FOOD-DEPRIVATION AFFECTS VERTICAL-DISTRIBUTION AND ACTIVITY OF A MARINE FISH IN A THERMAL-GRADIENT - POTENTIAL ENERGY-CONSERVING MECHANISMS

The effects of reduced food availability on the behavior of juvenile w alleye pollock Theragra chalcogramma were examined in laboratory exper iments designed to test for potential energy-conserving responses. Gro ups of juvenile fish were held on 1 of 6 ration treatments ranging fro m ad libitum to near starvation, and then vertical distribution and ac tivity levels were quantified in a 2.5 m deep water column under isoth ermal and thermally stratified conditions. Stratification resulted in a general shift to the upper, warmer layer in the 2 experiments employ ing a sharp thermocline at mid-depth, but the occurrence of fish in th e colder bottom layer varied with different ration treatments. Movemen t into cold water increased in intermediate ration groups compared to high ration groups. Since reduced temperatures should reduce metabolic costs, this behavior is consistent with our hypothesis that food depr ivation should invoke energy-saving behaviors. However, activity level s increased for fish held on intermediate rations, suggesting that the greater movement into cold water was a corollary result of increased searching for food. Fish in the lowest ration treatments had decreased activity levels, but also decreased their movement into cold water wh en a sharp thermocline was present, negating potential bioenergetic be nefits. In the third experiment, there was a gradual thermal gradient from surface to bottom rather than a sharp thermocline. Temperatures a ssociated with vertical positions of the fish were determined. In this experiment, clear energy-conserving responses to temperature were dis played by food-deprived fish; the average temperatures occupied by fis h on starvation rations were 3 to 4 degrees C colder than those of the higher ration groups. Based on the high Q(10) for metabolic rates of juvenile pollock, these reduced temperatures potentially conferred ene rgy savings of up to 34%, relative to the metabolic expenditures of fi sh on high rations. The contrast in behavior for the lowest ration gro ups between sharply stratified and gradually stratified conditions sug gested that the severity of the temperature gradient influenced the fi shes' ability to take advantage of cold water as an energetic refuge. The behavior of fish in the laboratory was consistent with prior obser vations in the Bering Sea, where juvenile walleye pollock remained in surface waters if food availability was high, but initiated vertical m igration into deeper, colder water with reduced prey densities. Result s of this study demonstrated a broad flexibility in the behavioral mec hanisms used by walleye pollock to deal with declining food levels. Th e initial response to food limitation was increased activity, indicati ve of greater searching behavior. With extended food deprivation, a sw itch to energy-conserving behavior was evident. The temperature respon ses of fish experiencing severe food limitation provided support for a bioenergetic hypothesis of diel vertical migration.