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Bass virus is spreading, but still a mystery

June 17, 2004


No one knows where largemouth bass virus originated.

It looks suspiciously similar to a virus found in tropical fish, and that would make sense because it was first found in Lake Weir in Florida, a state that has a lot of tropical fish farms and importers.

But that question has become moot now that the disease has spread to 17 states, including Michigan, the northernmost place it has been found. And for fisheries biologists, two questions have now moved to the top of the agenda: How do you stop or slow down its spread, and perhaps even more basic, is it really all that big a threat to America's most popular game fish?

After the virus was first seen in Florida in 1991, it took only nine years to show up in Lake George on the Michigan-Indiana border. (It caused its first large-scale fish kill in South Carolina's Santee Cooper reservoir in 1995.)

"What has us somewhat concerned is that we don't know how the virus works on the northern subspecies (of largemouth bass) or in northern waters," said Gary Whelan, the fisheries production manager for the state Department of Natural Resources.

The virus seems to kill adult bass, mostly fish bigger than two pounds, but Whelan said that could be because "we don't see the smaller ones that die."

The dead fish have a waxy brown or green substance in their swim bladders, and biologists say it doesn't affect humans.

"Wherever there has been an outbreak, it has killed about 10 percent of the largemouth bass population," Whelan said. "It first showed up in Lake George in 2000, and now we've confirmed it in 15 other lakes" out of 30 the DNR surveyed in the past two years.

The virus also has been found in smallmouth bass, bluegill and redbreast sunfish and crappies, as well as guppies and some minnows, but it doesn't harm those related species.

Some biologists don't think the virus is a new disease but rather newly identified, though many scientists think it is new to this country. Some scientists say the threat from the virus is more perception than reality. They point out that fish populations soon recovered even in the hardest-hit Southern lakes, and far more bass are killed each year by pollution and degraded water quality.

But bass are by far the nation's most popular game fish, and Phil Durocher, chief of the Texas fisheries division, said, "Anglers think it's serious, so it's serious."

Whelan said: "We're not going to cure it, but we do want to try to manage around it. We're going to look at sunfish, minnows, even suckers to try to discover the vector by which the disease is transmitted."

That question is still open. Some studies have found that the virus can live in open water for up to four hours, which means it can be carried in a fishing boat's live wells. But there is also evidence that it can be transmitted by the prey fish the bass eat. Fisheries scientists originally thought warm water temperature, higher than 80 degrees, might be the triggering mechanism for a fish kill by the virus. That theory was disproved when the Lake George kill occurred at a relatively cool water temperature of 72 degrees.

The DNR has asked anglers to take some steps that might help prevent the spread of the virus, things like washing down their boats and trailers between trips, avoiding moving fish or fish parts from lake to lake and refraining from releasing unused live bait into any waters.

The agency also asks that anglers handle bass as gently as possible and release them quickly, avoid keeping bass from long periods in live wells if they are to be released, minimize targeting largemouths from mid-July to mid-August, when water temperatures peak.

Even if those practices can't stop the spread of the disease, Whelan said, they can slow it while biologists get a better handle on the threat it presents and what they can do about it.

Whelan said the virus is an example of the kind of threat to Michigan waters that has a lot of biologists concerned.

"What worries me more about the exotics coming into the lakes that we can see, things like gobies and zebra mussels, are the ones that we can't see, like bacteria and viruses," he said.

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