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

Relations between annual abundance of three introduced fishes and 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 (K-n: mass at a given length) was monitored during spring, 1991-2013. Positive and significant relations were found between annual mean K-n and backwater densities of two common non-native, small-bodied, cyprinid species in downstream reaches of the 293 km study area. High mean K-n 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. No relation was found between mean K-n prior to spawning and subsequent abundance of Colorado pikeminnow young-of-the-year (YOY) 3 months post spawning, suggesting mean K-n was not a driver of fall YOY abundance. Based on predatory interactions with larval native fishes documented in the literature, red shiners were deemed detrimental to Colorado pikeminnow despite their food value. Sand shiners appeared benign: non-predatory and less important 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.