By Jon Hurdle

TRENTON, New Jersey, March 25 (Reuters) - New Jersey acted to save a rare shore bird on Tuesday, banning the harvest of horseshoe crabs whose eggs are an essential nutrient for the red knot on its 10,000-mile (16,000-km) annual migration.

New Jersey Gov. Jon Corzine signed a law that bans the horseshoe crab harvest until both the crab and red knot populations have returned to sustainable levels, as determined by the U.S. Fish and Wildlife Service.

The red knot stops on the New Jersey and Delaware beaches of the Delaware Bay each spring on its migration from the southern tip of South America to breeding grounds in Arctic Canada, one of the longest migrations in the avian world.

The bird's population has plummeted to around 15,000, well below the level of about 100,000 ornithologists believe is necessary for the species to sustain itself.

Many birds have recently been unable to find food because the crab population has been decimated since the early 1990s by fishermen, who use the crustaceans as bait.

At an average of 10 inches (25 cm) long and weighing about 4.7 ounces (135 grams), the birds will gorge themselves on the eggs, replenishing their fat reserves to complete their epic migration.

Some birds starve while others fail to reach the Arctic, or if they do, are too exhausted to breed, scientists say.

Dr. Larry Niles, a biologist who has worked on the red knot since the 1980s, said the law is the most important step yet to protect the bird, although it is still vulnerable to other factors beyond New Jersey's control, such as bad weather in its Arctic breeding grounds.

Niles said he expects the population to "skim along the bottom" for another two or three years but then show signs of recovery, thanks to the new law.

"If things go wrong in other places, at least we know we did the best that we could do," Niles said.

(Editing by Daniel Trotta and Sandra Maler)

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