Thermal Requirements of Bonytail Gila elgans: Application to Conservation Propagation and Thermal Regime Management of Rivers of the Colorado River Basin

We performed laboratory temperature trials on hatchery reared juvenile bonytail (Gila elegans) using the acclimated chronic exposure method. Bonytail were exposed to temperatures from 8-30°C for 112 d to determine the effect of temperature on growth, condition, body composition, and survival.  Survival was ≥ 98% for all treatments. The predicted temperature for maximum weight gain was 25.9°C, and the predicted temperature for zero weight gain was 14.2°C. Temperatures below 14°C depressed growth, temperatures from 14-20°C provided incremental growth, and temperatures from 22-26°C allowed accelerated growth. We hypothesize that propagation temperatures and/or seasonal river temperatures from 22-26°C would maximize growth of juvenile bonytail and might promote survival because bonytail would be less vulnerable to predation, have greater energy reserves, and increased metabolic efficiency. The threshold and optimal temperature we determined for bonytail were similar (within ±2.2°C) to threshold and optimal temperatures predicted for three other “big-river” Colorado Basin species: the humpback chub Gila cypha, Colorado pikeminnow Ptychocheilus lucius, and razorback sucker Xyrauchen texanus. Our research supports a hypothesis that a “big-river species thermal regime” exists for these coevolved large-bodied fish species. Our study might have value as a model for these four endangered species. Conservation propagation practitioners and river management operators might find this information beneficial in aiding the recovery of these species. This information might help identify and protect critical thermal habitat and could provide guidelines for the development of thermal control devices designed to restore more natural thermal regimes to river reaches of the Colorado River Basin.