pollock Theragra chalcogramma under controlled laboratory conditions

Abstract

Larval fish possess behavioral mechanisms which enable them to alter position in the water column to deal with vertical environmental gradients and select favorable conditions or avoid unfavorable ones. This study examined, under controlled laboratory conditions, behavioral responses of larval walleye pollock (4 to 8 mm) to various physical factors that may potentially play a role in vertical movements and distribution. Die1 periodicity in vertical distribution was evident and appeared to be under exogenous control. During the day, with light intensity at 70 FE m-' S-', larvae moved downward from the surface. As light intensity decreased during simulated evening twilight, larvae moved upward. At night, with no available light, larvae continued to swim upward, apparently the result of negative geotaxis. They remained near the surface until morning when they then began moving downward. Changes in vertical distribution occurred concomitantly with changes In activity, orientation and behavior. Formation of a vertical thermal gradient caused larvae to move upward and away from cold water. Turbulence at the surface, induced by an airstream, elicited an avoidance response and resulted in larvae moving downward. Findings are compared with information from field studies and suggest how behavioral responses to a number of integrated factors can determine the vertical movement and distribution of larval walleye pollock.