Mississippi Agriculture Commissioner Lester Spell recently announced his department has received approval from the United States Environmental Protection Agency for an emergency exemption on the herbicides Direx 4L and Karmex DF (both diuron) to control blue-green algae in catfish ponds.

“Our effort to make these diuron products available as pond management tools is vital to the health of our catfish industry as it gives producers the needed tools for controlling blue-green algae, a primary cause of off-flavor,” Spell said.

Mississippi catfish producers must balance their need to control blue-green algae with the desire to preserve pond plants that provide oxygen and keep water clean.

Several copper-based algaecides registered for use in catfish ponds are non-selective and kill both beneficial plants and non-beneficial plants in ponds.

Direx 4L and Karmex DF are selective herbicides and target the primary pest plants responsible for off-flavor. The species of blue-green algae responsible for significant off-flavor problems in catfish is Oscillatoria chalybea. This species releases a chemical compound known as MIB (2-methylisoborned) into the water.

Catfish absorb MIB into their bloodstream as water passes through their gills. The compound is transported by the blood and deposited into the flesh of the fish.

Low concentrations of MIB produce a musty-to-muddy off-flavor color, which inhibits the marketability of the fish.