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Minimizing catfish flavor problems.

When Huey Priest and Ray Roberts brought samples of their harvest-ready catfish to a Wisner, Louisiana, processing plant, they received grim news.

Flavor testers at the processing plant informed the two Wisner catfish farmers that their fish were off-flavor and not acceptable for processing.

"I'd say off-flavor is 50 percent of our production problems," says Priest, who is farming thirteen 10-acre catfish ponds. "I think we' ve pretty much licked disease, but we still have to deal with off-flavor."

Off-flavor in catfish is largely the result of poor water quality in ponds. In many cases, algae are the prime culprits because they emit natural chemicals, such as muddy-smelling geosmin,

Catfish tend to absorb these natural chemicals that are released into pond water when algae get sick or die.

The resulting problem costs Priest, Roberts, and other catfish farmers about $12 million a year in lost production. If samples of marketable catfish are turned down at the processing plant, catfish farmers have to wait until the off-flavor compounds naturally purge from their fish.

The two farmers have also found that the chances of getting off-flavor catfish are higher during warmer months.

"This time of the year, you don't have a whole lot of things you can do," Priest said during an interview at his farm last May in 90-degree-plus temperatures. "The only thing actually recommended is moving the fish from one pond to another.

"But I've never done that in the summertime because it stresses the fish," he continued. "When you stress your fish, you're going to get disease."

The more than 1,900 catfish farmers nationwide have very few means available to combat off-flavor and ensure that their market-size catfish consistently meet processors' stringent flavor quality standards.

However, Agricultural Research Service scientists are working on better ways for farmers to operate their ponds to reduce the incidence of off-flavors.

"Remote-sensing imagery provides a reliable estimate of algae activity in catfish ponds that can help farmers better manage their ponds," says Peter B. Johnsen, research leader of the project at ARS' Southern Regional Research Center in New Orleans. "The rapid growth and death of blue-green algae populations cause sudden deteriorations of water quality."

Johnsen and fellow researcher David F, Millie, a microbiologist at the center's Food Flavor Quality Research Unit, are working on ways to correlate images from aircraft-carried remotesensing equipment with various stages of algae growth and health.

The researchers began their studies in 1990 by obtaining readings from a sensor mounted on a NASA Learjet. The plane flew over Mississippi catfish ponds to obtain information on the algae pigments in the water.

The sensor recorded wave lengths that were ultimately translated into specific pigments. The color and amount of pigments indicate the type of algae present in the pond, their numbers, and health status, Johnsen says.

While remote sensing from satellites can detect the presence of algae, Millie says the images are recorded from too far away to provide a sufficiently clear picture to ascertain the health of individual algal types,

Some catfish farmers, like Priest and Roberts, currently have weekly water samples analyzed for off-flavor. But Priest says it normally takes 3 to 5 days to obtain results from these analyses.

And Johnsen says the condition of algae in ponds can change hourly-- depending on such factors as temperature, light levels, and nutrients.

As the research progresses, Millie envisions the use of sensors on small airplanes to collect information that could be read on a personal computer.

"Pilots could go up, fly over several farmers' ponds, return, and download the data onto a disk for use in a personal computer," Millie says. "Once mathematical calculations were made, the data could be communicated to farmers so they would know which ponds need to be treated and how often.

"We're talking about just a matter of hours, so the farmer might know that vet3, evening what treatment to start," he says. "The advantage to this system would be that it scans a lot of ponds and does it relatively quickly."

Scientists have measured several types of healthy algae, each of which shows a characteristic pigment "fingerprint." Variations in the pigments indicate the condition of the algae.

"It's rather like what happens to trees when they change colors in the fail," Millie says. "Different species rum different colors, revealing various pigments at different times.

"While the algae may have different colors, we can't see subtle differences with the naked eye," says Millie. "But the sensor looks at one specific band at a time and can register those differences.

"In addition to all the pigments that indicate specific algal types, the airborne sensor might also be able to detect deviation from a healthy fingerprint signature. So, if a particular alga . were to appear more yellow, for example, it might indicate stress and the likelihood of releasing off-flavor compounds."

Johnsen also envisions another use for the remote-sensing system-- monitoring oxygen levels in pond water.

"Algae produce oxygen during the day by photosynthesis but consume it at night," he says. "Since the system may be able to show fluctuations in oxygen amounts, it could help farmers manage the oxygen in their fish ponds."

If oxygen levels are low, information obtained from the sensor will be able to tell when, how often, and at what locations to aerate the ponds. Oxygen management is a key to fish production efficiency, Johnsen says, because low levels result in stress, low growth rates, and mortality.--By Bruce Kinzel, ARS.

Peter B. Johnsen and David F. Millie are in the USDA-ARS Food Flavor Quality Research Unit, Southern Regional Research Center, 1100 Robert E. Lee Blvd., New Orleans, LA 70124. Phone (504) 286-4421, fax number (504) 286-4419.
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Author:Kinzel, Bruce
Publication:Agricultural Research
Date:Sep 1, 1992
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