Forney et al. ; Abundance of cetaceans in California waters: aerial surveys 



23 



creased dramatically since the early 1980's. The 

 causes of this increase are not known, but it is pos- 

 sible that long-term oceanographic changes 

 (Roemmich, 1992; Roemmich and McGowan, 1994) 

 have resulted in a shift in the distribution of com- 

 mon dolphins into this area. This hypothesis is con- 

 sistent with the observed decline in population size 

 of the northern common dolphin south of our study 

 area (Anganuzzi and Buckland, 1994). 



Similarly, an apparent decrease in abundance was 

 seen in short-finned pilot whales, Globicephala 

 macrorhynchus . This species was commonly seen in 

 the Southern California Bight on surveys during the 

 late 1970's and early 1980's, 1 ' 2 but only one off-effort 

 sighting of four animals was made during our surveys. 



Our estimate of 304 humpback whales is roughly 

 half the recent estimate obtained from photo-identi- 

 fication studies. 4 This is quite surprising because 

 humpback whales, Megaptera novaeangliae , in the 

 California feeding population are expected to be in 

 waters off Mexico during the winter and spring sea- 

 son. However, it is possible that some animals had 

 already moved north into California at the time of 

 the sightings. Alternatively, the sighted animals may 

 have been part of the southeastern Alaska feeding 

 population that migrates southward to breed in Mexi- 

 can waters in spring (Baker et al., 1986). 



Previously published estimates for harbor porpoise, 

 Phocoena phocoena (Barlow, 1988; Barlow et al., 

 1988; Barlow and Forney, 1994) and gray whales, 

 Eschrichtius robustus (Reilly, 1984; Buckland et al., 

 1993b), are substantially higher than the estimates 

 presented here. This is probably because the defined 

 study area is not appropriate for the range of these 

 animals. Gray whales have a much larger range and 

 migrate through California waters (southward and 

 then northward) from roughly November to May. Our 

 estimate represents that portion of the population 

 which was migrating through California in March 

 and early April. Harbor porpoise are limited to a 

 narrow coastal band, and our transect lines only over- 

 lapped with this region at specific points. More appro- 

 priate abundance estimates for harbor porpoise are pub- 

 lished in Barlow (1988) and in Barlow and Forney 

 (1994). 



Comparisons with 1991 ship surveys 



Although a statistical comparison between these 

 winter and spring aerial survey estimates and the 



1991 summer and fall ship survey estimates (Barlow, 

 this issue) is precluded at this time because of dif- 

 ferences in the sizes of the two study areas, a few 

 patterns are noteworthy. Despite the differences in 

 seasonal timing and areal coverage, estimates of 

 abundance are very similar for several species. Simi- 

 lar estimates of abundance were obtained for total 

 common dolphins (306,000 vs. 246,000), northern 

 right whale dolphins, Lissodelphis borealis (21,300 

 vs. 9,340), bottlenose dolphins, Tiirsiops truncatus 

 (3,260 vs. 1,500), and sperm whales, Physeter 

 macrocephalus (892 vs. 756) (aerial vs. ship esti- 

 mates, respectively). More disparate estimates were 

 obtained for Pacific white-sided dolphins ( 122,000 vs. 

 12,300), Risso's dolphins, Grampus griseus (32,400 

 vs. 8,500), harbor porpoise (1,600 vs. 52,700), Dall's 

 porpoise (8,460 vs. 78,400), and total beaked whales, 

 Ziphius cavirostris and Mesoplodon spp. (392 vs. 

 3,230). 



It may be important to note that all cases in which 

 the ship estimates are substantially larger than the 

 aerial estimates are for species which spend a large 

 fraction of their time diving (harbor porpoise, Dall's 

 porpoise, and beaked whales). Such species could be 

 more easily missed by aerial observers owing to avail- 

 ability bias. In the case of Pacific white-sided dol- 

 phins and Risso's dolphins, the winter and spring 

 aerial estimates may be larger because of a seasonal 

 movement of animals out of Oregon and Washington 

 in winter. 5 Additional analyses, which account for 

 differences in geographic extent of the aerial vs. ship 

 surveys, are planned in the future. 



Bias 



There are several sources of potential bias in this 

 study. First, abundance estimates may be biased low 

 because animals are missed by aerial observers (per- 

 ception bias; Marsh and Sinclair, 1989). This is most 

 likely to be a problem with poor observation condi- 

 tions (high sea state or overcast conditions, or both). 

 We have attempted to estimate the magnitude of 

 perception bias in this study through the use of a 

 conditionally independent observer and have cor- 

 rected abundance estimates to reduce this effect. A 

 second source of downward bias, availability bias 

 (Marsh and Sinclair, 1989), is introduced because 

 animals that are submerged when the aircraft passes 

 overhead are not available to be seen. This effect is 



4 Calambokidis, J., G. H. Steiger, and J. R. Evenson. 1993. Pho- 

 tographic identification and abundance estimates of humpback 

 and blue whales off California in 1991-92. Final Contract Re- 

 port 50ABNF100137 to Southwest Fish. Sci. Cent., RO. Box 

 271, La Jolla, CA 92038, 67 p. 



5 Green, G. A., J. J. Braeggeman, R. A. Grotefendt, C. E. Bowlby, 

 M. L. Bonnell, and K. C. Balcomb III. 1992. Cetacean distribu- 

 tion and abundance off Oregon and Washington, 1989-1990. 

 Ch. 1 in J. J. Brueggeman (ed.), Oregon and Washington ma- 

 rine mammal and seabird surveys. Minerals Management Ser- 

 vice Contract Report 14-12-0001-30426 prepared for the Pacific 

 OCS (Outer Continental Shelf) Region. 



