Evaluating cumulative effects of within-day flow fluctuations and presence of non-native species on age-0 Colorado pikeminnow (ptychocheilus lucius) in nursery habitats of the Green River (Utah)

The recovery of the endangered Colorado pikeminnow (Ptychocheilus lucius) has been threatened by compounding effects of habitat alteration including water depletion, chemical contamination of water, hydroelectric operations, and the introduction of non-native species. I used 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 non-native red shiner (Cyprinella lutrensis). The model incorporates a spatially explicit nursery habitat, a temperature model, an invertebrate production model, and a model of fish competition, predation, and bioenergetics. I varied within-day flow fluctuation magnitude (0%, 1%, 5%, 10%, 20%, and 40% around mean baseflow) and red shiner density (0-5 fish/m^2) to evaluate combined stress on pikeminnow survival, defined as the proportion of initial fish surviving and growing to 30 mm or above at the end of a simulation. Simulations showed complex, non-linear results from the combined effects of flow fluctuations and red shiner density. Throughout all model experiments, red shiner density had noticeable impacts on pikeminnow survival. In the absence of red shiners, the magnitude of flow fluctuation had little impact on pikeminnow survival. As red shiner density was increased, however, additional stress from flow fluctuations became highly noticeable. There were physical characteristics of the nursery habitat that often mitigated the combined effect of the two factors, including mean depth and mean temperature. This study suggests that juvenile pikeminnow survival may be elevated by controlling population levels of non-native fish and the utilization of flow regimes that maintain suitable nursery habitat by providing adequate water depth, surface area, and temperature.