ONTOGENIC SHIFT IN GEOTAXIS FOR WALLEYE POLLOCK, THERAGRA-CHALCOGRAMMA FREE EMBRYOS AND LARVAE - POTENTIAL ROLE IN CONTROLLING VERTICAL-DISTRIBUTION

The behavioral capability of walleye pollock, Theragra chalcogramma fr ee embryos and larvae to control vertical distribution was assessed by examining buoyancy during resting and swimming orientation and activi ty as they developed in complete darkness from hatching to first feedi ng readiness (1 to 7 d post hatching at 6-degrees-C). Free embryos exh ibited positive geotaxis 1 d post hatching, actively swimming through a density gradient to remain in the lower water column. Activity incre ased with free embryo development and by 7 d post hatching, feeding-re ady larvae reversed their vertical orientation, now exhibiting negativ e geotaxis as they migrated to the upper water column. The results ind icate that even at the earliest developmental stages, walleye pollock possess the capability to control vertical distribution. Laboratory re sults are compared with patterns of vertical distribution observed in the sea.