The decline and endangerment of native fish has led to an increase in hatcheries that sustain and augment threatened and endangered species. In an effort to restore self‐sustaining populations, hatchery‐reared Bonytail (Gila elegans) are released into the lower Colorado River annually. Current augmentation protocols for Bonytail require a minimum stocking size of 300 mm total length due to greater apparent survival of larger individuals. The fish culture process of reaching this desired target size requires multiple years of growth, during which precocious individuals regularly spawn. Recruitment in production ponds increases densities, potentially reducing growth rates and adversely affecting culture conditions. Polyculture techniques have been shown to have synergistic effects and under proper conditions effectively control unwanted biota. In an effort to address the management implications of Bonytail recruitment on adult growth and survival, we tested the use of a historically co‐occurring piscivore, Colorado Pikeminnow (Ptychocheilus lucius), as a biological control agent. Colorado Pikeminnow effectively controlled Bonytail recruitment and had a positive effect on adult growth rates. Our results suggest polyculture of these two species can aid management actions by increasing augmentation efforts.
We characterized the sublethal physiological changes in bonytail Gila elegans subjected to consecutive 750-mg/L potassium chloride (KCl) and 25-mg/L formalin treatments for the removal of zebra mussel Dreissena polymorpha and quagga mussel D. bugensis veligers. Plasma cortisol, glucose, and osmolality were measured over 24 h and at 14 d posthandling after exposing bonytail to KCl and one net stressor (capture with a net), KCl plus formalin and two net stressors, and one or two net stressors without chemicals. Elevated plasma cortisol (322-440 ng/mL) and glucose (254-399 mg/dL) concentrations were observed in all treatments compared with the concentrations in control fish (plasma cortisol, 56 ng/mL; glucose, 43 mg/dL). While there were no detectable differences in plasma osmolality among the treatment and control fish, a difference was observed between fish that were handled once versus twice. Chemical effects of stress were not observed in any of the physiological responses when the KCl treatment was compared with the one-net stressor treatment or when the KCl plus formalin treatment was compared with the two-net stressor treatment. Cumulative responses, however, were observed between one net stressor and two net stressors for plasma glucose and osmolality but not for plasma cortisol. Plasma cortisol and glucose levels remained elevated at 24 h posthandling, indicating that bonytail had not completely recovered from the handling stressors and would benefit from a recovery period in protected refugia before being released.
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