TPW Releases Findings of Bass Virus Study

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April 5, 2001

AUSTIN, Texas - A virus responsible for killing largemouth bass on four popular Texas reservoirs and linked to nearly 20 die-offs nationwide is present in two-thirds of the state's watersheds, according to a Texas Parks and Wildlife study. Initial findings from the investigation were made public during a briefing of the TPW Commission here Thursday, April 5. State fisheries biologists, with the help of bass anglers who collected nearly half the fish specimens needed for the study, investigated 49 lakes during 2000 to determine the presence of Largemouth Bass Virus (LMBV). Scientists at the A.E. Wood State Fish Hatchery in San Marcos confirmed the virus in samples from 14 of the lakes. The infected bass populations reside in nine of 14 major river basins tested statewide. Only a small segment of the overall fish population in those water bodies -- 5 percent on average-tested positive for the illness and none of the affected fish appeared to be stressed or dying.

"What we now know is that this virus is not always a problem, not always lethal," said Dave Terre, TPW regional fisheries director in Tyler and Texas' representative on a multi-state, multi-agency team of researchers investigating the virus. "It's just out there in the environment and when conditions get right, we may have some fish die. It may be so minor that it's not noticeable, but when it impacts a fishery like Lake Fork or Sam Rayburn, it gets noticed."

The infected lakes in East Texas fall within the following river drainages:

Red River (Lake Texoma), Cypress (Lake Cypress Springs), Sabine (Lake Tawakoni, Lake Fork, Toledo Bend Reservoir), Neches (Lake Athens, Lake Palestine, Lake Tyler, Lake Nacogdoches, Sam Rayburn Reservoir) and Trinity (Lake Bridgeport, Lake Livingston).

The virus also occurs in Central Texas and affects the following river systems: Brazos (Hubbard Creek Reservoir, Lake Possum Kingdom, Lake Whitney, Lake Belton), Colorado (Lake Buchanan), Guadalupe (Canyon Lake) and San Jacinto (Lake Conroe).

Major river basins that tested negative for LMBV include the Arkansas (Canadian), San Antonio, Nueces, Rio Grande and the Sulphur. The TPW study findings illustrate the distribution and density levels of the virus in Texas, but researchers admit little else is known about LMBV. "We've completed the lab analyses of fish collected and wanted to make sure the angling community is aware of where we are at this point," said Terre. "This is not the end, just a chapter. There are a lot of variables we collected along with the fish that will take some time to synthesize." According to Auburn University scientist John Grizzle, LMBV is one of more than 100 naturally occurring viruses that affect fish but not warm-blooded animals. Its origin is unknown, but it is related to a virus found in frogs and other amphibians and nearly identical to a virus isolated in fish imported to the U.S. for the aquarium trade. Although the virus apparently can be carried by other fish species, to date, it has produced disease (death) only in largemouth bass. Researchers have discovered that LMBV can remain viable in water for at least three to four hours. "Tentatively, that means you could transport the virus in a livewell, bait bucket or in bilges," Terre said. "We also now know that bass can contract the virus from contact with other fish, but we still do not know how the virus is activated into a fatal disease, if there is a cure or a preventative." LMBV first gained attention in 1995, when it was implicated in a fish kill on Santee-Cooper Reservoir in South Carolina. Since then, the virus has been found in impoundments throughout the South and, during 2000, was confirmed as the source of a kill in Lake George on the Indiana-Michigan border. Often, the virus has been detected in bass that show no signs of illness, which suggests that some fish might be infected but not ever become sick. Biologists are uncertain about how long the virus has been present in Texas waters, but the pattern of occurrence suggests that unusually hot summer temperatures were a catalyst in the die-offs. According to lab analysis conducted in the summer of 1999, bass samples from Lakes Conroe and Toledo Bend tested positive for the LMBV in addition to those from Lake Fork and Sam Rayburn.

"The year the virus impacted Lake Fork, over half the bass sampled tested positive," said Terre. "That didn't mean 50 percent were dying, just that they carried LMBV. When we came back and tested again last year, only three percent of the Lake Fork bass we tested carried the illness. We don't understand the mechanics by which it becomes prevalent but it seems to happen just once in varying levels and outbreaks do not repeat." The good news, he noted, is that there has never been a fishery lost due to LMBV. "The only significant thing we've been able to determine is that when the virus occurs, angler catch rates, particularly for large fish, decline for a short period of time and then recover."

Because the virus appears to strike a fishery and move on, researchers say finding answers to its mysteries may take years and even if uncovered may be a mute point. "Who's to say this will even be a problem in 10 years or even if it'll be around?" asked Terre. "We need to be taking a look at fisheries in areas where the virus hasn't hit, like California and Canada, to determine if and where it already exists. This is a national problem that's getting more interest as more states are impacted." TPW has taken a leadership role in looking for answers, with it's A.E. Wood facility now geared up to accommodate lab analysis specific to LMBV, but Texas is not alone. "We obtained the equipment and expertise to test for LMBV," said Terre. "Also, we continued our investigation into hatcheries and into the kills on Fork and Rayburn. In addition to TPW, leaders in LMBV research include the federal Warm Springs, Georgia Fish Health Center, Mississippi State University, Louisiana State University and Auburn University. Sport fishermen have also taken a proactive role, with the Bass Anglers Sportsman Society playing host to two LMBV summits, where researchers from various states impacted by the virus gathered to share information.

"Bass are number one in Texas and other states look to us for leadership in bass management," Terre said. "They also look to us for leadership in studying the virus and we're doing it."