It began in January. At first, there were only a few. But as the weeks went on, more sea lion pups washed ashore. The dehydrated, emaciated pups showed up on Southern California’s beaches, tucked under trucks and lifeguard towers. One was found huddled in a flower pot.

In late January, scientists surveying Channel Island sea lion rookeries reported something worrying: Pups out there were in bad shape. By early February, regional marine mammal rescue centers were concerned.

The strandings hadn’t stopped. Instead, the pace was picking up.

Now, hundreds of these little animals have been admitted to rescue centers between Santa Barbara and San Diego. For a non-El Niño year, the numbers are much too high, too early. Something is going badly wrong offshore, and no one knows what it is yet.

"We're in the process of trying to understand what is actually causing this," said Sharon Melin, a wildlife biologist with the National Marine Fisheries Service. "The stranding centers in Southern California are being inundated with animals. It hasn't hit the northern centers yet."

As of Mar. 13, 517 pups had been admitted to five Southern California rescue centers. That total is higher than the total for some entire years, said Sarah Wilkin, regional strandings coordinator with the National Oceanic and Atmospheric Administration. “And we’re only two months and a week in.”

Rescue and Rehab

The stranded animals are about nine months old – most were born around June 2012. At this age, sea lion pups normally weigh between 25 and 30 kilograms (55-66 pounds). The animals coming ashore weigh about half that, Melin said. She's visited the island colonies several times recently, first in September and again in February, and noticed that the pups hadn’t gained much weight between visits. “Normally, they would have doubled their weight by February,” Melin said.

She and others suspect the pups have weaned themselves early and left their colonies. Not yet strong enough to find food on their own, they strand themselves on the mainland in a last-ditch effort to save energy and survive. Why they’re leaving home early is an open question.

>'The pups are hypothermic, dehydrated, and skinny.'

By the time the pups are rescued, many are too far gone to be saved, Wilkin said. Dehydrated, emaciated, and malnourished, those who can will spend several months in a rehabilitation facility, gaining weight and nourishment before being returned to the sea.

Rescue and rehab groups in the area are struggling to keep up with the onslaught. The pace of admissions is still accelerating, Wilkin said, noting that both Los Angeles County and Orange County admissions doubled last week.

So far, Marine Animal Rescue, based in El Segundo, has rescued 170 sea lion pups, said director Peter Wallerstein. "The pups are hypothermic, dehydrated and skinny," he said. Marine Animal Rescue brings those pups to the Marine Mammal Care Center in nearby San Pedro for treatment.

“We have admitted over 250 [pups] since January 1,” said MMCC’s director David Bard. “We normally have numbers in the teens for those animals.”

Once admitted, pups are examined, tested for disease, and started on a treatment program. Nutritional supplementation begins with clear fluids, then moves on to “gruel” – a mix of electrolytes, protein, sugar, and ground up fish – until, eventually, the pups are fed solid, fishy food. The process can take several months. “Overall, they’ve been responding very well,” Bard said, on Mar. 13. “We actually released four of them this morning.”

Right now, the San Pedro facility is caring for more than 100 pups. Down the coast, the Pacific Marine Mammal Center has more than 90. On Mar. 12, that facility declared a state of emergency after 18 rescues over two days threatened to overwhelm existing resources. Farther south in San Diego, SeaWorld reports more than 140 marine mammal rescues this year, the majority of which are California sea lions. In all of 2012, SeaWorld rescued 131 marine mammals.

“The good aspect to this is that emaciation and dehydration are something the rehab facilities are very experienced with, and doing a good job,” Wilkin said, reporting that mortality rates in the centers are relatively low, between 20 and 30 percent.

But the bad news is, the onslaught isn't over. Peak stranding season hasn’t happened yet. Historically, most sea lion strandings occur during April and May, when pups are weaned and have to find their way on their own. “We anticipate it will only get worse in the coming months,” Melin said.

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Double Punch

Such large numbers of strandings so early in the year are unusual, and suggest the situation offshore must be pretty grim. "When we see a big uptick like this, we know it's bad," Melin said. "There's something not right. We go out to the islands."

Melin's September visit to San Nicolas Island revealed that pup mortality rates were around 34 percent – about what one would expect for an El Niño year. By February, the rate had risen to somewhere near 50 percent. "By the time we get around to their first birthday, mortality might be as high as 60 percent, maybe even 70 percent," Melin said.

