SAN JOSE, Calif. — Two humpback whales that were found washed up dead on beaches along Monterey Bay earlier this month tested positive for domoic acid, a naturally occurring toxin produced by harmful algae blooms in the ocean.

Such algae blooms — which have been linked more commonly to deaths of sea lions, pelicans and other smaller animals — can worsen during marine heat waves like the one that has been affecting much of the Pacific Ocean off California all year from the Bay Area to San Diego, scientists said Monday.

“Hopefully this is a rare blip,” said Robin Dunkin, director of the Long Marine Laboratory Marine Mammal Stranding Network at UC-Santa Cruz. “But the frequency, geographic area and toxicity of domoic acid events is increasing.”

El Niño conditions now unfolding near the equator could lead to warmer-than-normal water temperatures across the Pacific through the fall and winter. Concerns are rising about the impact of similar domoic acid events on other whales and other ocean species.

“As the El Niño builds, you are almost certainly going to see more warm water move into our area and an increased risk of more algal blooms,” said Raphael Kudela, a professor of marine biology and marine algae expert at UC-Santa Cruz. “We are certainly going to be keeping an eye out for other strandings.”

Pacific humpback whales are some of the largest animals on Earth. They can grow up to 60 feet long, weigh 100,000 pounds and swim 10,000 miles a year through the ocean on a vast annual migration across the Pacific Ocean.

Both whales that washed up earlier this month were juveniles.

The first was discovered June 3 in Pacific Grove on the shoreline near Hopkins Marine Station, a scientific facility run by Stanford University. It was 24 feet long. The second was found two days later, about 15 miles north, on Sunset State Beach near Watsonville. It was 39 feet long.

So far this year, 62 whales have been found stranded on beaches along the West Coast. The majority, 52, have been gray whales, often emaciated, and some of which have been hit by ships around San Francisco Bay. The gray whale population rebounded in large numbers in recent decades after the United States banned commercial whaling in 1972 but has declined more recently, a trend scientists say is linked to changes in the amount of food they eat in the Arctic connected with climate change.

What was unusual about the two dead humpback whales, Dunkin said, is that both seemed to be properly nourished.

“It is pretty unusual to have whales, especially two humpbacks, strand so close together,” Dunkin said. “We didn’t see evidence of injury or orca attack and they otherwise seemed pretty healthy.”

Dunkin, who worked on the necropsy, or animal autopsy, of the Sunset Beach whale, said tests of the animal’s feces and stomach contents showed very high levels of domoic acid. She said she strongly suspects that domoic acid poisoning killed it.

The Pacific Grove whale had elevated levels of domoic acid also. It did not have a full necropsy, although researchers took samples of its blubber, skin, and feces, she said.

Additional tests will be done to learn more.

That’s important, said Dr. Padraig Duignan, director of pathology at the Marine Mammal Center in Marin County.

“The domoic acid is definitely something we should be concerned about but more work needs to be done,” he said.

“I’ve necropsied hundreds of whales. When they die from blunt force trauma, it’s easy to determine the cause of death. But much of the time, finding the cause of death is difficult. Most of the time you don’t find anything. It’s really hard to determine the cause of death of a whale.”

Tests should be done to look at brain and heart tissue in marine mammals to look for lesions that can prove the cause of death was domoic acid poisoning, he said.

Last year, harmful algae blooming off Southern California poisoned hundreds of dolphins and sea lions off Santa Barbara, Ventura, Los Angeles, Orange and San Diego counties, causing strandings along popular beaches. Some of the animals were able to be treated by wildlife rescue centers. Others experienced seizures on the beach and died or had to be euthanized.

That event was largely caused by changes in winds, which brought deep, nutrient-rich waters to the surface. Warm spells then fed rapid growth of the algae and the toxin it produces.

A similar event began in mid-May in Monterey Bay, Kudela said, and continued until the end of May. He has measured domoic acid levels with equipment on the Santa Cruz Municipal Wharf since 2002.

“The levels were the highest that we have ever recorded,” he said.

The domoic acid works its way up the food chain, often in anchovies and sardines, which are then eaten by pelicans, sea lions, whales and other animals.

“There have been whales that have tested positive for domoic acid,” he said. “But we generally don’t get mortalities. They are big. They have to consume a lot of fish with toxic algae. We get a lot of pelican and sea lion mortalities from domoic acid. But whales are rare.”

A marine heat wave off the California coast that has been ongoing all year, in which ocean temperatures have spiked to as much as 7 degrees hotter than average, can increase the risk of such blooms, he said.

When there are harmful algae blooms, health authorities sometimes put out warnings telling people not to eat mussels, clams and other types of seafood. Swimming in the ocean is usually safe, Kudela added.

The number and size of algae blooms has been increasing in the ocean. Some are triggered by nutrient runoff from farms on land, or sewage treatment plants. But climate change is also playing a role, many researchers suspect.

“A lot of things are changing,” Duignan said. “It’s very complex. A lot of it is climate driven, but it’s not something that’s easily predictable. It’s a dynamic, evolving situation. But in the last 10 to 15 years it has definitely gotten worse.”

Last week, researchers at NOAA declared that El Niño conditions are present at the equator, and that there is a 63% chance of a very strong El Niño in November through January. Very strong El Niño events, where ocean water is unusually warm, can affect weather patterns worldwide, causing droughts in some places and increasing the chance of rain in other places, including California. They also can send tropical fish species farther north and cause animal die-offs, like large numbers of seal lions that died along the shoreline during the last major El Niño in 2015.

“With this extreme El Niño, who knows what are going to see?” Duignan said.

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