CAUSES OF DECLINING SURVIVAL OF LAKE TROUT STOCKED IN US WATERS OF LAKE-SUPERIOR IN 1963-1986

Survival of the 1963-1982 year-classes of stocked yearling lake trout Salvelinus namaycush declined significantly over time in Lake Superior . To investigate possible causes of this decline, a Ricker model of st ock-recruitment was used to describe the catch per effort (CPE) of age -7 stocked lake trout in the Michigan, Minnesota, and Wisconsin waters of Lake Superior as functions of the numbers of yearlings stocked 6 y ears earlier (an index of density dependence), the density (CPE) of wi ld adult lake trout (an index of predation), and large-mesh (greater t han or equal to 114-mm stretch-measure) gill-net fishing effort (an in dex of fishing mortality). Declining CPE of stocked lake trout in Mich igan and Wisconsin was significantly associated with increasing large- mesh gillnet fishing effort. Declining CPE of stocked lake trout in Mi nnesota was significantly associated with increasing density of wild l ake trout. Declining survival of stocked lake trout may therefore have been caused by increased mortality in large-mesh gill-net fisheries i n Michigan and Wisconsin, and by predation by wild lake trout that rec ently recolonized the Minnesota area. We recommend that experimental m anagement be pursued to determine the relative importance of large-mes h gillnet fishing effort and of predation by wild lake trout on the su rvival of stocked lake trout in U.S. waters of Lake Superior.