Estimates of Abundance of Western/Southern Spotted, Whitebelly Spinner, Striped and Common Dolphins, and Pilot, Sperm and Bryde's Whales in the Eastern Tropical Pacific Ocean

摘要

Estimates of abundance for the western/southern stock of offshore spotted dolphins (Stenella attenuata), the whitebelly stock of spinner dolphins (S. longirostris), striped dolphins (S. coeruleoalba), three stocks of short-beaked common dolphins (Delphinus delphis), short-finned pilot whales (Globicephala macrorhynchus), sperm whales (Physeter macrocephalus) and Brydes whales (Balaenoptera edeni) in the eastern tropical Pacific Ocean are presented. The estimates are based on large-scale line-transect surveys carried out with oceanographic research vessels in 8 different years between 1986 and 2000. Searching for cetaceans was conducted primarily with pedestal-mounted 25x150 binoculars fitted with azimuth rings and reticles for angle and distance measurements. Aerial photography was used to improve observers estimates of group sizes. Estimates of abundance for each species or stock in each year were based on modified line-transect methods which included covariates to model the probability of detection. For the 15-year period, the low and high total estimates of abundance (in numbers of animals) were: 598,541 and 1,444,154 for western/southern offshore spotted dolphins, 243,812 and 1,067,540 for whitebelly spinner dolphins, 801,210 and 1,497,428 for striped dolphins; 56,207 and 642,465 for northern common dolphins; 180,460 and 731,652 for central common dolphins; 292,078 and 2,301,478 for southern common dolphins; 136,448 and 589,315 for pilot whales; 4,145 and 49,653 for sperm whales; and 3,364 and 14,413 for Brydes whales. Most species/stocks did not show significant changes between the 1986-1990 and 1998-2000 periods. Pilot and Brydes whales were significantly higher in the later period, but interpretation of changes in abundance are confounded by several factors, including movement in and out of the study area and a larger survey area in the later period.