Lake Michigan Offshore Reef Bathymetric Mapping: July 1, 2018 – October 31, 2019

The return of self-sustaining lake trout (Salvelinus namaycush) populations in Lake Michigan is a long-standing rehabilitation goal of the Illinois Department of Natural Resources (IDNR) Division of Fisheries and the Great Lakes Fishery Commission. Annual stocking of juvenile lake trout at sites lake-wide since 1965 has succeeded in building up spawning populations but until recently only minimal natural reproduction had been documented.Patterson et al. (2016) evaluated IDNR’s fall spawning assessment data and found similar abundances of mature lake trout in recent years at un-stocked Waukegan Reef compared to Julian’s Reef, the only site stocked annually within the Illinois management unit. These results, combined with high relative catches (~50%) of unmarked ‘wild’ lake trout adults at both reefs in recent years, have prompted the initiation of fall spawning assessments at additional offshore reefs in Illinois waters of Lake Michigan.Identifying and monitoring previously un-sampled lake trout spawning sites is essential for developing effective protection policy for critical habitat and adjusting stocking and harvest regulations for this important, native predator and sport fish. Attaining geo-referenced bathymetry data and developing accurate maps of potential spawning reefs is an important next step in the planning and implementation of lake trout spawning assessments at un-sampled reefs in Illinois. Thus, the objectives of this project were to 1) document the location and extent, 2) collect bathymetry data, and 3) create contour and three-dimensional bathymetric maps of offshore reefs in Illinois’ portion of Lake Michigan.Results of this project will aid IDNR in measuring the extent of lake trout rehabilitation success and identify important potential spawning habitats that may be targeted for future protection and/or enhancement.It will also lay groundwork for potential future research using side-scan sonar and telemetry to understand the use of spawning habitats by lake trout.