Evidence of natural reproduction of lake trout in western Lake Huron

Wild young-of-the-year (YOY) lake trout (Salvelinus namaycush) were taken in bottom trawls from Thunder Bay in 1984 and in every year from 1986 to 1993. All lake trout stocked in Lake Huron had one or more fins clipped (removed). The proportion of lake trout without clipped fins was significantly higher (p < 0.01) in spring gillnet assessments and sport catches from MH-2 (Thunder Bay area) than from other management districts in western Lake Huron. The proportion of the MH-2 survey catch that was unclipped was significantly higher (p < 0.001) among the 1984-90 year classes than previous cohorts. A spawning population of predominantly unclipped lake trout was discovered on Mischley Reef in Thunder Bay. The trawl catch of YOY and the increasing incidence of unclipped older lake trout in MH-2 suggest, therefore, that the lake trout stock is partly supported by reproduction. However, the mean age of unclipped lake trout in assessment catches did not increase appreciably in 1986-1993. Total mortality rates for lake trout in MH-2 during this period ranged near 50%. Sea lamprey (Petromyzon marinus) wounding rates were as high as 40 wounds per 100 lake trout. Alewife (Alosa pseudoharengus) abundance was high in June on near-shore reefs, but low on Mischley Reef where lake trout reproduced successfully. Development of wild lake trout stocks in Thunder Bay may be stymied by a combination of factors, including sea lamprey-induced mortality and predation on swim-up fry by alewives.