Potential predator-prey relationships between Bythotrephes longimanus and Cercopagis pengoi in Southwestern Lake Michigan

The introduction of multiple non-native predators to a lake may influence food-web dynamics in unpredictable ways, but in large lakes it is often difficult to conduct field experiments to test directly the impact of each new exotic species. Instead, a combination of field data, mathematical modeling, and laboratory experiments is often employed. In this study, we use this combination to address potential interactions between two exotic predators (Bythotrephes longimanus (formerly B. cederstroemi) and Cercopagis pengoi) in southwestern Lake Michigan. Specifically, we conducted laboratory experiments in which we varied the size of enclosure (10 mL, 100 mL, 1,000 mL, 10,000 mL) to determine how container size affects the outcome of predator-prey interactions. We also recorded the occurrence of both species in 2002-2003 and used a mathematical model to estimate potential encounter rates between these two species. Densities of C. pengoi ranged from 5 to 717 individuals/m³ at our sampling stations while B. longimanus was rare. The results from the laboratory experiments indicated that B. longimanus will eat C. pengoi. However, the predicted number of daily encounters between these two species in southwestern Lake Michigan was relatively small under most model scenarios. Moreover, our results also indicated that the size of the container and the species involved can significantly affect experimental outcomes. Hence, our results demonstrate that although the findings of laboratory experiments can provide insight into possible interactive dynamics, great care should be taken when extrapolating laboratory results to the field.