Cetacean Habitats in the Alaskan Arctic

Abstract

Marine mammals can be used as indicators of environmental productivity because they must feed efficiently and therefore aggregate where prey is plentiful. Three species of cetaceans, bowhead whales (Balaena mysticetus), gray whales (Eschrichtius robustus) and white whales (Delphinapterus leucas) migrate to the Alaskan arctic each year to feed. These species have distinctly different feeding modes and forage at dissimilar trophic levels. Bowhead whales filter zooplankton from the water column, gray whales siphon infaunal crustaceans from the benthos and white whales catch a variety of nekton including crustaceans, cephalopods and fishes. Line transect aerial surveys were conducted over the Alaskan Chukchi and Beaufort seas each late summer and autumn 1982–91. The resulting database, consisting of 634 flights, was post-stratified by survey type and sea state (Beaufort ≤04) to provide a database of cetacean sightings made along random transects during good survey conditions. Sightings made during connect and search legs of the survey, and in rough seas were excluded. Post-stratification resulted in a cumulative (1982–91) database of 276 754 transectkm of survey effort during which there were 554 bowhead, 608 gray and 831 white whale sightings. Habitat partitioning and variability in habitat use among cetaceans in offshore areas of northern Alaska is poorly defined. Available data suggest that cetacean distribution and abundance patterns can be quantified on the basis of water depth and surface ice cover, and that these indices can be linked to large-scale oceanographic processes. In summer, mean depth and percent surface ice cover were significantly different (p <0.001) among bowhead (900 m, 52%; n = 79), gray (40 m, 1%; n = 497) and white whales (1 314 m, 60%; n =146). All pairs were significantly different (p <0.003), except for bowhead–white whale ice cover (p <0.13). Similarly in autumn, mean depth and percent ice cover were significantly different (p <0.001) among bowhead (109 m, 22%; n = 475), gray (38 m, 7%; n = 111) and white whales (652 m, 52%; n = 685); all pairs were significantly different (p <0.001). In addition, mean depth and percent ice cover were significantly different (p <0.001) between summer and autumn for bowhead and white whale sightings. Currents are bathymetrically driven, and ice cover influenced by currents and wind, in the Chukchi and Beaufort Seas. The association of cetaceans with specific bathymetric and ice cover regimes provides a foundation for further investigation of inter-specific habitat selection, zones of productivity and insight to the role of cetaceans in Alaskan arctic ecology.