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 鈥� roughly four times as many as normal. While rehabilitation centres rescued as many as possible, the strandings were declared an 鈥渦nusual mortality event鈥� by the .
The cause has been a mystery, but one contributing factor has now been found: the sea lions鈥� , scientists from NOAA reported at a press conference on Monday.
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have shown that over the last 15 years, high-fat sardines have moved farther from shore, and their numbers are declining. This year鈥檚 annual spring survey reported very few sardines along the California coast, says Sam McClatchie of NOAA鈥檚 Southwest Fisheries Science Center in La Jolla.
Vanishing sardines
鈥淲e don鈥檛 know whether that鈥檚 because they have temporarily disappeared, as sometimes these fish do, or whether they鈥檝e moved out of our survey area,鈥� 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鈥檚 beaches, according to several biologists contacted by New 杏吧原创.
Stranded pup numbers are high again this year, albeit lower than last year. Already, 650 have been sent to rehabilitation centres 鈥� more than twice the normal number for this time of year, NOAA reported during the press conference. NOAA is now investigating whether this year鈥檚 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. 鈥淵ou shouldn鈥檛 get too fixated on the idea that all they鈥檙e feeding on is sardines, because that鈥檚 not correct,鈥� McClatchie says.
However, alternative food sources 鈥� such as squid and rockfish 鈥� 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 鈥� 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鈥檚 stranded pups. 鈥淲e don鈥檛 think they were the cause of the unusual mortality event, but they might have contributed to the severity of the impact,鈥� says Sarah Wilkin, coordinator of NOAA鈥檚 .
What does seem clear is that California鈥檚 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 Ni帽o could bring more disruption to the food supply later this year. In the past, El Ni帽o years have seen the highest numbers of sea lion deaths and strandings.
