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Technique detects algae potentially fatal to catfish

AGRICULTURAL RESEARCH Service scientists have developed a new method to detect the genetic material that algae need to produce a fish-killing toxin. The test was developed in response to several fish kills that occurred recently in the southeastern United States.

Scientists at the ARS Catfish Genetics Research Unit in Stoneville, Miss., identified the toxins and their source. The researchers were the first to confirm channel catfish mortalities resulting from toxin-producing algal blooms. A toxin-producing strain of freshwater blue-green algae, Microcystis aeruginosa, was the culprit behind the fish kills, according to microbiologist Paul V. Zimba.

Normally, M. aeruginosa produces oxygen necessary for fish respiration and removes toxic chemicals from the water. But Zimba says some strains of this alga can produce toxins that sicken or kill fish.

To determine the magnitude of the problem, researchers took water samples from 487 catfish farms in Alabama, Arkansas, Louisiana and Mississippi during an 11-day period in 2000. Thirty-one percent of the ponds contained algae with the genetic potential to produce toxins called microcystins. But less than 1 percent of the ponds had toxin concentrations above the international safe drinking water standard of 1 nanogram per milliliter, according to Zimba.

After the researchers identified the cause of the problem, they needed a rapid technique to determine if algae present in a given pond could produce toxins. They developed a polymerase chain reaction (PCR) test that detects a gene the algae need to produce the toxin. The gene is found only in toxin-producing strains.

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