Researchers with the Alaska Fisheries Science Center have determined that the population of Steller sea lions in the Western Aleutian Island is continuing to decline, a troubling trend, they say.
Within that western most region, the number of Steller sea lions has declined 94 percent in the last 30 years, including nearly 7 percent in 2016 alone, and nobody know for certain why this is happening.
Researchers say they are making progress, by ruling out many possible causes, while meanwhile science and management actions have contributed to other Steller sea lions populations increasing.
The species is made up of two distinct populations: the Eastern Stock and Western Stock. The Eastern Stock begins at Cape Suckling, Alaska and follows the coat to California. The Western Stock extends west of Cape Suckling, through the Aleutian Islands and into Russian territory.
According to AFSC research biologist Lowell Fritz, the Eastern Stock of Stellers is doing very well, particularly in southeast Alaska and British Columbia.
Overall, the Western Stock Steller sea lions have increased slightly this year, due to sea lions living in the eastern portion of the Western Stock’s rang- an area that has seen population increases since 2003.
But Stellers living in the western Aleutians have continued to decline west of Samalga Pass to Attu Island, said Center research biologist Katie Sweeney.
Research and management decisions by NOAA Fisheries and partners have taken since 1990 have made position impacts to Steller sea lion numbers in other areas. The Eastern stock had been listed as threatened under the Endangered Species Act since 1990 before rebounding and getting delisted in 2013.
Researchers say one of the most influential actions was ending a fishing practice that kept hundreds of tons of freshly caught fish semi-enclosed at the surface, awaiting processing.
The net full of fish would attract Steller sea lions. They would swim inside for a free meal. Then, when the processing ship came, the net was closed. Before that practice was phased out in the mid-1980s, scientists estimated that thousands of sea lions would be trapped and killed inside nets each year, which also affected pup populations. If a seal mother was killed, the pup would die of starvation.
The Alaska Department of Fish and Game led investigations into whether the animals were dying of disease, but after scanning for a variety of pinniped diseases, found no significant contribution factor.
The declines also prompted scientists to begin permanently marking some pups to better track individual animals throughout their lives. One of many benefits is that researchers know the animals from the western Aleutians aren’t simply moving east where populations are increasing. For the most part, Steller sea lions are staying put, researchers said.
Meanwhile, researchers are looking at other possibilities, including one theory that there is some kind of food supply shortage causing nutritional stress, leading to a slew of chain reactions.
For instance, female Steller sea lions care for pups longer than many other pinniped species.
“They’re good moms, Sweeney said. “In fact, a nursing pup may not wean for years. That’s in stark contrast to northern fur seal and harbor seal pups. They are weaned months after birth.
So researchers are asking now whether high maternal investment leading to decreased births and and subsequent decreases in recruitment in the western Aleutians.
“We see pups and the pups seem fat and healthy and the moms seem fat and healthy,” Fritz said. “Survival rates are pretty good. They are surviving until age two at the same rates where they are increasing. We just see way fewer pups. At Attu, we now only see 30 to 40 pups born each year, while back in the late 1970s, it was more than 600.”
If females in the west are putting extra effort into rearing pups, at the expense of giving birth to another, why are they doing so, scientists want to know.
Some clues may be coming from specially designed cameras built to withstand harsh Aleutian weather and sea lion curiosity. These remote cameras have shown photos of some pups being born, dying and eventually washed out to sea or being inadvertently buried by rocks. Scientists wonder if newborn pups are not surviving because females are not able to consume enough food to grow a healthy new pup.
In turn, females could be continuing to nurse older pups longer.
Other theories are that Steller sea lions are competing for food with killer whales and humans. Killer whales may be out-maneuvering Steller sea lions for fish, and so might some commercial fisheries. Another theory considers the impact of mammal-eating killer whales preying on sea lions.
Still another possibility is contamination. Research conducted over the last seven to eight years has shown high levels of mercury, but scientists don’t know if the contamination was recent or was always there, plus the mercury hotspots don’t match the declines, researchers said.