It's not just last year's pup population that's being affected. After giving birth in June, females spend the next months nursing, foraging, and reproducing. By this point in the year, many are probably pregnant again. But when food is scarce, females will sometimes abort a pregnancy and funnel all their resources toward the already growing juvenile. "We are seeing premature pups being born, up and down the coast. A lot of pregnancies are not coming to term," Melin said.

This means the sea lion population will take a double punch: reduced numbers of surviving pups from 2012, and fewer pups born in 2013. "It's two years of impact from something that we don't yet know," Melin said. "Thankfully, it's a healthy population. It weathers these kinds of things fairly well."

It's Not El Niño

But the cause of the mass stranding is still a mystery. Disease or an environmental perturbation affecting the food supply are the best guesses, though scientists are still in the early stages of the investigation. Next week, a team will return to San Nicolas Island and reassess the colonies there. Wilkin is working on applying to have the strandings declared “an unusual mortality event” by the National Marine Fisheries Service. That classification would free up funding and investigators, allowing scientists to move more quickly.

One possibility is that the population is dealing with an infectious disease. Morbillivirus, which causes distemper, and influenza have both sickened sea lion populations in the past, but scientists don't think this is likely because it’s not clear why only pups would be affected. Likewise, domoic acid, a toxin produced during algal blooms that can cause seizures and memory loss, seems an improbable culprit.

"If this was a disease process, you might expect it to be a little more across the board," Wilkin said. "It does seem to be pretty targeted to that age class."

Another possibility is that hookworms – which can infect pups until they’re about six months old – have weakened the pup population and left it susceptible to a second disease agent that is just now sweeping through. But while some pups are showing signs of having been infected with hookworms, it's not being seen at abnormally high levels, Melin said. "It does seem more likely to be food-related," she said.

>'Sea lions are usually pretty good at adapting.'

Warmer ocean temperatures, such as what happens during an El Niño year, can affect the food supply.

Those warmer waters dampen nutrient-rich seafloor upwellings. Without cold waters and added nutrients, prey species – phytoplankton, krill, and small fish – are scarce. Animals that eat those critters, such as larger fish, sea lions and sea birds, either move with the food toward colder water, or struggle. Scarcer food means sea lion mothers have a tougher time finding a meal for their pups. They may have to swim farther, dive deeper, and stay away longer, prompting pups to wean themselves and strike out on their own in search of fish.

“It's not that mom isn't coming back, she's just taking too long,” Melin said. "It takes a lot for a sea lion to leave its pups."

After a strong El Niño event in 1997-1998, rescue centers around the state saw elevated intake numbers, similar to what's occurring now except more broadly distributed. In spring 2009, an unpredicted halt in normal upwellings caused the food supply between Point Conception, north of Santa Barbara, and the Monterey Bay to collapse. "We had huge mortality of pups weaned that year," Melin said. "Close to 80 percent."

So far, this event doesn't fit into a pattern. The strandings are localized to southern California, and this isn’t an El Niño year. At least, not really: In summer 2012, a short-lived patch of abnormally warm surface water did settle off the Southern California coast. But that's cooled off now – and the sea lions stuck around.

"Sea lions are usually pretty good at adapting," Melin said, noting that biologists often monitor female sea lions and use them as a gauge of ecosystem health. "If the system starts changing or becomes out of whack, they're the one that are going to show the signs."

There are other hints that something more systemic is amiss in the Islands, namely the nesting numbers and success of brown pelicans in the Channel Islands National Park. Pelicans, like sea lions, are top predators. Both species tend to forage for the same fish, and their numbers tend to fluctuate in tandem. In 2004 and 2005, pelicans in the islands made roughly 6,500 nest attempts, said seabird biologist Laurie Harvey of the California Institute of Environmental Studies. Last year, out of several hundred nest attempts, only five pelican chicks fledged on Anacapa Island. "That ended up being the poorest reproduction year for pelicans on the Channel Islands since 1970," she said.

This year, though it's still early in nesting season, pelican numbers are fairly low, with fewer than a hundred nests on Anacapa. "We think that yes, it definitely looks like it's linked to the sea lion strandings," Harvey said. "Sea lions and pelicans feed primarily on coastal pelagic species like anchovies and sardines. What it's looking like is that the local availability of prey is insufficient."