Side-Scan Sonar Mapping of Lake Trout Spawning Habitat in Northern Lake Michigan

Abstract

Native stocks of lake trout Salvelinus namaycush were virtually or completely extirpated from the lower four Great Lakes by the early 1960s. The failure of early attempts to reestablish self-sustaining populations of lake trout was attributed partly to the practice of stocking hatcheryreared juveniles at locations and over substrates that had not been used in the past for spawning by native fish. Subsequent attempts to improve the selection of stocking locations were impeded by the lack of reliable information on the distribution of substrates on historical spawning grounds. Here we demonstrate the potential of side-scan sonar to substantially expand the data base needed to pinpoint the location of substrates where lake trout eggs, fry, or juveniles could be stocked to maximize survival and help ensure that survivors returning to spawn would encounter suitable substrates. We also describe the substrates and bathymetry of large areas on historical lake trout spawning grounds in the Fox Island Lake Trout Sanctuary in northern Lake Michigan. These areas could be used to support a contemporary self-sustaining lake trout population in the sanctuary and perhaps also in adjacent waters.