Abstract.- A total 3236 km of 

 boat and aircraft surveys was con- 

 ducted in the northern Gulf of Cali- 

 fornia in search of the vaquita Pho- 

 coena sinus in 1986-1988. Vaquita 

 were seen on 51 occasions, represent- 

 ing an estimated 96 individuals. 

 Forty-three porpoises (19 sightings) 

 occurred during 1715 km of vessel 

 transects, a rate of 2.51 individuals/ 

 100 km of surveys. The number of 

 sightings relative to the extent of the 

 survey emphasized the rarity of the 

 porpoise and is a cause for concern 

 regarding the vulnerability of the 

 population. All sightings of vaquita 

 occurred north of lat. 30°45'N, and 

 all but two sightings (96.1%) occurred 

 less than 40 km from San Felipe, 

 Baja California Norte. Porpoises 

 were observed in small groups (<3 

 individuals) in all but five sightings. 

 The mean group size was 1.9 ± 1.24 

 SD individuals, and the mode was 1. 

 All sightings occurred in water depths 

 of 13.5-37.0 m (j 26.1 ±6.18 SD), 

 with water clarity at these localities 

 ranging from 0.9 to 12 m. Calves 

 represented 9.37% of all individuals 

 sighted. Unconfirmed sightings sug- 

 gest that the porpoise is present in 

 the southern Gulf of California. Fish- 

 ermen interviewed in San Felipe, 

 Baja California Norte, were familiar 

 with vaquita, and some have caught 

 the porpoise in gillnets. 



Occurrence and Distribution 

 of thie Vaquita Phocoena sinus 

 in the IMortliern Gulf of California 



Gregory K. Silber 



Institute of Marine Sciences. University of California 

 Santa Cruz. California 95064 



Manuscript accepted 23 January 1990. 

 Fishery Bulletin, U.S. 88:339-346. 



The vaquita Phocoena sinus (Norris 

 and McFarland 1958) is among the 

 rarest and least understood of the 

 Cetacea. In the 30 years following its 

 initial description (Norris and McFar- 

 land 1958) very little new informa- 

 tion about the porpoise had been ob- 

 tained. Brownell (1986) reported that 

 only 45 confirmed records of vaquita 

 existed. These consisted of skulls and 

 skeletons (Noble and Eraser 1971, Orr 

 1969) and about 20 reported sight- 

 ings obtained between 1958 and 1986 

 (Norris and McFarland 1958, Norris 

 and Prescott 1961, Villa-R. 1976, 

 Wells et al. 1981). However, Brown- 

 ell (1986) argued that only four of the 

 sightings were valid. 



Increased study of the porpoise in 

 recent years has yielded new infor- 

 mation through descriptions of recov- 

 ered specimens (Magatagan et al. 

 1984, Brownell et al. 1987), reported 

 sightings (Robles et al. 1986, Vidal et 

 al. 1987, Silber 1988), and behavioral 

 accounts (Silber et al. 1988). Norris 

 and McFarland (1958) described the 

 range of P. sinus as occurring in the 

 upper Gulf of California and probably 

 extending south along the Mexican 

 coast, but Brownell (1986) main- 

 tained that the range was limited to 

 the upper Gulf of California. 



The vaquita population has been 

 impacted by gillnetting activities and 

 other forms of habitat degi'adation in 

 recent decades (Brownell 1983, Bar- 

 low 1986). The species continues to 

 experience mortality at an unknown 

 rate in gillnets (Brownell 1983, Silber 

 1988) set primarily for totoaba Toto- 

 aba rfiacdonaldi and various species 

 of shark. 



The objective for the initial year of 

 the present study (1986) was to locate 

 P. sinus, and subsequently (1987, 

 1988) to gather information on the 

 ecology and distribution of the por- 

 poise. Visual surveys were conducted 

 from boat and aircraft in an attempt 

 to determine the vaquita's distribu- 

 tion, range, and habitat utilization. 

 Reported here are the results of 

 three seasons of study on the vaquita 

 conducted in the northern Gulf of 

 California in the springs of 1986-88. 

 I have also included a discussion of 

 unconfirmed sightings in the south- 

 ern Gulf that may reflect the histor- 

 ical range of P. sinus. 



Materials and methods 



Vessel surveys 

 and sighting data 



Boat surveys for Phocoena sinus 

 were conducted in the northern Gulf 

 of California using an 8-m Boston 

 Whaler. A total of 1715 km of survey 

 transects was conducted on 71 days 

 during the three years (Figs. 1, 2a). 

 The study was conducted in the spring 

 months because (1) operational costs 

 and logistic (e.g., boat time) restric- 

 tions limited field work duration, (2) 

 conversations wath fishermen in the 

 upper Gulf in 1986 suggested that va- 

 quita were most abundant (perhaps 

 only present) in March-May, and (3) 

 the number of sightings that we ob- 

 tained (two porpoises in 485 km sur- 



Reference to trade names does not imply en- 

 dorsement by the National Marine Fisheries 

 Service, NOAA. 



339 



