DOHL ET AL.: COMMON DOLPHIN DISTRIBUTION AND ABUNDANCE 



tial sightability of large and small groups (discussed 

 below). Because we could not be sure that the 

 assumptions of the theoretical formula were met, 

 we also calculated the variance of population size 

 for the summer-autumn season, using a jackknife 

 estimator (Miller 1974; Burnham et al. 1980). 

 Pseudovalues of the area-wide population were 

 generated by sequentially deleting pairs of surveys 

 from the database. All sources of variance were con- 

 sidered in estimation of total variance: /(0), mean 

 group size, and spatial variability of sightings. 

 Because of the small number of perpendicular sight- 

 ing distances for winter-spring season (31), we were 

 unable to obtain a stable value of /(0), thus pre- 

 cluding the estimation of jackknife variance of that 

 season. 



Distribution maps were prepared using Surface 

 Display Library software (Dynamic Graphics, Inc., 

 Berkeley, CA). Contour lines, generated by linear 

 interpolation between density values assigned to 

 grid-cell centerpoints, were smoothed using a cubic 

 spline function. 



RESULTS 



Two distinct seasonal distributions were found for 

 common dolphins in the Southern California Bight 

 (SCB). In winter and spring months (January 

 through June), common dolphin sightings were 

 almost completely confined to the eastern part of 

 the SCB (Fig. 4). Within the area occupied, three 

 cells in the southernmost rank and one shore- 

 bounded cell north of San Diego showed significantly 

 higher density than the overall seasonal mean (P > 

 0.95 in all cases). In summer and autumn months 

 (July through December), common dolphin sightings 

 were widespread from Rodriguez Seamount and the 

 Patton Escarpment in the west to the mainland 

 shore in the east (Fig. 5). Cell density estimates in 

 this season were relatively homogeneous through- 

 out the area. Only a single cell in the San Diego 

 Basin could be shown to be significantly higher than 

 the seasonal mean at the P > 0.95 level. Neverthe- 

 less, we believe that the clustering of moderately 

 high-density cells east of Santa Catalina and San 



Figure 5.-Common dolphin distribution in the Southern California Bight, summer and fall, 1975-78. Density contours show 



animals/km 2 . 



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