Diel Migration Great Lakes Fish Communities: Forming Habitat Linkages, Promoting Habitat Coupling, and Consequences for Ecosystem Health and Function

Diel patterns of distribution of fishes in nearshore (15-80 m depth) and offshore (>80 m) waters of Apostle Islands region of Lake Superior were described using bottom trawls, mid-water trawls and hydroacoustic gear during day and night sampling along a series of transects targeting depths of 15-30 m, 30-60m, 60-90m, and 90-120 m.  Resulting distribution data revealed three types of diel migration: diel vertical migration (DVM), diel bank migration (DBM), and no diel migration. Applying our results to biomass estimates for Lake Superior revealed that DVM accounts for 80%, DMB 19%, and non-migration 1% of the total biomass in nearshore waters.  In offshore waters DVM accounts for 89%, DMB 3%, and non-migration 8% of the total biomass.  Our results suggest that diel migration by Lake Superior fishes is substantial and enhances linkage of shallow and deepwater and benthic and pelagic habitats.  The linkage of habitats by diel migration of fishes enhances the potential for habitat coupling, a condition where habitats become interconnected by energy and nutrient transfers.  Other Great Lakes with diminished DVM of fishes due to loss of native lake trout and ciscoes and reduced abundance of exotic pelagic fishes (rainbow smelt, alewife), in combination with benthification due to expansion of exotic dreissenid mussels, may have reduced potential for benthic-pelagic coupling and energy transfers compared to Lake Superior.