Linking emergent midges to alewife ( Alosa pseudoharengus) preference for rocky habitat in Lake Michigan littoral zones

Abstract

Alewife ( Alosa pseudoharengus) abundance and condition are in decline in Lake Michigan, where they not only exert negative effects on several native fish species but also support an economically valuable salmon sport fishery. We investigated feeding-related habitat selection for adult alewives occurring in Lake Michigan's littoral zone by comparing the relative abundance of alewives and non-biting midges (Chironomidae) on littoral rocky and sandy habitats. Both alewives and midges strongly favored hard over soft substrate, with approximately 22 and 23 times greater catch rates on rock than sand, respectively, from June through August 2005. Relative to sand, 92.2% of alewives and 94.8% of emergent midges were captured over rocky habitat. Gut content analysis suggested that the supplement of midge pupae created a benefit for alewives on rocky habitat and that midge emergence may be the basis for rocky habitat selection. Alewife consumption of midge pupae correlated with midge emergence events, frequency occurrence of midges in ale wife diets was significantly greater on rocky habitat, and alewives collected over sand were 1.5 times more likely to have empty stomachs than those collected over rocks. Our results indicate that alewives, the dominant pelagic forage fish in Lake Michigan, are seasonally encountered in littoral rocky communities, where midges are an important seasonal component of alewife diet.