Select Science—LMBV, the largemouth bass virus discovered in 1991, may have contributed to the high rate of mortality found in recent assessments at summertime bass tournaments. Studies suggest that confinement in livewells and release tanks may quickly spread the disease from infected to noninfected bass, and that released fish might spread it further within the wild population.

Dr. Hal Schramm and colleagues at Mississippi State University and Auburn University studied mortality at 12 tournaments on 8 reservoirs in 4 southeastern states during summer.* Although initial mortality was just 3 to 7 percent for different livewell procedures, by the end of a 5-day holding period, over 74 percent of bass held in floating cages or raceways had died.

At all these waters, some wild bass tested positive for LMBV. Over the holding period, prevalence of LMBV increased steadily and confined bass continued to die. Reference bass captured by electrofishing showed increasing incidence of LMBV while confined. They perished at rates similar to tournament-caught bass, though not subjected to weigh-in procedures.

The researchers concluded, “Largemouth bass virus is the only known factor that is consistently different than in previous studies that reported lower post-release mortality, and may explain the unprecedented high post-release mortality. Further, the rapid spread of LMBV and concurrent high mortality during post-tournament holding suggest that this virulent pathogen played a role in the post-release mortality in this study.”

They note that other tournament evaluations on waters known to have LMBV showed high mortality of reference bass collected by biologists to serve as a comparison and not subjected to tournament handling. Though this disease has not caused major fish-kills in the last few years, it has decimated top bass populations, including at Table Rock in Missouri, Sam Rayburn and Fork in Texas, and Grand Lake in Oklahoma. Populations in all these waters have rebounded since the outbreak, but not to pre-disease levels.

In this study, LMBV spread rapidly among bass in holding facilities, although a few individuals remained uninfected. Confining bass in livewells appears to quickly spread LMBV. Experimental holding facilities also spread the disease, and future investigations of tournament mortality in affected waters need new methods of evaluation that do not group fish.

How many bass would have remained disease-free and survived if released immediately after weigh-in, rather than being held together for longer-term observation? To what lengths should tournament organizations or resource agencies go to curb the spread of LMBV? Such questions remain, but it seems clear that where LMBV has been found, summer tournaments likely spread the disease as it’s passed from infected to uninfected bass in livewells and holding tanks.

Summer tournaments held on affected waters that involve standard procedures like livewells, holding tanks, or release boats may expect high mortality from LMBV that is likely to go undetected. This disease has affected bass fishing substantially and in ways we don’t yet fully understand.

*Schramm, H. L., Jr., A. R. Walters, J. M. Grizzle, B. H. Beck, L. A. Hanson, and S. B. Rees. 2006. Effects of livewell conditions on mortality and largemouth bass virus prevalence of largemouth bass caught in summer tournaments. N. Amer. J. Fish. Mgmt. 26:812-825.