Cetacean distribution and relative abundance on the central-eastern and the southeastern Bering Sea shelf with reference to oceanographic domains

Visual line-transect surveys for cetaceans were conducted in the central-eastern Bering Sea (CEBS) from 5 July to 5 August 1999, and in the southeastern Bering Sea (SEBS) from 10 June to 3 July 2000, in association with a pollock stock assessment survey aboard the NOAA ship Miller Freeman. Observers scanned for cetaceans with 25x (Big Eye) binoculars from the flying bridge (platform height = 12m) at survey speeds of 18.5-22 km h(-1) (10-12 knots). Transect survey effort was 1761 km in 1999, in a study area 196,885 km(2); and 2194 km in 2000, in a study area 158,561 km2. An additional 609 and 402 km of trackline was surveyed in 1999 and 2000, respectively, while in transit to or from pollock survey way points. Fin whales (Balaenoptera physalus) were the most common large whale, and Dall's porpoise (Phocoenoides dalli) the most common small cetacean in both regions. In the CEBS (1999), uncorrected cetacean abundance estimates were: 3368 (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). In the SEBS (2000), uncorrected abundance estimates were: 683 (CV = 0.32) fin whales, 102 (CV = 0.50) humpback whales (Megaptera novaeangliae), 1003 (CV = 0.26) minke whales, 9807 (CV = 0.20) Dall's porpoise and 1958 (CV = 0.21) harbor porpoise. These are the first estimates of cetacean abundance that can be directly compared between two regions of the eastern Bering Sea. Distributions of some species were associated with bathymetric features, and there were occasions when prey associations were obvious. For example, in the SEBS, fin whales occurred on the Middle Shelf (50-100 m) and on the Outer Shelf (100-200 m) near the Pribilof canyon, but in the CEBS fin whales occurred primarily on the Outer Shelf along the 200 m isobath (i.e. the Green Belt). Fin whales were sometimes associated with echo-sounder backscatter from a mixture of fish schools and zooplankton. Humpback whales were also seen on the Middle Shelf, near the 50=m contour where the Inner Front often develops. 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. (C) 2002 Elsevier Science Ltd. All rights reserved.