A bowfin (top) and rock bass caught in the St. Lawrence River near Clayton, N.Y. Rock bass are known to be vulnerable to a virus that causes anemia and hemorrhaging in fish but is harmless to humans. Since 2005, the virus has led to fish kills in Lakes Ontario, Huron, and Erie, in the St. Lawrence and Niagara Rivers. New research refutes the theory that ships introduced the virus. (Credit: Mohd Zafri Hassan)

CORNELL (US)—A virus that has killed large numbers of fish in several Great Lakes since 2005 may have been present for decades. The recent finding casts doubt on the theory that ships recently introduced the deadly virus.

“Viral hemorrhagic septicemia virus (VHSV) could be in a lake without killing fish,” says Mark Bain, associate professor of natural resources at Cornell University and lead author of a paper in the journal Public Library of Science One.

“Healthy fish can carry this disease at low levels,” adds Bain. “That means the eruption of fish kills from VHSV does not signal its arrival.”

After large numbers of fish inexplicably died in Lake Ontario in 2005, researchers at Cornell’s College of Veterinary Medicine identified the culprit as VHSV, which causes anemia and hemorrhaging in fish but is harmless to humans. It was the first time the virus had been documented in a Great Lake. The researchers had assumed that ships had recently introduced the virus.

The new study, however, reports that VHSV is prevalent in the waters and fish everywhere they tested in Lakes Huron, Erie, and Ontario, leading them to think the virus may have been living in fish undetected in the lakes for decades.

Until now, researchers had only tested samples from dead fish. The new study involved analyzing samples, for the first time, from live fish and water from 30 locations across the three Great Lakes, including 10 harbors, 10 boating centers, and 10 wild shorelines.

“We found it everywhere, not just around fishing harbors and boating centers,” says Bain. “We have no evidence that this pathogen is concentrated around shipping.”

The researchers do not know how the virus initially entered the Great Lakes, but VHSV has existed historically in the North Atlantic and in Europe.

Since 2005, large VHSV-related fish kills have occurred in Lakes Ontario, Huron and Erie, in the St. Lawrence and Niagara Rivers, and VHSV has been identified in Lake Superior and smaller lakes, including the western-most Finger Lake in New York, Conesus Lake.

Researchers have detected the virus in more than 20 Great Lakes fish species, posing a potential threat to New York’s $1.2 billion sport-fishing industry. Authorities have responded with strict regulations on boats and ballast water and on moving fish and bait minnows between lakes.

One theory why VHSV started killing fish in large numbers in 2005 is that warmer springs led to rapid rises in water temperatures, which stresses fish during spawning periods and makes them more susceptible to the virus, says Bain.

Another theory: Over the last 10 years, round goby fish—known carriers of the virus—have been spreading in the Great Lakes and may be shedding VHSV in the water.

“It’s the most infected fish,” notes Bain. “It may be that higher populations of round goby brought the disease to more prominence.”

The study was funded by the Great Lakes Protection Fund, the Northeast-Midwest Institute, the Great Lakes Research Consortium, and the Cornell University Agricultural Experiment Station.

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