Contrasting behavioral responses to cold temperatures by two marine fish species during their pelagic juvenile interval

Motor activity of juvenile walleye pollock, Theragra chalcogramma, and sabl efish, Anoplopoma fimbria, was monitored in the laboratory during high and low light levels under a changing temperature regime over a 5 d period. Wat er temperatures were ambient (12 degrees C) for the first 24 h of observati on, rapidly lowered to 3 degrees C for the next 48 h, then raised back to 1 2 degrees C for the final 48 h. We hypothesized that the fishes' behavior w ould either follow a simple bioenergetic response of lowered activity assoc iated with reduced metabolic rates at the colder temperature, or an avoidan ce response, with increased activity at decreased temperatures. Results for walleye pollock were consistent with a bioenergetic response, with activit y decreasing in the presence of cold water under both high and low light le vels, then returning to initial levels when temperatures increased. The res ponse of sablefish, in contrast, indicated avoidance of cold temperatures, depending on light level. During high light, when sablefish were typically highly active, cold water induced a slight but insignificant decrease in ac tivity. At low light, however, the presence of cold water caused a marked i ncrease in sablefish movement through the experimental tanks, with a seven fold increase in the measured index of activity. When water temperatures we re raised back to 12 degrees C, sablefish activity at low light returned to its normal, minimal level. The sharp increase in activity of sablefish in cold water, followed by a decrease in activity when the temperature was rai sed to pre-test levels, is suggestive of an avoidance response. The contras ting responses of the two species to thermal changes are consistent with th eir separate life history patterns and natural distributions.