Whale lovers find paradise at Boston Harbor

The vast Stellwagen Bank National Marine Sanctuary is one of the best whale-watching spots in the world, with a high concentration of humpback, sei and minke whales, as well as the endangered fin whales.

The humpbacks are the most popular, easily identifiable by their long fins that generate easy maneuverability and see them loved for their acrobatics.

More than 40 feet (12 meters) long and numbering 1,500 to 2,000 in the North Atlantic, visitors can watch them hunt sand eels using a technique known as "bubble netting," said New England Aquarium spokesman Tony LaCasse.

Two or three humpbacks will work together, encircle a school of sand eels, while one dives to the bottom to blow bubbles.

The eels will be fearful and make a tighter school, and another whale will dive in -- and swallow a giant mouthful of the little fish. Eels that escape are caught by seagulls, who perch on top of the whales, ready to ambush.

Harder to spot so close to the coast are sei whales, longer and slimmer than the humpback but no less spectacular.

Around 30 of the threatened species were visible in mid-April, a rare concentration so close to the mainland, LaCasse said.

Usually, only up to a dozen are spotted each season hunting down copepods, a group of small crustaceans, found in the waters of Cape Cod.

Only the luckiest get to see the right whale, which can weigh up to 70 tonnes. It's an almost extinct species, with only around 430 left in the North Atlantic after decades upon decades of hunting.

Boats are not allowed to approach closer than 500 yards (meters).