FORMATION AND MAINTENANCE OF AGGREGATIONS IN WALLEYE POLLOCK, THERAGRA-CHALCOGRAMMA, LARVAE UNDER LABORATORY CONDITIONS - ROLE OF VISUAL AND CHEMICAL STIMULI

Although planktonic marine fish larvae are often distributed in aggreg ations, the role of behavioral responses to environmental factors in t hese aggregations is not well understood. This work examines, under la boratory conditions, the influence of visual and chemical stimuli in t he formation and maintenance of aggregations in walleye pollock, Thera gra chalcogramma, larvae. Larvae were exposed to a horizontal gradient of light (visual stimulus), prey scent (chemical stimuli: squid/copep od and rotifer) or prey density (visual & chemical stimuli: rotifers). While larvae did not respond to prey scent, they did respond to a gra dient of light or prey, which resulted in the formation and maintenanc e of aggregations. Larvae moved into and remained in a zone of higher light intensity (0.56 versus 0.01 mu mol photons m(-2) s(-1)). Once en countering a patch of prey, larvae remained aggregated within the patc h to feed. In nature, movement of walleye pollock larvae in response t o selected environmental factors (e.g., gravity, light, temperature, t urbulence) may serendipitously bring them into contact with prey patch es, where they then could remain to feed as long as light intensity re mained at or above levels necessary for feeding.