LOS ANGELES (Reuters) - A warming ocean is taking its toll on California's popular and playful sea lions for a fourth straight year, with scientists now reporting lower birth rates depleting the population, and masses of young animals still washing up starving and stranded on beaches.
More than 2,000 emaciated California sea lions, mostly pups and juveniles, have been found beached dead and dying along the state's southern and central coasts since January, according to the National Oceanic and Atmospheric Administration (NOAA).
That is over twice the average number of strandings considered normal, although it is about half as many as the record 4,000 documented strandings that alarmed the public during the first six months of last year.
NOAA biologists believe declining births may account for the year-to-year reduction in mass strandings. Both trends are attributed to warming ocean temperatures along the Pacific Coast that have disrupted the marine mammals' food supply of sardines, anchovy and squid.
The phenomenon has no doubt put a dent in sea lion numbers, estimated at about 300,000 animals before beachings began to spike in 2013. The overall toll taken on the population has yet to be calculated.
The trend, while not considered an immediate threat to the species, could pose "pretty dire consequences" if it were to continue for a decade or more, said NOAA biologist Jeff Laake.
Known for their playfulness, loud barking and natural curiosity, sea lions are a familiar presence to California beachgoers and divers. They rank as one of the state's favorite coastal wildlife attractions.
Easily distinguished from their seal cousins by the presence of external ear flaps, sea lions are also considered an important species in the region's marine food chain.
Scientists believe a growing scarcity of natural prey around the animals' island rookeries off Southern California has forced nursing mothers farther out to sea in search of food, leaving their young behind to fend for themselves for longer periods of time.
When the malnourished pups venture off the islands to forage on their own, they end up carried off by currents and washed ashore on mainland beaches.
The disturbance in the food supply has also taken its toll on the animals' reproductive cycle.
Biologists visiting the Channel Islands off the Santa Barbara coast found a 40 percent decline in sea lion births from 2014 to 2015, NOAA officials said.
"The overall number of pups born was lower, which translates to an overall lower number of stranded pups," said Justin Viezbicke, the California stranding network coordinator for NOAA's National Marine Fisheries Service.
The falling birth rate is likewise driven by food-related stress on adult females. The more energy it takes to find prey, the harder it is for females to breed successfully or to carry fetal pups to term.
"It's all nutrition-based," Laake said.
Among pups that were born last year, more were succumbing to starvation and dying in the rookeries, further reducing the number of young animals that could make it far enough off the islands to become stranded on the mainland, Laake said.
This summer's annual pup count by scientists is just getting under way, he said.
Marine mammal rescue centers were overwhelmed with sea lion strandings in 2015, with some 4,600 animals washing up throughout the year, most before July.
More than two-thirds perished, either by the time they were found or during rehabilitation, Viezbicke said. Rescue teams were forced to leave some severely weakened animals to die on the beach. About 1,300 were nursed back to health and released.
Of the 2,000-plus young sea lions stranded so far this year, about 300 were found dead and the rest taken in for rehab care. How many will survive to be returned to the wild remains to be seen, Viezbicke said.
Also unclear is how long ocean conditions blamed for the crisis, declared an "unusual mortality event" by NOAA since 2013, will persist.
Rising sea temperatures that have displaced the animals' food are linked to a diminishment of the winds that normally help pull nutrient-rich, cooler water from the depths of the Pacific closer to the surface. Experts theorize the recent El Nino effect may have compounded the situation.
Reporting and writing by Steve Gorman; Editing by David Gregorio