Evidence for allochthonous prey delivery to Lake Michigan's Mid-Lake Reef Complex: Are deep reefs analogs to oceanic sea mounts?

Lake Michigan's Sheboygan Reef is contained in a refuge for lake trout (Salvelinus namaycush) restoration, and is one of several deep midlake reefs that historically were productive spawning and fishery grounds. The summits of these reefs are rocky and deeper than the photic zone. We propose that the basis of trophic support for lake trout and their prey is allochthonous and based, at least in part, on the principle of “topographic blockage” of descending vertical migrators (Mysis diluviana), as has been proposed for oceanic seamounts. We estimated slimy sculpin (Cottus cognatus) and Mysis densities via ROV (unmanned submersible) electroshocking and collected slimy sculpins for diet analyses via ROV and beam trawl. Slimy sculpin densities were greater or similar to that for its shallow-water (photic zone) congener, the mottled sculpin (C. bairdi), and not distinguishable from a random distribution. Mysis densities were highly contagious. The diet of the slimy sculpin was variable: Mysis typically predominated, followed by Diporeia and midge larvae. We argue that Mysis and Diporeia originate from off Sheboygan Reef because the summit of Sheboygan Reef is much shallower than the preferred daytime depth of Mysis and there is no suitable soft substrate for Diporeia, which is a burrowing amphipod. The probable mechanism for delivery of these two prey to Sheboygan Reef is topographic blockage.