Urban R. et al.: Megaptera novaeangliae in waters off the Pacific coast of Mexico 



1019 



■24° 



Baja California Peninsula 



-20° 



Clari6n Is. 



115° 



_] 



Revillagigedo Stock 



• San Benedicto Is. 



o 

 Soconx) Is. 



110° 



_J 



105" 



_J 



Figure 1 



Study area showing, by shading, the distribution of the Revillagigedo and coastal stocks. 



Materials and methods 



Individual identification 



Humpback whales were individually identified from 

 photographs taken by different institutions from 

 approximately December to March, along the main- 

 land coast of Mexico (1983-92), from mid-January 

 to March in Baja CaUfornia (1987-93), and at the 

 Revillagigedo Archipelago (1986-92) (Table 2). 



Although photo-identification data have been 

 available for the coastal stock since 1983 and for the 

 Revillagigedo stock since 1985, efforts before 1986 

 were limited in time and space, and also lacked con- 

 tinuity. Beginning in 1986, surveys were conducted 

 off Socorro Island for the Revillagigedo stock, and 

 near San Jose del Cabo or the mainland coast (or 

 both) for the coastal stock. No data were obtained 

 for either Revillagigedo or the mainland in 1993. 



The whales were identified individually through 

 photographs of the black and white pigmentation pat- 

 terns on the ventral surface of their flukes (see Katona 

 and Whitehead, 1981 ). Photographs were obtained with 

 35-mm cameras equipped with 200—300 mm lenses. 

 Although the film used varied by region and season, 

 most photographs were taken with black and white 

 Kodak Tmax 400 ISO pushed to 1600 ISO, ensuring 

 shutter speeds as high as 1/1000 of a second. 



On the basis of focus, angle, and light conditions, 

 all fluke photographs were judged to be either good, 

 fair, or of poor quality; only photos in the first two 

 categories were included in this study. Within these 

 quality levels, the whales showed at least 50% of each 

 fluke at a sufficiently vertical angle to allow the shape 

 of the trailing edge to be distinguished. Calves were 

 excluded from the analysis owing to their tendency 

 to show changes in pigmentation patterns during the 

 first year of life (Carlson et al., 1990). Selected pho- 

 tographs were compared visually by at least three 

 persons with experience in matching humpback 

 whales flukes photographs from both Universidad 

 Nacional Autonoma de Mexico and Universidad 

 Autonoma de Baja California Sur. 



Abundance estimation 



Abundance estimates were obtained by using an 

 eight-year period for the coastal stock, from 1986 to 

 1993, and a seven-year period for the Revillagigedo 

 stock, from 1986 to 1992. 



Estimates of population size were obtained by us- 

 ing mark and recapture models for both closed and 

 open populations, with each year representing a cap- 

 ture occasion. The time span of the study was seven 

 years; consequently, although calves were not in- 

 cluded in the analysis, it is inevitable that additions 



