People in Marina del Rey, Calif., were surprised when a sea lion pup wandered into their apartment complex. They named him Walter.

But getting help for Walter wasn’t easy. He’s just one sick pup among many ailing sea lions overwhelming marine mammal centers in California. Nearly 1,000 have washed ashore so far this year. Emaciated and dehydrated sea lions, mostly pups about 8 months old, have been admitted in record numbers to facilities up and down the California coast. It’s the third straight year of record strandings in the state.

Five hundred fifty sea lions were being treated statewide as of Feb. 18, according to National Oceanic and Atmospheric Administration Fisheries.

Rescuers are swamped.

Peter Wallerstein of Marine Animal Rescue works in much of Los Angeles County. There are so many strandings right now we cannot possibly pick up all the pups,” he said. “People just don’t seem to understand.”

Wallerstein’s two-person rescue team receives two dozen or more calls a day. He said he’d been rescuing marine mammals for 29 years and “for January and February, it’s the highest number of rescues I’ve ever seen.”

Earlier this month, researchers from NOAA’s National Marine Mammal Laboratory visited sea lion rookeries on the Channel Islands, where most of the United States’ sea lions breed, in a search for clues to the high number of strandings.

They measured and weighed pups and found them to be considerably underweight, with an average growth rate that was the lowest they had seen since they began monitoring in the early 1990s.

The pups’ weight was similar to that in 2013, the year of an “unusual mortality event” for sea lions.

In 2013, the Marine Mammal Care Center in San Pedro was treating two to three times as many California sea lions as usual. David Bard, operations director, said that in the first two months of this year, the numbers are double what they were at the beginning of 2013. Wallerstein remembers doing 25 rescues in the first two months of 2013. So far this year, he has rescued 117 marine mammals.

Scientists thought 2013 was an anomaly, said Shawn Johnson, director of veterinary services at the Marine Mammal Center in Sausalito. “But then it happened again last year, and now it’s happening again.”

NOAA Fisheries said higher-than-average sea surface temperatures in fall 2014 may be a factor.