BOSTON – About 30 miles off the coast of Cape Cod is a rare and beautiful spectacle, according to marine scientists: a herd of more than 50 endangered North Atlantic right whales feasting on a bounty of their favorite crustaceans.

Researchers say it is the most tightly clustered group they have spotted from aerial survey flights over the past decade, made all the more remarkable because fewer than 350 such whales are known to exist. And all the more worrisome, they say, because the whales are feeding in the path of oil tankers and cruise vessels using one of the East Coast’s busiest shipping lanes.

The National Marine Fisheries Service last week asked mariners to keep a “sharp lookout” and use caution in the federally designated critical habitat site, known as the Great South Channel. The agency also requested that fishermen remove or refrain from setting their gear in the area this month.

In the meantime, however, scientists and researchers say the whales’ precarious position – and the report of a dead calf off the Massachusetts coast last week – has added to the urgency of efforts in the United States and Canada to reduce the species’ often fatal encounters with humans.

“No ship captain wants to kill a whale, and we’re trying our best to get information to them,” said Amy Knowlton, a research scientist at the New England Aquarium, one of the leading research centers on northern right whales.

According to the Center for Coastal Studies in Provincetown, Mass., most of the right whale population is thought to feed on different banks in southeast Canada during summer and fall. In the late winter and early spring, calving females appear off the southeastern United States, while noncalving animals feed off the Massachusetts coast.

The hunting of right whales has been prohibited by international agreement for more than half a century, and they have been listed as an endangered species since 1970. Yet the whales’ survival remains uncertain after years of population decline caused by collisions with ships and their gear, and a lower than expected birth rate, among other factors, said Center for Coastal Studies senior scientist Charles Mayo.

The whales are vulnerable because they move slowly and spend extended periods of time at or near the surface, oblivious or slow to respond to approaching vessels. Calves have particularly limited diving ability.

Last summer, a right whale known as Churchill captured attention as rescuers unsuccessfully tried to free it from fishing gear caught in its upper jaw. He is thought to have died in September from an infection caused by the entanglement.

Ship collisions also have caused right whale deaths from massive wounds, including fractured skulls, severed tails and large propeller slashes, according to marine experts. Eighteen of the 53 known right whale fatalities from 1970 to 2001 have been attributed to ship collisions, scientists said.

Expecting to reduce collisions by as much as 80 percent, Canadian officials this year proposed shifting commercial shipping lanes in the Bay of Fundy, between Nova Scotia and New Brunswick, where three northern right whales are known to have been killed in ship collisions during the 1990s. Transport Canada, the government agency that regulates shipping, submitted a proposal in April to the International Maritime Organization to move the lanes east of a conservation area that is a primary whale feeding ground.

In the United States, federal officials have regulated the control and use of fishing gear and closed certain fishing grounds, with this year’s closures among the most controversial. They also launched a mandatory ship reporting system that requires captains of large commercial vessels entering right whale feeding and calving grounds to contact Coast Guard-operated shore stations for information about whale movements and collision avoidance procedures.

Federal funding devoted to northern right whale research has increased dramatically in recent years and now totals $6.85 million, of which two-thirds will be spent in the Northeast.

Federal officials are researching technology such as sonar systems and high-tech buoys that would warn the ships of whales nearby.

They also are considering other options on a port-by-port basis, including changes in commercial shipping routes and speed restrictions in certain whale habitats, said George Liles, a spokesman for the Marine Fisheries Service.

But “neither one of those is an easy solution,” he said.

That is especially true in the Great South Channel, which is about four times the size of Cape Cod Bay and where at least 700 ships pass through each year, along with smaller fishing vessels and pleasure boats. Vessels heading into the Port of Boston are required to use shipping lanes that cut through the critical habitat area, but ships headed to other ports are not.

Here and elsewhere, a careful balance must be struck between protecting the animals and addressing the needs of mariners to set schedules and avoid unnecessary costs, said Bruce Russell, co-chair of the National Marine Fisheries Service’s Ship Strike Committee and author of a federal report on right whales issued last year.

The National Marine Fisheries Service plans to create a report within several months commenting on the committee’s recommendations.

“Herding whales,” he said, “is like herding cats.”