Something fishy is happening along the northern section of North America’s Pacific coast. While the population of Steller’s sea lions is growing in Oregon, Washington and British Columbia, where groups of animals are recolonizing extirpated rookeries, it’s crashing in the Gulf of Alaska and farther west. “Protection measures and rates of recovery don’t line up,” says Andrew Trites, director of the Marine Mammal Research Unit at the University of British Columbia. “It appears something is going on.”

Changes to Canada’s Fisheries Act in 1970 protected the sea lions from the culling that had nearly wiped them out, but it wasn’t until 1983 that their numbers began recovering, and since then, the population has grown by an average of 4.7 percent per year. Over the same period, the worldwide population has crashed from 300,000 animals to fewer than 100,000.

These inverse population swings roughly coincide with a dramatic change in ocean conditions known as the Pacific Decadal Oscillation (PDO), which alters the prevalence of different fish species and, accordingly, impacts the diet of the Steller’s sea lion. Today, its preferred food items — oil-rich fish such as sand lance, herring and sardines — are common in British Columbia but rare in Alaska, where the sea lion feeds mostly on low-calorie pollock and cod. Trites believes that the high-oil fish in B.C. waters allows the female to wean her pups annually, while the Alaskan pups remain dependent on their mother’s milk for an extra year or more as they struggle to get enough nutrition to grow and stay warm on their own.

To test his “junk food” theory, Trites fed only pollock to captive Steller’s sea lions at the Vancouver Aquarium Marine Mammal Rescue Centre. “The animals got full before they reached their daily energy needs,” he says. “It would be as if you could eat only celery. You’d be full but still hungry.”

For corroboration, Trites looked to First Nations traditional knowledge and archaeological evidence. He found that Steller’s sea lion abundance has cycled for thousands of years, just like the PDO. “We tend to assume that what’s happening today in the natural world is humancaused,” says Trites. “In this case, it appears to be naturally driven.” But if global warming throws a wrench into the works, putting the PDO on pause, he adds, “the scenario we are seeing today may go on forever.”