Evaluating the Impact of Thermal Effluent on Sportfish Abundance in a Midwestern Cooling Lake

Coffeen Lake, a cooling reservoir in central Illinois, receives thermal effluent from an Ameren Energy Co. coal fire power station.  The goal of this project is to assess the impact of thermal effluent on sportfish distribution and to determine an effective sampling time for this reservoir.  Sampling was done during fall, winter, spring, and summer, starting in fall of 2010, and consisted of three-phase AC electrofishing from 5 separate sites.  Species sampled were largemouth bass, bluegill, white crappie, and channel catfish.  Catch per unit effort (CPUE) was used to assess relative density by site.  Fall 2010 produced high catch rates (426 fish/hr) of bluegill throughout the reservoir, while largemouth bass were found in highest numbers (70 fish/hr) farthest away from thermally impacted sites, compared to 22 fish/hr in the cooling loop.  Winter sampling resulted in significantly lower numbers of each species caught.  Spring 2011 sampling, which included fyke nets, produced the greatest relative densities for all species.  During both fall and spring, sites farthest from heated effluent had the highest catch rates for bass.  Temperature had a significant effect (P<0.05) on catch rates of bass; CPUE was found to decrease linearly with increasing water temperatures, regardless of season.  Overall, the discharge of heated effluent is impacting distribution and seasonal abundance of sportfishes in Coffeen Lake.