Here Today, Gone Tomorrow?

Largemouth Bass Virus Appears to be a One-Time Killer

Please note the publication date of this article. Statistics and seasonal information were accurate at the time of publication. Check links provided for the most current information.

By Larry D. Hodge

Published in Texas Fish & Game, May 2004

When largemouth bass virus (LMBV) killed thousands of bass in Sam Rayburn Reservoir in 1998 and Lake Fork the following year, some predicted those fisheries would suffer long-term declines. The doomsayers were wrong. Both lakes rebounded and continue to produce big bass.

How Bad Was It?

There definitely was a short-term effect. The year before LMBV struck at Rayburn, the lake produced six 13-pound-plus fish for the Budweiser ShareLunker program. The next year Rayburn got skunked, and in the four years following, only two ShareLunkers came from the lake. The story was similar at Fork, though not as dramatic. The year before the virus hit, the lake produced seven ShareLunkers, the year following only one. But Fork came back quicker, with 13 big fish being caught over the next two years.

The number of fish actually killed was small compared to the number of fish in the lakes. Only about 1,750 died in Sam Rayburn, and 4,800 in Fork. However, LMBV seems to target big fish, and the die-offs did have an impact. “The general feeling was that when a kill was occurring, the catch rates of large bass declined in both Fork and Rayburn,” says Dave Terre, the Inland Fisheries regional director from Tyler. Terre serves on a multi-state task force that meets annually to keep tabs on the virus. “The lower catch rates were confirmed by tournament results and creel surveys. The population data, though, showed no significant changes in the bass population. LMBV killed some fish, but after the first year, the fishing rebounded, and now I’d say catches are where they were before or even better.”

Terre also pointed out that severe drought caused low lake levels at the time of the kills, and changes in habitat and the possibility that fish with the virus may be less eager to bite may have affected catch rates as well.

How Have Rayburn and Fork Fared Lately?

Records kept by Sam Rayburn guide Will Kirkpatrick show there was a definite drop in the size of fish weighed in during the annual McDonald’s tournament. The top 80 bass weighed in on each of the three days of the tournament averaged 6.87 pounds for the four years preceding the LMBV kill. The year the virus struck in July, the average fell to 5.99 pounds and declined to 4.92 pounds the next year before hitting bottom in 2001, when the average was 4.80 pounds. The average climbed to 5.95 the next year and hit 6.07 pounds in 2003, prompting TPWD biologist Todd Driscoll to declare, “Rayburn is back.”

Richard McCarty, a guide on Lake Fork, says things are looking up there, too. “Lake Fork is coming back pretty good. Big fish are showing back up, and I think we will keep churning out our five to eight ShareLunkers a year.” Frank Hardy of Mineola caught the first lunker of the 2004 season from Fork, a 13.32-pounder that bit a homemade crappie jig on January 14.

During a trophy bass survey on Lake Fork from March to December 2003, anglers reported catching 1,736 bass weighing 7 pounds or more. Of those fish, 1,348 were weighed on scales, and 15 percent of them weighed 10 pounds or more. There are still plenty of big bass to be caught in Lake Fork.

What Do We Know About LMBV?

While it could in no way be called a flash in the pan, LMBV does appear to be a one-time killer. No lake has suffered two kills. “As far as we know, once you’ve had it, that’s it,” says Terre. “The theory is that the first time it occurs, it affects the fish it is going to, and the fish remaining are resistant to the disease. LMBV hit Rayburn hard in 1998 and Fork in 1999, and since that time we have had no LMBV mortality in either of the lakes.”

However, TPWD laboratory manager Loraine Fries cautions, “To date there have been no repeated kills, but it is unknown what may happen in the future. Fish immune systems are not very sophisticated, and it is not known how long an immune response will protect fish.”

The virus also appears to diminish over time, and not every fish infected with the virus becomes ill. “In 1999, 56.7 percent of 60 largemouth bass collected in Lake Fork tested positive for the virus,” Terre explains. “In 2000 only 3.3 percent tested positive. Now it is found in only 1 percent of the fish tested. Maybe the fishes’ immune system is fighting it, but we just don’t know.”

