IMIS

Visual line-transect surveys for cetaceans were conducted in the central-eastern Bering Sea (CEBS) from 5 July to 5 August 1999, in association with a pollock stock assessment survey aboard the NOAA ship Miller Freeman. Observers scanned for cetaceans with 25x (Big Eye) binoculars form the flying bridge (platform height = 12 m) at survey speeds of 18.5-22 km h-1 (10-12 knots). Transect survey effort was 1,761 km, in a study area 196,885 km2. An additional 609 km of trackline was surveyed, while in transit to or from Pollock survey way points. Fin whales (Balaeoptera physalus) were the most common large whale, and Dall's porpoises (Phocoenoides dalli) were the most common small cetacean. In the CEBS 1999, uncorrected cetacean abundance estimates were: 3,368 (CV = 0.29) fin whales, 810 (CV = 0.36) minke whales (B. acutorostrata), 14,312 (CV = 0.26) Dall's porpoise, and 693 (CV = 0.53) harbor porpoise (Phocoena phocoena). Non-pollock echosigns observed near cetaceans, some of which may have been cetacean prey, were not routinely identified during trawl sampling because the research focus was on pollock abundance assessment. Clearly, additional surveys and concomitant assessment of cetacean prey are needed to better define their role in the Bering Sea ecosystem. Such surveys, combined with measures of local hydrography and prey field should be the goal of future cetacean assessments. Purpose Cetacean distribution and abundance in the Bering Sea is poorly described, with even recent reviews of cetaceans' role in the ecosystem reliant on data from the commercial whaling era. Commercial harvests of baleen whales (mysticetes) were extensive in the North Pacific and Bering Sea, especially between 1835 and 1850 for North Pacific right whales and between 1965 and 1979 for fin and humpback whales. The effect of these large-scale removals on the marine ecosystem is largely unknown. Similarly, some species of toothed whales (odontocetes) are sometimes killed in the course of commercial fishing operations. Pelagic dolphins and Dall's porpoise were especially vulnerable during high seas driftnet fishing in the North Pacific in the 1980s, and once again the long-term ramifications of these removals are unknown. One reason for this uncertainty is the lack of data on current cetacean distribution and estimates of abundance in pelagic environments. Surveys to determine distribution and abundance are costly and, therefore, often confined either to coastal waters where the logistics are most practical, or to areas of the ocean where marine mammal mortality associated with commercial fishing is particularly high. The pelagic waters of the Bering Sea have not met either criteria and so are comparatively undersampled for cetaceans. Cruises were undertaken in association with the Alaska Fisheries Science Center/Resource Assessment and Conservation Engineering (AFSC/RACE) division to conduct visual surveys for cetaceans during the semi-annual acoustic trawl surveys for walleye pollock (Theragra chalcogramma) on the Bering Sea shelf. Biologists from the AFSC/National Marine Mammal Laboratory (NMML) were able to join surveys in the central-eastern Bering Sea (CEBS) in 1999, providing an opportunity to describe cetacean distribution and calculate abundance over a broad area of the Bering Sea shelf. Provisional results of the 1999 survey in the CEBS were presented, but without clear reference to the three hydrographic domains (coastal, middle shelf, and outer shelf) that delineate oceanographic and productivity patterns in the Bering Sea. Here, we present the first estimates of cetacean abundance that can be compared between two broad pelagic zones, as well as cetacean distribution and sighting rates relative to hydrographic domains and fronts commonly referred in papers describing oceanographic processes in the eastern Bering Sea.