Interactions Between Non-Native and Native Large River Planktivores

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 have received little attention despite the apparent relevance. To understand the interactions between silver carp and two native planktivores, bigmouth buffalo and gizzard shad, we used data from the Long Term Resource Monitoring Program (LTRMP) collected throughout the free-flowing stretch of the Middle Mississippi River from 1992-2008 to evaluate trends in population atributes of both non-native and native planktivores.  Our results suggest that the relative abundance of gizzard shad and bigmouth buffalo have declined precipitously over time while silver carp relative abundance has increased exponentially.  We also found that silver carp condition has remained fairly consistent over time while bigmouth buffalo and gizzard shad condition has declined, suggesting resource limitation for the native planktivores. To this end, our results suggest that potential negative interactions are occurring between non-native and native planktivores.  We suggest that future research should be directed at understanding the specific mechanisms that are structuring this relationship.  If silver carp are indeed negatively influencing native planktivorous fishes, management efforts should be directed at reducing silver carp abundance to subsequently rehabilitate native planktivores.