MISSISSIPPI STATE -- Catfish sales soared years ago when producers began to consistently deliver delicious fillets to consumers, but they continue to fight the off-flavor problem.

With today's tight markets and strong international competition, it is especially important for farmers to deliver a high-quality product to the marketplace as efficiently as possible.

Terry Hanson, an agricultural economist with the Mississippi State University Extension Service, said off-flavor is a very serious problem for the farm-raised catfish industry.

“Survey information indicates that every pond in the Mississippi catfish industry averaged seven off-flavor occurrences per year,” Hanson said. “Off-flavor prevents ponds from being harvested an average of 59 days per pond each year.”

According to a 2001 study by the Mississippi Agricultural and Forestry Experiment Station, these flavor-related harvest delays cost the industry in Mississippi an average of  $23 million annually from 1997 to 1999.

The ultimate goal is to reduce the possibility that consumers will buy off-flavor fish.

Craig Tucker, a MAFES researcher at the Thad Cochran National Warmwater Aquaculture Center in Stoneville, said off-flavor is caused by odorous compounds in the water or feed of catfish.

“Odorous compounds are absorbed across the gills or the gastrointestinal tracts of catfish and deposited in fatty tissue,” Tucker said. “They eventually are metabolized or excreted, but the meat will be off-flavor if the fish are harvested while the compounds remain in their bodies.”

Tucker said terms used to describe feed-related off-flavors include “fishy,” “rotten fish” or “decay/vegetal.” These can be caused when fish scavenge for foods other than the manufactured diets provided by the farmer. With good feed management, these off-flavors are not very prevalent, but it can take months for fish to purge these from their systems.

Tucker said common water-borne off-flavors are described as “musty” or “earthy.” Most are found in the summer and are algae-related. The good news is that these are easily treated in a few days to a few weeks.

“Musty off-flavor is the most common natural phenomenon,” Tucker said. “It's very difficult to change what nature wants. Nature wants blue-green algae in ponds, and it's very hard to manage them out.”

Jim Steeby, aquaculture specialist with the Mississippi State University Extension Service, said off-flavor most commonly occurs in the warm-weather months, and that also is the best time to treat the problem.

“Our treatment season starts in mid-April to May until water temperatures drop below 70 degrees, usually in late October,” Steeby said. “We encourage our growers to work on their off-flavor problems going into the fall because once the water temperatures get below 70, we frequently don't have any off-flavor compounds being generated, but they are still in catfish tissues.”

Catfish metabolism slows dramatically when water temperatures drop below 70 degrees. They eat less and any off-flavor compounds in their systems clear out very slowly.

Off-flavor problems caused by algae are corrected in a week or two in the summer when water temperatures are warm, but they can take months to correct when the water gets cold.

“If you have fish that are off-flavor, there is no instant way to fix it,” Tucker said. “You must solve the algae problem and allow the fish to purge.”

Tucker said the two types of blue-green algae most commonly at fault in off-flavor situations are MIB, produced by Oscillatoria perornata, and geosmin, produced by a species of Anabaena. Both types of algae are easy to identify.

“If the fish have an earthy, musty flavor, have your water checked under a microscope to see if you have the odorous algae,” Tucker said. “If you have the algae, you can treat it. If they're not there, you have to just wait.”

Tucker said it is important to check for the presence of the algae before treating the pond. Ponds can be cleared of algae faster than the catfish can purge the compounds from their systems.

“The off-flavor in fish may be from algae present a few weeks before you checked the pond's flavor,” he said. “If odor-producing algae are not present, using herbicides will not help.”

Copper sulfate is the most common treatment for the earthy/musty flavors caused by blue-green algae in ponds. This chemical can be difficult to dissolve.

“I think many people put copper sulfate out too fast and it's not properly dissolved,” Tucker said. “My guess is that only about half of the crystals are dissolved, and the rest sink into the mud and are wasted. If you don't have the time to put it out properly, you maybe should consider another method.”

Diuron is another chemical that, after a 10-year effort, should receive full registration this year to be used in catfish ponds.

“Diuron takes a long time to work, but is much less expensive than the alternatives,” Tucker said.