Quantification of historic lake trout (Salvelinus namaycush) spawning aggregations in Lake Michigan

We used commercial catch reports to determine site-specific characteristics of lake trout (Salvelinus namaycush) spawning aggregations in Michigan's and Wisconsin's waters of Lake Michigan before reproduction ceased in the 1950s. One hundred sites where annual catches exceeded 25 kg/year were identified. Two thirds of the catch was made in offshore waters and nearly half was made in the northeast sector of the lake. Catch was a better descriptor of the size of spawning aggregations than catch per unit effort (CPUE). CPUEs were not significantly different among onshore, offshore, and southern deepwater reefs. Spawning activity as measured by CPUE had peaked by the week beginning 29 October at all locations. Spawning aggregations were strongly clustered in the northeast section of the lake where Devonian rocks were subject to brecciation, i.e., fracturing and recementing following slumping. Areas zoned as refuges in the current lake trout rehabilitation plan account for 36% of the historical catch targeted at spawning aggregations. We recommend a refocusing of rehabilitation efforts in northern waters from sites where historical catches were modest to sites that produced the largest catches of lake trout aggregated for spawning.