U.S. Geological Survey researchers analyzing nearly 40 years of at-sea bird surveys in the Gulf of Alaska and Bering Sea say seabird density has dropped about 2 percent annually since 1975.

Research wildlife biologist John Piatt of the USGS Alaska Science Center says a 2 percent annual decline would translate into an overall decline in numbers and biomass of 50 percent or more through 2012.

He calls the preliminary results a significant decline.

Surveys were conducted in hundreds of ship transits.

Piatt says the decline may be tied to less food availability, a consequence of warmer ocean temperatures.

The review does not include counts at nesting colonies. U.S. Fish and Wildlife Service seabird researcher David Irons says colony data indicates a decline that leveled off after 2000.