Wildlife biologists are keeping a watchful eye on some recent Bay arrivals to see whether they turn out to bad neighbors — or a nice addition to the neighborhood.

The recent arrival is the double-crested cormorant, a fish-eating bird not found in the Bay until 1989, when it began nesting in Virginia. It began breeding in Maryland in 1991.

Since then, its numbers have boomed. About 1,000 pairs of the fish eaters were nesting in Maryland last summer, mostly in three large colonies. Virginia officials believe they had a similar number on the Bay and its tributaries.

The dark, streamlined birds are expert divers. Their webbed feet routinely propel them to depths of 8 to 20 feet — sometimes even farther — to make their catch. They can stay submerged up to 30 seconds, and their eyes provide excellent vision in and out of water.

It’s easy to cast them as a nice addition,” said David Brinker, a biologist with the Maryland Department of Natural Resources. “Of course, if you’re a pound-net fisherman, you might not feel the same way.”

Each bird can consume a pound or more of fish a day.

Being top predators, their populations were devastated by DDT and other contaminants in the 1960s and ’70s as adults, which can live up to 20 years, accumulated toxics from their prey over time. The chemicals caused reproductive failure; chicks that did hatch sometimes had crossed bills, club feet and other deformities.

In 1972, the birds was added to the list of species protected under the 1918 Migratory Bird Treaty Act for protection.

Since then, and with the ban on DDT, cormorant populations have rebounded dramatically. In the Great Lakes, where the birds had been hardest hit, the number of nesting pairs leaped from 89 in 1970 to 38,000 in 1991.

Nationwide, the number of breeding pairs has grown at an average rate of 6.1 percent a year from 1966-1994, reaching about 372,000 according to the U.S. Fish & Wildlife Service. It estimates the total number of cormorants — breeders and non-breeders — to be between 1 million and 2 million.

Its resurgence, though, has posed problems for aquaculture activities, which provide easy feeding grounds, in some regions. And in some areas such as the Great Lakes, cormorants have been increasingly villainized by sport anglers who complain that the birds are taking more than their share of fish.

Angry fishermen have at times taken things into their own hands. In 1998, nine men carried shotguns onto a 52-acre island used exclusively as a bird reserve in eastern Lake Ontario, and blasted more than 800 cormorants. The island had more than 8,000 active nests.

Because of rising concerns about cormorants, the USF&WS last November announced that it would develop a comprehensive national cormorant management plan. The plan could offer states and others more management options in problem situations, such as large-scale population control measures on breeding grounds, wintering grounds or migration areas in the United States.

Regional managers view the USF&WS action as a good move. Though cormorants are not a problem in the Bay for now, the new plan could give them flexibility to act if the situation changes in the future.

“Cormorants are an acceptable thing in the environment, but we don’t want them to get out of hand,” Brinker said.

It’s unclear whether cormorants were found in the Bay region before recent years. There is no record of them in the Bay for the past 100-150 years, Brinker said. Nor do colonial records mention the bird around the Chesapeake, but Brinker said it is possible colonists could have called it by another name.

Being newcomers, no one knows exactly what their impact could be.

During migrations, large numbers now pause around the Bay. Hundreds of thousands have been reported stopping at Fishermans Island National Wildlife Refuge near the mouth of the Chesapeake to eat fish before continuing their journey.

Often, the birds will try to grab fish trapped in fishing nets, only to become caught themselves and drown. In Virginia, there are several reports each year of beaches being fouled by hundreds of dead cormorants during migration season, said Donald Schwab, a wildlife diversity biologist with the Virginia Department of Game and Inland Fisheries.

Exactly how many fish cormorants eat has been a subject of increasing study — and could add a new wrinkle to the meaning of “multispecies management” when it comes to fisheries. “Usually, fishery managers don’t take that into account when they are allocating fish stocks and so on,” said Doug Forsell, a biologist with the USF&WS Chesapeake Bay Field Office.

Overall, though, studies worldwide generally have shown that cormorants take less than 10 percent of either the sport fish or forage fish that are available, and usually far less. But concerns stemming from the recent cormorant rebound in the Great Lakes have spurred a new spate of studies.

Brinker, though, said the number of cormorants needed to impact fish populations in the Bay would be “phenomenal.” In the Bay, the Maryland DNR has determined that cormorants eat mostly hogchokers and oyster toadfish.

A bigger concern may be their impact on other birds. Their large colonies could crowd out others using similar habitats, such as black ducks, egrets and herons. Those birds often seek out islands which offer habitat free of human disturbance where they can thrive. “If I’m watching for anything with cormorants, it’s competition with other colonial waterbirds,” Brinker said.

Not only can the sheer number of cormorants push other birds out, their wastes can denude an island, driving other birds off. Especially vulnerable are species like black ducks, which nest on the ground.

Vernon Stotts, a retired biologist from the former Maryland Game and Inland Fish Commission, believes cormorants were responsible for driving egrets and black ducks from one of the small remnants of Poplar Island several years ago.

“The cormorants began to nest and they just covered everything with feces and killed all the vegetation below,” he said. “The black ducks didn’t have any cover anymore and the egrets couldn’t stand all the guano that was falling all around them.”

That’s the sort of impact biologists want to watch for in the future — especially as major efforts are now under way to restore island habitats such as Poplar Island, which is being rebuilt with material dredged from shipping channels — at a cost of tens of millions of dollars.

Biologists don’t mind cormorants sharing such habitats — but they don’t want to see them take over. “We wouldn’t want them to take over habitat designed for other species,” Brinker said.

But Brinker doesn’t believe the relatively small cormorant population around the Bay has become a problem. And he cautioned against taking action too soon.

“I wouldn’t say they are displacing anything yet,” he said. “They have displaced species in other situations, and all we can do is sort of wait and see. At this low of a population, if you start reacting now, you risk driving cormorants from the Bay, and I don’t view that as a good alternative, either. I’d like to see some breeding here.”