20 



Fishery Bulletin 93(1), 1995 



species level. Four of these sightings were identified 

 as ziphiid whales, for which a combined abundance 

 estimate was calculated. The remaining 24 sightings 

 were treated separately in the analyses. 



300 400 500 600 700 



Perpendicular distance 



800 900 1000 



100 200 300 400 500 600 700 800 900 1000 



Perpendicular distance 



300 400 500 600 700 



Perpendicular distance 



800 900 1000 



Figure 4 



Distribution of perpendicular sighting distances (100-m 

 intervals; solid line) and Hazard model fit (dotted line) for 

 (A) small cetaceans in groups <10, (B) small cetaceans in 

 groups >10, and (C) medium and large cetaceans. 



The Hazard model provided adequate fits to the 

 perpendicular distance distributions for the three 

 species/group-size categories (Fig. 4). Estimates of 

 f\0) andg(O) are given for each group in Table 1. Al- 

 though the full transect grid was not completed in 

 either year because of poor weather, the resulting 

 estimates of abundance (Table 3) are the most precise 

 that have been produced to date for this area and sea- 

 son. CVs range from 0.24 to 0.49 for small cetaceans 

 and from 0.35 to 1.11 for large cetaceans. 



Discussion 



Comparisons with previous abundance 

 estimates 



Our abundance estimates (Table 3) can be compared 

 directly with estimates based on 1975-83 aerial sur- 

 veys, 12 which are likely to have similar biases. The 

 estimate of 8,460 Dall's porpoise, Phocoenoides dalli, 

 is similar to previous aerial survey estimates of 

 3,000-4,000 in winter and spring. 1 - 2 The current es- 

 timate of 122,000 Pacific white-sided dolphins, 3 

 Lagenorhynchus obliquidens, is greater than the com- 

 bined estimates of 26,000 (spring) to 33,500 (winter) 

 for central and northern California 2 and 5,300 ( Jan- 

 Jun) for southern California. 1 Our estimate of 21,300 

 northern right whale dolphins is less than the com- 

 bined estimates of 29,000 (spring) to 61,500 (winter) 

 for central and northern California 2 and 5,900 ( Jan- 

 Jun) for southern California. 1 The prior studies do 

 not give estimates of statistical precision for any of 

 the above species, but given the CVs of our estimates, 

 the above differences are not likely to be statistically 

 significant. 



In contrast to the species above, common dolphins, 

 Delphinus spp., appear to be much more abundant 

 at present than during the period 1975—83. The cur- 

 rent winter estimate (306,000; CV=0.34) is more than 

 an order of magnitude larger than the previous value 

 of 15,488 (CV=0.36; Dohl et al., 1986), and the 99% 

 log-normal confidence limits for these two estimates 

 do not overlap. Preliminary comparisons (Barlow, 

 unpubl. data) of 1979 and 1980 ship surveys with 

 the 1991 ship survey (Barlow, this issue) also show a 

 significant increase in common dolphin abundance. 

 Based on these two separate lines of evidence for 

 winter and summer conditions, the abundance of 

 common dolphins in California appears to have in- 



3 Although estimates for Pacific white-sided dolphins based on 

 the combined 1991 and 1992 survey data are over twice the 

 preliminary estimate of 46,000 from only the 1991 data (Forney 

 and Barlow, 1993), the new estimate lies well within the 95% 

 confidence limit of the previous value. 



