340 



Fishery Bulletin 88(2). 1990 



Figure I 



Number of days per week spent conducting transects 

 and observations of Phocoena sinus in 1986. 1987, and 

 1988. 



veyed) in February 1986 appeared to support informa- 

 tion gathered from fishermen. For these reasons, the 

 decision was made to concentrate survey efforts in 

 March-May, when the likelihood of encountering the 

 porpoise was believed greatest. 



During vessel transects, two to four observers posi- 

 tioned 3.5 m above the water surface searched with 

 unaided eyes and binoculars (7 and 10 x) the area in 

 front, and about 200-300 m on either side, of the ves- 

 sel's track. Observers also regularly scanned to greater 

 distances. During all surveys the vessel traveled con- 

 sistently 10-11 km/hour. Transects conducted in 1986 

 were restricted primarily to nearshore areas. In subse- 

 quent field work inshore surveys were augmented by 

 transects farther from shore. 



For each porpoise sighting, the number of individuals 

 seen was estimated and sighting locations were deter- 

 mined by triangulation from landmarks. Water depths 

 were obtained with a JRC color video depth sounder 

 or from a nautical chart. Water clarity (using a Secchi 

 disc) and temperature were measured at most sighting 

 locations. Sea states were obtained by visual estimate 

 according to the Beaufort scale. No surveys were con- 

 ducted when sea state exceeded Beaufort 3, and the 

 majority were conducted in sea states of 1. 



Aerial surveys 



Limitations imposed by time, weather, and financial 

 constraints prevented repeated vessel surveys south 

 of lat. 30°30'. However, aircraft surveys were con- 

 ducted as far south as lat. 29°34'N (Fig. 2b). A total 

 of 1521 km was flown on three days, 3-5 May 1988, 

 concentrating on the upper Gulf. 



Fishermen interviews 



Fishermen in San Felipe, Baja California Norte, and 

 in the vicinity of La Paz, Baja California Sur, were in- 

 terviewed in an attempt to obtain qualitative informa- 

 tion with regard to the natural history, range, and 



distribution of the vaquita. Questions were asked re- 

 garding type of fishing, target species, years of em- 

 ployment, and location of fishing. The men were asked 

 if they recognized vaquita after viewing photographs 

 of living and dead P. sinus and other odontocetes 

 common to the area. The interviews were intended 

 as general informational surveys of local knowledge 

 to direct future study of vaquita and preliminary 

 assessments of fishing effort and vaquita entanglement 

 rates. Information derived from interviews with fish- 

 ermen has been presented, although questioning 

 methodology and sample sizes were insufficiently 

 rigorous to draw definitive conclusions with regard to 

 vaquita distribution. 



Results 



Vaquita sightings 



Vaquita were seen on 51 occasions, representing an 

 estimated 96 individuals during boat and aircraft 

 surveys (Table 1). A total of 43 porpoises was seen 

 while conducting vessel transects, a sighting rate of 

 2.51 individuals/100 km surveyed. The remainder of the 

 sightings occurred while tracking porpoises, collecting 

 other types of data, or otherwise not engaged in visual 

 transects. The paucity of sightings relative to the 

 extent of the survey emphasized the rarity of the 

 porpoise. 



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 

 (Fig. 2a). Most sightings (94.2%) occurred in sea states 

 or 1 (Fig. 3). In 90.2% (n = 46) of all sightings, por- 

 poises were observed in small groups (< 3 individuals). 

 Mean group size was 1.9±1.24 SD individuals per 

 group (Table 1) and the group size mode was one (Fig. 

 4). For 86.3% (n = 44) of all porpoise sighting locations, 

 water depths ranged from 21 to 35 m (x 25.7 + 6.36 

 SD; overall range 13.5-37.0) (Fig. 5), and most sight- 

 ings occurred 11 to 25 km from the nearest shore 



