Science: Bass and Warm Water Discharges

CoffeenA common belief held by many anglers is that most bass tend to hang in and around the warm water discharge areas of thermally heated reservoirs, especially during the cold water periods. While this is probably the case in some reservoirs, it is turning out to not always be the situation in others. Researchers electrofished bass at Coffeen Lake, a cooling reservoir in central Illinois, and housing Ameren Energy Cos. coal fired power station.  The sampling was done during fall, winter, spring, and summer, starting in fall of 2010. Areas sampled included 5 separate sites that spanned the length of the reservoir, and covering both warm water and cool water sections. Fish species collected were largemouth bass, bluegill, white crappie, and channel catfish. Results included the following:

  • Catch per unit effort (CPUE) was used to assess relative density by site.
  • 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, presumably because many of the fish stayed in water deeper than could be efficiently electrofished.
  • Spring 2011 sampling produced the greatest relative densities for all species, again what you might expect since most fish are shallow to either feed or reproduce.
  • During both fall and spring, sites farthest away from the warm water dishcharge area had the highest catch rates for bass. Catch rates of bass were found to decrease linearly with increasing water temperatures, regardless of season.

The take home lesson here is that running right up into the warm water area of these cooling lakes, regardless of time of year fished, may not be the best decision for numbers of bass. Not only will you probably have less fishing pressure in the cold water areas, you will probably also catch more bass.

2 Comments
  1. you’re right. I fish that lake all winter every winter, 25-30 tournaments annually . cold water always produces the best, and the largest fish in that lake, there are exceptions during the spawn and the early summer, but for the most part, if you’re out of the cooling loop, you’re in the right area of the lake, also I might add, if they stall or idle the plant, the lack of current really slows things down, even in the cold water, that’s my 2 cents. Jim

  2. I’d like to read the full study. Curious about their sampling methods. Electrofishing is only sampling method? Tougher in current (discharge) and substrate may differ in discharge area (rock type and amount) thus affecting conductivity of water and electrofishing equipment settings… Just saying.

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