Bering Sea Helicopter Surveys for Ice-Associated Seals (2007-08)

In the spring of 2007 and 2008, researchers from the Alaska Fisheries Science Center conducted aerial surveys for ribbon, bearded, and spotted seals in the US sector of the Bering Sea. The surveys were conducted from helicopters based aboard the US Coast Guard icebreakers Healy and Polar Sea. Line transect surveys were conducted between approximately 09:00 and 16:00 (local solar time), which corresponds to the timing of peak seal haul-out probability. By local solar time, we mean that for each 15° of latitude west of 0 degrees longitude, one hour was subtracted; thus, we used UTC minus 11 h for Bering Sea observations which puts the sun overhead at approximately noon at these coordinates. Each flight had two to three observers and was flown at a target altitude of 118m (400 ft) at speeds of 80–95 knots. Only seals hauled out on ice were observed and recorded. The distance from each seal to the helicopter’s track line was calculated using a sighting bar mounted on each observer’s window. In all, 2214 seals were observed during approximately 73 h of survey effort covering 11819 km of survey line on 63 flights. Because ice conditions decayed markedly toward the end of the 2007 surveys, our analysis used only the first 27 flights. The number of ringed seal sightings was low, possibly due to a combination of a preponderance of survey effort away from near-shore areas favored by ringed seals and a greater tendency for ringed seals to be disturbed into the water by the helicopter, and therefore to be missed by observers.