Starving pups were fed at rescue centres (Image: Sarah van Schagen/The Marine Mammal Center)

When over a thousand starving sea lion pups washed up on the southern California coast last year, nobody could explain it. A picture of the causes is now emerging, and it looks like sea lions might face a similar fate this year.

By this time last year 1300 California sea lion pups had been stranded alive – roughly four times as many as normal. While rehabilitation centres rescued as many as possible, the strandings were declared an “unusual mortality event” by the US National Oceanic and Atmospheric Administration (NOAA).

The cause has been a mystery, but one contributing factor has now been found: the sea lions’ nutrient-rich food sources have moved, scientists from NOAA reported at a press conference on Monday.


Fishery surveys off the California coast have shown that over the last 15 years, high-fat sardines have moved farther from shore, and their numbers are declining. This year’s annual spring survey reported very few sardines along the California coast, says Sam McClatchie of NOAA’s Southwest Fisheries Science Center in La Jolla.

Vanishing sardines

“We don’t know whether that’s because they have temporarily disappeared, as sometimes these fish do, or whether they’ve moved out of our survey area,” McClatchie says.

He suspects that the long-term shift is caused by ocean currents that have pushed waters suitable for sardine spawning further from the coast.

Without their normal diet of high-fat sardines, some sea lion mothers were unable to adequately feed their pups. In an attempt to survive, the pups weaned early and struck out on their own. When the ocean proved too formidable a challenge, the exhausted animals hauled themselves onto California’s beaches, according to several biologists contacted by New Scientist.

Stranded pup numbers are high again this year, albeit lower than last year. Already, 650 have been sent to rehabilitation centres – more than twice the normal number for this time of year, NOAA reported during the press conference. NOAA is now investigating whether this year’s strandings are linked to those last year.

Pathogen link?

Sardine movement might only be one part of the story. Sea lions are opportunistic feeders and their other prey species are flourishing. As a result, some are hesitant to point too strongly at shifting sardines as the culprit. “You shouldn’t get too fixated on the idea that all they’re feeding on is sardines, because that’s not correct,” McClatchie says.

However, alternative food sources – such as squid and rockfish – are less nutritious, and may not be fatty enough to sustain some of the lactating mothers and their pups.

McClatchie and others think a number of factors combined to cause the fatalities. One of them may be the enormous size of the California sea lion population – it could be nearing the maximum number the habitat can support, and small perturbations in food supply could cause major ripples.

In addition, several pathogens were detected in last year’s stranded pups. “We don’t think they were the cause of the unusual mortality event, but they might have contributed to the severity of the impact,” says Sarah Wilkin, coordinator of NOAA’s Marine Mammal Health and Stranding Response Program.

What does seem clear is that California’s sea lions are headed for a rough stretch. There are predictions that an outbreak of bacterial leptospirosis could hit the population later this year, something that tends to happen every three to five years. And a looming El Niño could bring more disruption to the food supply later this year. In the past, El Niño years have seen the highest numbers of sea lion deaths and strandings.