Abstract. Two aerial line-transect censuses of cetaceans were conducted along the California coast during March-April 1991 and February-April 1992. The two sur-veys were designed to provide a combined estimate of cetacean abundance for winter and spring (cold-water) conditions; they com-plemented 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 south-ern California, and 185 km (100 nmi) off the coast of central and northern California. A primary team of two observers searched for cetacean species through bubble windows that allowed an unob-structed view to the sides and di-rectly beneath the aircraft. A third, conditionally independent observer searched through a belly window and reported animals that were missed by the primary team. Ap-proximately 7,069 km and 5,973 km were searched in 1991 and 1992, respectively, resulting in 253 sightings of at least 18 cetacean species (some animals could only be identified to higher taxa). Esti-mates of abundance and coeffi-cients of variation (in parentheses) for the most common small ceta-ceans are the following: 306,000 (0.34) common dolphins, Delphinus spp.; 122,000 (0.47) Pacific white-sided dolphins, Lagenorhynchus obliquidens; 32,400 (0.46) Risso's dolphins, Grampus griseus; and 21,300 (0.43) northern right whale dolphins, Lissodelphis borealis. Abundance estimates (and CV's) for the most common whales are the following: 892 (0.99) sperm whales, Physeter macrocephalus; 392 (0.41) beaked whales, genera Meso-plodon and Ziphius; 319 (0.41) humpback whales, Megaptera novae-angliae; and 73 (0.62) minke whales, Balaenoptera acutorostrata. The abundance of cetaceans in California waters. Part II: Aerial surveys in winter and spring of 1991 and 1992 Karin A. Forney Jay Barlow James V. Carretta Southwest Fisheries Science Center National Marine Fisheries Service. NOAA RO. Box 271, La Jolla, California 92038 Manuscript accepted 31 May 1994. Fishery Bulletin 93:15-26 (1995). California coastal waters are a pro-ductive and highly variable oceano-graphic region with a diverse ma-rine fauna. Coastal fisheries, prima-rily gillnet fisheries, cause the inci-dental death of a variety of marine mammal species (Barlow et al., in press). However, the impact of this mortality can only be evaluated if estimates of population size are available for the affected species. In the late 1970's and early 1980's, abundance estimates were obtained based on aerial surveys, 1 -2 but esti-mates of precision were not ob-tained for most species. Because of the age and uncertainty of these es-timates, the National Marine Fish-eries Service conducted aerial and shipboard surveys during 1991 and 1992. Based on evidence of season-ality in the abundance and distri-bution of some cetaceans (Leather-wood and Walker, 1979; Dohl et al., 1986), separate abundance esti-mates were obtained for winter and summer conditions. Two aerial sur-veys (March-April 1991 and Febru-ary-April 1992) were completed during cold-water conditions, and one ship survey (July-November 1991) was conducted during warm-water conditions (Barlow, this is-sue). The survey periods were cho-sen based on climatic atlases of the California coast which show that, on average, March and April have the coldest, and September and October the warmest sea-surface tempera-tures (U.S. Navy, 1977). Standard line-transect methods (Burnham et al., 1980; Buckland et al., 1993a) were used from both platforms. Pre-liminary abundance estimates were calculated after completion of the first aerial survey in 1991 (Forney and Barlow, 1993), but confidence limits were large. In this paper, we present combined abundance estimates for the 1991 and 1992 aerial surveys. Survey methods The methods used during the 1991-92 aerial surveys are described in detail by Forney and Barlow (1993) and Carretta and Forney (1993), and only a summary is presented below. The study area (264,270 km 2 ) 1 Dohl, T. P., K. S. Norris, R. C. Guess, J. D. Bryant, and M. W. Honig. 1978. Cetacea of the Southern California Bight. Part II of Summary of marine mammal and sea-bird surveys of the Southern California Bight area, 1975-1978. Final Report to the Bureau of Land Management, 414 p. [NTIS Rep. PB81248189.] 2 Dohl, T. P., R. C. Guess, M. L. Duman, and R. C. Helm. 1983. Cetaceans of central and northern California, 1980-1983: status, abundance and distribution. OCS Study MMS 84-0045. Minerals Management Ser-vice contract No. 14-12-0001-29090, 284 p. 15
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