FISHERY BULLETIN: VOL. 84, NO. 2 



Clemente Islands and west of San Nicolas Island 

 represents a real distributional pattern. 



Cell-density estimates and 95% confidence limits 

 are provided in Tables 1 and 2. Confidence limits 

 were calculated considering only sampling error due 

 to number of groups sighted (Equations (3) and (4)) 

 and not uncertainty in/(0) or mean group size. Sam- 

 pling error associated with the number of groups 

 sighted was the dominant source of variation in cell- 

 by-cell estimates of density, typically exceeding 

 variance of the/(0) term by three times and variance 

 associated with mean group size by five times. It 

 should be remembered that the density estimates 

 are mean values computed from pooled data col- 

 lected over a several month period in 3 successive 

 years. 



From these density estimates, we computed 



seasonal mean population size estimates. By cal- 

 culating population size as the sum of the numbers 

 in each 30' x 30' cell, we estimate a winter-spring 

 population of 15,448 animals. This figure is a mean 

 population occurring in the months of January 

 through June and includes months of higher and 

 lower numbers. Using Equation (5), we calculate a 

 theoretical winter-spring population size of 18,933 

 animals. This second estimate for the SCB, based 

 on pooled data, may be high because survey effort 

 was 6.7% greater in the higher density parts of the 

 study area in winter and spring. Based on Equation 

 (6), the coefficient of variation of the winter-spring 

 population was 36%. The coefficients of variation 

 for number of groups, f(0), and mean group size 

 were 16%, 8%, and 31%, respectively. The relatively 

 large variability in mean group size was due to a 



Table 1.— Relative abundance of common dolphins in the winter and spring. Mean density (animals/ 

 km 2 ) is provided for each 30' x 30' cell; latitude and longitude indicate center point of cell. Upper and 

 lower values are 95% confidence limits derived from the spatial variability of sightings along aerial transect 

 lines. 



Table 2.— Relative abundance of common dolphins in the summer and fall. Mean density (animals/km 2 ) 

 is provided for each 30' x 30' cell; latitude and longitude indicate the center point of cell. Upper and 

 lower values are 95% confidence limits derived from the spatial variability of sightings along aerial transect 

 lines. 



121°00' 120°30' 120°00' 



119°30' 



119°00' 118°30' 118°00' 117°30' 



34°30' 



34°00' 



33°30' 



33°00' 



32°30' 



340 



