Evaluation of fish spawning habitat at offshore reefs in southwest Lake Michigan using side‐scan sonar and underwater video

Abstract

Offshore reefs in the Laurentian Great Lakes provide critical spawning habitats for native lake trout. Though many historic spawning reefs have been documented, thorough evaluation of the habitat they contain is rare. After decades of stocking, natural recruitment of lake trout has increased dramatically in the Illinois waters of Lake Michigan since 2010, reinforcing the need for information about the viability and distribution of spawning habitats. Using side‐scan sonar and underwater video, this study sought to map bottom substrates at Lake Bluff Reef and Gumby's Reef, offshore of the northern Illinois coast of Lake Michigan, to describe and quantify the availability of substrates suitable for lake trout spawning. Substrates were manually delineated in ArcGIS using side‐scan sonar imagery mosaics. Underwater video was used to verify substrate classifications through direct observation. Suitable spawning substrate consisting of fractured bedrock debris and piled boulder/cobble was observed at both reefs. Underwater video revealed the presence of lake trout at both reefs in the weeks leading up to the spawning season. The largest congregation of fish was observed in the vicinity of the shallowest points, steep slopes, and the largest patches of spawning substrate. The distribution, amount, and condition of spawning substrate available at each reef was in line with others studied in the region and their likely importance for lake trout spawning was reinforced by observed aggregations of mature fish. This study has expanded the inventory of well‐documented offshore reefs in the Illinois waters of Lake Michigan, enhancing appreciation of what is coming to be understood as the Illinois Reef Complex. Knowledge of the distribution and status of habitats critical to the life history of native fishes in the Great Lakes are necessary for managers to properly assess, manage, and conserve lake trout and other fish populations.