In 1940 sardines were the most valuable fishing stock in the state. As the temperature of the ocean dropped the migrating sardine schools largely disappeared after 1950 from California waters and nearly all the canneries shut down. The ocean temperature has an irregular cycle called the Pacific decadal oscillation turned water temperatures colder in the mid-1940s, driving sardines southward and intensifying the pressures brought on by overfishing. As the ocean temperatures are cycling higher now there is some evidence that the sardines are starting to return.