For the first time, fishery regulators have established a cap on the amount of American shad and river herring that can be caught as part of the accidental bycatch in an ocean fishery.

The Mid-Atlantic Fishery Management Council in June approved a 236-metric ton limit on the amount of alosine species — American shad, hickory shad, blueback herring and alewife — that can be caught by trawl netters fishing for mackerel.

The council regulates fisheries that operate in federal waters, those more than three miles offshore, from North Carolina to New York.

Populations of American shad and river herring (blueback herring and alewife) are near all-time lows along the East Coast, and anecdotal evidence has suggested that large numbers are being killed by offshore fisheries targeting other species, such as mackerel.

Some estimates suggest the bycatch in federal waters actually exceeded the numbers caught in state waters — those within three miles of the coast and regulated by Atlantic States Marine Fisheries Commission. But the bycatch data is poor, and last year, the Mid-Atlantic Council took action to improve monitoring of offshore fisheries to get better information on the size of the bycatch.

Based on the bycatch estimates they do have, the council said the cap, which will take effect next year, would have forced an early closure of the mackerel fishery in two of the six years between 2005 and 2010, according to the council.

"This level should allow fishermen, who are likely in the best position to figure out how to avoid river herring and shad, to catch the mackerel quota if they can achieve a relatively low catch rate of river herrings and shads," said Jason Didden, fisheries management specialist for the council. Meanwhile, the New England Fisheries Council, which regulates catches in federal waters farther north, is also looking at measures to control shad and river herring catches.

In the Bay, a moratorium on shad fishing has been in effect since 1993 in Virginia and 1980 in Maryland. The ASMFC closed most remaining shad and river herring fisheries in state waters along the East Coast beginning this year.

"The council's action is a badly needed complement to the extraordinary measures the states are taking to restore river herring and shad and a good first step in getting at-sea mortality under control," said Pam Lyons Gromen, executive director of Wild Oceans and an adviser to the ASMFC on river herring and shad management.

"We owe it to the fishermen, businesses and local communities that once thrived, economically and culturally, on healthy river runs as well as to the health of the coastal ecosystem (the shad and herring) are such an important part of," Gromen said.

American shad and river herring are anadromous species, meaning they live most of their lives at sea along the coast, but return to their native freshwater rivers to spawn. They were historically among the most abundant species found in the Bay and other coastal rivers.

Spawning runs of shad in many Bay tributaries once numbered in the tens of millions, while runs of river herring may have numbered in the billions in some rivers. For much of the 20th century, American shad were the most valuable commercial fishery in the Bay. But a variety of factors, including overfishing, habitat degradation, pollution and construction of dams that blocked access to historic spawning areas have combined to send populations into a downward spiral.

River herring are being considered for listing under the federal Endangered Species Act, and this year only 12,733 American shad were counted at the Conowingo Dam on the Susquehanna River, the lowest number ever observed.