ANCHORAGE - The number of endangered Steller sea lions along a long stretch of Alaska coastline remains stagnant, federal scientists said Tuesday.
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Previous surveys had shown the western population of Steller sea lions - listed as endangered since 1997 - was growing at about 3 percent per year.
But the latest aerial survey conducted this summer shows the population is remaining the same, with some areas increasing and others decreasing, said Doug DeMaster, director of the National Oceanic and Atmospheric Administration's Alaska Fisheries Science Center in Seattle.
While bad weather and mechanical problems prevented flying over the westernmost portion of the survey area, the overall population appears to be about what it was in 2004, researchers said. Data gaps were filled by drawing on information gathered in 2006. The aerial survey has been going on since the 1970s.
"This year's count, while incomplete, supports the big-picture impression," DeMaster said.
Three NOAA scientists, accompanied by two pilots and a mechanic aboard a twin-engine Otter, conducted the aerial survey between June 9 and July 6. The survey area was from just east of Prince William Sound to Attu Island in the Aleutians.
The survey is done when the largest number of sea lions are onshore to breed and give birth.
Researchers used a camera mounted in the belly of the plane and pointed straight down to capture images of just over 26,000 sea lions, said Lowell Fritz with the center's Alaska Ecosystem Program. Scientists checked 260 sites for sea lions.
"If there were animals there, we would photograph them," Fritz said. "The numbers were basically unchanged if we added them up across the board."
The western population of sea lions probably stands at about 45,000 animals, down from an estimated 200,000 to 250,000 in the 1960s, Fritz said.
The survey shows sea lion numbers diminishing on the edges of the range. At the same time, sea lions in the central part from Kodiak Island to the eastern Aleutians - what was once the heart of their range - are doing better, Fritz said.
The eastern stock of Steller sea lions from southeast Alaska to the California coast is doing well, growing at an estimated 3 percent to 4 percent per year. Those animals estimated at between 45,000 and 51,000 are listed as threatened. They were not part of the survey, Fritz said.
The reasons for the decline in the western population remains unclear.
Modeling studies indicate there has been a fairly consistent drop in the birth rate since the late 1970s, Fritz said.
Scientists are looking at a variety of things. Disease or some kind of pollutant could be affecting reproduction, but there isn't much data to support that theory, Fritz said.
A once-prominent theory that killer whales were to blame also isn't panning out, he said. That's because research now shows that the survival rate for young sea lions - the ones most likely to be targeted by killer whales - has improved dramatically since 2000, Fritz said.
He suspects the lower birth rate has to do with the availability or distribution of fish. The key will be determining why that has occurred, whether it perhaps is connected to commercial fishing or climate change.
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