Factors Affecting Recruitment of Young Colorado Pikeminnow: Synthesis of Predation Experiments, Field Studies, and Individual-Based Modeling

Abstract

Predation experiments, field studies, and individual-based-model (IBM) simulations revealed factors that affected the survival and recruitment of early life stages of endangered Colorado pikeminnow Ptychocheilus lucius in the Green River basin, Utah and Colorado. Small-bodied, nonnative red shiners Cyprinella lutrensis attacked Colorado pikeminnow larvae an average of once per minute, and predation success approached 30% in laboratory aquaria. Attack rate was also high in mesocosm experiments; turbidity and alternative prey reduced predation success. Distributions of hatching dates derived from otolith daily increment analysis showed that large cohorts of Colorado pikeminnow larvae that hatched in the Green River in early summer had low survival to autumn and that the few survivors were fast growing. Larvae hatched in midsummer or later had higher survival. Autumn juveniles grew 12–73% faster than summer juveniles, which suggested differential mortality of slow-growing fish. The IBM simulations integrated size-dependent predator–prey relationships, Colorado pikeminnow life history information, temperature-dependent pikeminnow growth, Green River predator size-structure dynamics, seasonally variable Green River water temperatures, and turbidity and alternative prey availability effects; the simulations showed that red shiner predation interacting with environmental variables may significantly reduce age-0 pikeminnow recruitment in autumn. Recruitment and growth patterns from simulations and field observations were consistent and suggested that the IBM is useful in evaluating management scenarios. Experiments, field studies, and predictive modeling provided consistent evidence that interacting effects of predation and environmental variables, including flow fluctuations, may structure intra-annual growth and recruitment patterns of age-0 Colorado pikeminnow. Flow management to benefit growth and survival of young pikeminnow, particularly early hatching ones, and reduced nonnative predator abundance in Green River backwaters may enhance the Colorado pikeminnow populations.