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Black carp: Threat to aquaculture?

Only a tiny fraction of his business is in black carp, but Mike Freeze wants you to know it's terribly important. While many biologists fear the black carp — fear its ability to wreak havoc on the environment if accidentally loosed — it's currently a necessary component of aquaculture. Without the black carp, fish farmers say, not only would prices at the fish buffet skyrocket but several parasites would rule Delta ponds.

Freeze runs Keo Fish Farm in Keo, Ark. The operation encompasses about 1,200 acres of water and concentrates primarily on two species.

One species is the hybrid striped bass. Keo Fish Farm is a hatchery that sends bass fingerlings around the world — to China, Israel, Italy — as well as all over the United States. Once they've reached their new homes, the bass fingerlings are raised much like catfish until they're food-sized.

“The hybrid striped bass portion of our business probably makes up 45 percent of our gross income. Some folks prefer bass over catfish,” says Freeze, who is also an Arkansas Game and Fish commissioner.

Another 45 percent of Freeze's business comes from sales of sterile “triploid” grass carp — fish that eat aquatic vegetation. The grass carp, one of four species of Chinese carp, was first imported into the United States in the 1970s.

A short diversion

“Triploid” refers to a method (discovered in 1983) of manipulating the chromosome numbers of a fish to produce sterility. A normal organism is diploid, meaning it has a set of chromosomes from mother and father. To produce a triploid, fertilized eggs are subjected to a sort of shock: either temperature or pressure. The resulting triploid fish can't reproduce.

As part of the triploid grass carp program, most states allow such fish to be brought inside their borders only for vegetation control. This process is highly complicated and regulated. The U.S. Fish and Wildlife Service (FWS) is the agency that certifies such fish are sterile.

In 1995, the Triploid Grass Carp Certification Act allowed the FWS to run the program for a user fee. For every triploid grass carp Freeze sells, he must pay the FWS around 34 cents for certification.

“Every fish we sell will have a blood sample taken to make sure it's sterile. After we do that, the FWS sends someone to run a second set of tests. There's a quality control aspect to this. Once those tests are run satisfactorily, the FWS issues documentation allowing us to sell the fish.

“Between the bass and grass carp, that's 90 percent of our business. The other 10 percent is in ornamental fish, bait fish and a few acres of other things — including black carp.”

Freeze became interested in black carp in the early 1990s because problems with a parasite — the yellow grub — appeared in his hybrid striped bass ponds. An amazing pest, the parasite is transmitted through birds like great blue herons, egrets and other wading birds. When the birds defecate into pond water, the freed parasites swim around, searching for a snail. Once inside the snail, the parasite population explodes and leaves the snail in huge numbers. Fish are then targeted.

“The parasites, once they find a fish, burrow inside it and form a cyst about the size of a BB. Normally, these appear inside the flesh and if anyone cuts the nodules open he'll find a small worm inside. So, you can imagine the need to get rid of this parasite. No one wants to eat fish infected with live worms,” says Freeze.

Once inside the fish, the worms wait for a wading bird to consume its host. When the fish is eaten, the parasite then emerges in the bird's gut to start the cycle all over.

“In the fingerlings we raise, the problem is if the 1-inch to 2-inch fish get infected, a lot of times — because they're so small — they'll die. Sometimes we've had whole ponds of fingerlings that we've had to destroy because of yellow grub infestation. That's major money.”

The only way to control the yellow grub's life cycle is by removing the snails. It's impossible to get rid of the birds — all it takes is one landing near a pond for a short period. The fish can't be gotten rid of — that's where the business is. So that leaves the snails. How best to remove them?

“If there were chemicals to use against the snails, that would be great. But there isn't an approved (or non-approved, for that matter) chemical available for snail eradication while fish are in the pond.”

Enter the black carp, also known as the “snail carp.” In the early 1990s, with help from FWS, Freeze imported black carp from Israel and Taiwan.

“We wanted to use them as a biological control of snails. They work well stocked at a rate of five to 10 fish per acre. Since the carp are eating the snails, the fish are yellow grub-free. It works very, very well.”

A new catfish parasite

The black carp problem really started cooking when a new parasite (Bolbophorus confusus) showed up in Delta catfish ponds. This parasite, while sharing the same basic life cycle as the yellow grub, is a totally different species. It uses host pelicans and then snails before hitting the catfish.

