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Parasitic trematodes infest some catfish ponds

Fish biologists are encouraging farmers to monitor catfish for parasites that can seriously hurt pond production.

“Recently, there has been a resurgence in concerns over trematodes in ponds,” said David Wise, research leader for the applied fish health program at Mississippi State University’s Delta Research and Extension Center in Stoneville. “While the number of farms with severe infestations has decreased, mild to moderate cases remain widespread in many regions that produce catfish.”

This trematode parasite has three hosts in its life cycle: American white pelicans, ram’s horn snails and catfish. After detecting trematodes in Mississippi ponds in 1999, researchers developed effective control treatments and infestations tapered off until recently.

“While all trematodes have not been identified as a risk to production, the Bolbophorus species can cause catfish serious production and health-related problems,” Wise said.

“Severe infestations are easy to identify because they are generally associated with high mortality and very low feeding rates. Milder cases are harder to identify by casual observation. Missed diagnosis is one of the difficulties in managing the disease.”

Wise said even mild infestations can reduce feeding and make the fish more susceptible to other diseases. Left untreated, this disease can make catfish production unprofitable and has been a factor in the closure of several operations. However, because catfish recover completely by preventing new infections, the disease is manageable. In addition, there is no concern about the quality or safety of catfish to consumers.

“We consider this disease to be the most important production-related threat to the industry,” Wise said. “Control depends on breaking the life cycle of the trematode by discouraging pelican use of ponds and reducing snail populations.”

Jimmy Avery, Extension professor and aquaculture leader at the Thad Cochran National Warmwater Aquaculture Center in Stoneville, said several factors contributed to the growing and widespread nature of this problem.

“First, a larger than normal number of pelicans remained in the Mississippi Delta during the summer,” Avery said. “Second, the recent economic conditions have caused farmers to abandon many ponds, which tend to host many of these pelicans.”

Avery encouraged farmers to drain unused ponds if possible. Otherwise, adjacent producers should seek permission to harass pelicans out of the area.

Finally, Avery said, due to the decrease in severe infections, farmers had a false sense of security and believed that infections had subsided.

“Because of the difficulty in identifying the disease, growers need to make immediate evaluations of infestation rates in their ponds, beginning with fish experiencing unexplained drops in appetite,” Wise said.

Managing this disease requires a combination of bird harassment and snail control. Chemical treatments to reduce snail populations include the use of hydrated lime or copper sulfate.

“Use copper sulfate with extreme caution during the summer months due to risks associated with oxygen depletions,” Wise said. “Base treatment options on infection levels, availability of aeration equipment and the cost of chemicals. These are approved treatments and do not impact the quality or taste characteristics of the treated fish.”

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