This spring's shad runs brought remarkably mixed results-and some new concerns-in the Bay's tributaries, with some showing increases in migrating shad, while others stayed the same or declined.

Returning American shad reversed a long-running decline on the Susquehanna River. But that good news was partially offset by worries on the Potomac, which in recent years has had the strongest shad spawning run of any river around the Bay, or the East Coast.

An annual Potomac River survey caught 11.3 adult fish per net this spring-the third worst in the survey's 15-year history and the second straight year of declines on the river. Last year's index was 23.6. The 15-year average is 22.3.

Until the last few years, spawning runs on the Potomac River were showing an increasing trend-making it one of the few rivers along the East Coast that was not declining. In 2004, the survey hit a high of 39 fish per net. Fish spawned by that huge migration should now be mature and returning to spawn, but that doesn't appear to be the case, said Jim Cummins, a biologist with the Interstate Commission on the Potomac River Basin, who oversees shad restoration efforts on the river.

"I'm concerned, but I'm not overly worried yet," Cummins said. "These last two springs have really been cool and this one had a temperature spike in the middle."

Cummins speculated a sudden hot spell in late April may have spurred a mass spawning run that was partially missed by the monitoring effort. "Next year we'll try again," he said, "and hopefully, we will have better returns."

Shad are an anadromous fish, which spend most of their lives in the ocean but return to their native freshwater rivers to spawn.

Because they historically swam hundreds of miles upstream during huge spring spawning migrations-colonial accounts are filled with stories of early spring shad runs saving settlers from starvation-shad are viewed as a species that links watershed areas far upstream with the Bay.

As a result, restoring shad populations has been a major Bay Program goal, triggering watershedwide efforts to stock shad and clear their migration routes. More than 1,300 miles of rivers have been reopened to migration through the construction of fish passages or the removal of dams and other migration barriers.

Despite those efforts, shad numbers in the Bay-and much of the East Coast-have lagged in recent years, leaving the Potomac as one of the few bright spots coastwide.

A problem there could mean trouble for other rivers. The Potomac is the cradle for shad restoration around the Chesapeake. Hatcheries that support stocking programs in Maryland rivers, on the Susquehanna in Pennsylvania, and the Rappahannock in Virginia, all rely on eggs from Potomac fish.

That shortage contributed to a poor stocking year on the Susquehanna, said Mike Hendricks, a fisheries biologist with the Pennsylvania Fish and Boat Commission. Although a final count was not available, he said the total would be far below the annual target of 10 million fry. "We didn't get the eggs we were expecting," he said.

Still, there was some good news on the river, which contained the largest shad spawning habitat on the East Coast until it was blocked by large hydroelectric dams a century ago.

This spring, 29,272 shad were lifted over the Conowingo Dam, about 12 miles upstream from the mouth of the river. That was far below a peak of 193,574 seen in 2001, but it was the best showing since 2006, and sharply above the 19,914 that were lifted over the 100-foot-high dam last year.

"The fish passage at Conowingo was better than I expected," Hendricks said, especially as stocking efforts on the river have been poor for a number of years, meaning there were fewer juvenile fish to return and spawn. "I expected we would have much less coming back," he said.

Bob Sadzinski, a Maryland Department of Natural Resources fisheries biologist, said surveys suggested that shad runs were also a bit stronger in Maryland, where adult numbers seemed about 50 percent higher than last year-roughly the same level of increase as the Susquehanna.

But the good numbers may not pay off in a greater number of young fish. Sadzinski said he was concerned that the timing of above-average rainfalls in late May and early June could affect their survival. "These high-flow events can decimate the young of year," he said.

Stocking was still taking place in late June in Maryland's Choptank and Patuxent rivers, and in the Marshy Hope tributary of the Nanticoke. But egg collection efforts were "fairly average," said Brian Richardson, who oversees the DNR's hatchery program.

On Delaware's portion of the Nanticoke, Mike Stangl, a fisheries biologist with the state's Division of Fish and Wildlife, said the shad run appeared to be about the same as last year. But he said the state stocked a total of 700,000 3-day-old shad in the Upper Nanticoke and two tributaries, Deep Creek and Broad Creek-its best success ever.

Unlike most other stocking efforts, which strip eggs from fish to rear in hatcheries, the Nanticoke stocking relies on tank spawning, in which adult fish are placed into a 15-foot tank where females release eggs that are fertilized by males.

"We give them a little head start by protecting them from predators through the egg stage and the first three days," Stangl said.

The resulting young are released into the river. This year's release was up from 574,000 last year and 231,000 in 2007.

In Virginia, about 3.8 million shad fry taken from Pamunkey River fish were stocked in the James, which was below the 5 million target for the river. On the Rappahannock, 2.7 million fry were stocked, using eggs from Potomac River fish. That was a bit below the 3 million target.

In 2007, the Atlantic States Marine Fisheries Commission, a panel of state and federal fishery managers responsible for managing migratory fish, completed a stock assessment that concluded that American shad stocks along the East Coast are "currently at all-time lows."

No one is certain why shad, which have been the target of large-scale stocking efforts, fish passage projects and a host of fishing regulations along the coast, have not rebounded.

Shad have been subject to increasingly stringent coastwide management since the early 1990s. Their numbers increased in many places until the beginning of this decade, and have since declined almost everywhere along the East Coast.

The cause of the decline is unknown, although increased predation by striped bass and other fish, and the potential loss of shad in the bycatch of fisheries that take place in federal waters-those more than 3 miles offshore-are often mentioned as possible causes.

The ASMFC will consider potential new regulations for American shad when it meets in August.