DOHL ET AL.: COMMON DOLPHIN DISTRIBUTION AND ABUNDANCE 



single sighting of 2,450 animals; we choose not to 

 treat this observation as an outlier because the oc- 

 casional occurrence of very large groups is typical 

 of this species. 



For the summer-autumn season of greatest abun- 

 dance, the stock size estimate based on summing in- 

 dividual cell populations and the estimate derived 

 from Equation (5) were 57,270 and 46,675, respec- 

 tively. The theoretical estimate based on pooled data 

 may be low because survey effort was 7.8% greater 

 in the lower density parts of the study area in the 

 summer-autumn season (i.e., the offshore waters in 

 the west). The coefficient of variation computed 

 from the theoretical variance formula (Equation (6)) 

 was 17%. Coefficients of variation for number of 

 groups, /(0), and mean group size were 9%, 8%, and 

 11%, respectively. The jackknife estimator gave a 

 higher coefficient of variation for population size of 

 27%. Components of this estimate for number of 

 groups, /(0), and mean group size were 15%, 18%, 

 and 14%, respectively. Differences between the two 

 types of estimators may be due, in part, to the in- 

 herently conservative nature of the jackknife (Efron 

 1982), but probably result primarily from within- 

 survey correlation of variables. In addition, the jack- 

 knife estimate of /(0) relied on a smaller subset of 

 sighting distances measured only during summer- 

 autumn surveys (n = 81). 



DISCUSSION 



Even in an area as heavily utilized as the South- 

 ern California Bight, sightings of common dolphin 

 schools are not common events. For this reason it 

 was necessary to pool aerial survey data collected 

 over several months in each of three years to 

 describe their distribution in statistical terms. The 

 two seasonal views of common dolphin distribution 

 in the SCB are shown for contrast in Figures 4 and 

 5. It is apparent that the population makes season- 

 ally greater use of the SCB in summer and autumn 

 months. The months of greatest numbers, based on 

 sightings per km of trackline, were September 

 through November. During these months, the popu- 

 lation far exceeds the mean value of 57,000 and 

 probably approaches 100,000 animals. 



A potential source of bias in our mean population 

 size estimates was the differential sightability of 

 groups of various sizes. The detection function for 

 common dolphin sightings declined sharply beyond 

 about 1,650 ft (500 m), suggesting that mostly large 

 or conspicuous groups were seen at relatively great 

 distances. The Fourier estimator is robust to varia- 

 tion in sighting efficiency (Burnham et al. 1980). For 



comparison, the/(0) term of 2.29 for common dol- 

 phins was quite close to the/(0) estimate of 2.16 

 more recently obtained for 136 sightings of Pacific 

 white-sided dolphin schools on aerial surveys off- 

 shore of central and northern California (Dohl et al. 

 1983). However, variable sighting effectiveness may 

 also bias the estimation of mean group size. Holt and 

 Powers (1982) found that smaller groups of dolphins 

 were more likely to be missed on aerial surveys than 

 larger groups, resulting in a 25% overestimation of 

 mean group size. For our data on common dolphins, 

 we did not find a significant difference in mean 

 group size between sightings within the first 1,650 

 ft and beyond due to high variability in sightings size 

 (213 ± 46 SE, n = 65, compared with 308 ± 49 SE, 

 n = 50; F in3 = 1.94, P = 0.18). Nevertheless, our 

 calculations show that stratification of mean group 

 size by distance from the trackline (< 1,650 ft and 

 >1,650 ft) would result in an 18% decrease in mean 

 density values. 



The distribution shown for summer and autumn 

 can be viewed as a composite of monthly distribu- 

 tions. Common dolphin distribution expands from 

 the southeast into the central and western parts of 

 the SCB in late spring and early summer and 

 recedes toward the east and south in late autumn 

 and early winter. Common dolphin movement into 

 and out of the SCB appears to be temperature 

 related. As sea surface temperatures (SST) rise in 

 late spring-early summer, animals begin to be 

 sighted more often along the Coronado Escarpment. 

 Peak numbers of common dolphins were found in 

 open water regions of the SCB 3-5 wk after intru- 

 sion of the warmer waters. During cool-water 

 months, when SSTs down to 10.0°C were recorded 

 and the SCB-wide mean was 14.6°C, no animals 

 were observed in waters cooler than 14.0° C. 



Distributional patterns of the common dolphin 

 within the SCB may be changing. Hui (1979) ana- 

 lyzed data collected on Naval Ocean Systems Center 

 (NOSC) surveys from 1968 through 1976 and 

 showed no common dolphin sightings north of Point 

 Vincente (lat. 33°45'N) or west of approximately 

 San Nicolas Island. Our surveys in summer and 

 autumn months found 29.9% of all sightings and 

 30.8% of all animals occurred in the northern and 

 western portion of the SCB— an area largely un- 

 sampled by the NOSC surveys. Hui's results agreed 

 with those of Evan's (1975), who found only a small 

 fraction of the total sightings recorded on aerial and 

 shipboard surveys to occur in this northern and 

 western portion of the SCB; however, aerial sam- 

 pling effort in Evan's earlier study also favored the 

 inshore and southern portions of the SCB. 



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