Regions and Sub-regions of Lake Trout in the Main Basin of Lake Huron

Annual spring fishery independent surveys in the main basin of Lake Huron are analyzed in two steps. Over-year development and recent spatial pattern of lake trout stocks were first revealed based on origin (hatchery vs wild) and size compositions of lake trout and followed by analyses of spatial differences in abundance dynamics. During the 1980s and 1990s, while relative abundance of stocked recruits was similar lake-wide, adult abundance was much lower in northern Lake Huron than in southern Lake Huron. After recent major changes in Lake Huron ecosystem since the early 2000s, stocked recruitment declined lakewide, and wild recruitment increased lake-wide. In southern Lake Huron, the decline of stocked recruitment was especially abrupt, and the recruitment of wild lake trout eventually became weak. The abundance of adult lake trout declined with decreases in stocked recruitment in southern Lake Huron; but was stable in northern Lake Huron. The abundance of wild adults increased in both northern and southern Lake Huron, but the increase was more rapid in northern Lake Huron than in southern Lake Huron. Fisheries in both northern and southern Lake Huron are under similar management, and the harvest policy allows for adult annual mortality of 40-45%. The survey findings indicate that during the 1980-1990s adult mortality in northern Lake Huron must be much higher than southern Lake Huron and the mortality limit; recently, however, it must become much lower than southern Lake Huron and the mortality limit. The contrasts between northern and southern Lake Huron suggest that the current harvest policy is sustainable only when recruitment is stable and sufficiently high, such as in early years with very low post-stocking mortality of lake trout yearlings.