Discard Mortality of Summer Flounder in the Commercial Trawl Fishery

The purpose of this study was to gain perspective to improve and enhance fishery information about discard mortality for summer flounder in the trawl fishery.  To this end the objective was to evaluate actual trawl discard mortality within a research design for comparison to the assumed discard mortality rate currently used in the stock assessment.  The study goal was to determine discard mortality relative to tow time, total catch, fish size and the amount of time that fish were on deck.  Initially, ten scientific fishing trips consisting of tows of 1, 2 and 3 hours in duration were conducted.  Fish were culled at consistent time intervals into live and dead on deck.  Additionally, a sub-sample of live fish from the immediate cull and the delayed cull were held in an on board live system then transferred to an extended mortality monitoring net-pen system.  The overall summed discard mortality as determined by this project had a mean of 64.6% and a median of 78.7%.  A linear relationship of discard mortality to tow time and cull time existed.  For example, the mean discard mortality in the 1-hour tows was 58%, in the 2-hour tows it was 61% and in the 3-hour tows it was 77%.  Similarly related was the inverse correlation of mortality to cull time. Also, total catch was an important discard mortality factor but fish size was not. When comparing the parameter values of tow time, cull time and total catch as determined by this study it is apparent that shorter tows, faster culling and improving handling practices, while reducing per-tow total catch can definitively reduce summer flounder discard mortality. A modified Phase II component was also conducted where we only separated the fish into live/dead fraction on deck and did not save any live fish for extended mortality monitoring.  The Phase II component produced similar results.