Comparison of aerial survey procedures for estimating polar bear density: Results of pilot studies in Northern Alaska

The U.S. Marine Mammal Protection Act (MMPA) and International Agreement on the Conservation of Polar Bears mandate that boundaries and sizes of polar bear (Ursus maritimus) populations be known so they can be managed at opti mum sustainable levels. However, data to estimate polar bear numbers for th e Chukchi/Bering Sea and Beaufort Sea populations in Alaska are limited. We evaluated aerial line transect methodology for assessing the size of these Alaskan polar bear populations during pilot studies in spring 1987 and sum mer 1994. In April and May 1987 we flew 12,239 km of transect lines in the northern Bering, Chukchi, and western Beaufort seas. In June 1994 we flew 6 ,244 km of transect lines in a primary survey unit using a helicopter, and 5,701 km of transect lines in a secondary survey unit using a fixed-Ring ai rcraft in the Beaufort Sea. We examined visibility bias in aerial transect surveys, double counts by independent observers, single-season mark-resight methods, the suitability of using polar bear sign to stratify the study ar ea, and adaptive sampling methods. Fifteen polar bear groups were observed during the 1987 study. Probability of detecting bears decreased with increa sing perpendicular distance from the transect line, and probability of dete cting polar bear groups likely increased with increasing group size. We est imated population density in high density areas to be 446 km(2)/bear. In 19 94, 15 polar bear groups were observed by independent front and rear seat o bservers on transect lines in the primary survey unit. Density estimates ra nged from 284 km(2)/bear to 197 km2/bear depending on the model selected. L ow polar bear numbers scattered over large areas of polar ice in 1987 indic ated that spring is a poor time to conduct aerial surveys. Based on the 199 4 survey we determined that ship-based helicopter or land-based fixed-wing aerial surveys conducted at the ice-edge in late summer-early fall may prod uce robust density estimates for polar bear populations in the Chukchi/Beri ng and Beaufort seas.