BOSTON -- A rare adult blue whale was spotted over the weekend 15 miles off Gloucester during a research cruise.

Researchers at the Whale Center of New England said that they were startled to find the whale on Sunday.

During the nearly 45 minutes that the whale was observed, its limited movement around the area suggested that the animal was feeding on a localized prey source. Despite their huge size, blue whales specialize in feeding on plankton, especially krill. The whale was seen in the same vicinity as approximately a dozen feeding humpback whales.

Whale Center staff took photos of both sides of the animal, where each individual whale has a distinctive pigment pattern used to identify individual whales. The photos will be contributed to an ocean-wide catalog of blue whales, coordinated by Canadian whale researchers, which contains 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.

Because of their great size, they were the first target for modern whalers and were endangered by the 1930s. Currently, they are thought to number less than 10,000 worldwide. While there is no good population estimate for the North Atlantic, it is thought that less than 2,000 individuals survive today.

Blue whales typically occur in colder waters than those found off the New England coast, typically off of Iceland, Norway and northern Canada. The first recorded sighting off New England was in 1981, and they were seen only sporadically in the next two decades.

In the past few years, sightings have been slightly more regular, although still rare. In 2002, five different whales were seen over a two-week period in September. Last fall, three different individuals were seen off the Maine and New Hampshire coast in the fall. Few sightings have been recorded this close to the Cape Ann shoreline.

Images Of Whale