Researchers have found a critical piece of evidence in solving the unexplained die-offs of smallmouth bass in the Susquehanna River— a largemouth bass virus.
Researchers from Michigan State University found the virus itself doesn’t kill the fish. However, at higher water temperatures, it can weaken their immune system and allow other infections to spread, which ultimately finish off the fish.
Since 2005, the Pennsylvania Fish and Boat Commission has documented outbreaks of disease in smallmouth bass that left them with open sores and lesions. Commission Executive Director John Arway said the timing of the deaths remains unexplained.
“Was there a certain set of environmental factors that led the virus to proliferate? We don’t know.”
Intersex fish – adult males with female eggs in their testes – have also been documented since the early 2000s. The problems have threatened Pennsylvania’s multi-billion-dollar recreational fishing industry.
In 2015, the state Department of Environmental Protection released a study, which zeroed in on several causes: endocrine-disrupting compounds and herbicides, as well as pathogens and parasites.
Arway said the new research about the virus is helpful, but many questions remain.
“It’s an important piece of the puzzle, but it’s a complicated puzzle,” he said. “We were also studying the impacts of the algae blooms — causing oxygen levels that were so low they alone could have a stressful impact. There is a combination of factors in why those fish died.”
He says the smallmouth bass population has been steadily recovering.
“We’ve put catch and release regulations on, so we’ve protected the older bass from harvest. And a couple of good years with higher flows, and cooler temperatures have allowed those fish to spawn,” he said. “Right now, the fishery season is going fairly well.”
This story is produced in partnership with StateImpact Pennsylvania, a collaboration among The Allegheny Front, WESA, WITF and WHYY to cover the commonwealth's energy economy.