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How increased rainfall can affect PA fish populations

Apr 23, 2019
A critical issue for spawning and egg and juvenile fish survival is high-flow rainfall events, which in the springtime is the biggest threat to smallmouth bass, according to Shannon White

A critical issue for spawning and egg and juvenile fish survival is high-flow rainfall events, which in the springtime is the biggest threat to smallmouth bass, according to Shannon White, a Penn State PH.D. candidate studying the adaptive capacity of coldwater fish.

“For them, a really heavy rain event has the potential to completely destroy a nest,” she said. “If you’re talking about bass, you’re talking about the potential for reduced reproduction, or at least reduced reproduction for those eggs.”

But even more devastating is the potential for entire populations of fish to collapse, White said. It’s a reality that could occur at a higher rate as catastrophic flooding events become more and more frequent, something that White has observed in the Loyalsock Creek in Lycoming County, where she conducts much of her research.

Hurricane Irene and Tropical Storm Lee brought historic flooding to Loyalsock Creek in 2011, as waters exceeded flood stage by nearly 8 feet, according to the NWS, the worst flooding to hit the area to date.

When White began her research around 2016, the creek experienced another major flooding event. Although this one wasn’t as bad as the 2011 flood, she observed the complete collapse of brook trout populations, where there would go from being 100 fish at a site to two.

While fish are typically pretty resilient and able to recover from such flood events — and can even benefit by the creation of new habitat — White said that the increased frequency of these “50-year” or “100-year” floods is making that recovery more difficult.

“It’s when we have an increased occurrence of these really, really big flood events that we start seeing more long-term issues or more long-term effects on the populations,” White said. “This fish populations can usually recover, but it may take a few years. But if they’ve recovered, gotten a little unstable and this happens again, and that kind of pattern continues to happen — which we are expecting with climate change — then eventually they can’t recover from it so much.”

Once a fish population initially recovers, it typically has decreased genetic diversity, White said, making it more susceptible to future disturbances or disease.

“That’s the kind of stuff that’s hard to see,” White said. “The population might be large, but it could be kind of like a sitting duck, in terms of it only takes one maybe fairly small disturbance for it to completely wash out again.”

That point, White said, is when the fish populations — particularly brook trout, the focus of her research — may not come back.

Another flooding-related issue for fish, White said, is increased stream temperatures that can result from floodwaters running off pavement in more developed watersheds — particularly in the summer.

Pennsylvania’s state fish — the brook trout — is one of the more susceptible fish populations to warmer water, White said. Typically preferring water temperatures in the mid-60s, all it takes is for rainfall to increase stream temperatures by a few degrees to put stress on the fish.

“When you have increased stress, it generally decreases reproduction and decreases population sizes,” she said. “Then we’re back to having decreased genetic diversity.”

To view the complete story from the Centre Daily Times, go to