EFFECTS OF SIMULATED TRAWLING ON SABLEFISH AND WALLEYE POLLOCK - THE ROLE OF LIGHT-INTENSITY, NET VELOCITY AND TOWING DURATION

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 a lterations in behavioural, physiological and mortality indices in juve nile walleye pollock Theragra chalcogramma and juvenile and adult sabl efish 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 dur ation. At a light intensity which simulated daylight at depth in clear ocean water (0.5 mu mol photons m(-2) s(-1)), walleye pollock juvenil es 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 de creased to <0.002 mu mol photons m(-2) s(-1), fish became entrained in the meshes of the net and exhibited significant alterations in feedin g behaviour, predator evasion and increases in plasma cortisol concent rations. Marked increases in stress-induced mortality also occurred, i n 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 mu mol photons m(-2) s(-1)) and were much more resistant to post-capture stress. Towing of net-entrained fish fo r 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 alter ations in feeding and cortisol with feeding recovering to control leve ls by 6 days and cortisol by 3 days; mortality was 25%. When adult sab lefish 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 n o mortality observed after 30 days. The results demonstrate the value of using laboratory-based behavioural and biochemical indices to ident ify factors that may potentially affect post-capture survival among di fferent species of fish. (C) 1997 The Fisheries Society of the British Isles.