Lake trout spawning habitat suitability at two offshore reefs in Illinois waters of Lake Michigan

In Illinois waters of Lake Michigan, Julian's and Waukegan reefs were once-productive offshore commercial fishing sites. Currently, both reefs are stocked and naturally reproduced lake trout aggregate at these reefs during the spawning season. Attempts to document natural reproduction of lake trout at deep-water spawning sites in southwestern Lake Michigan have been hampered, in part, by a lack of detailed information on suitable spawning habitat and imprecise placement of egg collection devices. We developed high-resolution substrate and bathymetric maps for Julian's and Waukegan reefs using geo-referenced bathymetry readings, sidescan sonar, and underwater video. Spawning activity was evaluated at suitable and unsuitable habitat using egg traps deployed during the 2009 and 2010 spawning seasons. Sidescan sonar data allowed identification of suitable substrate at Waukegan (1%) and Julian's (2%) reefs as well as previously undocumented Waukegan South Reef (6%). Small, discrete areas with suitable spawning habitat (substrate and slope) were found and in total constituted < 1% of all hard surfaces mapped at both the Waukegan Reef complex and Julian's Reef. No eggs were collected either year, due in part to difficulty sampling small, localized patches of suitable spawning habitat and extensive coverage of Dreissena and Cladophora species. In the future, use of this high-resolution habitat data combined with more precise egg or fry sampling equipment will allow for a more comprehensive evaluation of natural reproduction at these once-productive offshore reefs.