Lancaster, Pa. — A college laboratory has solved what has been a maddening 13-year mystery: what killed so many of the prized smallmouth bass in the lower Susquehanna and Juniata rivers, beginning in 2005.
The answer – a shocker to Pennsylvania and federal investigators – is largemouth bass virus.
It’s a disease that was known to be present in Susquehanna smallmouths but had been ruled out early on because it was thought it couldn’t harm them.
Turns out it can be lethal when the shallow water near the banks of the river where young bass roam becomes stagnant and hot, turning into a soupy cauldron of deadly pathogens.
In that environment, the virus does not kill the bass, but it allows lesions and ugly sores where bacteria and fungus settle in, proving fatal.
Concerns remain, but there have never been so many big bass in the Lower Susquehanna for anglers.
The discovery was made by scientists at Michigan State University who injected smallmouth bass from a Pennsylvania fish farm with the virus and also proved that mortality was heightened when the fish were in summerlike warm water.
The surprise discovery comes even as the next generation of bronzebacks seem to have built up a semblance of immunity to the virus.
Fishing for smallmouths – aided by a moratorium on keeping fish – has rebounded mightily the last several years and is today as good as it was in the early 1990s, considered the golden years.
There have been several consecutive years of promising numbers of bass hatchlings.
“The thought is it (the virus) has kind of run its course, for the most part,’’ said Geoff Smith, the Susquehanna River biologist for the Pennsylvania Fish & Boat Commission. “We’ve seen low levels of the disease since 2012 and reduced mortality.’’
Before anglers get too confident that the danger has passed after all these years of handwringing, however, Smith has several caveats.
One, the virus may well erupt in a couple of other Pennsylvania smallmouth fisheries – the Delaware and Allegheny rivers – at any time under warm-water and low-flow conditions. Climate change may increase that likelihood, Smith said.
Also, while Susquehanna smallmouths may have built up a resistance, the virus also could mutate and come back with a vengeance.
In addition, the Fish & Boat Commission still worries what role high levels of nutrients running off farm fields may have on water conditions and stressing fish.
“We’ll never know how much of the mortality is from largemouth bass virus alone,’’ Smith cautioned. “All that could play into additional stress, but we’re not sure how at this point.’’
Moreover, bass with sex organs of both males and females continue to turn up with no defined cause found. “It does appear to be unrelated” to largemouth bass virus, Smith said.
“It’s another concern out there that we’re still trying to figure out. In the course of this investigation, we’ve uncovered a lot of other not-so-nice things in the Susquehanna that need to be cleaned up.’’
Namely, biologists are concerned about a class of chemicals known as endocrine disrupters that can interfere with hormone systems. They are likely behind the male bass having egg cells in their testes.
Possible sources could be pharmaceuticals flushed down toilets that end up in sewage plants, runoff of herbicides and pesticides from farm fields and suburban yards, as well as household chemicals.
Still, if you’re a smallmouth bass angler, it’s hard not to be encouraged.
Fish & Boat Commission sampling of bass by electrofishing is showing catch rates similar to what they were in those boom years of the early 1990s.
And even before the moratorium, more anglers were practicing catch and release than ever before, so more big bass are out there.
“If we keep things going the way they’re going, we should see that for quite some time unless something like this virus crops up again,” said Smith.