Investigations into Recent Declines in Survival of Brown Trout Stocked in Lake Charlevoix and Thunder Bay, Lake Huron;Research rept

摘要

Sharp declines in the Thunder Bay, Lake Huron and Lake Charlevoix brown trout fisheries prompted investigations into the causes of brown trout failures in these waters and possible solutions. Both Thunder Bay and Lake Charlevoix are located in the northern part of Michigan's Lower Peninsula. Test netting and diet studies of predators and prey in Thunder Bay during 1990 showed that piscivorous fish, particularly walleyes, consumed recently stocked brown trout, but that spawning aggregations of alewives during June appeared to buffer predation on stocked trout by offering ample alternate prey. The stocking date for brown trout, which had been early May, was therefore changed to mid-June in 1992. Two strains of brown trout, Wild Rose and Seeforellen, were selected for field evaluation based on evidence of satisfactory lacustrine performance elsewhere, and programmed for testing in Thunder Bay and Lake Charlevoix. These strains were also compared with Plymouth Rock strain, which had been stocked in both systems prior to the study. Both Seeforellen and Wild Rose strains produced greater returns and faster growth than Plymouth Rock strain. Seeforellen and Wild Rose strains were similar to each other with respect to returns to creel, growth rates, and longevity in the fishery. However, there was evidence that Seeforellen strain produced slightly better results in Thunder Bay. The brown trout fishery in Thunder Bay rebounded with successful stockings in 1991-95, but declined again as a consequence of poor survival of trout stocked after 1995. The short-term recovery was attributed to the later stocking window and deployment of the new test strains of brown trout. The brown trout failure after 1995 appeared to be caused by declining alewife abundance, which essentially 'closed' the June stocking window. Predation on brown trout was probably exacerbated by an 8.4-fold increase in double-crested cormorant Phalacrocorax auritus numbers in the Thunder Bay area between 1989 and 1997. Offshore stocking had no measurable effect on survival of the 1996 and 1997 yearling cohorts$ both nearshore and offshore treatments survived poorly in both years. The brown trout niche in Thunder Bay appeared to be tenuous. Zooplankton and other prey were scarce for age-1 brown trout. Diet of age-1 brown trout was chiefly terrestrial insects because the trout were too small to utilize the abundant adult alewives. Longevity of brown trout in the fishery was relatively short and few survived past age four. Thus, once recruited at age two to a size sufficient to feed on alewives, brown trout contributed to the fishery only one or two more years. Two successive year class failures therefore were sufficient to cause collapse of the fishery. Unless alewives recover, predators decline, or another prey species (such as the round goby, a recent invader) alter the food web in a way that favors brown trout, the niche for put-grow-take brown trout management of Thunder Bay may have disappeared.