Catfish are naturally resistant to native parasites — hence catfish have little worry with the yellow grub. But the new parasite is an exotic, and there's no clue where it came from. This parasite not only makes catfish inedible, it will also kill the fish. First discovered about eight years ago in Louisiana, several fish farms went out of business because of the pest.

“Well, after a bit, they began finding this parasite elsewhere in the Delta. Fish farmers — especially in Mississippi — began scrambling to find black carp to protect their catfish. Until the late 1990s, Mississippi's Department of Agriculture required only triploid black carp in ponds. When this parasite cropped up, though, there was a real concern that there wouldn't be enough triploid black carp to go around the 100,000 acres of Delta catfish ponds. So for one year, they allowed the sale of non-sterile black carp to Mississippi catfish producers.”

The uproar ensues

When such actions were allowed, says Freeze, it created a tremendous uproar up and down the Mississippi River. Many states became concerned (and remain so) that there would be an escape of the fish. If it did escape, the argument went, the black carp would harm endangered species. As over two-thirds of U.S. freshwater mussel species are endangered or threatened and the fish are capable of eating up to 4 pounds of mollusks daily, says Freeze, “that is a legitimate concern.”

In both 1996 and 2000, a joint effort between different government agencies assessed risk associated with the black carp. The report said that all black carp to be used should be certified as triploids. The agency also said the carp shouldn't be used for control of zebra mussels, which was an idea early on.

With that assessment, everything was quiet for a while. But a group called MICRA — Mississippi Interstate Cooperative Research Agency — soon stepped in. MICRA is an organization with agency representation from every state with water that flows into the Mississippi River. The group petitioned the FWS to list the black carp as an “injurious species.”

Under the Aquatic Nuisance Species Act, the FWS can choose to list a species as injurious. If they do that, the listed species may no longer be transported across state borders and can no longer be imported or exported from the country.

The comment period on the proposed listing ended Sept. 30. All parties are still waiting to see what FWS' decision is.

“If they list the black carp in such a manner, we'll still be able to use the black carp in Arkansas as long as the Arkansas Game and Fish Commission says it's okay. Mississippi has brood stock (although they've yet to produce offspring because the fish must be six years old to spawn), and they'll be able to work with black carp already living in their borders. But transporting black carp from here to there or back won't be allowed. Each state will be an island,” says Freeze.

If FWS does what he expects and lists the black carp as injurious, one of the things Freeze admits to being concerned about is Mississippi producing triploid black carp properly. The procedure for producing sterile black carp is different from grass carp, he says.

“In 2000, I believe, some diploids were stocked mistakenly as sterile. The potential for something to go awry in that instance is frightening.”

With several others, Freeze has gone to Washington, D.C., and met with the director of the FWS.

“We explained the situation, we explained that FWS helped bring the black carp in, that FWS was a part of the risk assessment. We pointed out that if this fish was listed as injurious, it would mean overlooking FWS' own risk assessment. Plus, because states will become ‘islands,’ the threat of viable, diploid black carp being stocked would be infinitely worse.”

Freeze says the bottom line is that through an “injurious” listing, the FWS would be exacerbating what they're trying to prevent.

“I think that may be why there's been no ruling yet. This is very complex and hopefully FWS is paying close attention to it. I'd guess they're going to list the black carp as injurious and take their chances. But nothing is certain.”

No good alternative

Aquaculture needs the black carp until a replacement — whether chemical or otherwise — can be found to deal with the yellow grub, say fish farmers.

“There's nothing sacred about the black carp. Once we have some other way to control snails, who cares if it's here or not? Those who want the black carp gone say we can use copper sulfate and citric acid to deal with the parasite. Well, that only works in certain environments. You need very hard water, and it will kill 80 to 90 percent of the snails. That's not bad — but the black carp completely eradicates the snails. And if you still have 10 or 20 percent of the snails that escape treatment, that's enough to infect fish thousands of times over.”

If the FWS insists on listing the black carp as injurious, Freeze and others have asked that designation only apply to diploids.

“They need to still allow us to trade and ship the sterile black carp. At one time, they actually certified triploid black carp. Then, suddenly, they said they weren't in that business any longer. We asked why that was the case and were told, ‘We feel that by certifying the triploids, we're advocating the use and dispersal of them.’ Common sense went out the window in this whole thing.”

e-mail: [email protected].

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