Imagine a classroom of thirty students. Then, imagine them slowly leaving until only four or five are left. You would probably wonder why.
In a similar scenario, a sea lion population mysteriously declined 85 percent over three decades. For a population that diminished so rapidly, finding where sea lion bodies went would answer many lingering questions. But, there are no bodies to be found; not from starvation, not from disease, not even a single body part on the beach. They have just disappeared. With an average male weighing just under a ton, where did they go?
The Northern or Steller sea lion is divided into an eastern and a western stock. The eastern stock lives south of the Alaskan coast, extending down to California. The western stock lives in the Gulf of Alaska.
Both sea lion stocks have seen a net decrease in population, but the western stock has rapidly declined over the past 30 years. Meanwhile, the eastern stock, which is listed as threatened, has steadily increased 3 percent each year over the past 30 years, said Tom Gellat, program leader of the Alaska Ecosystem Research Program.
The western stock hit its lowest numbers in the ‘80s, a dramatic decline of 15 percent per year, said Lowell Fritz, research fishery biologist at the Alaska Fish and Science Center. The population dropped 5 percent per year in the ‘90s.
"For a population like this, it’s like dropping off a cliff," Fritz said.
Before the decline, 250,000 western sea lions roamed the sea. At its lowest population in the year 2000, only 40,000 to 45,000 were left, Fritz said. In 1990, the National Marine Fishery Service listed the western sea lion as threatened, and then endangered in 1997, Fritz said.
No one knows why this population has seen such a dramatic decline, but many theories have surfaced. It’s different each decade, Fritz said. Before the decline started in the ‘70s, there was no commercial fishing around the western sea lion habitats. Then, in the ‘70s, competition between fisheries depleted the sea lions food source; cod, herring, pollock, Atka mackarel and other fish. Without prey to feed on, the western sea lion growth rate slowed. Incidental net catches of sea lions also played a part in their decline.
Another sign the population was in distress was that the females started giving birth at later ages and fewer times in a life span.
Typically, a female can give birth around the age of four, Fritz said, but most likely around five or six. Females can give birth once a year and produce up to 20 sea lion pups in a lifetime, but instead they were giving birth every other year, and then every third year. Additionally, the small number of females hinders the ability for the population to bounce back.
In the late ‘70s and early ‘80s, a climate change warmed the water, which changed the distribution of many of the fish sea lions eat. This increase in temperature, accompanied by a change in food distribution is directly related to a decrease in reproductive and survival rates of the western sea lion, Fritz said.
"The decline in birth rates has not rebounded as much or as fast as it should have," Fritz said.
The last theory involves increased consumption of western sea lions by killer whales, because they are the only ones that eat them, Fritz said. But, sea lions make-up only 10 percent of their diet.
The decline in sea lions has not had a rippling effect on any other animal, including the killer whale, Fritz said.
Others disagree, in the late ‘90s, the sea otter population suffered from a dramatic decline as well, causing the whale population to relocate to the Gulf of Alaska. The decline in sea otters forced the whales to feed on the western sea lions and other seals, according to an article in Science Magazine. The decline in the western sea lions could have a similar effect on whale eating habits.
After researching the possible reasons for the decline scientists found some solutions. Protected habitats were established, marking off the areas identified as critical to the sea lion’s recovery, Fritz said. These areas are not closed to use, but fishing is highly regulated and managed by the National Marine Fishery Service.
"It’s challenging finding a solution to satisfy both economy and habitat," Fritz said.
Another practice aiding the recovering population is monitoring sea lion activity. However, extensive monitoring is extremely difficult, Gellat said. During the summer, aerial photos are taken of sea lion colonies lying on rook reefs or large rocks in Alaska, a place where pups are born and counted. Summer is when western sea lions breed and give birth.
Another way to track the population is to collect information on their life history or attributes, Gellat said. This is accomplished by addressing the questions of how long they live, how many pups a female can have in a lifetime and at what age females give birth. This way, if a female gives birth later than normal, researchers can figure out why, Gellat said.
Since the year 2000, the population of western sea lions has stabilized and in some cases has even increased slightly, Fritz said.
"We will continue to learn more," Gellat said. "We will never know why they declined so rapidly. We just hope to find out what’s going on."
The decline of stellar sea lions remains a mystery. The population’s recent increase gives researchers time to uncover the answers, and to design ways to prevent potentially more devastating declines in the future.