Deadly fish virus returns to northern Michigan hatchery

By Marshall Lee Weimer

A deadly fish virus thought long gone from the Great Lakes has reemerged in a northern Michigan lake trout hatchery.

The pathogen, epizootic epitheliotropic disease virus (EEDv), causes lake trout to produce heavy mucus and hemorrhages at the eyes. The skin of infected fish turns blotchy, bloated and sore-ridden. Eventually they die.

The Marquette State Fish Hatchery lost around 100,000 fish in 2012 to the disease, according to a new publication in the American Fisheries Society. Similar losses happened again in 2017.

The threat to wild-caught trout appears minimal, experts say. But the disease could decrease how many of the fish get stocked by the Marquette hatchery.

That would mean fewer fish for anglers to catch in certain regions of Michigan.

Extreme weather, increased water turbidity and high densities of fish could contribute to the outbreak, according to the study co-authored by Thomas Loch, an assistant professor of pathobiology and diagnostic investigation at Michigan State University.

A key question Loch and his colleagues pursued is whether the disease is already present in wild lake trout.

“A lot of the work we are now doing is to try and figure out the distribution of this virus,” Loch said.

In the 1980s, the disease killed 15 million juvenile lake trout reared at Great Lakes fish hatcheries, the study reports. Managers removed the fish from the contaminated hatcheries and disinfected them thoroughly.

The virus was thought to have been long expunged from the region until the 2012 and 2017 outbreaks.

Loch suspects that fish collected from the wild, which provide eggs and sperm for reproduction in hatcheries, already had the virus without the Department of Natural Resources knowing about it. The agency began changing the sites of where they would collect the eggs and sperm, and then artificially bred these lake trout in the hatcheries.

While the virus was present in certain wild populations, those fish didn’t break out with the disease. But once introduced into the Marquette hatchery,  this “created an environment where, over time, the virus could cause more problems,” leading to an outbreak of the disease, Loch said.

What that means is as yet unclear.

“The impact of the disease will vary,” said Gary Whelan, the fisheries division program director for the Department of Natural Resources.

“This virus is another pathogen that we must manage around and will incur fisheries management costs from those efforts,” he said. “It will cause issues at times for the number of lake trout available in some waters, but the disease effects are not annual as the stressors do not occur every year.”

Those stressors include heavy rainfall and large amounts of sediment in the water.

One worry: Climate change could increase frequency of heavy rain and other abnormal weather, which could increase the frequency of disease outbreaks.

“With some of these more recent events, almost every time they have been preceded by a significant, quote unquote, abnormal weather event,” Loch said. “Most of these hatcheries use surface water, and when you have these large weather events, you get this large slug of turbidity and sediment.”

Lake trout need clear, low-sediment laden waters to thrive, Whelan said. An increased frequency of large storms, could trigger the reemergence of dormant diseases like EEDv.

“If climate conditions trend to more extreme events, there will be more storm related stress issues in the water supply of this facility,” he said.

Loch agreed: “That may be why the virus is becoming more frequently seen.”

The distribution of the virus among wild lake trout is unclear, he said, “But it is important to know. If fish test positive, they can’t be used to stock hatcheries.”

Although the disease doesn’t appear to kill wild fish, Whelan said.

“While EEDv could decrease the number of hatchery fish available for stocking in those years when the disease occurs, we have many self-sustaining lake trout populations in Great Lakes waters and in a number of inland waters,” he said. “So the direct effect will be on the numbers of lake trout available for stocking.”