Already homely when in the best of shape, catfish infected with trematodes are flat-out ugly, says Larry Dorman. And consumers don't like ugly.
“There's a marketing issue with trematodes,” says the Arkansas Extension fisheries specialist. “If you have a fish with a few trematodes that makes it through a processing plant, the consumer at the grocery store isn't going to be too happy. You know, if you get home and find little critters in your supper, it'll turn the consumer off in a heartbeat: ‘Worms? Yuck!’”
Trematodes aren't at all harmful to humans. Few, however, can get past the cysts caused by the tiny reddish worms. Dorman says a fish infected with trematodes is akin to a hail-damaged car: it'll run just as well but just doesn't look good.
First found in Montana trout some 40 years ago, trematodes showed up in southern Louisiana catfish ponds in the mid-1990s. The parasite was found in Mississippi and a few eastern Arkansas ponds in 1999.
“For a while, ponds in Arkansas with the problem really hugged the Mississippi River — trematodes didn't seem to move far from the river,” says Dorman. “Unfortunately, that's no longer the case. Last year, I found a trematode-infected pond in the central part of Chicot County (in southeastern Arkansas). It was well away from the Mississippi River. Since it has reached that far inland, that leads me to believe trematodes could be anywhere in that county.”
It only takes one hyperactive or travel-happy bird to spread the parasite even further, Dorman points out.
“We've had situations where very heavy populations of trematodes in fingerling fish cause the fish to go off feed. And the fish wouldn't begin to feed again. The infestation was so heavy; the fish were stressed and had no appetite.”
In such cases, even though the fish rarely die, they won't grow either. That means the producer is left, for all practical purposes, with a dead pond.
“Production depends on those fish eating and growing. Trematodes, no matter how you look at it, are a money-loser for farmers.”
Currently, Arkansas doesn't have the same problems with trematodes as Mississippi does — but it could get as bad. Dorman doesn't know why the situation isn't as bad already. Perhaps, he suggests, the state doesn't have as many “loafing areas” around ponds for parasite-transmitting birds.
And, as if they didn't already have enough to do, producers really need to pay attention to water that's near their ponds: lakes, sloughs, anywhere birds (especially white pelicans, the main avian vector of the parasite) can rest without being harassed. “If your pond is close by, big birds will be by for a visit and they may bring trematodes with them. If you see pelicans on your ponds, you've got to get them off fast. If you don't, the ponds will inevitably end up with trematodes. Producers also need to watch out for the ram's horn snail. They are pretty big as snails go — bigger than the end of my thumb.”
An adult trematode typically lives in a pelican's gut (Dorman is open to the possibility it lives in other fish-eating birds like herons, egrets or cormorants). When the pelicans defecate, trematode eggs are released.
“In some cases, eggs can get into ponds just from pelicans dipping their beak into water,” says Dorman. “These eggs hatch into miracidia which reach the intermediate host, the ram's horn snail. One miracidia might infect a snail and then reproduce thousands of times — it's not a one-to-one proposition. The miracidia mature in the snail and release larval trematodes called cercariae that infect catfish. Pelicans then eat the fish to complete the cycle.”
Once you have a problem or to help prevent one from developing, Dorman advocates stocking ponds with sterile black carp. A proper ratio of the carp per water acre (typically 10 to 20 carp) will lessen your snail problems. The carp eat snails like popcorn, says Dorman.
To lessen snail populations even further, producers may also want to stock grass carp, says Dorman. These fish eat aquatic vegetation, thus depriving the snails a main food source.
Dorman began Extension work in the early 1980s. While large fish-eating birds were around then, they weren't as much of a bother. That has changed as their populations have increased in the past decade.
“Now, an environmentalist or conservationist may think that population increase isn't a problem. But from a producer's standpoint, it's certainly a problem when these birds fly away with an expensive fish crop in their bellies and leave behind parasites.”