A Tale of Two Lakes: Contrasting Patterns of Diel Migration in Lakes Superior and Michigan

We compared diel migration patterns in the Lake Superior fish community.  We identified three behaviors: diel vertical migration (DVM), diel bank migration (DBM), and no diel migration. Application of these species-specific behaviors to the Lake Superior nearshore fish community for the period 1989-2010 predicted that 77% of the biomass exhibited DVM, 22% exhibited DBM, and 1% showed no diel movement patterns.  In offshore waters, DVM accounted for 84%, DBM 4%, and non-migration 12% of total fish community biomass.  Application of diel migration behaviors to the Lake Michigan fish community for the period 1973-2010 yielded different results that changed over time.  In Lake Michigan nearshore waters, DVM accounted for 53-88%, DBM 10-27%, and non-migration 1-21% of fish biomass.  In offshore waters, DVM accounted for 34-49%, DBM for 0-1% and non-migration for 51-66% of fish biomass.  Diel migration observed in Lake Superior represents historical natural patterns of the native fish community, whereas those in Lake Michigan appear due to extreme variation in recruitment of native bloater, declining recruitment in non-native alewife and rainbow smelt, and to the effects of recent invasions of non-native dreissenid mussels and round goby, which have disrupted food web dynamics.  The prevalence of diel migration by Lake Superior fishes 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.  Great Lakes fish communities with diminished diel migration 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 mussel, have reduced potential for benthic-pelagic coupling and energy transfers compared to Lake Superior.