It's been a busy but ultimately disappointing spring for the crew that runs the big fish lift at Conowingo Dam.

Since the beginning of April, the aquatic elevator near Darlington has hoisted more than a million finny creatures up the 94-foot wall holding back the Susquehanna River, helping them on their annual spawning run upriver. The lift is so crowded at times with migrating fish that technician John Lahr has to count them by tens as they swim pell-mell by an underwater viewing window, headed away from the dam like morning commuters late for work.

But only a tiny fraction of those have been American shad, the once-abundant fish for which the $12 million lift ostensibly was built. In all, 12,733 had hitched a ride upriver, the fewest seen since the facility began operating 22 years ago. With the run waning, the lift shut down for the year on Monday.

"It's very puzzling," said Michael Hendricks, fisheries biologist with the Pennsylvania Fish and Boat Commission, who's been tracking the species' travails for decades. The number of American shad taking the lift each spring peaked at nearly 194,000 in 2001 and has been on a downward slide since. "We're not sure why," Hendricks added. "I wish I knew."

The lift's dismal performance comes as the hydroelectric dam's operator, Exelon, faces pressure to remedy the environmental harm caused by the massive barrier on the Chesapeake Bay's largest tributary. With Exelon seeking federal permission to continue producing power there for another 46 years, environmentalists and others say the company ought to be required to take significant — and potentially costly — new steps at Conowingo to enhance fish passage, improve water quality and generally restore some of the river's lost ecological vitality.

"This is a once-in-a-generation opportunity," said Mark Bryer, Chesapeake Bay program director for The Nature Conservancy.

Exelon has applied to the Federal Energy Regulatory Commission to renew its operating license for Conowingo and is in negotiations with officials from Maryland and Pennsylvania and with federal fish and wildlife agencies. Until those talks are resolved, all parties have agreed not to discuss publicly what the Chicago-based power company might do to improve passage of shad. But Kimberly Long, environmental program manager for Exelon Power, pointed out that shad runs up and down the East Coast are down, suggesting that the troubles may not be limited to Conowingo.

"I don't think anyone has pinpointed one reason for the decline in the American shad population," she said in an interview last week at the dam.

The parties are also talking about how to restore American eels, another species in trouble, to the Susquehanna, where they could play a role in helping clear up the river's murky water. And potentially the most costly and contentious of all, they're expected to grapple over what Exelon might do to help deal with the buildup of sediment behind the dam, which threatens to foul the bay if a major storm washes much of it through the dam.

But figuring out how to get more American shad upriver is particularly challenging, if only because it's not clear why they've declined. With a few exceptions, shad runs on rivers from Maine to Florida are depressed to practically nonexistent. It's a sad state of affairs for a species that's been dubbed the nation's "first fish" for its role in feeding Europeans who settled the East Coast. Shad once thronged the bay's rivers each spring, making them the Chesapeake's most important fishery. So many of the succulent but bony fish were caught on the Susquehanna with huge nets stretched across the river that farmers wound up using some to fertilize their crops.

Such harvests eventually proved unsustainable, and the annual catch in Maryland fell from a peak of 7 million pounds in 1890 to just 24,000 pounds in 1980. Worried state officials finally banned fishing for them, a moratorium that has never been lifted. Maryland biologists estimate that there may be 112,000 American shad in the river below the dam, but that population also appears to have declined over the past decade, said Karen Capossela, who participates in annual sampling of them for the state Department of Natural Resources.

The loss of the American shad's spawning areas, which on the Susquehanna reached all the way to Binghamton, N.Y., is believed to be a big factor in the fish's decline. Shad are anadromous, meaning they spend part of their lives in the salty Atlantic Ocean before swimming up coastal rivers to spawn in fresh water.

Conowingo, built in 1928, was the last of five dams on the river, just 10 miles from where it empties into the bay at Havre de Grace.

Many once thought the lift built on the eastern end of the dam would turn the fish's fading fortunes around. The largest of its kind in the nation when opened in 1991, it was hailed at the time as a "phenomenal achievement" and "a giant step forward" for reviving the Susquehanna's depleted populations of shad and related river herring. For the next 10 years, the numbers of shad passed upriver gradually grew, and the operators of the four upriver dams also built fishways to reopen long-closed spawning waters.

Biologists say it's not clear what's behind the slide over the past decade, but there are several suspects. Oceangoing trawlers fishing for herring or mackerel appear to be harvesting shad accidentally during the time they roam the Atlantic. Efforts are under way to get a better idea how big that "bycatch" is, said Kate Taylor, who coordinates shad fishery management for the Atlantic States Marine Fisheries Commission.

A few rivers — including, notably, the Potomac — have seen their shad population rebound and remain relatively strong. Taylor said researchers are trying to figure that out as well.

Many shad populations might be hurt by development and pollution in their river spawning areas, Taylor said. She said it has also been suggested that other species have benefited from the decline in American shad and are now keeping them from rebounding, competing with them for food or habitat. At Conowingo, 90 percent or more of the fish lifted are gizzard shad, a related species that doesn't roam as much.

Even so, Taylor said, dams and other blockages that remain on many rivers may be a factor in keeping the coastwide shad population at an all-time low. The Potomac and the Delaware, another river with a better shad run than the Susquehanna, are "much less dammed," Bryer said.

"There's no doubt that we need much better fish passage on the Susquehanna to restore the [river's] stock, and it all starts at Conowingo," Hendricks said.

In talks leading up to the negotiations, state and federal agencies have laid out a range of options for trying to improve fish passage, ranging from tweaking current operations at a cost of less than $1 million up to replacing both existing fish lifts at the dam, estimated to cost a combined $24 million. Besides the large lift on the east side of the dam, there's an older, small one on the west side primarily used to catch shad for use by Pennsylvania in stocking its hatchery, which some have suggested be upgraded to handle more eels.

By comparison, Exelon's annual budget for operating Conowingo and the nearby Muddy Run pumped storage hydroelectric facility is $17.5 million. The company spends about $160,000 a year operating the fish lift at Conowingo.

Some question whether bigger, better or more lifts can do an adequate job of reopening a dammed river. In a paper published this year in the journal Conservation Letters, seven biologists concluded that less than 3 percent of spawning American shad are able to get past all the dams blocking three East Coast rivers, including the Susquehanna.

It might be time to admit that fish can't be restored to a dammed river using lifts or ladders, they wrote, or even by augmenting the run with fish produced in hatcheries.

"The data speak for themselves," said co-author Karin Limburg, a fisheries scientist at the State University of New York in Syracuse who focuses on shad in the Hudson River, another population in decline. "People ought to face the music and remove some mainstem dams. … It really comes down to what society wants — does society want species to just slip away?"

No one's seriously proposing to remove Conowingo Dam, though, as its 572 megawatts of power-generating capacity help balance out electricity supply to the Mid-Atlantic grid, particularly when demand is at its peak. Moreover, the 14-mile-long "pond" of water backed up behind the dam furnishes cooling water for the Peach Bottom nuclear power plant, also operated by Exelon. And it's a backup source of drinking water for Baltimore, which has tapped it occasionally when severe droughts drained the metro area's reservoirs.

Conowingo's removal "would cause a number of impacts to other users," Bryer acknowledged. "If we can strike a better balance and get ... better environmental performance out of the lower river while getting electricity generation, I think that would be a solution many would be excited about."

tim.wheeler@baltsun.com