Diversity and disturbance: Modeling how exotic species affect juvenile pikeminnow response to river flow fluctuations

Background/Question/Methods

The recovery status of the endangered Colorado pikeminnow (Ptychocheilus lucius) has been influenced by compounding effects of habitat alteration by hydroelectric operations and the introduction of non-native species. I utilized an individual-based model to examine the response of juvenile (age-0) pikeminnow to the interacting effects of dam induced within-day flow fluctuations and the presence of the non-native red shiner (Cyprinella lutrensis).  The model incorporates a spatially explicit nursery habitat, a temperature model, an invertebrate production model, and a fish action model in which the two species of fish compete for the same food resource and potentially eat each other. The virtual nursery environments were created from physical measurements taken during the years 2004-2010 from several distinct sites in the Green River, Utah, downstream of the Flaming Gorge dam. I varied flow fluctuation magnitude (0%, 1%, 5%, 10%, 20%, 40% around mean baseflow) and red shiner density (0-5 fish/m²) to evaluate combined stress on pikeminnow success in different physical environments.

Results/Conclusions

A series of simulations showed complex, non-linear results from the combined stress of flow fluctuations and red shiner density. Throughout all experiments, red shiner density had noticeable impacts on pikeminnow success.  In the absence of red shiners, the magnitude of flow fluctuation had little impact on pikeminnow response. As red shiner density was increased, however, additional stress from flow fluctuations became highly significant. Across simulations, there were physical characteristics of the nursery habitat that often mitigated the combined effect of the two stressors, including mean depth and mean temperature.  This study suggests that juvenile pikeminnow survival may be elevated by the continuation of aggressive non-native fish removal and the utilization of flow regimes that reduce the magnitude of within-day flow fluctuations while maintaining suitable nursery habitat characteristics.