A group of scientists say it is time to admit that efforts to restore migratory fish on the Bay's largest tributary have failed, and time to consider a solution that many believe is unthinkable: Remove the Conowingo Dam.

To be sure, that's not likely to happen anytime soon. Negotiations are under way on a new license that would allow the dam's continued operation for as long as 46 years.

Nonetheless, the authors of a recent paper in the journal Conservation Letters say a serious debate about the future of the Conowingo and other dams on the Susquehanna is needed if shad and other migratory fish are to be returned to what was historically their largest spawning habitat on the East Coast.

The paper, which also examined passages on the Connecticut and Merrimack rivers in the Northeast, found little evidence that existing fish passages could get significant numbers of fish upstream on rivers with multiple dams blocking the mainstem.

"It may be time to admit failure of fish passage and hatchery-based restoration programs and acknowledge that significant diadromous species restoration is not possible without dam removals," they wrote. Diadromous fish are species such as sturgeon, striped bass, shad, river herring and eels, which make long distance migrations between the salty ocean and fresh coastal rivers.

Their conclusions are disputed by biologists working to get shad past Conowingo and three other hydroelectric dams located on the first 55 miles of the Susquehanna.

"I think they want to pull the trigger a little too soon," said Mike Hendricks, a fisheries biologist with the Pennsylvania Fish and Boat Commission. "Things haven't gone as well as we had hoped, obviously. But we would like to give it a little more of a shot than they would."

He and other biologists increasingly recognize that fish passages are typically a poor substitute for an open river. In the Bay watershed, dam removal has emerged as the preferred method to reopen rivers to fish migration. The Bay Program recently reported that 148 miles of river were opened to migratory fish in 2011, mostly through the removal of dams.

But taking out the 100-foot-high Conowingo, the largest dam in the Bay watershed, has never been seriously considered.

"The reality is, in the Susquehanna those dams are not going to go away," said Steve Minkkinen, who heads the U.S. Fish and Wildlife Service Maryland Fisheries Resource Office, and has been involved with fish passage issues on the Susquehanna. "Do you give up on shad?"

"If we don't keep trying to develop new and better technologies, we are never going to get there, and we are never going to get better passage," he added.

The Susquehanna once hosted phenomenal spring runs of shad and river herring — anadromous species that spend most of their lives in the ocean but return to their native rivers to spawn. Stories from colonial times tell of settlers at Cooperstown, NY — 444 miles upstream from the Bay — being saved from starvation by the arrival of herring after a difficult winter.

But a series of large hydroelectric dams closed the river to such migrations. York Haven, completed in 1904, was built 55 miles upstream from the Bay. Holtwood Dam, completed in 1910, closed the river at the 24-mile mark. Then, in 1928, the Conowingo Dam closed the river just 10 miles from its mouth. (Safe Harbor, located between Holtwood and York Haven, was constructed later, in 1931).

Utilities that own the dams spent tens of millions of dollars in the 1990s building fish elevators to lift migrating shad and herring over the first three dams, while a fish ladder at York Haven — the smallest of the dams — opened in 2000.

Biologists had hoped those passages would eventually allow 2 million shad a year to get upstream. Instead, fewer than 3 percent of the shad lifted over Conowingo have made it past all the dams. In 2001 — the best year — 16,200 shad made it past all four. Fewer than 1,000 a year have made it that far since 2006, and none made it in 2011.

Ironically, more got upstream when the utilities paid to have the shad at Conowingo captured and then trucked around the dams. In 1995 alone, 56,674 shad were moved by truck.

John Waldman, a biology professor with Queens College of the City University of New York, and a co-author of the paper, called it "profoundly saddening" that the most successful movement of fish on the river in recent decades was powered by an internal combustion engine.

He cited one historical account of 15 million shad and herring being caught in a single net on the river prior to dam construction, requiring a "constant parade" of wagons and several days to empty. "Compare that with three-digit figures getting upstream these days," Waldman said.

