With several weeks remaining in the migration season, the number of American shad captured at the Conowingo Dam on the Susquehanna River through May was nearly twice last year's record mark - the latest sign that the Bay's shad are enjoying their best spawning run in years.

"I've been working the past week just keeping track of all the records we are breaking," William Peirson, an environmental specialist for Conowingo Dam owner PECO Energy, told Alliance for the Chesapeake Bay members in mid-May. "This is really overwhelming."

The good news was not confined to the Susquehanna. Virginia officials completed their most successful year stocking shad on the James River and other Bay tributaries. A new stocking program began on the Potomac River this year. And in July, the Maryland Department of Natural Resources' annual estimate of the upper Bay shad population is expected to show record numbers as well.

But the rebound on the Susquehanna, which once was the largest shad spawning ground on the East Coast, is particularly encouraging because of the huge investment being made there. Utilities that own four large hydroelectric dams are spending tens of millions of dollars to construct large "fish lifts" that will open hundreds of miles of spawning habitat for shad.

By the end of May, 58,500 shad had been counted at Conowingo, the southernmost dam on the river. With several weeks to go, that far surpassed last year's record of 32,330.

"I don't know where they're all coming from," said Richard St. Pierre, Susquehanna River Coordinator for the U.S. Fish and Wildlife Service. "We kind of guessed this year that the run might peter out early because it started so well, but it's still going."

By late May, about 1,000 fish a day were still being collected at the dam. "It wasn't that long ago that 1,000 fish for a season was really good," St. Pierre said.

The strong shad rebound comes two years after a sudden - and still unexplained - drop in the shad spawning population, both in the Bay and along most of the East Coast. The strong spawning run this year appears to indicate that the one-year setback did not significantly affect what had generally been an improving trend.

Shad are an anadromous fish that spend most of their lives migrating along the Atlantic Coast but return to their native freshwater rivers to spawn. For many decades, shad was the most valuable fish harvested in Maryland and Virginia.

But catches declined dramatically in the 1970s. Overfishing, pollution and the construction of dams that blocked historic spawning habitat have all been blamed for playing a role in the shad decline.

To help the populations recover, Maryland closed the Bay to shad fishing in 1980, and Virginia closed its portion of the Bay in 1993. In addition, efforts have been made to improve water quality in spawning areas, build fish passages and bolster the stock with millions of hatchery-reared fish.

Efforts are under way in all the Bay states to reopen rivers to spawning shad. A fish passage at Bosher's Dam in Richmond, the last major blockage to fish migration on the James River, is to be completed this fall. When it opens, more than 130 miles of additional spawning habitat will be available, allowing fish to migrate all the way to Lynchburg.

On the Potomac River, plans developed by the Army Corps of Engineers call for the completion of a fish passage at the Little Falls Dam, located just upstream from Washington, D.C., by 1997. That will open 10 miles of spawning habitat between the dam, constructed in 1959, and the Great Falls - a stretch of river that historically was an important nursery area for young shad. Great Falls is the natural boundary to fish migration on the Potomac.

The biggest hurdle is on the Susquehanna River, where four major dams in the first 60 miles of the river have closed huge amounts of spawning habitat to migrating fish since the beginning of the century. Before the dams, fish swam as far north as New York.

PECO Energy, owner of the Conowingo Dam, in 1991 completed the construction of a $12 million fish lift designed to carry migrating shad and river herring over the 100-foot-high obstruction.

The construction of fish lifts has begun at the next two upstream dams: Holtwood, owned by Pennsylvania Power & Light Co., and Safe Harbor, owned by Baltimore Gas and Electric and PP&L. Both are expected to be open in 1997. A fish lift at the fourth dam, York Haven, owned by Metropolitan Edison, is to open by 2000. Altogether, the utilities are expected to spend more than $30 million on lifts at the three dams.

Until the lifts are completed, fish captured at the Conowingo fish lift are being trucked north of the other dams and then released so they can spawn.

In addition to the returning fish, the spawn will be bolstered by better-than-normal production at the Pennsylvania Fish and Boat Commission's hatchery on the Juniata River, a tributary of the Susquehanna. St. Pierre said this year's production of hatchery-reared shad could top 10 million - a level not seen in years.

In Virginia, meanwhile, "we had a bumper run as well," said Tom Gunter, American shad restoration coordinator for the Department of Game and Inland Fisheries.

Commercial fishermen hired by the Virginia Marine Resources Commission caught more than 2,100 shad, which yielded 17.4 million eggs. Those eggs were taken to hatcheries operated by the state and the U.S. Fish and Wildlife Service, which produced 7.4 million shad that were stocked in Virginia rivers. That was a dramatic increase from the 2.1 million stocked last year.

"We probably could have gone as high as 10 million fry [young shad] this year, but unfortunately we didn't have the hatchery space to handle all the eggs that we were taking," Gunter said. "At one point, we actually had to shut down the egg-taking process because there was no hatchery room right at the peak of the spawn."

Gunter said the future of the program is uncertain, though, because there are few state funds available for shad restoration or hatchery operations after this year. Department of Game and Inland Fisheries biologists have estimated that the shad stocking program, now 2 years old, should operate for 10 years to restore a viable shad population within the James River basin.

The first shad stocking program on the Potomac River began in late May. In an effort headed by the Interstate Commission on the Potomac River Basin, about 500,000 juvenile shad will be released above the Little Falls Dam.

Maryland's efforts to stock shad are continuing this year as well, and the state DNR expects that a total of about 3 million shad, cultured from native fish, will be put in the Patuxent and Nanticoke rivers and the upper Bay. "We hope we're going to boost the restoration effort, at least in the Nanticoke and the Patuxent," said Pete Jensen, director of the DNR's Fisheries Division.

Hatchery-reared fish are stocked in rivers where fisheries managers want to build up populations. For reasons not fully understood, the juvenile fish will become "imprinted" with that river and will return to spawn when they are 3 to 5 years old. As those offspring return and reproduce, officials hope the number of "wild" fish that return will increase and surpass the number of hatchery-reared shad.

St. Pierre said a major reason for this year's shad run was the weather - low flows and cool temperatures created ideal conditions for the spawning fish. But the numbers reflect a continued - and gradually accelerating - increase in the number of shad which has gone on for nearly a decade except for a one-year decline in 1993.

"We had a few hundred fish in the early '80s; they produced a few thousand fish in the mid '80s which have produced a few tens of thousands of fish in the early 1990s," St. Pierre said. "So we're seeing that progression of growth. If you take that to the next step, we should be seeing a few hundreds of thousands of fish in the next couple of generations."

Another measure of the stock's health, the number of shad in the upper Bay, will not be available until July, but Jensen said that "it's clear it's going to be a record year."

Last year, the Maryland DNR survey counted 129,000 shad - a 2.5-fold increase over the 1993 number, though still short of the 141,000 counted in 1991. The Maryland count is significant because it can trigger a reopening of a catch-and-release recreational shad fishery when the population reaches 500,000. "We could easily pass the 200,000 to 300,000 mark in the next couple of years," Jensen said.

When Maryland imposed a moratorium on shad fishing in its part of the Bay in 1980, the population was less than 10,000.