Normally, a whale watch cruise from Boston Harbor might yield two or three close sightings, even on good days. But this month, whale watchers have been treated to a rare abundance of humpback whales, regularly spotting a dozen, sometimes far more.
On Wednesday, 40 whales were spotted on a single three-hour tour to Stellwagen Bank, known as one of the world’s most active marine sanctuaries.
“The past few weeks have been exceptional,” said Laura Howes, director of marine education and conservation for whale watch trips run by Boston Harbor Cruises.
What appears to be drawing the acrobatic creatures, along with a range of other aquatic life, is an influx of eellike fish called sand lance. The oily fish, also called sand eels, are a favorite food source of humpback whales, who gobble them up by the tens of thousands.
Sand lance were mysteriously absent last year from the Stellwagen Bank National Marine Sanctuary, a rich feeding ground for whales about 25 miles east of Boston, just north of the tip of Cape Cod.
But this spring the forage fish has returned in force. No one knows quite why, but its resurgent ranks have spurred a feeding frenzy of whales, seals, and basking sharks.
“It’s been an exciting few weeks,” Howes said. “It’s a great time to come out.”
It is difficult to estimate how many humpbacks have come to Stellwagen Bank this season, but specialists say the concentration is unusually high. There have been about 900 individual humpbacks documented in Stellwagen’s waters over the years.
Humpback whales, which are about the length of a school bus and weigh 40 tons, use teamwork to round up the tiny prey, blowing nets of bubbles to scare schools of the fingerwide fish into tighter groups. Given the chance, a whale might eat a ton of sand lance each day, Howes said.
With its sandy bottom and relatively shallow waters, Stellwagen Bank is a perfect habitat for sand lance, which feed on plankton and burrow into the seabed to hide from predators.
The wealth of sand lance, which travel in schools of tens of thousands, provides easy, calorie-rich food for the whales, which live in the waters off Massachusetts during the spring and summer before heading to warmer waters.
Sand lance are a critical component of the oceanic food chain, specialists said. But their populations seem to be highly cyclical and little is known about their life cycle or precisely why their populations fluctuate.
David Wiley, research coordinator at the sanctuary, is leading a survey of the sand lance population this week, seeking to document how changes in sand lance populations influence marine life.
“That’s one of the big mysteries,” Wiley said. “We really don’t know.”
Jesus Pineda, senior scientist at the Woods Hole Oceanographic Institution, said the whale resurgence comes after several down years and is probably connected to the rebound in sand lance.
“That’s mostly what drives the distribution,” Pineda said. “You will find very few whales where there are no sand lance.”
Pineda said sand lance are under siege from all corners, with whales, sharks, and an array of fish relying on them as food.
But when their numbers are decimated, their predators look elsewhere, giving the writhing creatures a reprieve, he said.
“I think that’s part of the story,” he said.
For whale watching outfits, the abundance has been cause for rejoicing. A Provincetown group saw 12 humpbacks Wednesday, to the delight of a group of schoolchildren, and a boat out of Gloucester spotted 15, including a breaching calf.
“People are blown away,” said Jim Douglass of Cape Ann Whale Watch in Gloucester. “It’s been great.”
Douglass is quick to thank the sand lance for their sacrifice.
“That’s what makes the whales happy,” he said. “And that makes us happy.”
Chad Avellar, who gives private whalewatching tours from Provincetown, said he must have seen 25 whales Wednesday, too many to cover in one tour.
“We couldn’t get to all of them,” he said. “No way.”Peter Schworm can be reached at email@example.com. Follow him on Twitter @globepete.