SELECTIVITY FOR ZOOPLANKTON PREY BY FATHEAD MINNOWS AND BROOK STICKLEBACKS

Wetlands in the Prairie Pothole Region of North America are harsh environments for fish, and only two planktivorous species are common in these ecosystems: fathead minnows (Pimephales promelas) and brook sticklebacks (Culaea inconstans). Given their similarities, competition between these species may be high, especially for food resources. We assessed potential competition between fathead minnows and brook sticklebacks, as well as their influence on zooplankton communities, by measuring their selectivity for zooplankton prey in controlled laboratory experiments. Minnows and sticklebacks of varying size were placed in aquaria with mixed zooplankton assemblages to assess selectivity for copepod and Daphnia prey. Sticklebacks of all sizes showed strong selectivity for Daphnia, while minnows were less selective, and prey selectivity varied with fish size. Selectivity of prey was not related to relative prey abundance for either minnows or sticklebacks. Although minnows were less selective, their mass-specific consumption rates were significantly lower than consumption rates for sticklebacks. Our results indicated that effects of predation on zooplankton may differ between these two fish species, with minnows potentially affecting greater numbers of taxa. Additionally, generalist feeding on zooplankton prey by fathead minnows may be advantageous in habitats with low Daphnia densities, as is common in prairie wetlands with dense fish populations.