UNIVERSITY PARK, Pa. -- This year's drought means it will probably take several years for wild fish populations in Pennsylvania streams to recover, says a fisheries biologist in Penn State's College of Agricultural Sciences.
"The drought alone has stressed fish populations, but we'll also see fewer young trout emerge from nests this spring because of the severe winter weather," says Robert Carline, adjunct professor in the Pennsylvania Cooperative Fish and Wildlife Unit at Penn State.
Low stream flows crowd fish into smaller living spaces, increasing competition for food, Carline explains. Once fish become stressed, they become more susceptible to disease. Lower water also exposes fish to predatory birds.
During warm weather droughts, higher water temperatures can be a hardship for cold water fish like trout. But in winter, "anchor ice" -- a layer of slush which forms on stream bottoms during low flows and prolonged cold spells -- can harm overwintering eggs. Anchor ice also damages the invertebrates that fish eat, such as mayflies and worms.
"Brown and brook trout spawn in the fall. They deposit their eggs in pits in the stream bottom," Carline says, "then cover the nests with several inches of gravel. Stream water moving through the gravel supplies the eggs and embryos with oxygen, and carries away carbon dioxide and other wastes. Anchor ice impedes this process.
"With the recent severe cold weather, we expect fewer young brown and brook trout to emerge in late February and early March," he says. "In nests with prolonged anchor ice, all of the eggs may die."
Fish in small streams will be most affected, he says.
Drought effects also are exaggerated by urban development, Carline explains. When drainage patterns are disrupted, less water infiltrates the ground to recharge the groundwater, and groundwater constitutes the base flow of streams.
"During low flows, the stream flow reflects the size of the groundwater reservoir," he says. "Because groundwater levels are lower in watersheds with a lot of development, we're seeing lower base flows than we have historically."
As stream beds become exposed, Carline advises against removing the rocks or woody materials. Woody materials provide hiding places and refuge for fish during high flows. Also, many organisms that fish eat attach themselves to debris. "Fish production is directly related to the amount of woody materials in the stream," he says. "Some people call it trash -- we call it habitat."
After the drought, algae and attached plants should recolonize quickly, followed by insects and other invertebrates. Fish recovery will take a little longer, because repopulation relies on fishes moving in from upstream and downstream.
"Hatchery trout generally aren't well-adapted to our streams and most don't reproduce successfully," Carline says, "so it would be almost impossible to replace lost wild trout by stocking."
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EDITORS: For more information, contact Bob Carline at 814-865-4511.
Contacts: Kim Dionis KDionis@psu.edu 814-863-2703 814-865-1068 fax