Age‐Specific Estimates Indicate Potential Deleterious Capture Effects and Low Survival of Stocked Juvenile Colorado Pikeminnow

Abstract

AbstractHatcheries and stocking programs have become necessary to repatriate or augment populations of imperiled fishes worldwide. Over nearly two decades, millions of endangered juvenile Colorado Pikeminnow Ptychocheilus lucius have been stocked into the San Juan River (Colorado, New Mexico, and Utah); however, recruitment of these individuals to adult life stages (age ≥6) remains low. Using a mark–recapture data set collected from annual riverwide electrofishing efforts between 2003 and 2016, we investigated apparent survival and capture probabilities of stocked Colorado Pikeminnow to identify age‐specific bottlenecks contributing to this lack of recruitment. With relatively high capture rates, which averaged between 0.34 and 0.39 for the first 2 years after an individual's first encounter, our results indicated that survival was consistently less than 0.25 for young age‐groups (i.e., ages 1–3), and no appreciable increase in survival occurred until fish had been in the river for at least 3 years (i.e., age ≥4+). Although age and capture effects were confounded for most age‐groups, capture appeared to reduce apparent survival for age‐2 fish by approximately 50%. The confounding effects of age, a completely hatchery‐origin population, and extensive environmental alterations to the San Juan River make it difficult to disentangle factors associated with this overall reduced juvenile survival.