Viral hemorrhagic septicemia (VHS) affects 34 species of fish, including walleye and bass. Its emergence has resulted in large-scale mass mortality events throughout the eastern Great Lakes and cost millions of dollars in management efforts. The virus is considered to be the most significant freshwater fish health threat in the world and has a history of large-scale fish kills. It is transmitted fish-to-fish from close contact of contaminated water or reproduction.
The strain of VHSV in the Great Lakes (VHSV-IVb) is particularly active in cool waters (5-20C) and can survive outside of the host at ideal temperatures (~10C) for weeks. The virus is transmitted fish-to-fish from close contact of contaminated water or reproduction. The virus can also be transmitted on the surface of the egg to be passed from parent to progeny.
What it affects
In the Great Lakes, 34 species of fish have been found positive from natural infections. This includes many of the popular game fish species such as Walleye, Muskellunge, Salmonids, and Bass. The reservoir species (e.g. maintains virus persistence in the environment) in the Great Lakes appears to be another invasive species, the Round Goby.
Where it's found
VHSV has been confirmed in all of the Great Lakes, in some inland waters of Michigan (Budd Lake), New York (several finger lakes), and Wisconsin (Lake Winnebago). The virus has not been detected in inland waters of Minnesota, however Lake Superior is known to be infested.
How it spreads
The virus can spread from one location to another via direct fish/water movement (i.e. connected waters), transfer of infected fish (i.e. stocking, baitfish), and contaminated water and gear (i.e. boaters, anglers). Proactive measures to test fish to certify freedom of disease for live fish transfer has been an important management strategy limiting the spread of VHSV in the region.
What it looks like
As the name describes, the virus can cause internal and external bleeding which in severe cases leads to organ failure and death. However, it is important to note that clinical lesions can vary among susceptible species. For example, walleye can develop severe hemorrhagic lesions, while muskellunge appear fairly normal despite being highly susceptible to low levels of virus.
Fish kill events in Minnesota are widespread, and may provide clues about VHS and other ecosystem issues. Reporting these fish kill events is an easy and effective way to do your part to help protect Minnesota waters. The online, user-friendly database is available now at http://z.umn.edu/fishkill. It simply asks for the date, the location of the fish kill, and other basic information of any fish kill you observe. Once reported, fish kills are triaged and, if appropriate, trained biologists and students will collect samples to diagnose the cause of mortality. Do your part to protect our waters by reporting fish kill events that you see.