The Effects of Increased Winter Temperatures on Native Fish Reproduction in the South Platte River

During winter, effluent discharge from wastewater treatment facilities can warm stream temperatures above ambient levels, but the effects of warmer temperatures on native fish reproduction are largely unknown. We examined the spawning success of two fish species resident in the South Platte River downstream of an effluent discharge north of downtown Denver Colorado. In laboratory experiments, we exposed johnny darters (Etheostoma nigrum) and fathead minnows (Pimephales promelas) to three temperature regimes for four months to bracket both elevated winter temperatures typical of the South Platte River below the effluent discharge (16˚ and 20˚C) and the winter temperature standard set by the state of Colorado (12˚C). Johnny darters spawned at all three temperatures during the winter, and did not spawn again in the spring. Fathead minnows spawned under the 16˚ and 20˚C temperature regimes but not the 12˚C temperature regime during the winter. Fathead minnows that had not spawned in the winter at 12˚C did spawn during the simulated spring warm-up period. Fathead minnows that had spawned during the winter at 16˚ and 20˚C spawned again in the spring, but had reduced fecundity. Real time PCR analysis for vitellogenin mRNA production supported the spawning data.