Steller sea lions captured by a hexacopter. (Photo courtesy NOAA Fisheries)

Every summer, biologists visit Alaska to count Steller sea lions. The western stock of the population has been in decline for nearly 40 years — hitting a low in 2002. The count helps determine whether sea lions stay on the endangered species list, which puts costly restrictions on the commercial fishing fleet. Even after decades of research, the reason for the decline is still a mystery.

It’s 10:30 at night and the Tiglax has just pulled into Dutch Harbor. Biologist Tom Gelatt is finishing up a two week stint with his team. They visited 35 sites to count sea lions from the furthest west point on the Aleutian chain to Unalaska.

A map of survey locations. (Courtesy NOAA Fisheries)

“My job is to try and figure out how many there are, where they are, and if there are declines, where are those threats? What are the things that are limiting the recovery of Steller sea lions?” Gelatt said.

He collects data the old fashioned way, by hand counting sea lions. But Gelatt prefers using aircraft — high tech drones or a twin otter plane — to capture photos of rookeries and haulouts. His big question: Why is the population of Steller sea lions in the western Aleutian Islands declining while the animals are doing well in other parts of the state?

“We honestly do not know why because the animals that we see out there appear to be quite healthy,” Gelatt said. “They’re quite large. We don’t see dead, dying animals on the beach. We just don’t see the numbers. They just aren’t there.”

The decline of Steller sea lions dates back to the 1970s. Over 24 years, the population of the western stock plummeted 75 percent. In 1997, the federal government listed Steller sea lions west of Kayak Island, near Cordova, under the Endangered Species Act.

Today, the eastern stock is rebounding — at a rate of about 3 percent a year since the 1980s — and those sea lions are no longer on the endangered species list. But sea lions in the west are still struggling. And Gelatt says the count is a factor that determines whether the animals stay endangered.

“Under the Endangered Species Act, there are some pretty stiff protection measures in place to protect Steller sea lions which often compete with the fishery,” Gelatt said. “So everyone wants to know how many there are.”

Until the population rebounds, federal restrictions — like prohibiting fishermen from harvesting the full allowable catch — will stay in effect.

According to John Gauvin, with the Alaska Seafood Cooperative, those restrictions have a big financial impact on fishermen in the region.

“The measures in place are in the tens to 20 million dollars of revenue reduction, (that) would be my guess,” Gauvin said.

Gauvin says the fisheries that take the biggest hit are for Atka mackerel and Pacific cod. He says the fishing industry is not opposed to restrictions, but he also says right now there isn’t a lot of evidence that fishing is having any negative impact on sea lions.

Holding a hexacopter drone before it takes flight. (Photo courtesy Kristen Campbell/NOAA Fisheries)

For the most part Gelatt agrees; Gauvin is probably right. There is no hard evidence commercial fishing is to blame for the sea lion decline. He says lots of factors could play a role.“The fishing industry will do what it can to protect Steller sea lions, but at the same time we aren’t convinced that fishing is what’s affecting sea lions out there and the science is hardly straightforward on that,” Gauvin said.

“Obviously there are predators like killer whales that take Steller sea lions,” Gelatt said. “There’s also issues with perhaps climate change and the change of habitat, and the availability of prey, the warming of the oceans — something that may have nothing to do with commercial fisheries.”

And it’s hard to pinpoint a culprit because all those threats may be acting separately or together. But Gelatt says the mystery keeps him coming back.

“It’s interesting that we have an animal that has been studied for a long time and we know a lot about,” Gelatt said. “Yet because they’re so difficult to work with and so far away, some of the basic ecology questions that we know about other large mammals are really difficult to get.”

On this trip, Gelatt’s team has amassed vast amounts of data, including more than 300,000 photos, that they have to sift through.  So for now, he says we all have to stay tuned.