"Clearly we are seeing steep declines still way out west in the western Aleutians," said Lowell Fritz, a biologist at the National Marine Mammal Laboratory in Seattle.
For instance, only 43 sea lions were counted this summer at Buldir Island at the end of the Aleutians, half the number of four years ago. In the 1980s, thousands were spotted, he said.
Fritz said Wednesday the western population stretching from Yakutat to the far western Aleutian Islands is "hanging in."
"We have not seen a total blinking out of an area yet," Fritz said.
The National Marine Fisheries Service conducted its aerial survey - the first complete survey since 2004 - over a two-day period in June when adult and juvenile sea lions are most likely to be onshore to give birth and breed.
The survey found that Steller sea lion numbers in the western population are either stable or declining slightly. The eastern population continues to increase.
The survey found the biggest declines were in the central and western Aleutians, where populations declined 30 percent and 16 percent respectively. The western population is now thought to be about 45,000, down from perhaps 250,000 in the late 1960s and early 1970s. The population was listed as endangered in 1997.
The eastern population, which extends from Alaska's Cape Saint Elias into California, now stands at between 45,000 and 51,000 animals. The population was declared threatened in 1990. It has more than doubled.
Most of the world's sea lions live along Alaska's vast coastline. There are believed to be about 16,000 across the Bering Sea off Russia's coast.
About $160 million has been spent on Steller sea lion research in recent years with the goal of stopping the declines. While scientists do not know why sea lions in some areas of Alaska are in trouble, certain things have become less plausible, Fritz said. Those things include killer whale predation, disease and contaminants, he said.
What scientists have determined is that the number of sea lions being born appears to be down. "Something is preventing them from getting born," he said.
Scientists are looking closely at nutrition as a factor to determine if females are getting enough food to give birth and nurse a pup from the previous year.
Global warming and more short-term changes in the Bering Sea also could be factors, he said.
Dave Benton, executive director of the Marine Conservation Alliance, a coalition of commercial fishing groups, said it is important to note that since 2004 there was a 7 percent increase in sea lions in the eastern Aleutians, the area where most of the commercial fishing takes place.
Despite no-fishing zones around rookeries and haul-outs, Benton said sea lion numbers continue to decline in the central and western Aleutians.
"We need continued scientific research to determine those factors and help rebuild these stocks," Benton said.
Fritz said scientists are assessing the impacts of commercial fishing on sea lions, which he said remains a potential high threat to the animals. A biological opinion is due next year.
For more information visit the Stellar sea lions page on the NOAA Web site.
By Associated Press Writer Mary Pemberton