Shad and herring also fare poorly getting past multiple dams on the Connecticut and Merrimack, the paper said. But Waldman said the location of the Conowingo Dam — just 10 miles upstream from the river mouth — essentially closed the entire drainage to fish. Dams on the Merrimack and Connecticut are farther upstream. "We have hurt the Susquehanna more than any other river on the East Coast," he said.

Even if the Conowingo were removed, fish would still have to deal with the others. Biologists say there is little usable spawning habitat until shad get past at least the first thee dams.

Fish not only have to get past the dams, they need to do so rapidly. Spring migration runs are triggered by temperature and other climatic cues. Fish have a limited amount of time to reach suitable spawning habitats once they begin heading upstream. When they encounter fish passages, migration is delayed as fish wait for elevators to lift them over each of the first three dams, then climb a ladder at the fourth.

"You have a better opportunity to restore fish with only one or two dams, that's for sure," Hendricks said.

But he said some passages can be effective. Studies have shown that once fish that get over Holtwood they can cover the seven miles to Safe Harbor and be lifted upstream in less than 24 hours. Safe Harbor is the best performing fish passage on the river, routinely passing more than 70 percent of the shad lifted over Holtwood.

"We look at Safe Harbor as an example of how good things can be," Hendricks said. "It's a matter of figuring out why Safe Harbor works so well, and trying to duplicate that at other projects."

When dam operators were originally required to build passages, Hendricks said, it was assumed that elevators and ladders would be effective when completed. As a result, the operating licenses — which typically last for decades — contained no requirements that poorly performing passages be improved.

Hendricks said biologists want to include performance standards for passages in dam operating licenses as they come up for renewal, including the one to be negotiated for Conowingo this year. They want dam operators to monitor performance, make tweaks, and potentially major modifications to passages, if standards are not met. "This is definitely not a build-it-and-walk-away situation this time around," Hendricks said.

Excelon, which owns Conowingo, declined to comment for this story.

The paper's lead author, J. Jed Brown, of the Mashdar Institute of Science and Technology in Abu Dhabi, who has previously worked with fisheries in the mid-Atlantic, said it was possible that improvements at the passages could get more fish up the river. But, he cautioned, when there are multiple dams involved, the number of fish that make it upstream will always be limited by the poorest performing passage.

"For the improvements to be successful and benefit the Susquehanna, all four dams need to be efficient in passing fish," Brown said. "Three good fishways and one bad one will still be a problem."

Further, he noted, the passages can get only some fish upstream. They are not designed for others, like sturgeon, striped bass or eels.

He and the other authors acknowledged there are social, economic and political issues with removing large dams. Hydropower is a renewable energy, dam removals are expensive, and sediments stored behind dams have to be dealt with — something that would be a particular problem at Conowingo, where sediment storage capacity has nearly been reached.

But the authors said that large dams once considered unlikely to be removed are being taken out in other places, particularly on the West Coast.

Innovative solutions have also been found on the East Coast to help fish and meet green energy goals, the article reported. On the Penobscot River in Maine, for instance, two dams were purchased and are being removed while a bypass for fish is being built around the third. The dam operator was given the opportunity to increase generation at other existing dams to make up for most of the lost production.

"I'm not blanket against either hydropower or fish passageways per se, but the point of the paper was to wake people up and say that this is an issue that needs to be looked at," said Karin Limburg, a co-author of the paper and a biology professor at the State University of New York College of Environmental Science and Forestry in Syracuse.

She noted that populations of many species that historically migrated up East Coast rivers are in dire shape, at least partially because dams block access to large portions of their historic habitat. Atlantic sturgeon, shortnose sturgeon and Atlantic salmon are listed as endangered species, while river herring and American eels are under review for such listing. American shad numbers are at an all-time low.

"It's fallen out of people's memory that these were once live, functioning rivers," Limburg said. "We advocate a position of bringing these rivers back, but it will probably mean some clever negotiations to remove some of the dams."