More than 3,000 starving sea lion pups have washed up on California’s beaches since January—easily 15 times more than in a normal year.

“It’s unprecedented,” says Sarah Wilkin, national marine mammal stranding and emergency response coordinator for the National Oceanic and Atmospheric Administration .

And those are the lucky pups. The situation on California’s Channel Islands, where more than 90 percent of the U.S. sea lion population congregates to breed and nurse young, is even worse than in other parts of the state.

The influx of weak, sickly pups—3,110 as of May 20—has overwhelmed rehabilitation centers for the third year in a row . From San Diego to north of San Francisco, rescuers are busy retrieving pups (sometimes from city streets and oceanside bars ), fattening up the ones that can be saved, and releasing them back into the sea [see video]. During the worst month, March, more than 1,000 pups rolled ashore , more than rescuers would normally see in an entire year.

Scientists blame this year’s stranding on a lack of food for the pups, courtesy of a warm water blob that has settled off the West Coast. But the reasons for the two previous mass stranding events, in 2013 and 2014, aren’t yet obvious. (See " Warming Pacific Makes for Increasingly Weird Ocean Life .")

“We’re thinking it’s kind of the same root cause that keeps playing out, in that the animals are unable to find prey,” Wilkin says. “But the mechanism behind what is limiting the prey or keeping the prey away may have changed slightly over the three-year period.”

Hunting for Food

Most of the pups washing ashore this year were born in June 2014. Typically, sea lions stay with their mothers for about a year, nursing and learning how to catch fish on their own . But starting in January, droves of pups began leaving their island nurseries. Scientists suspect that’s because the sea lions’ normal food fish, such as sardines and anchovies, aren’t there.

When fish are scarce, mother sea lions have a hard time feeding their pups. Normally, nursing moms stay close to the Channel Island nurseries and only go on short foraging trips. But when the fish move away, those trips get longer and longer—so long that some waiting pups become hungry enough to take their chances on their own.

“The few pups that have enough strength to leave the rookeries and make it to the mainland get recorded as strandings,” says NOAA’s Mark Lowry , who monitors sea lion population numbers and also keeps track of what they’re eating . “There’s a lot of death out there.”

View Images Rescuers and rehabilitation centers have been working hard to help more than 3,000 starving sea lion pups that washed ashore this year. But not all of them make it. These dead pups will be put into cold storage until they can be necropsied at The Marine Mammal Center in Sausolito, California. Photograph by Peter Dasilva, EPA/Corbis

When Lowry visited two of the main U.S. sea lion nurseries in April, he found a grim scene. In addition to beaches full of starving or dead pups, there were dead adult female sea lions and many aborted fetuses.

“Adult females are taking a hit now,” he says. “What I’m predicting is that pup production—when they start being born in June—is going to be really low.”

Empty Bellies

To figure out what sea lions are eating—or not eating—Lowry has to track the end result: their poop. Four times a year, he visits the islands of San Clemente and San Nicolas, off the coast near Los Angeles. There, he spoons fecal material into a bag, targeting samples from adult females.

Back at the lab, he sifts through the poop to find the fishy bits, which help him figure out the species and sizes of fish the sea lions are eating. In a normal year, he would find plenty of sardines, anchovies, and market squid. In years where food is scarce, he tends to see uncommon prey species, such as bottom-feeding flatfish.

Samples from the 2013 stranding and from earlier this year look like what he’d expect when normal prey are scarce: “a hodgepodge of stuff.” The 2014 samples will have to wait for more funding.

“I have about 2,000 samples in the freezer right now,” he says. “I’m way behind because I don’t have any help.”

Clues to Causes

It's clear that the pups' main problem is a lack of food, rather than disease, but it's less certain what's wiping out food supplies from year to year, and whether the problem will persist. This year, the most likely culprit is that warm water blob, which has disrupted food webs and might also be killing masses of seabirds. But that blob wasn’t there during the earlier strandings, when oceanographic conditions looked much more normal.

It’s also uncertain whether prey populations have died out or simply relocated. In either case, sardine populations are low enough this year to close sardine fisheries off the West Coast .

Sea lions are considered sentinel species, which means they’re used as indicators of ocean health—and the indications aren’t looking good.