Flow regime affects availability of native and nonnative prey of an endangered predator

Abstract

Understanding how altered flow regimes mediate interactions among native and nonnative species is necessary for the conservation of aquatic systems. Anthropogenic alteration of natural flows and establishment of nonnative fishes coincided with near extirpation of Colorado pikeminnow ( Ptychocheilus lucius) from the San Juan River, NM, USA. Despite major efforts to re-establish this species, recruitment of young individuals into the adult population has not been documented in several decades. A potential reason for apparent recruitment failure is that modified flow regimes and nonnative species have affected reproductive success of native prey, thus limiting potentially critical resources for young (<age-2) P. lucius. To assess the susceptibility of native and nonnative fishes as prey, we used field and mesocosm experiments to quantify gape limitation, functional response, and prey preferences of age-1 P. lucius. These observations were used to parameterize an individual-based predation model to predict the availability of native and nonnative prey for age-1 P. lucius across 3 years with different levels of spring discharge. Whereas gape dimensions of age-1 P. lucius were limiting for deeper-bodied nonnative prey, consumption rates decreased with prey length for both native and nonnative prey. Thus, in early summer prior to spawning of nonnatives, age-1 P. lucius showed a strong preference for smaller age-0 native prey in predation experiments. The predation model predicted that in moderate and high flow years, a large proportion of the diet of P. lucius was native prey, despite the numerical dominance of nonnative red shiner ( Cyprinella lutrensis). In low flow years, however, nonnative C. lutrensis are predicted to dominate the diet of age-1 P. lucius. Because discharge from a major impoundment on the San Juan River partly regulates the abundance and temporal availability of native and nonnative prey, it is possible dam output might be used as a management tool to maximize prey availability for endangered P. lucius.