NOTE ON THE SUBPOPULATIONS OF LAKE TROUT 

 IN THE GREAT LAKES 



By 



Paul H. Eschmeyer -' 



1/ 



The definition and recognition of sub- 

 populations of lake trout ( Salvelinus namaycush) 

 in the Great Lakes is a problem of considerable 

 magnitude and interest that has been little in- 

 vestigated. Opportunity for study has become 

 increasingly restricted with the dwindling of 

 populations following the invasion of the sea 

 lamprey (Petromyzon marinus) . Lake trout are 

 nearing extinction in LaKes Michigan and Huron; 

 commercial fishery statistics indicate a pro- 

 gressive decline in numbers in Lake Superior. 

 It appears certain that some subpopulations 

 will be or have already been exterminated. 



In Lake Superior, where the species 

 probably attains its greatest diversification, 

 different subpopulations are isolated genetical- 

 ly in part by their characteristic spawning 

 seasons and localities. Breeding seasons of 

 different subpopulations (including the siscowet, 

 a form given subspecific ranking, Salvelinus 

 namaycush siscowet) cover a span of at least 6 

 months (June through November) . Some spawn 

 on rocky bottoms in the open lake at depths of 

 less than 20 fathoms; others spawn at 50 to 80 



fathoms, apparently sometimes on soft bottoms; 

 still others are reported by Ontario workers to 

 enter streams to spawn, particularly along the 

 east shore of the lake . Tagging studies con- 

 ducted by both Canadian and United States 

 agencies have revealed a marked tendency of 

 adults to return during successive years to the 

 spawning grounds on which they were tagged. 



In addition to differences in spawning 

 seasons and localities, wide variation occurs 

 in size at sexual maturity among different sub- 

 populations. Chemical analyses of flesh samples 

 have shown a large difference in fat content 

 between lake trout and siscowets . 



The various subpopulations of lake trout, 

 in Lake Superior at least, are clearly real. Al- 

 though representatives of some of these may be 

 recognizable by physical characteristics, others 

 unquestionably are not so identifiable . The sig- 

 nificance of their existence to the management 

 of the species is great, and there is urgent 

 need for research on various problems which 

 they present . 



1/ Assistant Director, Institute for Fisheries 

 Research, Michigan Department of Conservation, 

 University Museums Annex, Ann Arbor, Michigan. 

 Formerly Fishery Research Biologist, Great 

 Lakes Fishery Investigations, U.S. Fish and 

 Wildlife Service, Ann Arbor, Michigan . 



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INT.-DUP. SEC . WASH., D.C. 19 9J9 



