Researchers are puzzled why hundreds of frail and starving sea lion pups continue to be washed ashore in California beaches.

Rescuers have been netting over 250 sick juvenile sea lions from San Diego to San Francisco Bay since January.

Most of these ill and hungry pups were seven months old and were supposed to be nursing with their mothers in Mexico or the Channel Islands.

This year is the third year in a row when massive numbers of young sea lions have been dying off. Alarmingly, the 2015 figures also happen to be the worst.

More pups are being taken into rehabilitation facilities this year than in 2013, when the National Marine Fisheries Service declared a rare "unusual mortality event." If this continues, there could be long-term effects on sea lion population, experts believe.

"They just look very emaciated, very underweight," said Erica Donnelly-Greenan from the private and non-profit organization Marine Mammal Center. "You can see the bones under their skin. You can get a visual of their ribs."

The pups that turned up on the beaches were very malnourished; they weighed only half of their ideal weight.

Experts say that the pups that go to the center actually weigh just between 15 and 16 pounds; the ideal weight should, however, be from 35 to 40 pounds.

The exact cause of the massive die-offs remains a mystery. Scientists say it is possible that the warmer waters off the coast force the mother sea lions to venture farther, in search for food, and leave their pups behind for too long.

Marine Mammal Center director of veterinary medicine Shawn Johnson said that because the young sea lions are starving and desperate, they jump into the water and swim. Unfortunately, the young animals are too small and too weak to catch their own food.

Justin Viezbicke from the NOAA Fisheries in California also said that the number of sea lions may have hit a natural ceiling, with habitat and food no longer enough to support more animals.

"These animals are a puzzle piece for us," Viezbicke said. "They provide insight into what's going on in the environment ... These things could come back and affect humans."

The NOAA is also looking at the possibility that the recent die-off may be associated with changes in the ocean conditions or other problems at sea. Some also think that the pups suffer from a yet unidentified disease.