Toccoa Tailrace Trout Fishery Decimated By Warm Water

Nick Carter | October 31, 2010

The Tennessee Valley Authority’s (TVA) deep drawdown of Blue Ridge Lake to more than 60 feet below full pool for dam repairs this summer has all but wiped out the trout population in the Toccoa River tailrace below the dam, rendering one of the state’s premier trout fisheries almost barren.

An electrofishing survey was taken the week of Oct. 11-16, and WRD Fisheries Biologist John Damer said there was an 83.6 percent decline in fish numbers compared to a survey taken in June, before the drawdown. The two samples were taken using the exact same methods at the same nine locations spread over 15 miles of the fishery from the dam to McCaysville.

“We assume a lot of that is due to mortality from the high temperatures, but there’s also a chance that some of the fish could have emigrated or swam out of the tailwater into some of the tributary streams because they have been a little cooler,” said John. “There’s not really any good way to tell how many fish were lost because of mortality and how many were lost because of emigration, but either way, they aren’t there now.”

John explained that trout do not thrive in water warmer than 70 degrees, and during the drawdown water temperatures at the dam spiked as high as 78.17 degrees. The irony here is that cold water from the bottom of Lake Blue Ridge and a diffuser that bubbles oxygen into the water is what creates suitable habitat for trout in the first place. However, with such a large release of water during the hot and dry months of summer, the cooler water at the bottom of the lake was sucked through the turbines early, and warmer layers that would have normally been well above the intake were released into the river.

The sample suggests the larger trout were hit hard. In the June survey, 10 trout longer than 18 inches were sampled. There was only one 18-inch fish in the October sample.

The latest survey also revealed fish in a stressed state. John said on average the fish were at about 80 percent of the ideal weight-to-length ratio. He said there’s a good chance some of these fish will not survive much longer.

A final interesting statistic in the survey, which supports generally accepted theory, is that brown trout fared far better than the rainbows in the high water temperatures. In the pre-drawdown survey, browns accounted for 41.8 percent of the total sample, and in the October survey that figure rose to 71.9 percent.

Dissolved oxygen levels, another critical water-quality factor, have remained largely within the acceptable range, said John. There was a 12-hour period when the dissolved oxygen diffuser was shut down to facilitate dam repair, but John said warm water was a far greater threat to the fish.

The good news is we appear to be over the hump for the season. Water temperatures at the dam have fallen back to about 70 degrees, and cool weather will bring temperatures lower. WRD plans to stock the river this fall to provide for a winter fishery, but the details have not yet been nailed down.

The remaining wild card in the equation is next summer. TVA has estimated the project will be completed and the lake refilled by August 2011. Delays in construction or insufficient rain to refill the reservoir next spring could both result in too little water to maintain cool-water discharges next summer. TVA has stated the reservoir will not have to be at full pool in order to provide those necessary discharges.

John said it will take years to return the Toccoa to its former glory as a trout fishery. But on the bright side, this year’s deep drawdown could improve the long-term health of the fishery. The repairs eliminate the need for periodic drawdowns conducted about every five years, and the oxygen diffuser will be upgraded.

The habitat is still there, and the aquatic insects on which trout feed will not experience long-term damage, said John. Also, since the Toccoa is primarily hatchery supported, there is not much concern over loss in reproduction. One thing that would speed the recovery is if riverside landowners continue feeding trout, as they have done for years.

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