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Virus Blamed for Lake Fork Bass Die-Off

AUSTIN, Texas--The deaths this summer of up to 4,000 largemouth bass on one of the nation's premier trophy bass fisheries, Lake Fork, were most likely caused by a virus similar to one implicated in fish kills in four southeastern states, according to an investigative report released this week by Texas Parks and Wildlife (TPW).

The findings point to several stress-related factors that may have influenced the onset of an iridovirus strain similar to the Largemouth Bass Virus (LMBV) that affected adult bass almost exclusively on several popular fisheries in the Southeast. But the report draws few conclusions other than to suggest the Lake Fork fish kill may be an isolated event that is coming to an end.

"This is good news and bad news," said Dr. Larry McKinney, TPW senior director for aquatic resources. "The bad news is that we have the problem. The good news is that we now know where to focus our efforts and the fact that where this virus has occurred in the South, the impact has been short-termed and the fishery has quickly rebounded."

The virus was first isolated in 1995 from Santee-Cooper Reservoir, a popular bass fishery in South Carolina, and has since been confirmed in bass populations on 12 reservoirs in Alabama, Georgia, South Carolina and Mississippi. Texas was added to the list after results from extended virological testing detected a similar virus in a number of dead and dying bass collected on Lake Fork in June and July.

With help from local anglers, fishing guides, area merchants and the Sabine River Authority, TPW fisheries biologists and members of the department's Kills and Spills Team of contaminants biologists have been investigating this relatively small but significant largemouth bass die-off since reports of dead or dying fish surfaced on Lake Fork in late June. Most of the dead fish measured 16 to 23 inches long and were found scattered throughout the main body of this 27,000-acre reservoir in Northeast Texas.

Anglers have reported lesions and sores on bass at Lake Fork and this was also confirmed by TPW sampling. These sores and lesions were not the cause of death, but may be symptomatic of stressful conditions. These same stressful conditions could also make largemouth bass more susceptible to the LMBV. Until recently, when extended virology testing revealed the virus, officials were at a loss as to the cause of this die-off.

"There has been a lot of speculation about the cause of the kill, which illustrates how passionate anglers can be when a precious resource like Lake Fork appears threatened. This has been a great team effort between TPW and Texas anglers to get to the bottom of a puzzling and frustrating issue," McKinney noted. "We will continue to look at water quality and other related factors, (all of which can be stresses that initiate LMBV outbreaks) but these findings will help us to better focus our future efforts."

To learn more about the virus, TPW biologists will be teaming with counterparts in other southeastern states where similar studies are being conducted. Coincidentally, one such project was completed in July by researchers at Mississippi State University's College of Veterinary Medicine, in cooperation with the U.S. Army Corps of Engineers and the Mississippi Department of Wildlife, Fisheries and Parks.

The Mississippi study was spurred by an LMBV outbreak on Sardis Reservoir last September that resulted in the deaths of about 3,000 adult largemouth bass. To determine the extent of the virus in other lakes, researchers analyzed bass samples from 11 popular bass fisheries in that state. Findings indicated significant levels of LMBV in two lakes, less than 10 percent incidence in two others and no virus present in samples of other popular game fish tested. Conclusions similar to Mississippi's were reached in South Carolina following a LMBV outbreak that killed about 1,000 adult bass on Santee-Cooper during the summer of 1995.

"We're planning to launch a study to see if this virus is present anywhere else in Texas," said Phil Durocher, TPW director of inland fisheries. "If someone finds a cure or a preventative for this virus, then we can take appropriate action. Right now, there's nothing we can do to prevent this from happening again other than to continue to address water quality and other stress factors. It's like the flu, it's out there and if fish are stressed and susceptible, they're probably going to get it."

Biologists have not been able to confirm that a die-off last summer on Sam Rayburn Reservoir was linked to the Lake Fork incident, but are submitting more samples to try and confirm their suspicions. "We didn't find LMBV in the samples analyzed last year, but that doesn't mean it's not there," said Dave Terre, TPW regional fisheries director in Tyler. "What we've observed at Lake Fork this year shows similarities to what happened at Sam Rayburn Reservoir. Both die-offs occurred at the same time of year and the outward behavior of affected fish was the same. Dying fish at both lakes were observed struggling at the surface and had enlarged swim bladders. This is a clinical sign of LMBV."

To assist state fisheries biologists, the Southeastern Cooperative Fish Disease Project Diagnostic Laboratory at Auburn University has become the control center for LMBV research for the region. The Auburn facility is set up to analyze cultures for LMBV and to coordinate findings from the various states involved. Fish samples collected recently from several popular Texas fisheries, including Lake Fork, Toledo Bend and Sam Rayburn have been sent there for analysis to determine if LMBV is present. Results from these samples are expected this fall. Biologists have also submitted samples from state fish hatcheries to see if the virus is carried in broodstocks.

While a cure for the virus is unknown, researchers have identified common fish-stressing conditions in all instances where LMBV has been documented, including high fishing pressure and extreme temperatures. Other underlying factors that may be responsible are not known. "We have not discovered any specific contaminants that could have caused this disease," said Durocher. "The question that keeps coming up is 'Why just adult bass?' because pollution usually hits all fish. This may be something that occurs naturally or it may be related to other introduced factors. That's what we hope to find out.

"The good news is that we went back out on Fork the other day with the support of 10 public observers in six boats and found only one dead bass," Terre went on to add. "There were an estimated 450,000 bass caught on Fork last year. We're not downplaying the significance of this threat, but it puts this die-off in perspective. We appreciate the support that local anglers have given our investigations. We all recognize the value and importance of this special lake and our work together has been a reflection of this."

Although TPW crews are continuing to monitor Lake Fork, anglers who observe a fish kill are encouraged to contact Dan Jones with the TPW Kills and Spills Team at (903) 566-2518 or their local TPW fisheries biologist. The TPW report, authored by Jones, will be made available to the public on the agency's Web site at  www.tpwd.state.tx.us .

 

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