American shad swarmed back to the Susquehanna River this spring, eclipsing last year’s record spawning run by mid-May, with several weeks still to go in the run.

As of May 19, more than 184,000 shad had been passed over the Conowingo Dam, the first obstacle encountered by migratory fish. In 2000, the previous best year on record, 153,546 fish were carried over the 100-foot-high dam by a mechanical “fish lift.”

The three upstream dams had all surpassed last year’s mark as well. Typically, the spawning run continues into early June.

Shad are anadromous fish that spend most of their lives migrating along the Atlantic Coast but return to their native river to spawn. The shad population in the Bay and along much of the East Coast has been severely depressed for decades, a victim of pollution, overfishing and the closure of spawning grounds by dams and other barriers.

Shad fishing has been banned throughout the Chesapeake. Rebuilding the population has been a major goal of the Bay Program, with the Bay states stocking tens of million of young shad each year while also building fish passages to reopen historic spawning areas.

Historically, the Susquehanna River was the largest East Coast spawning grounds for the shad, but most of it has been closed for nearly a century by the four large hydroelectric dams located within the first 70 miles of the river.

In the past decade, the companies that own the dams have spent nearly $60 million to build passages. Fish lifts — mechanical elevators — were built at Conowingo, Holtwood and Safe Harbor, the first three dams encountered by migrating fish. And last year, a fish ladder was completed at the smaller York Haven dam, the last structure encountered by fish in the lower river.

As of May 19, about 94,000 shad had been lifted over Holtwood, nearly 74,000 were passed over Safe Harbor and more than 13,000 had swam through the fish ladder at York Haven. Those numbers surpassed last year’s totals for each facility.