– Once a year in June, sea lion mothers on the Channel Islands give birth to a pup. For the next 11 months, the mothers swim off to forage for days to provide food for themselves and milk for their progeny.

But in early 2015, calorie-dense sardines and anchovies — the best food — were hard to find. The mothers were forced to dive deeper and swim farther, and by the time they returned to the islands, they didn’t have much milk to offer.

The ocean, which in years past had been full of food, was emptier than before. The hungry pups struck out prematurely that winter, and wound up stranded on mainland beaches in record numbers, emaciated and starving.

As El Niño bears down on California this year, worse is expected.

Marine mammal care centers are preparing for a rash of strandings. They do so even as some marine biologists and ocean advocates warn that such a compassionate response is little more than a futile attempt to wrap a Band-Aid on an oceanwide problem that could last decades and may have been worsened by overfishing.

“In the end, it’s shortsighted to believe you’re going to save the sea lions simply by rescuing them, rehabilitating them and then sending them out when there’s not really enough food to go around. From a humane point of view, it makes sense, but it’s really only addressing the symptom rather than the root cause,” said Geoff Shester, the California campaign director for Oceana, an environmental group.

No one knows how well the sea lion pups fare after rehabilitation, because “once you release them, the odds of seeing them again are really low, whether they make it or not,” said Sharon Melin, a National Oceanic and Atmospheric Administration wildlife biologist who studies sea lions.

In a normal year, the Pacific Marine Mammal Center in Laguna Beach will rehabilitate 100 to 120 sea lions. In 2015, the center rescued a record-setting 534 sea lions, including more than two dozen that had re-stranded. The center is braced for even more this year.

“That’s a high number, but it shows there’s something wrong out in the wild,” said Keith Matassa, the executive director at the mammal center.

Historically, El Niño events such as the one in California now caused pup production to plummet. But three years ago, it wasn’t an El Niño season, and births were very low. Something else was going on. It’s what El Niño does to sea lions’ food supply that makes them suffer.