No abstract

Laboratory apparatus which simulated capture of fish in the cod-end of a towed trawl was used to induce post-capture stress as measured by alterations in behavioural, physiological and mortality indices in juvenile walleye pollock Theragra chalcogramma and juvenile and adult sablefish Anoplopoma fimbria. Differences in resistance to net entrainment varied between species with the severity of stress and the potential for recovery depending on light intensity, net velocity and towing duration. At a light intensity which simulated daylight at depth in clear ocean water (0·5 mol photons m 2 s 1 ), walleye pollock juveniles were able to maintain swimming in nets towed at 0·65 m s 1 for 3 h with no discernible effects on behaviour or mortality. However, when net velocity was increased to >0·75 m s 1 or light intensity was decreased to <0·002 mol photons m 2 s 1 , fish became entrained in the meshes of the net and exhibited significant alterations in feeding behaviour, predator evasion and increases in plasma cortisol concentrations. Marked increases in stress-induced mortality also occurred, in some cases after a delay of 6 days and eventually reaching 100%. In comparison with walleye pollock, sablefish juveniles became entrained in the meshes of the net at higher velocities (>0·92 m s 1 ) or lower light intensities (<0·0004 mol photons m 2 s 1 ) and were much more resistant to post-capture stress. Towing of net-entrained fish for 15 min caused no detectable changes in feeding and cortisol and for 2 h, no changes in feeding although mortality increased from 0% for 15-min tows to 19% for 2-h tows. Towing for 4 h caused significant alterations in feeding and cortisol with feeding recovering to control levels by 6 days and cortisol by 3 days; mortality was 25%. When adult sablefish were towed for 4 h followed by 15-min exposure to air, feeding was inhibited 6 days after towing, but recovered within 30 days with no mortality observed after 30 days. The results demonstrate the value of using laboratory-based behavioural and biochemical indices to identify factors that may potentially affect post-capture survival among different species of fish. 1997 The Fisheries Society of the British Isles

The effects of reduced food availability on the behavior of juvenile walleye pollock Theragra chalcogram~na were examined in laboratory experiments designed to test for potential energyconserving responses. Groups of juvenile fish were held on 1 of 6 ration treatments ranging from ad hbltum to near starvation, and thenvertical distribution and activity levels were quantified in a 2.5 m deep water column under isothermal and thermally stratified conditions. Stratification resulted in a general sh~ft to the upper, warmer layer in the 2 experiments employing a sharp thermocline at middepth, but the occurrence of fish in the colder bottom layer varied with different ratlon treatments. Movement lnto cold water increased in intermediate ration groups compared to high ratlon groups. Since reduced temperatures should reduce metabolic costs, this behavior is consistent with our hypothesis that food deprivation should invoke energy-saving behaviors. However, activity levels 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 when a sharp thermocline was present, negating potential bioenergetic benefits. In the third experiment, there was a gradual thermal gradient from surface to bottom rather than a sharp thermocline. Temperatures associated with vertical positions of the fish were determined. In this experiment, clear energy-conserving responses to temperature were displayed by food-deprived fish; the average temperatures occupied by fish on starvation rations were 3 to 4°C colder than those of the higher ration groups. Based on the high Qlo for metabolic rates of juvenile pollock, these reduced temperatures potentially conferred energy savings of up to 34 %, relative to the metabolic expenditures of fish on high rations. The contrast in behavior for the lowest ration groups between sharply stratified and gradually stratified conditions suggested that the severity of the temperature gradient influenced the fishes' ability to take advantage of cold water as an energetic refuge. The behavior of fish in the laboratory was consistent with prior observations in the Bering Sea, where juvenile walleye pollock remained in surface waters if food availability was high, but initiated vertical migration into deeper, colder water with reduced prey densities. Results of this study demonstrated a broad flexibility in the behavioral mechanisms used by walleye pollock to deal with declining food levels. The initial response to food limitation was increased activity, indicative of greater searching behavior With extended food deprivation, a switch to energy-conserving behavior was evident. The temperature responses of fish experiencing severe food limitation provided support for a bioenergetic hypothesis of die1 vertical migration.

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.