Blue whales, the world's largest animal, are being seen in droves off the 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.
"I tell people it's a once-in-a-lifetime chance," marine biologist Nancy Black told the Santa Cruz Sentinel newspaper.
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 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," Mike Sack, co-owner of Sanctuary Cruises in Moss Landing, told the Monterey County Herald newspaper.
