Incorporating Applied and Basic Approaches to Evaluate the Effects of Asian CARP on Native Fishes

Understanding fish community interactions is a critical component of large river ecology, especially the interactions of nonindigenous fishes on native fauna.  Non-native large bodied planktivores are capable of posing deleterious effects to large river ecosystems. In particular, the effects of silver carp on native planktivorous large river fishes such as bigmouth buffalo and gizzard shad have received little attention despite the apparent relevance. As such, the objective of this study was to understand the interactions of silver carp with bigmouth buffalo and gizzard shad (i.e., native planktivores) in the free-flowing stretch of the Upper Mississippi River (a.k.a., the Middle Mississippi River, MMR).  Specifically, we used data collected for the Long Term Resource Monitoring program (LTRM) throughout the MMR from 1992-2008 and from controlled experiments to evaluate trends in both non-native and native planktivores population attributes.  Our results suggest that non-native planktivore relative abundance has increased while relative abundance of native planktivorous fishes has declined. Specifically, the gizzard shad and bigmouth buffalo have shown precipitous declines in abundance while silver carp have dramatically increased. As a quantitative measure of a limiting resource, we found that silver carp condition has remained fairly consistent while bigmouth buffalo and gizzard shad condition has declined. Correspondingly, the results of our controlled mechanistic experiments suggest that silver carp may be able to exclude native fishes via exploitative competition. Based on the results of this study, silver carp are likely negatively influencing native planktivorous fishes (e.g., creating a limiting resource through competition), and because of this management efforts should be directed at reducing silver carp abundance to subsequently rehabilitate native planktivores.