Ending months of input and speculation, the U.S. Fish and Wildlife Service on Tuesday added to its list of threatened species a small shorebird called the red knot, whose round-trip migration of nearly 20,000 miles includes stops on Delaware Bay every spring to refuel on horseshoe crab eggs.

The bird's dramatic decline has been blamed by many on aggressive harvesting of the crabs, which are used as bait in other fisheries. However, the red knot is the first bird whose listing identifies climate change as a principal threat to its survival, said Dan Ashe, director of the Fish and Wildlife Service.

Noting that more than half of all shorebird species in the United States are also in decline, Ashe said the designation sends "a clear message about the threats that species like the red knot are facing from the effects of a changing global climate system."

Those threats include sea-level rise that is eroding red knot habitat, and temperature changes that throw the bird out of sync with food resources.

The red knot's grueling migration takes it to Delaware Bay just as horseshoe crabs are coming ashore to spawn, and the Arctic as summer's bloom of insects is emerging.

Conservation groups praised the decision. It is expected to raise awareness about the need for conservation efforts and enhance research programs, according to the Fish and Wildlife Service. Eventually, a recovery plan identifying specific ways to help the bird will be developed.

"It creates protection by law, which is the greatest thing that we could hope for," said Larry Niles, a New Jersey biologist who in the 1990s was among the first to notice the shrinking population of birds visiting Delaware Bay.

As head of the Endangered and Nongame Species Program for the New Jersey Department of Environmental Protection, Niles initiated studies of the red knot throughout its primary range, not only on the bay but also on its breeding grounds in the Arctic and its wintering grounds in Tierra del Fuego at South America's tip.

Ashe and other officials said they did not expect big changes on the Delaware Bay as a result of the decision, but that more funding might be available to continue conservation efforts. Perhaps chief among them, especially after Hurricane Sandy, is beach replenishment.

The beaches that provide good habitat for the crabs are in or near bayshore communities that received minimal post-Sandy restoration funds. But last spring, money to restore crab-spawning beaches washed away by Sandy came from many sources - benefiting not only the crabs and the knots, but also other shorebirds and, ultimately, humans, Niles said.

"You end up with your beaches being used by birds and crabs for one month a year," he said, "and by people for another 11 months."

David Mizrahi, New Jersey Audubon's vice president for research, said the listing "puts all the pieces in place" so that conservation can proceed "without the kinds of battles that were required in the past."

Since the 1980s, the red knot population has fallen by about 75 percent in key areas, including Delaware Bay and Tierra del Fuego, the service said.

In 2004, as the crab harvest came under scrutiny, the Atlantic States Marine Fisheries Commission, a regulatory agency, placed limits on the Delaware Bay crab harvest - the first time a fishery was restricted out of consideration for a bird, officials said.

The commission recently adopted 2015 levels that were the same as 2014's. However, states can institute more restrictive measures, and New Jersey has a harvest moratorium.

Mike Litchko, a Cape May County commercial fisherman who used to harvest the crabs, contended in an interview that the data are flawed and disputed "the whole concept that fishermen have been pulling the food from the plate for these birds."

Ashe said he did not necessarily expect new limits on the crab harvest, or on the use of crabs for biomedical purposes. Their blood contains a substance that can indicate contamination of medical equipment. Most bled crabs are returned to the bay alive, but an estimated 10 percent to 30 percent die anyway.

Mizrahi said that harvest limits might be reconsidered. "A lot of that remains to be determined," he said, adding that both practices would "come into more focus as a recovery plan is developed."

In 2005, several conservation groups petitioned the Fish and Wildlife Service, asking for an emergency listing of the red knot. It was denied. But in 2006, the bird was placed on a "candidate" list - indicating that it warranted protection, though there were other higher priorities. In 2013, the service proposed listing the bird.

In a public comment period that followed, the service received more than 17,400 responses. Many were "supportive form letters," the service said. Others raised questions about the adequacy of horseshoe crab management, the impacts of wind turbines, and other issues.

The decision announced Tuesday will take effect 30 days after it is published in the Federal Register, expected to be Thursday.

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