A warmer Pacific Ocean might sound like a dream if you're planning a summer vacation along the California coast. But for sea lions it's turning out to be a nightmare. Hot ocean temperatures are disrupting the food chain, leading thousands of hungry sea lion pups to wash ashore hungry and dehydrated.
Sea lion strandings have been high since 2013, when more than 1,100 of them came ashore. Almost 2,500 have washed ashore this year, mostly on southern California beaches from Santa Barbara in the north to San Diego in the south, according to the US National Oceanic and Atmospheric Administration (NOAA).
At this time in 2012, fewer than 150 pups had been stranded.
Sea lions are known to sun themselves along the shore. But the NOAA numbers represent pups that have washed up sick and without their mothers.
Female sea lions leave their pups on islands for days a time while they hunt. Scientists at NOAA believe that unusually warm ocean temperatures have driven key prey like sardines and anchovies farther north than where sea lions traditionally hunt their prey. That's forced the mothers to swim greater distances and stay away from their pups longer in an effort to find food.
"Some of the pups are striking out their own because they're so hungry and they're not getting enough from their mothers," NOAA's Michael Milstein told VICE News. "And [they're] not making it very well."
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The Pacific Marine Mammal Center in Laguna Beach has rescued 380 animals this year, nearly all of which are sea lions, a spokesperson told VICE News. They're currently treating 127 pups, the vast majority of which are suffering from malnutrition and dehydration.
About 300,000 California sea lions live and breed in the waters along the West Coast, from Mexico up through southern Canada. The population usually includes about 50,000 pups, which are generally born in June and stay with their mothers for a year.
In September, scientists weighing pups on San Miguel Island off the coast of Santa Barbara found that they were about 19 percent below their normal weight, weighing in at 31 pounds instead of a healthy 38 pounds.
"We certainly expect there's some that don't make it far enough to be stranded," Milstein told VICE News.
Sea surface temperatures off the coast of California have increased between 1 and 1.5 degrees Fahrenheit since the beginning of the 20th century, according to the US Environmental Protection Agency. That's in part due to warming sea surface temperatures that occur every two to seven years along the eastern edge of the Pacific Ocean, a phenomenon called El Nino.
But the variation in temperatures has been greater than usually, says Toby Garfield, an oceanographer at NOAA. Warming in the Gulf of Alaska in early 2013 and off the coast of Baja California last year combined to produce El Nino-like conditions in the North Pacific before they were seen at the equator, a reversal of the usual pattern.
That's affected the nutrient levels in the water and changed the distribution of a group of tiny crustaceans known as copepods, which form the basis of the food chain that sea lions rely upon. In warmer waters, copepods tend to be less nutrient-rich, which means even the food the sea lions can find is providing less nutrition.
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"We think that lack of energy is sort of cascading up through the trophic system," Garfield told VICE News. "It could be that nursing mothers of sea lion cubs are having to go longer and farther to feed their pups. … The whole energetics of the system seems to be out of balance."
It's not only affecting sea lions. In the Pacific Northwest, tens of thousands of tiny seabirds called Cassin's auklets have washed ashore, apparently starved to death. As many as 100,000 may have died since October.
California saw a spike in sea lion pup strandings in 2009 during an El Nino, with nearly 2,500 pups stranded. But only 185 had washed ashore by the end of March.
"We're on pace to go beyond those numbers," Milstein told VICE News. "There's always a few that struggle, but this year is far more pronounced than what we've typically seen."
Follow Laura Dattaro on Twitter: @ldattaro