Over 31,000 salmon euthanized after disease detected at Michigan hatchery

• The DNR euthanized over 31,000 fish that were infected with bacterial kidney disease at a fish hatchery in Wexford County 
• The disease is caused by the pathogen Renibacterium salmoninarum
• Infected fish are typically treated with antibiotics, which were administered but ultimately unsuccessful 

The Michigan Department of Natural Resources announced Tuesday morning that over 31,000 Atlantic salmon in Wexford County were euthanized after testing positive for bacterial kidney disease, also known as BKD. 

“Having to make the decision to dispose of these diseased fish hurt, but it was clearly the right thing to do,” said Ed Eisch, division assistant chief for DNR Fisheries, in a press statement. 

Bacterial kidney disease is a chronic infection caused by the bacterium Renibacterium salmoninarum. Common symptoms include abdominal fluid build-up, swelling, muscle ulcers, anemia and blood blisters. The DNR says the disease is believed to be responsible for the decline in Great Lakes Chinook salmon in the mid-1980s.

The disease was found in fish during a routine inspection in early April at the Harrietta State Fish Hatchery in Wexford County, and confirmed by the Michigan State University Aquatic Animal Health Lab. 

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“The Atlantic salmon fishery is highly valued, but first and foremost, we have a public trust responsibility to protect the aquatic resources of the state of Michigan. Stocking fish known to be actively suffering a disease outbreak would be counter to that,” Eisch said. 

A 28-day antibiotic treatment was completed on May 17 but “was not fully effective” according to the DNR. After the treatment, signs of active disease appeared in another group of fish. 

The treatment is typically administered when the bacterium is first found in fish, Eisch told Bridge Michigan.  However, since the fish had already contracted BKD, they were more resistant to treatment. 

In the last decade, there haven’t been many cases where fish had tested positive for bacterial kidney disease but more cases where fish had the bacterium Renibacterium salmoninarum, Eisch said. Those fish were successfully treated, but this case is different. 

“These fish were sick enough that a significant portion of the fish were not feeding well,” said Aaron Switzer, DNR fish production program manager. “That means that the antibiotic, which was mixed in with their feed, was not being eaten at the rate necessary to eliminate the pathogen.”

Between 20 million to 30 million fish are stocked in state waters each year, according to the DNR. Over 9 million individual fish had been stocked by the fall of 2023.

Observations of infected fish were first recorded in 1999 and have been found in several waters across the Upper Peninsula, west and northern Michigan, according to the National Oceanic and Atmospheric Administration’s Great Lakes Aquatic Nonindigenous Species Information System. The last observation was recorded in 2008. 

While the 31,000 fish euthanized was significant, it’s not enough to impact Atlantic salmon populations for Michigan anglers, Eisch said. 

If an angler happened to catch an infected fish, it would still be safe to consume as long as it's cooked properly, as most pathogens or parasites that infect fish won’t survive at the temperatures that humans maintain, Eisch told Bridge.