The Steller sea lion, whose unexplained declines in Alaska have worried scientists and conservationists in recent decades, has seen an equally mysterious population drop on Russia's extreme eastern lands this year, researchers said.
Even as their worldwide numbers dwindled, sea lions — large playful members of the eared-seals family — had established new reproductive rookeries on Russia's Commander Islands, the country's farthest eastern outcropping of land, the islands' Komandorsky Nature Reserve said in a statement. But this summer, the number of new pups born at the rookeries was the lowest in more than 20 years of continuous observation, the statement said.
By the end of the sea lions' reproductive season in early July, only 163 pups were born on the main colony in a southeastern part of the Commander Islands, the statement said, citing a report by Sergei Ryazantsev, a researcher from the Kamchatka branch of the Pacific Institute of Geography that has been monitoring the rookery.
"The reasons for the sea lion population drop on the islands are unknown, and the decline in their number causes concern both among scientists who are studying it and among nature preserve employees," the statement said.
The report adds a new twist to a tragic global mystery that has affected sea lions' main habitat in the world, Alaska, since the 1970s: the numbers of the animals have been dropping, and nobody knows why.
The Gulf of Alaska and Aleutian Islands accounted for 74 percent of the global population in 1970s, but the number shrank to 56 percent by 1989, according to the U.S. National Park Service. The rate of the decline reached an alarming 15 percent per year in the late 1980s, according to the Alaska Department of Fish and Game.
Adding to the mystery, the decline affected only one of the two genetically different stocks of sea lions known in the world: the so-called western stock that lives in areas from California through southeast Alaska, according to the U.S. park service.
The sea lions on the Commander Islands and the eastern seaboard of the Kamchatka Peninsula also belong to the western stock.
The other, so-called eastern stock stretches from Prince William Sound further west.
As researchers race to stop the sea lions from becoming extinct and to figure out what is causing the population decline, several theories have emerged.
One hypothesis blamed a warming trend of the mid-1970s in the Gulf of Alaska and the Bering Sea that caused the numbers of small oily fish to drop and be replaced by less nutritional species of fish, causing an unhealthy change in sea lions' diet, the park service said.
"The change would be as if a human diet switched from steak and eggs to popcorn," the statement said.
Another hypotheses blamed fishermen who competed with Steller sea lions for resources, prompting a slew of regulations that created "no trawl" buffer zones around rookeries, making more pollock rich with nutritious roe available to female sea lions when they were pregnant, the statement said.
Researchers believe that the regulations may have helped slow or stop the sea lions' decline around Alaska after 2000. An initial, modest 5.5 percent increase in the western stock was reported in 2000-2002, and the numbers rose slightly further in the next two years, the U.S. National Park Service said.
In the 1960s, the number of sea lions around the world reached between 250,000 and 300,000, according to the U.S. National Park Service. However, only about 39,500 sea lions of the endangered western stock remain, according to the most recent data published in 2006, the park service said.