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Summary
One of the most significant issues affecting marine fisheries management
today is the mortality of fish that are discarded after capture or that
escape from fishing gear. Fish are released after capture (bycatch)
because of harvest restrictions: number, size or sex limits, or
incidental catch as nontarget species. Fish escape from gear as a
result of gear modifications causing unwanted fish to be excluded or
released prior to landing. Mortality rates of bycatch and escapees are
generally unknown and constitute a large source of uncertainty in
estimates of overall fishing mortality. Measurements of bycatch amounts
and bycatch mortality rates for individual fisheries are necessary for
improvements in present management schemes.
Systematic bycatch research conducted over the past 10 years
in the laboratory of the Alaska Fisheries Science Center's
Fisheries Behavioral Ecology Group has
examined the interacting capture stressors (towing in a net and
hooking), light conditions, temperature, and exposure to air in walleye pollock, sablefish, Pacific halibut, and lingcod using changes in
behavior, blood physiology, and mortality as measures of stress. Fish
were
exposed to bycatch stressors by various means. Fish were towed
under light or darkness in nets attached to two rotating
arms in a tank to simulate capture in a codend. Fish were hooked on lines and held in darkness
to simulate capture by longline. After exposure to capture
stressors, fish were placed in lighted tanks with controlled seawater
temperature to simulate exposure to increased
temperature associated with thermoclines. Fish were placed in lighted
tanks without water in temperature controlled rooms
to simulate exposure to air at increased temperature.
Past efforts at understanding and reducing bycatch mortality have focused on modifying fishing gear to avoid capture of potential bycatch and to reduce physical injury to fish that are caught in fishing gear. Fish experience stress from physiological injury and behavioral deficits that may not be readily apparent to human observers, but may result in significant direct or indirect mortality in discards and escapees. Bycatch mortality is linked to environmental and biological factors and their interactions, which have not been previously investigated in any detail. Research in a laboratory setting under controlled conditions allows for a systematic investigation of bycatch stressors and furthers our understanding of key principles of bycatch mortality.
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