Swinburne Island, a small, manmade hunk of land off the coast of Staten Island, was once used to quarantine immigrants who arrived at Ellis Island suffering from contagious diseases; later it became a base for military training exercises. Its decaying brick huts, rotting pier, and complete isolation may harm its prospects for development, but at least one group of New Yorkers finds it an attractive piece of real estate.

Migrating harbor seals, once an extremely rare sight, have made the rocky island their winter home for the last several years before returning to their native Maine in summer.

Yesterday, scientists, students, and volunteers left on a boat from Kingsborough Community College in Manhattan Beach, Brooklyn, to study the creatures during the third annual seal count. The trip’s organizer, Paul Sieswerda, curator of the New York Aquarium, said such record-keeping will help biologists learn the new seal community’s size and migration schedule.

The return of seals to city waters after a decades-long absence is primarily attributed to population growth that has expanded their range. The 1972 Marine Mammal Protection Act, which outlawed the killing of seals, helped boost their numbers.

“Fisherman would shoot them because they were competitors for fish,” Mr. Sieswerda said. “In the old days, there used to be a bounty on seals’ noses that hunters would collect, but with better protection the population has grown.”

As the boat slowed to a halt about 50 feet off Swinburne Island’s coast yesterday, black flashes began to appear and disappear in the water. Harbor seals bobbed their heads out of the water like submarine periscopes and examined their visitors.

“They’re very curious, they’ll come right up and look us,” Mr. Sieswerda said as the seal count participants eagerly shouted out more, and increasingly closer, sightings. The animals quickly surrounded the boat, sometimes popping out of the water as near as 10 feet from the ship before retreating underwater, where they can stay for as long as five minutes before coming up for air.

After a half hour, Mr. Sieswerda called an end to the count, putting the final number at eight, consistent with the nine spotted in 2007 and 10 in 2006.

“It’s interesting how they’re coming back to New York City,” he said. “In a giant metropolis, having nature return is a good thing.”