Hundreds of dead California sea lion pups were found on a small island south of San Francisco, alarming experts who are unable to explain the huge wave of fatalities.
The disturbing discovery on Año Nuevo Island in Monterey Bay coincided with similar observations as far south as Mexico, the Los Angeles Times reported.
The mysterious deaths came just days after more than 1,000 sea lions were counted at Pier 39 at San Francisco's Fisherman's Wharf, a popular tourist attraction, after being drawn there by large schools of anchovies and herring earlier this month.
Patrick Robinson, director of the University of California at Santa Cruz's Año Nuevo Reserve, told USA Today that he spotted about 50 bodies on May 7, with the number skyrocketing to 288 during a May 14 aerial survey.
"This is definitely an unusual event, but we don't yet know the cause," Robinson said.
UC Santa Cruz veterinarian Megan Moriarty said the dead sea lions included spontaneously aborted fetuses and stillborn or dead pups.
"In a typical year, one might expect to see five to 10," Moriarty told the Los Angeles Times.
In addition, researchers saw living pups that were malnourished and adult females that had experienced difficult births, Moriarty said.
Necropsies showed the dead pups weren't "developed enough to breathe on their own," Moriarty said.
"They'll sometimes live for a couple days at this point, but can't nurse and don't have the motor skills to hold their head up or nurse effectively," Moriarty said.
Testing for bird flu and domoic acid — a toxin produced by algae that can kill sea mammals — was reportedly underway.
A spokesperson for the National Oceanic and Atmospheric Administration said it was "tracking initial reports from biologists" about the surge in pup deaths while awaiting the results of annual surveys of sea lion birthing grounds in Southern California's Channel Island and elsewhere.
The California sea lion population comprises about 250,000 animals and is generally considered healthy, spokesperson Michael Milstein said.
"Premature births are not uncommon with sea lions, and often become numerous during El Niño periods when pregnant sea lions need to swim farther in search of shifting prey species," Milstein said. "This can leave them malnourished, leading to premature births of their pups."
