The dark, undulating mass beneath La Jolla, California’s waters on Tuesday may have been mistaken for an oil spill, but it was actually comprised of millions of tiny northern anchovies. According to Fox 5 San Diego, students from the Scripps Institution of Oceanography were able to capture video of the anchovy swarm as it massed off the coast of San Diego near Scripps Pier.
“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,” bystander Robert Monroe told the Los Angeles Times. “It’s like watching the motion of a lava lamp.”
Scripps researchers said it was the largest collection of fish they have seen in the area for over 30 years, with the school measuring over 100 meters wide in some places. Students collected some of the six-inch fish for study but the swarm dissipated later on in the evening.
The Northern, or Californian, anchovy is generally found in tightly-packed schools between 18 and 300 miles out to sea. They do not usually come so close to shore, especially in the relatively warm temperatures that scientists recorded on Tuesday.
“Even veteran fisheries oceanographers were amazed,” the institute wrote on its Facebook page.
Graduate students Julia Fielder, Bonnie Ludka, and Sean Crosby were among the first to swim out to the swarm and recorded the fish up close on a GoPro camera. You can see that video below: