Lake Trout Rehabilitation in Lake Michigan

     Lake trout Salvelinus namaycush were extirpated from Lake Michigan by 1960, and this extirpation has been attributed to predation by sea lamprey Petromyzon marinus in conjunction with overfishing.  To rehabilitate the lake trout population in Lake Michigan, a major stocking program was initiated in 1965, and between one and three million lake trout yearlings (or yearling equivalents) have been stocked into Lake Michigan each year since that time.  During 1965-1984, most of these hatchery reared lake trout were stocked nearshore and were of the Marquette strain, a strain of lean lake trout from Lake Superior.  Two offshore refuges were established during 1984-1985.  The Northern Refuge encompassed a complex of shallow-water (< 20 m deep) spawning reefs, whereas the Southern or Midlake Refuge included four large deep-water (45- to 80-m) spawning reefs.  Fishing for lake trout continues to be prohibited in both refuges.  Since 1986, about 50% of the lake trout stocked into Lake Michigan have been stocked into the offshore refuges, and Seneca Lake and Lewis Lake strains of lake trout have also been stocked.  Relatively high lake trout spawner densities have been observed at several locations throughout Lake Michigan during the 1990s and 2000s.  In addition, annual survival rates of adult lake trout exceeding 60%, the target level for rehabilitation, have been estimated for several regions throughout the lake.  However, recruitment of wild fish to the adult lake trout population has been very low to date.  The overall leading cause for this low recruitment is believed to be interference by alewives Alosa pseudoharengus, via predation on lake trout fry and via thiamine deficiency complex (TDC), with lake trout reproduction.  At certain locations, other causes included insufficient diversity in ages of lake trout spawners, insufficient lake trout spawner abundance, and predation on lake trout eggs.