Algae and bacteria, which thrive in the nutrient-rich ponds, produce compounds that cause fish that eat them to taste woody, rotten, like sewage, or similar to diesel fuel.

Paul Zimba, a microbiologist at ARS' Catfish Genetics Research Unit in Stoneville, Miss., studies the biology of blue-green algae, called cyanobacteria, and other species that are the sources of geosmin, 2-MIB, and other off-flavor compounds. The catfish industry's most common management system may contribute significantly to the growth of off-flavors, according to Zimba.

In the multiple-batch system, hatched fish eggs, or sac fry, are reared in nursery ponds. Once they reach the fingerling stage, they're moved up to ponds where they grow to market size. The restocking process can continue for several years with several different ages of catfish present at any one time and without the ponds being drained.

The researchers found that water in older ponds contained higher densities of off-flavor-causing zooplankton than water in newer ponds. Older ponds may be best suited for holding sac fry and fingerlings, according to Zimba. The larger zooplankton in these ponds could serve as fry food, while younger ponds would be better suited for growing out fish and purging potential off-flavors.

Zimba and Steven J. Thomson, an agricultural engineer in Stoneville, are also studying a remote sensing technique that may detect unwanted algal species in production ponds before the problem gets out of hand.

They and their collaborators shot digital video during low-altitude flights over ponds to obtain unwanted algae's unique color profiles. They're working to understand the specific relationships in catfish ponds to predict occurrence of harmful algae.

ARS is the U.S. Department of Agriculture's chief scientific research agency.