A tiny parasite is threatening further damage to an already hurting catfish industry. Transmitted by the American white pelican, the trematode species Bolbophorus confusus is an internal parasite that can kill young catfish and fingerlings.
“It's a widespread problem, but it's not on every single farm in every single pond,” says David Wise, a researcher at the Thad Cochran National Warmwater Aquaculture Center in Stoneville, Miss. The severity of infestation appears to depend on the availability of the parasite's preferred hosts — the American white pelican and the ram's horn snail.
According to Wise, the trematode parasite lives as an adult in the pelican, which sheds the trematode's eggs infecting any snails present in the pond. From there, the parasite is able to penetrate the skin of the catfish, which can then be eaten by pelicans, continuing the trematode life cycle.
Originally identified about 40 years ago in a trout species in Montana, the parasite was first found in catfish in Louisiana in the mid-1990s, and was documented in the Mississippi-Arkansas Delta in the summer of 1999.
“While controlling pelicans populations is easier said than done, it's important to do the best you can to keep them off your farm,” says Wise. Catfish producers must also try to control snails through weed management, because weeds provide a habitat for snails. In fact, controlling snail populations seems to be the best line of defense currently available to producers.
Available control treatments include hydrated lime using a 50-pound bag to cover about 75 feet of levee, or lime slurry at a rate of 4 pounds per gallon, or 20 gallons per 100 feet of levee. In both cases, it is recommended producers concentrate the lime in the pond margins, because that's where the snails likely are located.
“With a dry powder application of lime, it is very important to wear a respirator mask to keep it out of your lungs,” says Wise. “The lime slurry is a little more difficult to put out and may have to be put out in bulk. That could limit its availability because you may not have companies in your area that truck it in by the large load.”
A treatment that's easier to put out, is a little cheaper, and more convenient is copper sulfate, but it may be toxic to blooms, he says. For those growers who choose to apply copper sulfate, Wise recommends a rate of 10 pounds copper (Cu) and one pound of citric acid per 60 gallons of water spread over 250 feet of pond shore.
The probable long-term solution, Wise says, is biological control using either black carp or red-ear sunfish.
Before beginning any treatment regimen, however, growers are advised to assess each pond's potential for infection by the catfish parasite. Risk factors to consider include location of the farm, pond structure, water grass population, weed density and pelican population history.
“Basically, stay on top of the situation,” says Wise. “Sample ponds for snail numbers before treatment. Check the transmission rate in sentinel fish, and then check snail numbers and transmission rates again seven days after treatment. Don't just treat a pond without evaluating that pond.”
Wise recommends beginning control treatments if the following conditions exist, in decreasing order of priority: ponds with active infection; ponds with past history of pelican pressure; ponds feeding poorly with snails present; ponds with high numbers of snails present; or ponds with no evidence of infection and a low number of snails. The last situation, he says, is simply a matter of treating for piece of mind.
“The good news is it looks like if we get the catfish out of an infested environment and get them in another clean pond, there seems to be little, if any, impact on production,” he says. “Infected fish cannot transmit the disease to other fish, and other birds cannot transmit the disease. In fact, there is no known risk of spreading the disease through the movement of fish to areas that are free of trematodes. The fish do not transmit the infection, the pelicans do.”
Those producers interested in initiating a bird dispersal program are urged to contact USDA's Wildlife Services at 662-325-3014.
“We are not a regulatory agency. We will not show up on your place and write a ticket. That's not our job. Our job is to help producers get the permits they need to legally handle predatory birds,” says Harris Glass with USDA's Wildlife Services in Starkville, Miss.