Consumption of two exotic zooplankton by alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) in three Laurentian Great Lakes

Introductions of the zooplankton Bythotrephes longimanus and Cercopagis pengoi into the Great Lakes have drawn attention surrounding their suitability as prey for zooplanktivorous fishes. We used gut contents and stable carbon isotopes to quantify differential consumption and selection of the exotics by alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) in Lakes Erie, Michigan, and Ontario. The exotics were more often consumed by alewife (up to 70% of gut content biomass) than by smelt (up to 25% of gut content biomass). Measured stable carbon isotope ratios of fish and ratios predicted from mixing models confirmed that the "snapshot" descriptions of diet through gut contents were representative of longer-term diets. While B. longimanus generally was selected for (14 of 17 sampling dates), C. pengoi was not a preferred prey item. Cercopagis pengoi was sometimes a large component of alewife diet because of its high densities in the environment. The exotic zooplankton are more important for alewife than for smelt, and consumption of the cladocerans varies throughout the growing season and among lakes, generally related to patterns of exotic abundance. Effects associated with consumption of the exotics should be high in alewife-dominated systems invaded by B. longimanus or large numbers of C. pengoi.