SANTA CRUZ, Calif. — Blue whales, the world’s largest animal, are being seen in droves off the Northern California coast.

The ocean giants are being lured by an increase this year in their favorite food — shrimp-like creatures known as krill, biologists said.

In Monterey Bay, whale watching tour operators have reported seeing dozens of blue whales and droves of humpbacks.

Marine biologist Nancy Black said that basking sharks, the world’s second-largest shark species, have also made an appearance.

“I tell people it’s a once-in-a-lifetime chance,” she said.

Blue whale sightings close to shore are generally rare.

The animals, which were nearly hunted to extinction a century ago and remain endangered, tend to stay further out to sea while migrating north. They can also stay under water for more than 20 minutes.

There are only about 10,000 blue whales left worldwide, with 2,000 in the northeastern Pacific Ocean.

Marine biologists say a biological chain reaction is likely behind the whale boom.

Strong northwest winds have brought to the surface more of the cold, nutrient-rich waters from the deep. This roiling brings phytoplankton up to the surface, which multiply in the sunnier environment, and are in turn feasted on by krill.

Blue whales can eat about four tons of krill every day.

“We are seeing every single day humpbacks feeding on krill, and most days, blues feeding,” said Mike Sack, co-owner of Sanctuary Cruises in Moss Landing.