Scientists and animal-lovers along the West Coast worry as 2015 has already turned into the most dangerous winter for new-born sea lions in recent years.

In the first six weeks of 2015 alone, rescuers from San Diego to San Francisco have already discovered over 250 weak and starving sea lion pups – many more than were rescued during the same period in 2013, a year that the National Marine Fisheries Service recognized as an “unusual mortality event” in the beloved marine mammals. Most of the pups are around seven months old and starving by the time they are picked up on the coast.

Usually, sea lion pups nurse with their mothers until they are about 11 months old, when they become strong enough to swim and hunt on their own. The pups washing ashore should still be at their nursing sites in the Channel Islands or Mexico, according to marine scientists. Experts theorize that rising temperatures in the waters off the coast may leave the sea lion mothers’ usual fishing grounds barren, forcing them to travel further for food and leave their starving pups behind.

The Marine Mammal Center, based in Sausalito, has already rescued 120 young sea lions this year – more than the non-profit helped in the first four months of 2014 combined. “They just look very emaciated, very underweight,” said Erica Donnelly-Greenan, who manages the Marine Mammal Center’s rescue station along Monterey Bay, where dozens of the starving pups have been recovered. “You can see the bones under their skin. You can get a visual of their ribs.”

Shawn Johnson, director of veterinary medicine for the Center postulates that the abandoned pups become so hungry and desperate that “they just jump in the water and swim and get pulled to the coastline.” The young sea lions would quickly become overwhelmed by currents because “they’re just too small and weak to dive and catch fish.”

However, there is not yet any conclusive proof that this is the cause of the “mass die-offs” of sea lion pups. Scientists have not yet ruled out the presence of an unidentified disease, although they admit it is unlikely. This is the third year in a row – and, so far, the worst – of young sea lions washing ashore in large numbers in emaciated and weakened states.

The passage of the Marine Mammal Protection Act in 1972 helped boost declining sea lion numbers after centuries of the animals being hunted for their pelts and blubber. The current population is estimated at about 300,000, according to the National Oceanic and Atmospheric Administration. However, Johnson worries, “if this continues, there will be some long-term effects on the sea lion population.”