Stress and Delayed Mortality Induced in Pacific Halibut by Exposure to Hooking, Net Towing, Elevated Seawater Temperature and Air: Implications for Management of Bycatch

Abstract

The extent of stress and eventual mortality in Pacific halibut Hippoglossus stenolepis that resulted from simulated capture by hooking or towing in a net, followed by abrupt exposure to warmer seawater temperature and air, were determined under laboratory conditions. Abrupt exposure to 16°C seawater and air after either method of capture increased capture-induced stress, with accompanying mortality of 33% for hooked fish and 78% for fish towed in a net. Moreover, these deaths occurred as long as 30 d after experimental treatment, suggesting that delayed mortality should be considered in any study of Pacific halibut bycatch mortality. Stress induced by hooking or towing in a net followed by air exposure was reflected in cessation of negative phototaxis and feeding, both of which were resumed after 5 d with no mortality occurring. The results of this study clearly show that seasonal increases in temperature associated with thermoclines and deck conditions have the potential for markedly increasing the mortality of Pacific halibut that might otherwise survive capture and release in colder seasons. Strategies for effective management of Pacific halibut bycatch need to include consideration of seasonal temperature increases and how this factor might increase mortality.