A group of professors and students swimming in the waters off La Jolla, California were completely shocked to find swarms of anchovies in the water on a balmy, summer day.

The shoal measured about 325 ft. (100 meters) long and 50 ft. (15 meters) wide - the largest seen in 30 years. It looked like a daunting shadow of the Pacific sea. According to researchers, the school of fish could have had about one to 100 millions of fish.

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

Marine biology grad students and professors just took a dip when they were surrounded with the huge school of fish. One student used a GoPro camera to capture the massive bait fish and even a leopard shark. The fish normally live much further out to the sea and it is baffling experts why they came very close to the shore.

"Schools like this exist throughout the region, but I don't know why they butted up right against the surf," said professor David Checkley of Scripps. "A school this size and this immensity, it's rather difficult to know why." He said it is improbable that the fish were looking for their normal food of zooplankton.

The anchovy population in California has been recently rising and it could be due to the cooling of the Pacific Ocean. This is an effect called the Pacific Decadal Oscillation, which is a natural climate phenomenon that turns most of the Pacific to colder temperatures. The water temperature was 74 degrees Fahrenheit which experts said was too high for anchovies which prefer cool water. It could be the warmest water the species was seen. The anchovy population in California has been low in the last 20 years.

Last May, a shoal of anchovies showed up in Marina del Ray, Southern California but its stopover had been tragic. Due to lack of oxygen, the anchovies were suffocated after being trapped in the harbor.

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