Potential spawning habitat for lake trout on Julian's Reef, Lake Michigan

Julian's Reef is an historical spawning ground for lake trout (Salvelinus namaycush) in southwestern Lake Michigan. It is a designated lake trout refuge and is the focus of lake trout restoration efforts in Illinois waters of the lake. We studied the reef to determine its potential as spawning habitat for stocked lake trout. We used side-scan sonar and a remotely operated vehicle equipped with a video camera to survey and map 156 ha of lake bed on the southeast portion of the reef where an earlier study revealed the presence of loose-rock substrate potentially suitable for use by spawning lake trout. Our survey showed that the substrate on the reef that most closely resembled that described in the literature as suitable for spawning by stocked lake trout in the Great Lakes was rubble patches with interstitial depths greater than 20 cm. These nibble patches occupied about 2 ha of the 13 ha expanse of bedrock and rubble substrate near the reef crest in the surveyed area. We estimated that these rubble parches, if fully used by spawning lake trout, could accommodate egg deposition by at least 1,300-3,300 2.7-kg females.