Whale watchers in Massachusetts have been treated to a bounty of sightings of humpback whales, likely drawn close to shore by an abundance of one of their favorite types of fish, experts say.

On some days this past week as many as a dozen sightings have occurred during whale-watch cruises, which normally might provide two to three on a normal day, and on May 21 one lucky boatful of watchers saw 40 whales in a three-hour cruise to the Stellwagen Bank, a marine sanctuary located around 25 miles east of Boston that is known for its whale activity.

"The past few weeks have been exceptional," says Laura Howes, director of conservation and marine education for Boston Harbor Cruises, which organizes whale watch trips.

The humpbacks are apparently taking advantage of bountiful numbers of a fish called a sand lance, also known as sand eels, which are attracted to the underwater plateau that is the Stellwagen Bank area because of its sandy bottom

The oily eels, which travel in schools that can number in the tens of thousands, are a preferred food source for humpback wales, which cooperatively hunt them by blowing streams of bubbles that herd the schools into ever-tighter groups.

An adult humpback whale can easily down a ton of the calorie-rich 6-inch sand eels in a day, Howes says.

An estimated 900 of the humpbacks, which can be as long as a school bus and grow to reach 40 tons in weight, live in ocean waters off Massachusetts in the spring and summer, then head south to warmer regions.

The sand eels have been conspicuous by their absence in the last several years, and their rebound is likely the reason for the whale numbers being seen this year, says Jesus Pineda, a scientist at the Woods Hole Oceanographic Institution.

"That's mostly what drives the distribution," he says. "You will find very few whales where there are no sand lance."

The resurging numbers of the fish, one of the main preys in the region, have also attracted seals and basking sharks eager to take advantage of the marine buffet.

Whale-watching tour operators throughout Massachusetts are just happy the fish are back and that the whales have followed.

"That's what makes the whales happy," says Jim Douglass, who helps run Cape Ann Whale Watch in Gloucester. "And that makes us happy."