No abstract

Grass pickerel (Esox americanus vermiculatus) was collected from 16 streams and rivers in Indiana for diet analysis to determine the effects of this small-bodied esocid on lotic ecosystems. The 128 fish used in the analysis were divided among tlwee size classes-57-95 mm, 96-150 mm, and >I50 mm. Grass pickerel exhibited an ontogenetic shift in prey type with increasing sizc. The small size class had a diet dominated by fish, the intermediate size class had a diet dominated by both fish and crayfish, and the large size class had a diet dominated by crayfish. Insects also made up 22% of the volume of small and 29% of the volume of the intermediate size classes, whereas the large size class consumed only 1 % insects by volume. Calculated relative importance index (RI) showed that grass pickerel diet shifted from one dominant in fish (RI = 53) at the small size class to one dominated by crayfish (RI = 57) at the large size class. The grass pickerel was opportunistic in its diet choices and fed on increasingly larger items as its total length increased. This choice by grass pickerel in small streams suggests that it is feeding as a top predator and structuring the fish communities in these ecosystems.