Pacific marine heatwave upends Steller sea lion winter diets

SEWARD, Alaska (KTUU) - Steller sea lions aren’t the easiest mammals for scientists to monitor. Rookeries are often on remote rocky outposts like Chiswell Island near Resurrection Bay, where remote monitoring cameras were set up about 25 years ago to study the marine mammals’ population trends.

But the cameras only go so far when it comes to researching marine mammals like Steller sea lions.

A recent study authored by Alaska SeaLife Center scientist John Maniscalco in the journal “Global Ecology and Conservation” studied Steller sea lions up close. By examining the scat remains of the creatures during and after the Pacific marine heatwave of the mid-2010s, Maniscalco found that the heatwave had a profound effect on the sea lions’ winter diet.

“It is the first study that has looked at Steller sea lion diets this deeply and consistently over several years at a time and especially over years where we saw this significant change in water temperatures,” Maniscalco said.

Prior to the heatwave when the waters were relatively cooler, Maniscalco found the Stellar sea lions preyed on shallow-water fish like capelin.

But as the warming trend spread across Pacific waters to the coast of Alaska, the sea lions began diving deeper for their prey; the capelin were nowhere to be found.

“They either went deeper, and were inaccessible to the sea lions, or they just didn’t reproduce and just weren’t out there in big numbers in the water column where the sea lions typically forage for them,” Maniscalco said.

Smaller, less nutrient-rich species like polychaetes, sand lance, sculpins and skates took the place of the fish. Maniscalco said researchers once thought that a highly-diverse diet was a good thing, but that’s only true in the summertime.

“During the winter months, things are different,” he said. “If they have a highly-diverse diet, it could mean that they’re just scrounging around for whatever they can find.”

The study looked at the winter diet of the Steller sea lions for several reasons. The majority of past studies on Steller sea lion diets have been centered on the summer. Maniscalco said the winter diet is crucial to the survival of the species because it’s when they are most vulnerable. Females are often nursing offspring from the previous summer as well as pregnant with a new pup for the coming summer.

“There’s a lot of physiological stress on these animals during that time of year, and at the same time, there’s there’s not necessarily a lot of food available for them during the winter months,” Maniscalco said.

Maniscalco said the study provides key insights on what may have contributed to Steller sea lions being placed on the endangered species list in the first place, namely, the disappearance of fat-rich fish like capelin. Between the late-1970s and early-2000s, the population of Steller sea lions in Western Alaska declined by about 80%.

But it’s not just Steller sea lions that require capelin, but seabirds like kittiwakes and murres as well.

“When capelin don’t do well, nothing else does well. So if there were one canary in the coal mine, I’d say, (it’s) capelin,” Maniscalco said. “We really need to keep an eye on what capelin are doing out there.”