524 



Fishery Bulletin 105(4) 



0.5 in Beaufort 2. We assumed that g{0) for minke 

 whales in Beaufort to 2 would be the same as for 

 small groups of delphinids (0.846), but minke whales 

 are very difficult to detect and an overestimate of this 

 parameter would lead to an underestimate of minke 

 whale abundance. 



Physeteridae The estimated abundance of sperm whales 

 is temporally variable off California (Table 7), but the 

 two most recent estimates (2001 and 2005) were mark- 

 edly higher than the estimates for 1991-96. Following 

 the 1997-98 Nifio, giant squid (Dosiclicus gigas) have 

 been more frequently observed off northern California 

 and Oregon, in particular beginning in 2002 (Pearcy, 

 2002; Field et al., in press). Sperm whales are known 

 to forage on giant squid, and their increased abundance 

 within our study area may have been related to the 

 increased availability of this prey species in recent 

 years. Compared to baleen whales, sperm whales are 

 found in larger groups, and fewer groups were seen on 

 each survey, both of which contribute to more variable 

 estimates. Also, the sperm whale population is likely to 

 extend outside the study area, at least during certain 

 times of the year. Of 176 tags that were implanted in 

 sperm whales off southern California in winter, only 

 three were later recovered by whalers (Rice, 1974); of 

 these three, one was recovered outside the study area 

 (far west of British Columbia). It is likely that at least 

 some fraction of the population is absent during part of 

 the year, and that fraction may vary with oceanographic 

 conditions. This pattern of distribution differs from the 

 situation with humpback whales; the majority of the 

 humpback population appeared to be feeding in U.S. 

 west coast waters during the time of the surveys. The 

 density of sperm whales estimated in our study for the 

 California Current (1.7 per 1000 km^) is similar to the 

 worldwide global average for this species (1.4 per 1000 

 km-; Whitehead, 2002) but is less than recent estimates 

 for waters in the eastern temperate Pacific (.3-5 per 1000 

 km-; Barlow and Taylor, 2005) and around Hawaii (2.8 

 per 1000 km2; Barlow, 2006). 



Dwarf and pygmy sperm whales are seldom seen 

 by people because of their offshore distribution and 

 cryptic behavior. Nonetheless, the estimated number of 

 individuals found off the U.S. west coast exceeds the 

 number of some much more commonly seen species, 

 such as killer whales. 



Ziphiidae Although they are rarely seen, approximately 

 7000 beaked whales were found in west coast waters — a 

 number that exceeds that documented for baleen whales. 

 The absence of California sightings for two beaked whale 

 genera (Mesoplodoii and Berardius, Table 7) since 1996 

 is disconcerting, especially in light of recent discoveries 

 about the susceptibility of this group to loud anthropo- 

 genic sounds (Simmonds and Lopez- Jurado, 1991; Cox et 

 al., 2006); however, weather conditions were less favor- 

 able for the detection of beaked whales during the more 

 recent surveys (Fig. 1) and it is unclear whether this 

 may have played a role in their apparent decrease. The 



distributions of all beaked whale species extend outside 

 the study area, and it is likely that some individuals 

 move in to and out of the study area as habitat changes. 

 An analysis of trends in beaked whale abundance should 

 include consideration of these effects. 



Previous abundance estimates 



Estimates presented in this study differ, typically by a 

 small amount, from previous estimates from the 1991 

 survey (Barlow, 1995) and preliminary estimates from 

 the 1993 (Barlow and Gerrodette, 1996), 1996 and 2001 

 (Carretta et al., 2006). and 2005 (Forney, 2007) surveys. 

 The differences are primarily due to differences in the 

 stratification and in the use of multiple covariates in 

 the line-transect modeling. Both modifications should 

 result in more precise estimates of cetacean abundance. 

 In addition, some of these previous estimates did not 

 include group-size calibration for individual observers, 

 and therefore our estimates corrected a small negative 

 bias present in those earlier estimates. The principle 

 weakness of the current analysis is the small sample size 

 for several rare species. However, we believe it is better 

 to include all species for completeness and to properly 

 quantify uncertainty in the estimates for rare species. 



Acknowledgments 



We thank the marine mammal observers (W. Armstrong, 

 L. Baraff, S. Benson, J. Cotton, A. Douglas, D. Ever- 

 hardt, H. Fearnbach, G. Friedrichsen, J. Gilpatrick, J. 

 Hall, N. Hedrick, K. Hough, D. Kinzey, E. LaBrecque, 

 J. Larese, H. Lira, M. Lycan, S. Lyday, R. Mellon, S. 

 Miller, L. Mitchell, L. Morse, S. Noren, S. Norman, C. 

 Oedekoven, P. Olson, T. O'Toole, S. Perry, J. Peterson, 



B. Phillips, R. Pitman, T. Pusser, M. Richlen, J. Quan, 



C. Speck, K. Raum-Suryan, S. Rankin, J. Rivers, R. 

 Rowlett, M. Rosales, J. C. Salinas, G. Serra-Valente, B. 

 Smith, C. Stinchcomb, N. Spear, S. Tezak, L. Torres, B. 

 Troutman, and E. Vasquez,), cruise leaders (E. Archer, 

 L. Ballance, E. Bowlby, J. Carretta, S. Chivers, T. Ger- 

 rodette, P. S. Hill, M. Lowry, K. Mangels, S. Mesnick, 

 R. Pitman, J. Redfern, B. Taylor, and P. Wade), survey 

 coordinators (J. Appier, A. Henry, P. S. Hill, A. Lynch, K. 

 Mangels, and A. VonSaunder), and officers and crew who 

 dedicated many months of hard work collecting these 

 data. T. Gerrodette was the chief scientist for the 1993 

 survey. J. Cubbage and R. Holland wrote the data entry 

 software. Data were edited and archived by A. Jackson. 

 Areas within the 20-m depth contour were calculated by 

 R. Cosgrove. J. Laake provided his R-language code for 

 fitting the multiple-covariate line-transect models. This 

 article benefited from the reviews and comments by M. 

 Ferguson, L. Thomas, and the Pacific Scientific Review 

 Group. Funding was provided by National Oceanic and 

 Atmospheric Administration and the Strategic Environ- 

 mental Research and Development Program (SERDP). 

 This work was supported in part by the Monterey Bay 

 Sanctuary Foundation. 



