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Understanding Fish Bycatch Discard and Escapee Mortality

  see caption for explanation

Circular tank and net-towing apparatus at the Center's Newport laboratory for simulation of fish towed in a trawl codend.


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.  


Fisheries Behavioral Ecology staff inserting sablefish into a net to be towed in the laboratory.  

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.


  photo of sablefish (see caption for explanation)

Sablefish often are more durable than other fishes when subject to trawl stressors.

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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