Environmentally sound procedure: Treatment promises to reduce off-flavor

A short-term study of a common catfish pond treatment suggests copper sulfate is an environmentally sound procedure for controlling off-flavor problems.

John Hargreaves, Mississippi Agricultural and Forestry Experiment Station aquaculture biologist with the Thad Cochran National Warmwater Aquaculture Center at Stoneville, Miss., completed a study of the effects of copper sulfate applications to catfish ponds over three years.

“Copper sulfate is most often used to kill the blue-green algae that cause the most common off-flavor in catfish,” Hargreaves said. “It has even been used to treat off-flavor in reservoirs used to supply drinking water. Copper sulfate is one of the few approved chemical tools that we have available to us.”

Many farmers now use the chemical diuron to control blue-green algae. The Environmental Protection Agency and the Food and Drug Administration granted catfish producers an exemption last year to use it. However, these agencies reconsider the exemption every year and renewal is not assured. Copper sulfate is an approved chemical and its use is not regulated by these agencies.

Without an approved means of killing the algae that cause off-flavor, producers with the problem must hold harvest-ready fish until their flavor becomes acceptable. Money is lost feeding fish to maintain weights and from deaths caused by diseases while the fish are held.

When producers add copper sulfate in low doses, it kills enough of the blue-green algae that cause off-flavor, but does not completely eliminate the algae. Algae in ponds are desirable because they add oxygen to the water and remove some kinds of fish waste such as ammonia.

Hargreaves' study looked at ponds that had been treated with copper sulfate at industry-recommended doses for three years and measured the concentration of copper in pond sediments. This research was part of a larger project related to an assessment of treatment methods to reduce off-flavor in catfish.

“We applied the copper sulfate according to a regime suggested by catfish farmers,” Hargreaves said. “We found that it does not leave the pond. We could account for all the copper that was added to the pond. It ends up in a number of different forms in the soil. Over time, copper changes from forms more readily available to those that are less readily available.

“The main finding was that the copper ends up in the soil in forms that are generally not available to enter the food chain,” Hargreaves said. “Preliminary data on the toxicity indicated that concentrations of copper measured in the sediment caused no problems.”

Researchers tested for possible toxic effects of the copper by exposing two sensitive test organisms to sediment from ponds that had been treated with copper sulfate for three years. They found no problems when they exposed an aquatic invertebrate and cattail roots to the soil with elevated copper concentrations.

Results of this study suggest that older ponds with 10 to 15 years of copper sulfate applications would likely have high levels of copper in the sediment. It is likely that none of this copper will leave the pond and that it will have no negative effects on the pond's plants, fish or the environment.

Hargreaves said further research is needed to test this hypothesis.

“In the time frame of the industry as it exists today, this was a fairly short-term study,” Hargreaves said. “We don't know the long-term effects of copper sulfate use, but our limited data suggest that the effects of long-term copper sulfate applications will be confined within the pond.”

Results of this research are being published in an upcoming issue of the Journal of Environmental Quality.

Bonnie Coblentz writes for Mississippi State University Ag Communications.

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