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Diel migration patterns of fishes in nearshore (15–80 m depth) and offshore (>80 m) waters of Lake Superior were examined to assess the potential for diel migration to link benthic and pelagic, and nearshore and offshore habitats. In our companion article, we described three types of diel migration: diel vertical migration (DVM), diel bank migration (DBM), and no diel migration. DVM was expressed by fishes migrating from benthopelagic to pelagic positions and DBM was expressed by fishes migrating horizontally from deep to shallow waters at night. Fishes not exhibiting diel migration typically showed increased activity by moving from benthic to benthopelagic positions within demersal habitat. The distribution and biomass of fishes in Lake Superior was characterized by examining 704 bottom trawl samples collected between 2001 and 2008 from four depth zones: ≤40, 41–80, 81–160, and >160 m. Diel migration behaviors of fishes described in our companion article were applied to estimates of areal biomass (kg ha−1) for each species by depth zone. The relative strength of diel migrations were assessed by applying lake area to areal biomass estimates for each species by depth zone to yield estimates of lake-wide biomass (metric tonnes). Overall, species expressing DVM accounted for 83%, DBM 6%, and non-migration 11% of the total lake-wide community biomass. In nearshore waters, species expressing DVM represented 74% of the biomass, DBM 25%, and non-migration 1%. In offshore waters, species expressing DVM represented 85%, DBM 1%, and non-migration 14% of the biomass. Of species expressing DVM, 83% of total biomass occurred in offshore waters. Similarly, 97% of biomass of non-migrators occurred in offshore waters while 83% of biomass of species expressing DBM occurred in nearshore waters. A high correlation (R2 = 0.996) between lake area and community biomass by depth zone resulted in 81% of the lake-wide biomass occurring in offshore waters. Accentuating this nearshore-offshore trend was one of increasing estimated total areal biomass of the fish community with depth zone, which ranged from 13.71 kg ha−1 at depths ≤40 m to 18.81 kg ha−1 at depths >160 m, emphasizing the importance of the offshore fish community to the lake ecosystem. The prevalence of diel migration expressed by Lake Superior fishes increases the potential of fish to link benthic and pelagic and shallow and deepwater habitats. These linkages enhance the potential for habitat coupling, a condition where habitats become interconnected and interdependent through transfers of energy and nutrients. Habitat coupling facilitates energy and nutrient flow through a lake ecosystem, thereby increasing productivity, especially in large lakes where benthic and pelagic, and nearshore and offshore habitats are often well separated. We propose that the application of biomass estimates to patterns of diel migration in fishes can serve as a useful metric for assessing the potential for habitat linkages and habitat coupling in lake ecosystems, and provide an important indicator of ecosystem health and function. The decline of native Lake Trout and ciscoes and recent declines in exotic Alewife and Rainbow Smelt populations in other Great Lakes have likely reduced the capacity for benthic-pelagic coupling in these systems compared to Lake Superior. We recommend comparing the levels and temporal changes in diel migration in other Great Lakes as a means to assess changes in the relative health and function of these ecosystems.

Diel patterns of distribution of fishes in nearshore (15–80 m depth) and offshore (>80 m) waters of the Apostle Islands region of Lake Superior were described using bottom trawls, mid-water trawls, and acoustic gear during day and night sampling. These data revealed three types of diel migration: diel vertical migration (DVM), diel bank migration (DBM), and no migration. DVM was expressed by fishes migrating from benthopelagic to pelagic strata and DBM was expressed by fishes migrating horizontally from deeper waters in the day to shallower waters at night while remaining within the benthopelagic stratum. Most fishes that did not exhibit diel migration showed increased nighttime densities as a result of increased activity and movement from benthic to benthopelagic strata. Rainbow Smelt (Osmerus mordax), Cisco (Coregonus artedi), Bloater (C. hoyi), Kiyi (C. kiyi), juvenile Trout-Perch (Percopsis omiscomaycus), and adult siscowet (Salvelinus namaycush siscowet) exhibited DVM. Lake Whitefish (C. clupeaformis), lean Lake Trout (Salvelinus namaycush namaycush), and juvenile siscowet exhibited DBM. Adult Trout-Perch and adult Pygmy Whitefish (Prosopium coulteri) exhibited a mixture of DBM and DVM. Burbot (Lota lota), Slimy Sculpin (Cottus cognatus), Spoonhead Sculpin (C. ricei), and Deepwater Sculpin (Myoxocephalus thompsonii) did not exhibit diel migration, but showed evidence of increased nocturnal activity. Ninespine Stickleback (Pungitius pungitius) exhibited a mixture of DVM and non-migration. Juvenile Pygmy Whitefish did not show a diel change in density or depth distribution. Species showing ontogenetic shifts in depth distribution with larger, adult life stages occupying deeper waters included, Rainbow Smelt, lean and siscowet Lake Trout, Lake Whitefish, Pygmy Whitefish, Ninespine Stickleback and Trout-Perch. Of these species, siscowet also showed an ontogenetic shift from primarily DBM as juveniles to primarily DVM as adults. Across all depths, fishes expressing DVM accounted for 73% of the total estimated community areal biomass (kg ha−1) while those expressing DBM accounted for 25% and non-migratory species represented 2% of the biomass. The proportion of total community biomass exhibiting DVM increased with depth, from 59% to 95% across ≤30 m to >90 m depth zones. Along the same depth gradient, the proportion of total community biomass exhibiting DBM declined from 40% to 1%, while non-migrators increased from 1% to 4%. These results indicate that DVM and DBM behaviors are pervasive in the Lake Superior fish community and potentially provide strong linkages that effect coupling of benthic and pelagic and nearshore and offshore habitats.