THE ABUNDANCE OF CETACEANS IN CALIFORNIA WATERS .2. AERIAL SURVEYS INWINTER AND SPRING OF 1991 AND 1992

Two aerial linetransect censuses of cetaceans were conducted along the California coast during March-April 1991 and February-April 1992. The two surveys were designed to provide a combined estimate of cetacean abundance for winter and spring (cold-water) conditions; they compleme nted a summer and fall ship survey in 1991. The study area (264,270 km 2) extended about 278 km (150 nmi) off the coast of southern Californi a, and 185 km (100 nmi) off the coast of central and northern Californ ia. A primary team of two observers searched for cetacean species thro ugh bubble windows that allowed an unobstructed view to the sides and directly beneath the aircraft. A third, conditionally independent obse rver searched through a belly window and reported animals that were mi ssed by the primary team. Approximately 7,069 km and 5,973 km were sea rched in 1991 and 1992, respectively, resulting in 253 sightings of at least 18 cetacean species (some animals could only be identified to h igher taxa). Estimates of abundance and coefficients of variation (in parentheses) for the most common small cetaceans are the following: 30 6,000 (0.34) common dolphins, Delphinus spp.; 122,000 (0.47) Pacific w hitesided dolphins, Lagenorhynchus obliquidens; 32,400 (0.46) Risso's dolphins, Grampus griseus; and 21,300 (0.43) northern right whale dolp hins, Lissodelphis borealis. Abundance estimates (and CV's) for the mo st common whales are the following: 892 (0.99) sperm whales, Physeter macrocephalus; 392 (0.41) beaked whales, genera Mesoplodon and Ziphius ; 319 (0.41) humpback whales, Megaptera novaeangliae; and 73 (0.62) mi nke whales, Balaenoptera acutorostrata.