What is known about LMBV is comforting. It’s caused fish kills in only five Texas lakes, even though the virus has been confirmed in 23 of the state’s reservoirs: Sam Rayburn, Toledo Bend, Conroe, Fork, Athens, Texoma, Cypress Springs, Tawakoni, Palestine, Tyler, Nacogdoches, Bridgeport, Livingston, Hubbard Creek, Possum Kingdom, Whitney, Belton, Buchanan, Canyon, Bastrop, Richland Chambers, Fayette County, and Ray Roberts. The five lakes that have suffered kills due to the virus are Rayburn (1998), Conroe (1999), Toledo Bend (1999), Fork (1999), and Bastrop (2002).

Early on, LMBV seemed to be a problem only in the South, and high summer temperatures were suspected of being a possible trigger. The virus was first found in Florida in the 1980s, though it was not associated with a kill. That came in Santee-Cooper Reservoir in South Carolina in 1995. Since then, however, LMBV has been found in more northern states such as Indiana, Michigan, Illinois, and Vermont.

How Does LMBV Kill Fish?

Largemouth bass virus attacks the swim bladder in fish. This affects their ability to control their buoyancy and maintain their position in the water column. Eventually they float to the top, where they can be affected by all kinds of stresses, including high temperatures and lower oxygen levels.

Most bass infected with LMBV appear normal. Those with the full-blown disease, however, may have trouble swimming and remaining upright. They may also appear bloated.

What Is Being Done About LMBV?

LMBV is spread by fish-to-fish contact and also through the water; there is speculation it might also be spread by insects. The virus can survive for several hours in water. While the virus appears to kill only largemouth bass, the virus can be carried, and perhaps transmitted, by other species of fish, including smallmouth bass, spotted bass, bluegills, redbreast sunfish, white crappie, and black crappie.

TPWD takes special care not to spread LMBV through its stocking programs. “We have found LMBV in some of our hatcheries, so we will not stock fish in lakes known not to have LMBV from a hatchery that has had it,” Terre says. “Plus, we screen samples of all the fish that come out of our hatcheries.”

Through the efforts of TPWD, the U.S. Fish and Wildlife Service, and organizations such as B.A.S.S., LMBV is the subject of ongoing research projects. “In the last two years there has been a lot of organized effort to study LMBV,” says Terre. “Scientists get together once a year to share information. “There’s been a lot of research on how LMBV is transmitted, and that has led to advisories to anglers on what they can do to prevent spreading it.”

How Can Anglers Help?

Anglers can help avoid spreading LMBV by doing the following things.

  1. Thoroughly clean and dry livewells, boats, trailers, and equipment between fishing trips. You can kill the virus by filling livewells for at least five minutes with a solution of ¼ cup of chlorine bleach per gallon of water. Dry the livewell completely before putting fish in it again to avoid harm to fish.
  2. Do not move fish or fish parts (including bait) from one body of water to another.
  3. Handle bass as gently as possible when catching and releasing them; stress seems to increase the likelihood LMBV will be fatal.
  4. Hold weigh-in tournaments during the cool months and use “paper” tournaments during the warm months.
  5. Report dead or dying fish to TPWD at (512) 389-4848.

Is It Safe to Eat Fish Infected with LMBV?

Yes. LMBV is not known to infect any warm-blooded animals. However, always thoroughly cook any fish you eat, and never use dead or dying fish for food.

What Lies Ahead?

“The main thing to remember is that LMBV is not something to worry about,” Terre says. “We’ve only had kills in 5 lakes out of the 23 where the virus was found. We need to keep a sharp eye on the situation, but we are not as worried now about its impact on fishing. Texas has had kills in two of its most valued fisheries, Lake Fork and Sam Rayburn Reservoir, and both those lakes are fine now. Five or 10 years from now we will understand LMBV a lot better.”

In the meantime, go fish.


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