Researchers caught a rare glimpse of an adult blue whale during a cruise aboard their 42-foot research vessel, The Mysticete, on Sunday morning.

Staffers from the Whale Center of New England were on a standard cruise conducting research in an area about 15 miles off the Gloucester coast, where whale-watch boats usually don't go when they spotted the blue whale, according to Mason Weinrich, the center's executive director and chief scientist.

Blue whales are endangered and generally occupy colder waters than those found off New England; they are usually found off Iceland, Norway and northern Canada.

Weinrich said a food source likely lured the whale this far south. Despite their huge size, blue whales mostly feed on plankton, especially krill, a small shrimp-like crustacean, Weinrich said.

During the nearly 45 minutes that the whale was observed, its limited movement around the area suggested that it was feeding. The blue was seen in the same area as about a dozen humpback whales, which were also feeding.

"How long it stays depends on how good the food source stays," he said.

During the sighting, Whale Center staff photographed both sides of the animal. Each whale has a pigment pattern distinct from others. These photos were e-mailed Monday to an ocean-wide catalog of blue whales, the nonprofit Mingan Island Cetacean Study, coordinated by Richard Sears and Canadian whale researchers. The center hopes to discover something of the whale's history from the database, which counts more than 300 individual whales.

Blue whales are the largest mammals, and possibly the largest animals, ever to live. The largest blue whale ever accurately measured was 102 feet, although animals longer than 90 feet are rare in the North Atlantic. Weinrich, who has experience with North Atlantic and North Pacific blue whales, said the whale spotted Sunday was clearly an adult 70 to 80 feet long.