Abstract

Dall’s porpoise in the western North Pacific Ocean suffered incidental mortality from the gillnets of the Japanese high seas salmon fishery until 1987 and mortality from the high seas squid gillnet fishery until 1992. Their abundance is estimated from line transect data gathered during 1979/89. Correction factors are estimated for the bias in abundance estimates because of the attraction of animals to the survey vessel and the failure to detect all animals close to the trackline. Data from experimental surveys carried out in 1983 and 1984, using a helicopter to detect schools ahead of the ship before responsive movement occurs, are analysed. The multiplicative correction factor for the 1983 survey is 0.314 (SE 0.131) and for the 1984 survey, 0.130 (SE 0.050). Animals were sighted at shorter distances in the 1984 survey than in the 1983 survey. A linear combination of these correction factors is proposed which adjusts for the observed distribution of perpendicular distances. After applying the resulting corrections, a conservative estimate of population abundance for the western North Pacific Ocean of 141,800 animals is obtained, with 95% CI (83,100; 241,700). Possible biases in this estimate are discussed.