Environmental links to alosine at-sea distribution and bycatch in the Northwest Atlantic midwater trawl fishery

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The bycatch of river herring (Alosa pseudoharengus and A. aestivalis) and American shad (A. sapidissima) by midwater trawl vessels targeting Atlantic herring (Clupea harengus) and Atlantic mackerel (Scomber scombrus) has become a possible threat to their conservation. Most bycatch occurs during winter when the regional distributions of these five species overlap. The life histories and habits of these species suggest that there are times and areas during winter where overlap is low and bycatch could be reduced. This study identified environmental conditions associated with the presence of river herring and American shad during winter in the US Mid-Atlantic area by testing the hypothesis that fish were detected in an equal proportion to the amount of samples taken within an environmental condition. River herring were associated with colder temperatures and relatively shallow depths, while American shad appeared to be associated with a broader habitat. To determine whether these associations could be used to reduce bycatch, the proportion of effort and catch of target and bycatch species under the conditions associated with bycatch species presence was quantified in a fishery-dependent dataset. Targeting Atlantic mackerel in waters with surface temperatures above 6°C appeared to be the most effective advice for reducing river herring bycatch.

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