Warmer Pacific waters have disrupted feeding patterns and forced about 2,900 pups ashore, with rescue shelters full and distressing scenes along state’s beaches

This article is more than 5 years old

This article is more than 5 years old

The influx of sick, malnourished sea lion pups on to California’s beaches is abating, but experts have warned of a repeat next year unless the Pacific Ocean swiftly cools.

About 2,900 pups have struggled ashore so far this year – 17 times the average number for strandings, said Jim Milbury, a spokesman for the National Oceanic and Atmospheric Administration (NOAA).

The statistic underlines a crisis in offshore rookeries where warming waters have disrupted feeding patterns, separating pups and yearlings from their mothers and forcing them to fend for themselves.

“It’s dropped off but we’re still receiving sea lions,” said Laura Scherr, of the Marine Mammal Center in Sausalito. “They’re coming in very emaciated – really just a bag of bones near death.”

Pups were swimming very long distances, using up blubber, leaving them vulnerable to pneumonia, parasites and other ailments, she said. “They’re very sick and lethargic.”

The surge has overwhelmed rescue shelters up and down California, forcing some to leave stranded sea lions on the sand.

Desperation drove some of the animals further inland, including a 20lb pup which Mendocino county sheriff’s deputies found on Highway 1, south of Fort Bragg. It reportedly rubbed against their legs for attention.

Another, described as an emaciated yearling, was rescued near Skyline Boulevard in San Francisco, more than 1,000 feet from the ocean.

Such scenes have astonished and alarmed the public. When rescue shelters proved unable to respond to sightings, blaming full capacity, some people attempted their own – illegal – rescues, lifting the creatures with bare hands or wrapping them in coats. Several people reported being bitten on the face, hands and arms.

The number of pups coming ashore has fallen from a peak in March, said Justin Viezbicke, NOAA’s Fisheries California Stranding Network coordinator.

“It’s definitely decreased,” he said, speaking while inspecting a beach in San Diego. The reduction reflected the finite number of sea lions, not improving conditions, he said. “There are only so many pups that are born.” Viezbicke said there were no estimates for how many had died.

Facebook Twitter Pinterest Volunteers Rex Ivanchich and Doug Hailey from the Marine Mammal Center wrangle a stranded fur seal in to a carrier on Ocean Beach in San Francisco. Photograph: Peter Dasilva/EPA

Even with an unusually high mortality rate scientists said California’s sea lion population, totalling about 300,000, was stable.

Experts blamed the strandings on warming waters which drove fish away from sea lions’ traditional hunting grounds, forcing nursing mothers to swim greater distances and leave offspring for days, even weeks.

“The pups end up starving so they leave the rookeries to forage but they don’t know how and end up on California’s beaches,” said Mary Beth Steen, director of development for the Pacific Marine Mammal Center in Laguna Beach, which has rescued more than 400 sea lions.

Coastal waters are 2-3C (3.6-5.4F) higher than normal because cold northern winds have slackened since spring 2014, said Nate Mantua, an expert in climate and aquatic ecosystems at the University of Washington.

“The weak winds are disrupting the cooling mechanism.” The phenomenon was too recent to be attributed to climate change, said Mantua. “This is probably a fluke of natural variability. It’s not a pattern that’s expected to become the new normal.”

Even if cold winds return soon it may be several seasons before water temperature and fish levels revert to normal. “We’re preparing for the same again next year,” said Scherr, of the marine mammal centre. Viezbicke, of NOAA, agreed: “We can expect to see more high stranding rates.”

Seventeen young sea lions recovering at the Laguna Beach mammal centre suffered an additional trauma when someone filled their pool with chlorine – an apparently intentional act which police are investigating.

An unknown person, or persons, entered the facility on the night of 27 April and is believed to have poured the chemical directly into the pool or into its filtration system, wounding the animals with corneal ulcerations, or eye burns. Ten have recovered and the rest are still being treated.

“It’s really depressing for everyone. Who would do this?” said Steen.