Scientists say the size of a massive school of anchovies seen migrating along the Southern California coastline is something they haven't seen for at least than 30 years.

The giant shoal of Northern Anchovies seen darkening the ocean waters off of La Jolla north of San Diego was 45 feet wide and stretched for more than 320 feet, they say.

"It was remarkable. From a distance it looked like an oil slick and you think 'What happened?' and then you get up close and it's amazing," said Robert Monroe of the Scripps Institution of Oceanography. "It's like watching the motion of a lava lamp."

Anchovies normally live far out at sea, and why the shoal would come so near the coast is unknown, experts said.

"It is rare to see so many anchovy abutting the surf zone," Scripps Professor Dave Checkley said. "More usually, schools are seen hundreds of yards to many miles offshore."

The shoal may have contained anywhere from a million to 100 million fish, he estimated.

The ocean temperature at the time, 74 degrees, is unusually high for the anchovies that normally choose cooler water, Scripps researcher Phil Hasting said, and in fact was "pretty much the warmest water the species has been reported in."

Still, the North Pacific overall has been transitioning to cooler temperatures and that could account for a rises in anchovy numbers, which also fluctuate naturally over decades, Scripps researchers say.

The shoal disappeared after spending much of July 8 off La Jolla, but whether the continued a migration either north or south or simply moved farther off shore is unknown, Checkley said.

Northern anchovy have been fished off the coast of California since the beginning of the 20th century for human consumption, as bait, and for fishmeal and oils, according to the National Oceanographic and Atmospheric Administrations.

The anchovies can grow to 6 or 7 inches and feed on plankton, although that's probably why the La Jolla shoal moved so closed to shore, the Scripps experts said.

"I don't think we'll be able to ferret it out ... But for history's sake, Scripps has collected specimens for preservation," Hastings said.

The giant school of anchovies attracted numbers of California sea lion, for which the small fish are a favorite prey, and there was evidence that leopard sharks were also taking advantage of a sudden abundance of a food source, the researchers said.