ANDOVER, Ohio -- A mystery lurks in the dark waters of Pymatuning Reservoir.

On a black September night, a jon boat rigged with electrical equipment chugs out of a launch on the Ohio side of the 14,000-acre impoundment straddling the Ohio-Pennsylvania line. From metal poles on its bow hang wire tentacles that could be props from a creepy Tim Burton movie. Attached is a techy looking apparatus powered by a loud on-board generator. When the tentacles touch the water ...

Zap!

The electro-fishing survey is part of a collaborative biological study conducted by the Ohio Division of Wildlife and the Pennsylvania Fish and Boat Commission.

The big mystery on the biologists' minds: Where are all the walleye?

Beginning in the late 1980s, Pennsylvania (and later Ohio) stocked millions of young walleye in the jointly managed lake. But in 2001, survey results began showing a puzzling decline in walleye numbers. In 2005, the annual survey found none.

Like frustrated fishermen tired of getting skunked, last year the biologists changed tactics. Instead of stocking quarter- to half-inch fry, the agencies primed Pymatuning with 16 million walleye fingerlings, hatchery grown to 1 to 2 inches. PFBC added another 78,000.

With four boats from Ohio and one PFBC boat near Linesville, Pa., Division of Wildlife fisheries biologist and project manager Matt Wolfe says he hopes the electro-fishing survey finds some fish.

"Anything's better than the big zero we got in '05," he says, firing up the dual 40-horsepower Evenrudes. "We have to stock. Natural reproduction on this lake is probably next to nothing. The state agencies -- we and the Fish and Boat Commission -- are providing the walleye to the anglers. It's basically a put-and-take fishery."

The problem is poor spawning habitat. When the reservoir was built in the 1930s, walleye spawned abundantly on the gravelly bottom. Decades of sedimentation, however, have buried the walleyes' ability to reproduce. Wolfe said stocking is cheaper than the costly habitat development it would take to rebuild spawning grounds.

Wolfe, who grew up in Jeannette and worked briefly for the PFBC, said biologists from both states are still unsure of what gobbled up some 20 million walleye fry in 2005. Why didn't the massive predation register on earlier surveys? Was the total loss of a year class of walleye a mere statistical anomaly, disease or something else?

"Before, we just stocked fry, fry, fry," he says. "Now we're trying to stock fewer but bigger fish and hoping for better results. The fry, all you do is raise them three or four weeks in hatchery jars, which costs practically nothing. Fingerlings take a couple of months. You transfer them to hatchery ponds and feed them and have zooplankton blooms and enrich the ponds with nutrients ... You're talking 15 cents to 20 cents per fish."

The 600,000 walleye fingerlings stocked this year by Ohio and Pennsylvania cost $90,000 to $120,000 to raise, he said. As in Pennsylvania, Ohio fisheries management is paid for mostly through angler dollars. No money comes from the state's general fund; about 5 percent comes from federal reimbursements.

The jon boats motor to shallow waters in five predetermined sections of the lake. With electrodes breaking the surface, each boat cruises slowly taking seven "transects," or measurements, lasting 15 minutes. The tentacles create a 400-watt teardrop-shaped electrical charge that extends about 5 feet below the hull.

Many of the temporarily stunned fish don't float. But among the thousands of gizzard shad and alewives and dozens of bass, perch, carp, bluegills and crappies that drift to the surface surrounding Wolfe's boat, some immature walleye emerge. They're quickly scooped into 10-foot nets and dumped into a tank on the boat. While Wolfe pilots and records notes, Ohio fisheries biologists Curt Wagner and Chris Aman take length measurements, scrape off scale samples and bag the collected walleye for further study.

After 120 total minutes of stunning, the five boats have collected over 330 half-year-old walleye -- their best catch since 2000 -- suggesting that the new stocking method is working. A gill net survey of deeper water later this year will count mature walleye.

"There's a lot of [walleye] in here, but they're old and smart," says Wolfe, motoring back to the launch. "A lot of anglers are trolling Hot n Tots or big old crankbaits. Fish that are eight or nine years old have seen every Hot n Tot pattern imaginable. You've got to switch up. Try something different."

On the south side of Pymatuning, says Wolfe, anglers are shooting white spinnerbaits imitating the flash of alewives. And Wolfe says he's done well trolling worm harnesses in 6 to 7 feet.