Side-scan Sonar and Underwater Video Observations of Potential Lake Trout Spawning Substrate at Two Previously Un-described Offshore Reefs in Southwest Lake Michigan

Substantial numbers of naturally reproduced lake trout have been collected in fall gillnet surveys in Illinois waters since 2012, after decades of rehabilitation efforts. The results of these surveys, which occurred at known historical spawning locations (Julian’s and Waukegan Reefs), prompted interest in a more thorough accounting of lake trout spawning habitat in southwest Lake Michigan. Using information from historical reports and anglers, bathymetric maps were created for a series of offshore reefs from 2017-2019 to aid future gillnetting surveys. Fall gillnet surveys at Lake Bluff 10-Mile Reef in 2019 produced substantial numbers of lake trout, 55% of which were wild fish. Thus, Lake Bluff 10-Mile Reef and Gumby Reef (located near Julian’s Reef) were singled out for more in-depth surveys in 2020. Using a combination of underwater video and side-scan sonar, we sought to characterize the substrate available at these two reefs to understand their suitability as spawning habitat and compare the characterized spawning habitat with previous surveys done at Julian’s and Waukegan Reefs. We observed a variety of substrate types at these two reefs, including large cobble with significant interstitial spaces, massive bedrock outcrops and cliffs, and areas of sand and clay with scattered cobble. Video survey showed nearly all hard surfaces were encrusted with a layer of quagga mussels and both lake trout and lake whitefish present over harder substrates during the spawning season. The results of this research contribute to our knowledge of potential spawning habitat for lake trout and key habitats to study for egg and fry survival in the Illinois waters of Lake Michigan.