ESPN

KISSIMMEE, Fla. — As he spoke to state BASS conservation directors about fish health, John Grizzle delivered some good news:

"We didn't hear of any new largemouth bass virus [LMBV] kills in 2004," said the expert on fish health from Auburn University. "But people still are out there looking."

Grizzle added that many unknowns still exist regarding the virus, including its cause, how it spreads, and what prompts it to suddenly evolve from a virus to a lethal disease.

Fisheries affected by LMBV did suffer a downturn, the scientist explained, with higher mortality seen in larger fish, along with reduced growth rates and lower weights observed in affected populations overall.

"But bass populations do recover," he said.

In addition to offering an LMBV update, Grizzle provided a general look at fish health. First, he explained why diseases occur and why it's important to determine the cause of disease in wild fish.

They occur either because the "injurious agent" changes or resistance decreases. With the former, the amount might increase or the environment might become more hospitable for its strengthening. In the latter, a skin or gill injury might make a bass vulnerable, as could weakening of its immune system through stress.

Determining causes, he said, helps prevent spread of disease and reduces unnecessary concern. If fish are suffering from low dissolved oxygen in a pond or small lake, for example, resource managers could remedy that by adding aeration.

Grizzle classified low dissolved oxygen as a noninfectious disease, along with "toxicants" such as nitrogen, phosphorous, pesticides, herbicides, and other man-made chemicals.

Infectious diseases include parasites, bacteria and viruses. Some parasites, such as roundworms, and a few bacteria can affect humans.

But viruses, such as LMBV, do not affect humans, Grizzle emphasized.

"There are pathogens out there [in other countries] that aren't here yet," he added. "We still need to worry about importing them."

One of those is a virus found, thus far, only in Taiwan. It kills bass and other species, with a 30 to 50 percent mortality rate among fish infected.