By Steven Maier
Conditions might be right in parts of the Great Lakes for another outbreak of a deadly fish virus.
Large parts of the lakes have environments that could support a new outbreak of viral hemorrhagic septicemia, or VHS, according to a study recently published in the Journal of Fish Disease. The same virus was responsible for large fish die-offs in the region from 2006 to 2009, including a 2006 event that killed several thousand muskies in Lake St. Clair, according to the Michigan Department of Natural Resources.
The areas suitable for the virus are often shallow spots without much vegetation and that see lower maximum surface temperatures, according to the study. VHS requires cool water (50 degrees Fahrenheit or colder) and low levels of salt and minerals.
The scientists looked at data from previous outbreaks to identify the types of environments where the virus thrives.
“Basically, where it’s happened before can lead to where it can happen in the future,” said Nick Phelps, co-author of the study and director of the Minnesota Aquatic Invasive Species Research Center in St. Paul, Minnesota.
The team used satellite data to identify areas that fit the bill for the virus, said Luis Escobar, a research associate at the University of Minnesota and the study’s co-author. It found suitable environments in each of the Great Lakes including large parts of Lake Michigan and Lake Huron and most of Lake Erie.
Most of the die-offs occur in the fall or spring, when the water hits the right temperature and the fish are physically stressed by the changing seasons, Phelps said.
VHS can cause fish to bleed from their eyes, sides, heads and fins, according to the Minnesota Sea Grant. Other symptoms include bulging eyes, fluid-filled chest cavities and death.
The extent of the symptoms varies by fish, Phelps said. Some fish, like muskie, will almost certainly die if they contract the virus, while others can appear healthy.
“Mortality events have ranged in size from just a couple fish to several hundred tons of fish,” Phelps said. “It just depends on the species involved.”
The virus was discovered in the Great Lakes in 2005 when fish began to die in Lake St. Clair, which straddles Michigan and Ontario, Phelps said. Examination of frozen fish samples revealed the virus was present as far back as 2003. VHS is similar to viruses found in the North Atlantic and Pacific oceans and was probably introduced to the Great Lakes via frozen bait or ballast water.
Now that they’ve identified the places where the virus can thrive, they can focus their efforts to prevent more die-offs, Escobar said.
“We only have so much money and time to look for things,” Phelps said. “We just made the haystack smaller.”
Lake Superior is the only one of the large lakes with no recorded VHS die-off events, and scientists are still trying to discover why, Phelps said. It may be that the fish populations aren’t dense enough to spread the virus, that most of the the lake is too cold or that the native fish aren’t particularly susceptible.
Recording an event also requires someone to see it, Phelps said. In such a big lake, even a large group of floating fish is easy to miss.
Despite the lack of documented findings, the study found that much of the coast of Lake Superior is highly suitable for VHS.
The study wasn’t done to alarm anyone, Escobar said, but to help identify areas that need further study to keep the virus contained.
There are places that can host the virus outside of where it already exists, Phelps said. This knowledge will help keep infected or especially vulnerable fish from being transported to and from these “hot spots” and places where the virus is known to be present.
“It’s easy to get complacent when you’re not seeing mass die-offs,” he said. “A study like this, I think it shows that the risk remains, and we need to be aware of that.”