Behavioral responses of juvenile walleye pollock Theragra chalcogramma Pallas to light, thermoclines and food: possible role in vertical distribution

Abstract

Two major causes of mortality in early life stages of marine fish are starvation and predation, both of which may be influenced by vertical position in the water column. This study examined under controlled laboratory conditions the behavioral response of juvenile walleye pollock Theragra chalcogramma Pallas (60–90 mm) to light, thermoclines and food, factors which have been correlated with their vertical distribution in the sea. Juveniles generally showed a preference for low light intensities and low reflectance. However, when given a choice between two very low intensities equal to full moonlight and quarter moonlight, preference shifted towards higher intensity. When cold water (3 ° C) was introduced into the bottom of the experimental tank to form a thermocline, the fish avoided it by swimming upward and remaining in the warmer upper water column (9 ° C). When food was introduced below the thermocline, the motivation to feed caused the fish to descend transiently into cold water thereby altering their vertical distribution. The results are consistent with findings from previous field studies and suggest how behavioral responses to light, temperature and food availability may determine vertical distribution.