A sick and malnourished California sea lion pup sits in an enclosure at the Marine Mammal Center in Sausalito, Calif., on Feb. 24, 2015. (Justin Sullivan/Getty Images)

California's sea lion population has dramatically increased since the 1970s, going from just 50,000 to 340,000. But another, more troubling, trend has taken hold in recent years: record numbers of starving pups.

Researchers with the National Oceanic and Atmospheric Administration believe they've found "compelling evidence" that explains the spike in malnourished pups, which they published Wednesday in the journal Royal Society Open Science.

To make a long story short, mama sea lions in southern and central California have terrible food options, and their babies are suffering as a result.

Researchers analyzed data from 2004 to 2014 on sea lion pup strandings as well as surveys of fish populations in likely hunting waters for pregnant and nursing sea lions. They found that pups weighed less when sardine and anchovy levels dropped – even if there was an abundance of squid and rockfish. The reverse was also true.

Sardines and anchovies have more calories and higher fat content than squid and rockfish, which makes the latter the equivalent of sea lion junk food.

This "shift from high to poor quality" food for breeding female sea lions means pups aren't getting enough to eat, "ultimately flooding animal rescue centers with starving sea lion pups," the study authors write. They weren't able to collect data that shows how low-quality food affected the quantity and quality of the sea lion mothers' milk.

"Sea lions are opportunistic feeders that switch and increase the diversity of their diet under conditions of scarcity," the authors write. They emphasize that limited food supply doesn't just happen when El Niño conditions make waters warmer.

It's not exactly clear what's behind the decrease in high-quality food for sea lions, and what role is played by the environment, fishing or the high resource demands of an increased sea lion population, the researchers note. But given there's been a decline in quality and abundance of food over a large area during a decade, that suggests that the food trends "are environmentally-driven," the researchers write.

"In the near term, we expect repeated years with malnourished and starving sea lion pups," the researchers write.

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