Reports of dead fish floating in Lake St. Clair have Michigan's Department of Natural Resources investigating a possible outbreak of viral hemorrhagic septicemia, a virus researchers describe as "very contagious."
Symptoms can include bloody patches on the fish's skin. The virus, also known as VHSV, can affect more than 30 species of fish in the Great Lakes.
VHSV has been found in several Great Lakes since at least 2003, according to the Michigan Department of Natural Resources. Gizzard shad are particularly affected by the virus. However, because the symptom of bloody patches is shared with other pathogens, Michigan authorities say more testing is needed to confirm if VHSV is the culprit.
Ontario tests show negative
Tests run by Ontario's Ministry of Natural Resources and Forestry indicated that gizzard shad found in Lake Huron and the St. Clair River in late March had preliminary reports testing negative for VHSV. Further lab tests are still being carried out.
The ministry has also received more recent reports from mid-April of dead fish in Lake St. Clair, with the cause of death still undetermined.
The virus is not always fatal despite there being no effective treatment, according to Gary Whelan, research program manager with the Michigan DNR fisheries division.
"The good news is that most fish are not going to die from this," he said. "We will have to keep an eye out what effects this will have on fishery management and fish populations."
Whelan added that U.S. authorities are asking members of the public to avoid cross-contaminating other lakes or rivers.
"We have been asking our public not to move live fish between water bodies. That's really important because we don't want to infect new water bodies with this virus," he said.
Boaters are also cautioned to make sure they empty bilges and live wells before leaving a boat launch.
The virus does not infect humans or other mammals, and should not affect whether the lake is safe for swimming.