Grove City, Pa. — State fishery biologists are stocking Lake Wilhelm in Mercer County with largemouth bass, in a rare move aimed at bolstering a population decimated by gizzard shad.
Using stock cultivated at the Linesville hatchery, biologists began putting fingerling largemouths into Wilhelm two years ago, after gizzard shad took their toll on bass, crappies and bluegills, the lake’s key species.
The stockings will continue at least through next year, in an effort to restore the lake’s predator-prey balance and improve the size of panfish, said Pennsylvania Fish & Boat Commission biologist Al Woomer.
We’re trying to keep up the bass numbers, and we’re hoping to put pressure on the gizzard shad,” he said, noting the stockings involve nearly 50,000 largemouths.
While there are big bass in the lake, anglers have had a tough time competing with all the natural forage, and small bass have been scarce. “When gizzard shad explode in abundance, it can affect bass recruitment,” said Woomer. “Young bass have dropped off because of the shad.”
A type of herring, gizzard shad increasingly are showing up in lakes across the state, presumably having been introduced by anglers who use them as baitfish. They can grow fast and achieve lengths of 10 to 12 inches.
After about 4 inches, only muskies and some of the biggest bass and walleyes can eat them. Their populations multiply rapidly, too, until they out-compete other species for habitat and food.
State biologists netted one gizzard shad at Wilhelm in 2004, and 2,000 shad in the same net location six years later. In the spring of 2013, 48 percent of all fish captured in trap nets at Wilhelm were gizzard shad.
Surveys correlated with angler reports about the sudden and dramatic decline of Wilhelm’s gamefish.
“There’s a whole suite of effects shad have directly and indirectly on a fishery,” Woomer said. “Young bass and bluegills need zooplankton, but the shad eat it out of existence.
“The zooplankton feed on phytoplankton, so if the zooplankton is gone, the phytoplankton goes nuts, and you get algae blooms. The algae in turn shut out sunlight for the rooted vegetation. So the impacts are felt from the bottom to the top of the food chain.”
Even a gizzard shad die-off at Wilhelm this past winter wasn’t enough to put a dent on shad numbers, said Woomer. “It might have knocked some back, but they weren’t going to disappear. Wilhelm is a very fertile lake. It’s shallow and has the kind of habitat shad like.”
It may be too soon to determine if the bass stockings are having a significant effect, but Woomer saw some encouraging signs this year, he said.
“There are still algae, but there was more rooted vegetation in the shallow areas of the lake, and the water color was more green than brown, which is an improvement over past years.”
He also saw nice numbers of bluegills and panfish along the shoreline. “They were small. They’re still not back to the great sizes of other years. But, hopefully, the crappies are taking advantage of young-of-year gizzard shad.”
In time, gizzard shad could become less of a nuisance on Wilhelm, which has happened at a few other lakes, he said.
“When they first explode in a fishery, gizzard shad can have all kinds of effects, but we’ve also seen them become assimilated over time. In Pymatuning and Shenango, they’ve become just one part of the general fish community.”
Gizzard shad once existed in Conneaut Lake, Pennsylvania’s largest natural lake, but none were captured in the most recent survey, Woomer said. “So they’ve been known to exit a fishery, too.”