Evidence for buoyancy regulation as a speciation mechanism in Great Lakes ciscoes

We present evidence for a hypothesis that the deepwater ciscoes (Coregonus spp.) of the Laurentian Great Lakes were selected for buoyancy-regulation traits that facilitated planktivory on Mysis relict, a crustacean that undertakes extensive diel vertical migration (DVM). We assessed the vertical distribution and behavior of bloaters (C. hoyi) in Lake Michigan with bottom and mid-water trawls in late summer 1987 and with acoustics in August 1995. Biomass of bloaters on the lake bottom was reduced by 84% at night. Mean size of bloaters increased with depth as did the reduction in night biomass. Juvenile bloaters (