SAN DIEGO - A school of California anchovies so big it rivals anything seen off the San Diego coast in three decades was spotted off La Jolla by researchers with the Scripps Institution of Oceanography, it was announced Tuesday.
"Multimillions" of the finger-sized fishes prized on pizzas and in animal feed -- also called Northern anchovy -- created a dark-blue band in shallow waters just off the coast when first spotted Monday.
"At first some thought it was algae," said Philip Hastings, a marine biology professor at UC San Diego's Scripps Institution of Oceanography. "We could see schools extending all the way from the point in La Jolla to as far as you could see to the north of us for several miles."
Divers told Hastings the school of fish stretched 15 feet beneath the surface of the water.
"They said the school swam over them and it was like suddenly night," he said. "It just completely blocked out the sun, there were so many fishes above them."
Scripps scientists say they haven't seen such an aggregation in more than 30 years, but were unclear why the large school moved into shallow waters off the coast.
Hastings said the northern anchovy is known to cluster in schools, but rarely comes as close to shore as it did on this warm summer day. The water temperature was close to 75-degrees, but this type of anchovy usually prefers cooler water.
"It's not really clear if the high water temperature affected their behavior, causing them to come into shore or not," said Hastings. "It seems more likely that warmer water would force them off shore, so it's really kind of a mystery as to why they've aggregated here in such massive numbers."
Jenn Moffatt, who works at the Birch Aquarium, saw the massive school of fish and decided to go for a swim.
"It was an amazing experience," she said. "Thousands and thousands of fish, and when you dive down into the school, they respond to you, they swim off in all sorts of directions, they'll swim around you, behind you. You can't see where they start and where they end."
Hastings said the school swam north and was mostly dissipated Tuesday evening.
"The schools part as swimmers or sea lions" pass through, he told City News Service. "Leopard sharks were feeding on them this morning."
He had no explanation for the huge swarm 15 meters to 100 meters wide and extending from Scripps Pier to a distant point north.
Hastings said he doubted the mystery of their appearance will be solved.
"I don't think we'll be able to ferret it out," he said. But for history's sake, Scripps has collected specimens for preservation.
The band of 6- to 7-inch fish, which feed on plankton, was first spotted Monday by lifeguards, Hastings said. Hastings called the stragglers that remained Tuesday "bait balls."
Photos showed surfers paddling out for a view below, and video was posted on the Scripps website.
According to FishWatch.gov, maintained by the National Oceanic and Atmospheric Administration fisheries service, more than 20 species in the Engraulidae family are marketed as "anchovy," and Northern anchovy is the species found and commercially harvested off the West Coast.
Northern anchovy is harvested mainly for use as bait in other fisheries and sometimes processed into fish meal, "but it once supported a multimillion-dollar fishery as catch was sold for human consumption, for bait, and for reduction into meal, oil and soluble protein," according to the NOAA.
Northern anchovy has been fished off the West Coast since at least 1916, the agency says.