A virus strain responsible for the deaths of sheephead in Lake Ontario last year was as the cause of the massive die-off of freshwater drum, or sheephead, in western Lake Erie, a state biologist said yesterday.
Jeff Tyson, fisheries biology supervisor with Ohio's Sandusky Fish Research Unit, said investigators were able to identify relatively quickly the pathogen viral hemorrhagic septicemia because of an earlier experience with a fish kill in the Bay of Quinte in Lake Ontario in spring of 2005.
The U.S. Fish and Wildlife Service says viral hemorrhagic septicemia results in heavy fish mortality, a fact that was evident last month in Lake Erie when dead fish floated to the shores, creating a stink as the fish rotted on beaches.
Mr. Tyson said 2005 was the first time the virus was documented in fresh water.
The primary victims in the Lake Erie deaths were sheephead, a less-desirable type of fish. Other species were found dead as well, but the role of the virus may be limited, he said.
Separate strains of the North American virus have been identified in salmon and related saltwater species on the east and west coasts. A different strain has hurt the fish-farming industry in Europe.
"There may be other types we don't know yet," he said.
Viral hemorrhagic septicemia was found in a muskellunge caught from the Michigan side of Lake St. Clair in the spring of 2005, raising the possibility the strain is spreading, said Gary Whelan, fish production manager for the Michigan Department.
"We really don't know a lot about it," Mr. Whelan said from his office in Lansing. "It's pretty new, or it may have been here but we just have not detected it before."
He said learning the cause of death of a fish or other creatures can be difficult because the virus isn't always the primary cause.
Fish kills generally occur in spring and often are attributed to fluctuations in water temperatures. The fact that the winter was not particularly cold may have contributed to the increase of bacteria, which survived an extensive freeze, Mr. Tyson said.
The discovery of the infection in the muskie in Lake St. Clair may prompt state biologists who are raising game fish there for release in the wild may have to reconsider where they obtain their breed fish, and the source of water for the pens, Mr. Whelan said.
The virus has been found in the wild in fish such as mummichog, stickleback, striped bass, freshwater drum, and brown trout, researchers in Michigan said, saying there's no indication it constitutes a threat to public health at this point.
The North American VHS strain appears to be much less virulent to salmon and trout than the European VHS strain, Mr. Whelan said.
Finding the cause after a fish has died and decomposed presents quite a challenge in finding a cause of death, compared with examining a live, sick fish, biologists say.
"The disease does not express itself until something else attacks the immune system," Mr. Whelan said.
"Fish in the wild that are dead could have a lot of beasties in them. There's an awful lot of bacteria everywhere," he said.
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