Astute readers of the Water Line know, the starting point for zebra mussel infestation in North America was the Great Lakes. Recently, another “invader” has grabbed the attention of fishery managers in that region, Viral Hemorrhagic Septicemia (VHS). There are still a lot of unanswered questions about VHS and only time will tell if this infectious virus spreads to other regions, like the zebra mussel.
There are four known strains of VHS, three of which are found in Europe and impact freshwater trout and turbot. The fourth strain has historically infected herring along the Pacific and Atlantic coasts of North America. The VHS strain found in the Great Lakes Region is genetically similar to that fourth strain. This genetic similarity could mean that the virus was introduced into the region from the coast by boat, spawning fish, or even a bird carrying a diseased fish.
Information concerning the ecology of this new viral strain is limited, creating management challenges for those fisheries effected by VHS. What is known is that this strain of VHS can infect many different fish species. Michigan reports that VHS has been confirmed in 16 coolwater species and 3 coldwater species of fish, while the University of Wisconsin Sea Grant Institute states that 45 different fish species are known to be susceptible to VHS. Some of the species on the Wisconsin list include: black crappie, bluegill, channel catfish, largemouth bass, rainbow trout, rock bass, smallmouth bass and walleye.
The VHS virus can cause large scale hemorrhaging, both internal and external (though external symptoms are not always present). In the end the fish usually dies from internal organ failure relating to the internal hemorrhaging. Interestingly, not all fish that contract VHS die. If there are no stressors other than the virus, a fish may survive by creating antibodies. Often these surviving fish become carriers of the virus. As mentioned, the virus seems to be most deadly when fish are already stressed; such as times of sudden temperature change or during the spawn. The virus also seems to be worse when water temperatures are in the 40-55° F range.
Another factor that can increase the severity of a VHS outbreak is high fish density. High density is usually a minor concern in lakes and rivers, but is a larger concern in fish hatcheries. High densities of fish in a relatively small area create stress and could lead to a severe outbreak.
Currently VHS related fish kills have been limited to some of the Great Lakes and the Upper Midwest (Lake Huron, Lake Ontario, Lake Erie, Lake St. Clair, Lake Winnebago, St, Lawrence River, Niagara River and St. Clair River). Research is needed concerning the transport of VHS, it’s life history, and how well it survives out of water. Once some of these questions are answered scientists will be able to estimate the threat and rate of infection. The biggest concern for Missouri is likely regarding VHS and fish hatcheries (both public and privately owned). The density in these hatcheries is high, increasing fish stress and making them more susceptible to the virus.
It is the responsibility of all lake users to keep this virus out of our lakes and to minimize the impact of the virus, should it arrive.