A recent whale watching cruise from Boston Harbor to Stellwagen Bank -- a federally protected marine sanctuary located at the mouth of Massachusetts Bay -- featured 40 whale sightings, all humpbacks.

Cruise passengers can usually consider themselves lucky if the number of whales spotted outpaces the number of seasick tourists hunched over their barf bags.

But lately, there's no time to be sea sick, as the waters off Massachusetts' coast have been littered with humpback whales.

Minke whales, fin whales and North Atlantic right whales have also been spotted recently.

"The past few weeks have been exceptional," said Laura Howes of Boston Harbor Cruises.

The large pods of humpbacks have apparently arrived in pursuit of an influx of sand lance, a small eel-like bait fish. Humpback whales eat roughly one million small fish, such as the sand lance, every day.

Humpbacks rely heavily on lance fish, but their population is highly cyclical -- and marine biologists don't really know why.

"But what is driving this sand lance population is one of the big mysteries," said Dave Wiley, a scientist at the Stellwagen Bank National Marine Sanctuary.

Wiley and others are hoping new research can offer clues as to what is behind the surges and lulls in sand lance numbers.

Whale watching tour companies, on the other hand, are just thankful.