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 



