Peter Wallerstein realized something was seriously wrong when a sea lion pup turned up seven miles inland at a cellphone store in California.

The number of the starving animals stranded along the southern California coastline has been rising since January, but usually they were just found on beaches. Now they were everywhere. And there were hundreds of them.

And, while the immediate crisis seems to have abated, experts are scratching their heads over what could have caused a tidal wave of sick and malnourished animals over the last two or three months.

"I wasn't too alarmed in the beginning," 61-year-old Wallerstein of Marine Animal Rescue, who has been saving the mammals for nearly three decades, told AFP.

"But when the numbers got to be higher, where we're getting 75 to 100 calls a day and finding animals at the Carson Verizon store and under cars, finding them all over the place, one after another, it kinda put a red flag up."

Carson, 20 miles south of Los Angeles, lies several miles back from the ocean-front.

"It had to swim miles and miles up the flood control canal, cross a couple of roads, almost get hit by a car. The sheriff's deputies called me about 11:30 at night, saying 'Hey, we've got a sea lion at the Verizon store'."

Stranded sea lion pups are nothing unusual in these parts -- dozens of them are cared for by rescue centers along the coast every year, when they struggle to forage for themselves after being weaned from their mothers.

But usually rescuers don't start seeing them until April.

"What happened this year was, we started seeing those pups that should have still been with their mother, showing up as early as January, at six months of age," said biologist Sharon Melin of the National Marine Fisheries Service.

"It has been increasing ever since," she told AFP from Seattle, adding that there were two main theories -- either disease running through the population, or shortage of food, both of which are being intensively investigated.

Neither can explain this year's unexpected surge in strandings.

"What made this event really unusual is that the age and class of animals is very specific, and the fact it started so early," said Melin, adding there was apparently "no large scale regional event" that could have caused the problem.

Wallerstein said that, at the height of the crisis, he was working round the clock responding to calls about stranded pups.

After catching the forlorn animals, he mostly took then to the Marine Mammal Care Center in San Pedro, situated on a picturesque headland looking out over the sparkling Pacific.

The center's director David Bard, told AFP the baby sea lions coming in were on average half the normal weight of 50 to 60 pounds.

"Typically in the first three months of the year we'll see between 50 and 80 animals. This year by the end of March we had well over 400 admitted," he said.

The sea lions are housed in an array of pens. The smallest and weakest are under constant surveillance in a separate unit, while others are moved to bigger pens with deeper pools, to be cared for and fed as they recover.

Some, inevitably, don't make it. "As with any hospital there are some that don't survive," he said, adding: "If we have an animal that is undergoing pain and suffering and we can't cure it some of those animals are euthanized."

Back out on the beaches, Wallerstein recalls how, at the worst point in the crisis, there were too many rescued sea lion pups, and not enough space to house them.

"We were like a paramedic without a hospital. It really hurt, we were having to relocate animals that I knew were in trouble" to less exposed beach areas, but with no backup care. "As rescuers we had no other option," he said.

Jim Milbury of the National Oceanic and Atmospheric Administration (NOAA), based in Long Beach, said that between January and March, 1,098 sea lion pups were stranded on the coastline between Santa Barbara and San Diego.

That compares to a historic average of 131. In Los Angeles County alone, the number stranded is 412, compared to 48 on average for the first three months of previous years.

Wallerstein said call outs have dropped but biologist Melin, in Seattle, said numbers may begin to increase again.

"We would expect that it might have a little lull now, but the proper weaning time is right now so normally the stranding centers will get an uptick in stranding," she said.

"If the event is ending, then maybe that number is going to be normal. If it's continuing. We would expect maybe another increase."