(CNN) -

Who ordered the anchovies?

That's what beach-goers may be asking after a huge swarm of the oily fish descended on the shallow waters of La Jolla Shores, California, this week.

"It is rare to see so many anchovy abutting the surf zone," said Professor Dave Checkley of the Scripps Institution of Oceanography. "More usually, schools are seen hundreds of yards to many miles offshore." The surf zone is the area in which waves break on shore and humans normally swim.

Schools of anchovy can range from hundreds to millions of fish, said Checkley, who gave this school "a very rough estimate" of between 1 million and 100 million anchovies.

"In my approximately 30 years at the SIO starting in 1970, I've not seen this before," Checkley said.

Changing ocean temperatures may be the cause of the anchovy influx. The North Pacific is currently transitioning into a cooler state, which is favorable to anchovies, said Checkley. He added that, while this particular phenomenon is unusual, anchovy populations naturally fluctuate on the scale of decades.

Scientists weren't the only one taking note of the army of anchovies. The California sea lion, which preys on the small fish, seemed "happy as a clam," Checkley said.

"Human beach-goers and surfers similarly (were happy), enjoying a rare sight of an abundance of schooling fish within easy swimming distance," said Checkley. "For those able to snorkel among the anchovy, it is a wonderful experience ... fish, fish, fish!"

By Wednesday, the school had departed the surf zone near La Jolla Shores. According to Checkley, the anchovies could have moved farther offshore just as easily as they could have slid along the coast to the north or south.