August 23, 2011
MADISON -- Recent test results show that healthy-looking yellow perch in Wisconsin waters of Lake Michigan were infected with VHS virus even though there was no fish kill.
That positive VHS result comes four months after the deadly fish virus was confirmed as the cause of a fish kill that left thousands of gizzard shad floating in the Milwaukee harbor ship canals.
Together, those 2011 findings show that VHS persists in Lake Michigan and remains an active threat to fish in the big lake and in nearby inland waters and fish farms, and that anglers and other boaters need to continue to follow the rules to prevent spreading VHS and other aquatic invasive species, Wisconsin fish health experts say.
Fish health specialists also say VHS seems to be following a common path that infectious disease takes in fish.
"We expect that VHS will periodically recur in the Great Lakes, much the same as other animal and human diseases cycle over time," says Sue Marcquenski, the Department of Natural Resources fish health specialist.
Michigan test results show that VHS returned in 2011 to an inland lake, Budd Lake (exit DNR) after three years of looking for, but not finding the virus. VHS caused a die off in that lake in late April and early May 2011 of largemouth and smallmouth bass, bluegills, and pumpkinseed sunfish.
VHS, or viral hemorrhagic septicemia, does not affect people nor pets, but can infect several dozen species of fish and cause them to bleed to death.
Dr. Tony Goldberg, a UW-Madison veterinary epidemiologist conducting a VHS study in Lake Winnebago says, "I often get asked, 'Why do we have to worry about VHS any more -- it's gone?' The answer is that infectious disease can cycle. You can get peaks of infection every few years, and then valleys. Just because we don't see a disease for a few years doesn't mean it's gone. We may simply be in one of these down cycles and we could be on the verge of an upcycle.
Young gizzard shad (top) and young silver carp (bottom) are hard to tell apart. VHS rules will help prevent bait harvesters from accidentally introducing silver carp to another water.
"It's also important to realize that we shouldn't expect VHS to simply disappear. Invasive viruses are like other invasive species -- zebra mussels or Asian carp, for instance. Once they're here, they are almost certainly here forever and we're not going to be able to go back to the way things were," he says. "It would be a mistake to let our guard down."
Goldberg is one of the principal investigators in a multi-year study underway in the Lake Winnebago system to see whether the fish virus is still a threat there and to develop a faster, cheaper test to detect its presence. VHS was first detected in Wisconsin in Winnebago System waters in 2007.
The yellow perch tested for VHS were collected in June from the Green Can spawning reef offshore of Milwaukee. DNR was conducting its annual spawning assessment and wanted to test the fish for VHS because "we were observing low numbers of male yellow perch in the survey and those males were not sexually mature," says Brad Eggold, DNR southern Lake Michigan fisheries supervisor.
VHS work was done in conjunction with other tests. The Wisconsin Diagnostic Veterinary Laboratory in Madison notified DNR of the positive VHS result earlier this month.
The finding marks the second time VHS has been found in spawning yellow perch at the Green Can reef. The first detection was from fish sampled June 5, 2008.
VHS testing was not done in the intervening years of yellow perch from Lake Michigan; once the virus has been confirmed in a particular fish species from Lake Michigan, DNR directs its limited surveillance testing dollars to monitoring other waters to see if VHS has spread.
The yellow perch lab results show that VHS virus is still present and being shed by yellow perch during spawning. The prolonged spring and cooler early summer water temperatures may have created an extended window for VHS to infect fish in 2011, Marcquenski says.
Despite the yellow perch testing positive for VHS, DNR received no reports of dead or dying fish from anglers, Eggold says. "So at this time it's hard to speculate on the impacts of VHS on the yellow perch population although having a continued threat from an invasive like VHS is not good for the fisheries in Lake Michigan," he says.
Alewivesthat washed ashore Lake Michigan beaches earlier this summer do not have VHS, test results show. It may be a seasonal pattern, Eggold says.
FOR MORE INFORMATION CONTACT: Sue Marcquenski - (608) 266-2871 or Brad Eggold - (414) 382-7921