An Evaluation of Bycatch in Saginaw Bay, Lake Huron's Commercial Trap Net Fishery

Commercial fishing bycatch can represent a significant source of uncertainty for the accurate assessment of fish populations. Bycatch and bycatch mortality rates in the Saginaw Bay, Lake Huron commercial trap net fishery are currently unknown.  Throughout the 2010 fishing season, we quantified bycatch in trap nets by observing normal fishing practices onboard commercial vessels.  Though all non-target species incidentally caught were evaluated, we emphasized capture of walleye Sander vitreus and lake trout Salvelinus namaycush because these species are important foci of Great Lakes management.  Specifically, we used generalized linear models to relate the number of individuals of each species caught to environmental conditions and fishing practices.  We also counted the number of discarded morbid (floating) walleye and lake trout to obtain baseline mortality estimates.  For both lake trout and walleye, bycatch was seasonal and was dependent on trap net soak time.  Walleye bycatch was highest in May, but lake trout bycatch was highest in July and August.  Longer soak times resulted in higher bycatch rates for both walleye and lake trout.  Walleye bycatch morbidity increased with trap net depth and larger target catches, whereas lake trout morbidity increased with target catch and surface water temperature.  These results fill a significant knowledge gap necessary to effectively manage Saginaw Bay’s fishery resources.