Shark encounters and sightings along California's coast are at their highest level in decades, scientists say, warning that warmer waters mean beachgoers will have to be on the lookout for the predators all summer.

The latest near-deadly encounter came in late May when a 52-year-old woman was mauled by a shark near Los Angeles, prompting beach closures for several days.

Chris Lowe, director of California State University's Long Beach Shark Lab, said there have been more sightings of great white sharks this year than in the previous 30, and that even hammerheads have been spotted along the coast.

He said the El Nino weather pattern of the last two years has led to unusually warm waters in the eastern Pacific, meaning the sharks and other species are not migrating -- and are moving closer to shore.

"They are going to hang out near our beaches all summer," Lowe told AFP.

The high water temperatures are having an effect on other marine life as well, with whales, dolphins, seals and sea lions increasingly spotted off the coast.

Other non-indigenous marine life has also been reported, including manta rays and hundreds of thousands of red tuna crabs that washed ashore in southern California in May.

Even a striped dolphin, normally found in tropical waters, was spotted in northern California this year.

More worrisome are highly venomous yellow-bellied sea snakes, normally found in the tropical waters of the Pacific or Indian oceans, several of which have washed ashore in southern California.

"There have been whales staying in the area all year round rather than the usual three months," said Elliott Hazen, of the National Oceanic and Atmospheric Administration, noting they normally migrate up to Alaska to feed and down to the warmer water off Mexico to breed.

- Memories of 'Jaws' -

Hazen said a spike in the anchovy population was one reason three to four times more humpback whales had stayed in the Monterey Bay area in the last two years.

The presence of the veritable marine menagerie along the California coast has sparked a huge uptick in business for whale-watching tour operators.

"The whale-watch industry has been booming for sure, with increased sightings," Hazen said.

Meanwhile as the hot summer weather sets in and people hit the beaches, everyone is on the lookout for sharks.

Authorities have even deployed drones to detect the predators.

But experts point out that although the increased sightings and attacks may bring back memories of "Jaws," which scared a generation of movie-goers out of the water, the rate of shark attacks has actually gone down per capita in recent years.

"You are far more likely to die driving to the beach than from a shark," Lowe said.

According to the International Shark Attack File (ISAF), an unprecedented 98 unprovoked shark attacks were recorded worldwide in 2015, 59 of them in the United States, mainly in Florida.

Significantly, only six fatalities were recorded overall, including one in Hawaii.

Apart from blaming El Nino, scientists attribute the rising number of shark sightings and incidents to an increase in human activity in the world's oceans.

"Some people are in the water every day," said Lowe, adding that an upsurge in aquatic sports also means that more people wander far from shore and take more risks.