MISSISSIPPI STATE -- Catfish farmers have economic evidence that battling the most common cause of off-flavor with copper sulfate brings higher profits.

Research shows that adding copper sulfate to catfish ponds to kill blue-green algae greatly reduces problems with off-flavor. By treating to keep the fish on-flavor, producers can expect higher profits.

Terry Hanson, aquaculture economist with the Mississippi Agricultural and Forestry Experiment Station, did an economic analysis of nine ponds treated with copper sulfate and nine ponds left untreated. Before the research began, the decision was made to harvest in August when most off-flavor problems occur.

"We found that when copper sulfate is used, we can have an annual net return of $848 per acre per year of profit above cash costs. This profit does not include such costs as pond construction, land prices, machinery and equipment, and depreciation," Hanson said. "In the control ponds without treatment, we got an annual net return of $569 above cash costs."

When harvest-ready catfish are found to be off-flavor, they must be held in ponds until flavor improves. Getting a catfish back on-flavor can take from a few days to several months. During this time, producers must continue to feed the catfish to maintain market weight, and many die during this wait.

"Treating ponds with copper sulfate stabilized catfish production, mainly by reducing delays in harvesting, which in turn reduced the losses of fish to infectious diseases," Hanson said.

Key to managing the off-flavor problem is applying the copper sulfate correctly.

"Copper sulfate is a crystal, which means if you just throw it in a pond, it dissolves very slowly and falls in the mud where it is inactive," Hanson said.

Research found that the best way to apply copper sulfate is to place it in a burlap bag inside a second burlap bag which is placed 20 feet behind the pond's aerator. The bags keep the crystals suspended so they can dissolve, and the aerator circulates the copper sulfate across the entire pond. The treatment rate developed specifically for ponds in the Mississippi Delta is five pounds per acre per week when water temperatures are above 70 degrees. Water quality may differ in other areas of the state, such as east Mississippi, and this treatment can be ineffective or dangerous to catfish. Catfish farmers should check with their local aquaculture extension specialist before treating ponds with copper sulfate.

"The research shows the catfish farmer that if you use this application method and application rate, your off-flavor occurrences should be greatly reduced," Hanson said.

Hanson performed his economic analysis on research conducted in the late 1990s by Craig Tucker, MAFES aquaculture researcher at the Thad Cochran National Warmwater Aquaculture Research Center in Stoneville. Tucker's research demonstrated that copper sulfate applied at the prescribed rate effectively controlled blue-green algae.

"In 1995 a Mississippi catfish farmer told us of his success at managing blue-green algae using frequent, low doses of copper sulfate. Later that year, we initiated a controlled study to determine the effectiveness of this practice," Tucker said.

Tucker initially didn't think the procedure would work, but flavor checks provided some of the most clear-cut experimental data he had ever seen. The results were well replicated, too.

"The ponds we were treating with the low rate of copper were mostly on-flavor, but the ponds we weren't treating had half or so of the fish off-flavor," Tucker said. "The first year the data looked pretty good, but we still thought it was a fluke. The second year was more convincing and by the third year, we were sure."

The economic data now available on this treatment option, show it to be highly cost effective.

A second, newer option is to use Diuron in the ponds to treat the blue-green algae. However, this substance is controlled by the U.S. Environmental Protection Agency which must grant an emergency exemption each year it is used. Diuron is a liquid that is easier to administer to ponds than the copper sulfate in crystal form.

In 1997, losses to off-flavor were estimated at $22.7 million and $23.2 million in 1998. In 1999, the first year Diuron was approved for use, off-flavor losses dropped to $14.7 million.

"We think Diuron had a large part in that. Diuron was a major management difference not available to farmers in 1997 and 1998," Hanson said.