Thousands of dead gray and white fish washed up on the sand of Manresa State Beach in Aptos Friday morning, according to NBC affiliate KSBW.
Monterey Bay marine biologist Giancarlo Thomae told KSBW he thinks the fish died from asphyxia after the fish swam too close to shore.
"When too many are packed inshore, they deplete the oxygen. That is why the Santa Cruz harbor aerates the upper harbor," Thomae said to KSBW.
But a NOAA biologist told KSBW the fish may have been trapped in squid boat nets and dumped by the crew.
It is illegal for commercial fisherman to keep any fish caught in squid nets, according to rules enforced by the California Department of Fish and Wildlife.
Researchers are still trying to determine the cause of the dead fish.