Ecological Shifts in Resource Use by Bloaters in Lake Michigan

Abstract

In late August-early September 1979 and 1980, both juvenile (80–145 mm fork length) and adult (> 145 mm) bloaters Coregonus hoyi were abundant in bottom trawls from Lake Michigan, but few young-of-year bloaters (<80 mm) were caught near the bottom despite small cod-end mesh in the trawl. Along the bottom intersected by the metalimnion, juveniles were more abundant in warmer water than adults. Diets of both juveniles and adults were 99% benthic prey by biomass. Young-of-year bloaters were most common at the surface during the day and in the metalimnion (off the bottom) at night. They ate primarily small cladocerans and cyclopoid copepods. These ontogenetic shifts in resource use by bloaters may reduce interactions among size groups. Further, since alewives Alosa pseudoharengus became abundant in Lake Michigan in the 1960s, bloaters have assumed benthic habits much earlier in their lives. Our dietary data indicate that young bloaters now abandon small pelagic zooplankton for larger benthic prey at much smaller sizes than formerly; though we did not age our fish, available age-size data suggest this switch occurs at age 1 +, as much as 2 years earlier than before alewives became abundant. Adult bloaters also eat less zooplankton now than they did before the alewife population expanded. This shift in bloater life history toward earlier occupancy of the benthic habitat in the presence of alewives provides reasonably strong support for the hypothesis that alewife competition has been an important influence on the structure of the current Lake Michigan fish community. Received March 17, 1984 Accepted August 24, 1984