Body condition variation in a riverine piscivore: have small non-native cyprinids benefited an endangered fish?

Abstract Effects of three introduced fishes on body condition of an endangered piscivore, Colorado pikeminnow Ptychocheilus lucius, in the upper Colorado River were assessed to aid prioritization of non-native fish control efforts. Relative condition (Kn: mass at a given length) was monitored during spring, 1991–2013. Positive and significant relations were found between annual mean Kn and backwater densities of two common non-native, small-bodied, cyprinid species in downstream reaches of the 293-km study area. High mean Kn in spring followed years when fall densities of fathead minnow Pimephales promelas and red shiner Cyprinella lutrensis were high. No such relation was found with a third species, sand shiner Notropis stramineus. These three species have dominated zero-velocity habitats since their introduction in the 1960s. There was also a significant relation between mean Kn and low antecedent river flow levels, likely because fathead minnows and red shiners flourish under such conditions. No relation was found between mean Kn prior to spawning and subsequent abundance of young-of-the-year 3-months post spawning, suggesting mean Kn was not a driver of annual reproductive success or larval survival. Based on predatory interactions with larval native fishes documented in the literature, red shiners were deemed detrimental to Colorado pikeminnow persistence despite their food value. Sand shiners appeared benign: non-predatory and unimportant as food. Fathead minnows, however, might be beneficial. They appear non-predatory on fish larvae while providing an important food source for various Colorado pikeminnow life stages. Managers need to weigh these benefits when considering control efforts.