Phytoplankton Prey Selection by Hypophthalmichthys molitrix Val. (Silver Carp) in a Lower Mississippi River Backwater Lake

Abstract

Hypophthalmichthys molitrix (Silver Carp) are invasive and abundant in the Mississippi River system, where they consume phytoplankton. There is concern that Silver Carp may influence phytoplankton community structure with cascading effects on other trophic levels. Information is needed regarding Silver Carp phytoplankton-consumption rates and prey selection to assess their potential impact on the food-web in the river. We investigated Silver Carp diets in a backwater lake of the Lower Mississippi River in order to quantify phytoplankton prey selectivity. We made measurements on 4 dates over a 2-y period, which spanned a range of hydrologic connectivity between the lake and the river and a variety of fish sizes. We quantified selection by comparing phytoplankton community composition in the lake to prey in foreguts of captured Silver Carp using Vanderploeg and Scavia's (1979) relativized selection index. With a possible exception of diatoms on 1 date, there was no relationship of sample date or fish size on prey selection. However, there was a consistent pattern in prey selection: euglenoid algae were positively selected, selection of colonial algae and diatoms was variable, and flagellates and filamentous cyanobacteria were negatively selected. Results are discussed in the context of a conceptual model for Silver Carp phytoplanktivory that incorporates the roles of habitat selection, prey availability, prey capture and processing, and digestive physiology.