Scientists believe that a fish die-off that numbered in the tens of thousands in a Southern California harbor on Saturday was due to a lack of oxygen.

The northern anchovies, which were spotted by California Department of Fish and Wildlife staff earlier in the week heading towards the harbor, are thought to have possibly been trying to avoid a predator in the area. The Los Angeles Times reports the fish likely got trapped in the Marina del Rey harbor and died after all the oxygen was used up.

By Sunday morning, the harbor's surface was covered in the rotting fish, which fed local sea lions and other aquatic animals, but required the Los Angeles County Department of Beaches and Harbors to conduct a cleanup. In all, about six tons of dead fish were pulled from the water.

DFW spokeswoman Janice Mackey said, "While the sight of so many dead fish may be startling to some, this is not considered to be too unusual."

While it was mostly anchovies that died in the harbor, officials also found other fish, including stingrays, according to The Associated Press. In 2011, fish die-offs due to lack of oxygen occurred at Ventura Harbor and Redondo Beach.

Still, samples of the fish were collected so a more detailed analysis could be done in a lab.

image via Twitter from Dale Lezon