This malnourished juvenile sea lion pup was rescued by the Marine Animal Rescue on April 5, 2013 in the San Pedro area of Los Angeles, California. Kevork Djansezian/Getty Images Young California sea lions have been struggling for years. In 2015 alone, thousands of emaciated pups washed ashore, overwhelming rescue efforts.

Researchers have been searching for clues on the pup's demise ever since and now, they may be one big step closer to understanding its source.

In a new study published March 2 in the journal Royal Society Open Science, scientists have found that the emaciated pups are suffering because their mothers aren't eating enough high-quality food.

And while the team is not entirely sure what's bottoming out their nutrition, they think it's a trend that is unlikely to reverse itself any time soon.

California sea lions have been in a bad place for a while, but their situation has reached epic proportions in the past year.

In the six months between January and June 2015, an unprecedented 3,000 emaciated and starving sea lion pups washed up onto California's beaches. By the end of that year, the Marine Mammal Center in Sausalito near San Francisco had counted a staggering 4,200 stranded California sea lions — the worst the center has seen in their 40-year history.

Scientists have scrambled to understand why these pups are declining in droves while conservation centers care for the thousands they've taken in.

The new study suggests that their poor health is a result of their mothers eating the oceanic equivalent of "junk food," as Discovery News calls it.

Sea lions eat a wide variety of fish and squid, but not all food items stack up equally on their food pyramid. Sardines and anchovy pack the biggest nutrition and calorie punch, whereas rockfish and squid are the cheaper, less-preferred snack.

Think of sardines and anchovy as the eggs and avocados of the sea lion food world, whereas rockfish and squid are the less wholesome breads and pastas.

Sick and malnourished sea lion pup named Tough is anesthetized for an examination at the Marine Mammal Center on February 24, 2015 in Sausalito, California. Justin Sullivan/Getty Images

Scientists have noted in the past that when these species of fish go through natural environmentally-driven fluctuations, so does sea lion health.

During the El Niño of 1982 to 1983, for example — which is a natural ocean-warming weather phenomenon — sea lion births and birth weights declined while the number of pup deaths increased.

After analyzing a decades-worth of data on the availability of forage fish in California sea lion habitats, the team found that sea lion mothers are being forced to eat less nutritious rockfish and squid because higher-calorie sardines and anchovy populations have nosedived.

Warmer-than-normal waters in the Pacific Ocean in the past two years, which have worsened due to the present El Niño phenomenon, is a big offender. The prevailing theory has been that the California's sea lions' preferred prey are swimming north to cooler waters.

But while this new study suggests that limited sea lion food for nursing mothers is linked to warmer waters, that's likely not the only culprit.

Despite their current struggle, California sea lion numbers have exploded in the past 40 years — growing from a population of 50,000 to 340,000. With more sea lions hunting for food, their prey become more scarce.

Michael R Perry/Flickr

The study doesn't conclude what exactly is causing this dip in availability of nutritious food, but it's likely a combination of multiple factors in the environment, including warmer waters, fishing impacts, and sea lion population growth.

And unfortunately for the sick pups, this downward trend probably won't right itself in the near future, the authors noted in the paper.

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