WELLFLEET — Every spring, horseshoe crabs arrive on the sheltered beaches of Cape Cod, crawling up onto the shore to lay their eggs in the sand. For years, during this spawning season, they have been targeted by “hand harvesting” fishermen, who collect the arthropods. Some are taken as bait and others for their blood, which is used as a biomedical resource. This month, new Marine Fisheries Advisory Commission regulations have made taking horseshoe crabs off-limits for a period each spring.

The new regulations ban horseshoe crab collection between April 15 and June 7, which corresponds to the spawning season. This year, however, the new rules are set to go into effect on April 26. With an additional lunar restriction based on older rules still in effect from April 21 to 25, the ban essentially begins this Sunday, the 21st.

According to Mark Faherty, science coordinator at Mass Audubon’s Wellfleet Bay Wildlife Sanctuary, the new restrictions put Massachusetts “somewhere near the middle of the pack” in protecting horseshoe crabs. Other states including New Jersey and Delaware go much further in safeguarding these animals, he said.

Historically, horseshoe crabs were abundant on Cape Cod. But two factors, poorly regulated fishing and habitat degradation, have caused their population to plummet. Today, Monomoy National Wildlife Refuge off Chatham is the only place they still spawn in numbers on Cape Cod, said Faherty.

The lack of regulation reflects the fact that horseshoe crabs were once considered a pest species. Today, scientists and conservationists view them as an important part of the ecosystem of the Atlantic shore. Their eggs in particular are a source of food for migratory shorebirds, including the federally threatened rufa subspecies of red knots.

The crabs are also an attractive food source for a valuable marine snail called the whelk. And the whelk fishery’s harvest of horseshoe crabs for bait is the primary source of horseshoe crab mortality.

Their blue blood is used to test for the presence of bacterial toxins in vaccines and other products, and the process of “milking” these crabs kills 15 to 30 percent of them. Mortality in the bait harvest is 100 percent, Faherty noted.

Before this spring, the only restrictions on harvesting crabs during spawning season were on the five days around the new and full Moons, when horseshoe crabs spawn in the largest numbers. But the crabs can spawn on any day during their spring breeding season. Under the previous rules, fishermen “could just hammer them,” Faherty said, during the remaining days of the Moon’s cycle.

With spawning season providing easy harvesting conditions, fishermen ended up taking the crabs that are most important to the animals’ population: egg-laying females. “That’s not sustainable management,” said Faherty. “That’s an eradication program.”

The horseshoe crab decline has had a big effect on the shorebirds that rely on them for food, with red knots, for instance, declining by 75 to 94 percent since the 1980s.

Previous attempts to institute a spawning season ban in Massachusetts had failed, most recently in 2023, when the commission rejected a similar ban. This time, however, the board seemed to agree with environmental advocates, voting 6 to 1 with one abstention to institute the ban.

During the advisory commission’s deliberations on the new rules, more than 2,600 people submitted written comments in support of the change, according to Mass Audubon. “People love horseshoe crabs,” Faherty said. “It’s not the olden days when people would kill them.”

Faherty said he hopes the new rules will encourage whelk fishermen to move on from using horseshoe crabs as bait and eventually end the hand harvest in Cape Cod Bay.

Shelley Edmundson, a marine biologist and executive director of the Martha’s Vineyard Fishermen’s Preservation Trust, is researching alternate baits for the snail. An abundant mollusk called the propeller clam and a mixture of the clam’s guts with the invasive green crab are being tested for that purpose, and preliminary results show the latter may be just as effective as horseshoe crabs are at attracting whelk, the Vineyard Gazette reported last August.

Faherty said he thinks some results of the new rules will be noticeable right away, with more crabs surviving the spawning season. Because horseshoe crabs take a decade to mature, however, it will be 10 years before this generation of now-protected juveniles returns to shore to lay eggs.

Another sign of success Faherty expects to see is an increase in the numbers of red knots, ruddy turnstones, and other shorebirds that eat horseshoe crab eggs in Cape Cod Bay in the spring. He said that at the Monomoy Refuge, where there is no horseshoe crab harvest permitted, hundreds of knots still gather every spring, so there is reason to suspect a similar trend could occur in Cape Cod Bay.