Short-Term Mortality of Discarded Finfish Bycatch in the Gulf of Maine Fishery for Northern Shrimp Pandalus borealis

Abstract

Although numerous studies have detailed the level of bycatch and discard in commercial fisheries, few studies have documented mortality of bycatch that is discarded after capture. We studied the short-term mortality caused by landing and by handling and sorting activities, as well as the levels of avian predation, imposed upon four finfish species—American plaice Hippoglossoides platessoides, winter flounder Pleuronectes americanus, witch flounder Glyptocephalus cynoglossus, and pollock Pollachius virens—that are prominent discards in the Gulf of Maine fishery for northern shrimp Pandalus borealis. Mortality caused by capture and on-deck sorting varied among the four species. Mortalities of American plaice, witch flounder, and pollock were correlated positively with on-deck sorting time. but mortality of winter flounder was not. Mortality of winter flounder was less than 10% for all time periods of on-deck sorting tested (from 15 to 60+ min). Mean size of fish, maximum depth of tow, duration of tow, and air and water temperatures also were correlated with on-deck sorting mortality for at least one of the species tested. Seabirds consumed moderate to high proportions of pollock, witch flounder, and American plaice that were living when discarded. Seabirds ate only 6% of discarded winter flounder before these fishes sank or swam into the water column. The rate of seabird predation upon discards was correlated directly with time spent on deck during sorting for American plaice, witch flounder, and pollock. Differences in mortality among species caused by capture and handling and by avian predators probably are due in part to characteristics of the seasonal fishery, not just species-specific resistance to stress. Management strategies that (1) minimize the time fishes spend on deck during sorting and (2) restrict the fishing season to times when air and water temperatures are cooler and when shrimp tows are conducted in relatively shallow water may reduce the level of bycatch mortality caused by capture and handling and by avian predation on discarded fish.