Carretta et al. Abundance and depth distribution of Phocoena phocoena off northern California 
35 
of 0.05, these two estimates would not have 
been significantly different, but the power to 
detect such a difference was low (0.13). This 
result highlights recent criticisms of statistical 
significance testing (Johnson, 1999) and under- 
scores the importance of estimating statistical 
power to aid in decision-making. Regardless, 
real differences exist between aerial and ship 
estimates obtained in 1995, as evidenced in 
Figure 5, A and B, which may be due to a 
number of factors. 
Animals may have moved out of the study 
area between the time of the two studies. Sea- 
sonal movements of harbor porpoise are known 
to occur on the Atlantic coast (Polacheck et 
al., 1995), and in Glacier Bay, Alaska (Taylor 
and Dawson, 1984). In California, the situa- 
tion is less clear because polluntant evidence 
suggests limited latitudinal movement along 
the coast (Calambokidis and Barlow, 1991) and 
genetic stock structure remains unresolved 
(Chivers 2 ). Several researchers have reported 
highest porpoise densities during late summer 
and autumn throughout California (Barlow, 
1988, Monterey Bay; Calambokidis et al. 7 , cen- 
tral California; Dohl et al. 8 , California-wide; 
Goetz, 1983, northern California; Sekiguchi, 
1995, Monterey Bay). Barlow (1988) and Seki- 
guchi (1995) observed the lowest harbor por- 
poise densities in Monterey Bay during winter. 
Forney (1995, 1999) reported harbor porpoise 
abundance was negatively correlated with pos- 
itive sea-surface temperature anomalies in the 
Monterey Bay region in autumn, a finding that 
suggests that animals move in response to 
changing oceanographic conditions. Pyle and 
Gilbert (1996) reported harbor porpoise sight- 
ings near Southeast Farallon Island in central 
California only from March to November, even 
though observers searched year-round over 
a ten-year period (1982-92). Collectively, the 
data suggest small-scale seasonal movement 
of harbor porpoise along the California coast. 
Most cetacean species in California show sea- 
sonal trends in distribution and abundance 
(Forney and Barlow, 1998) and despite the re- 
striction of harbor porpoise to neritic habitat 
7 Calambokidis, J., C. Ewald, G. H. Steiger, S. M. 
Cooper, I. D. Szczepaniak, and M. A. Webber. 
1990. Harbor porpoise studies in the Gulf of the 
Farallones. Final contract report CX 8000-8-0001 
to the Gulf of the Farallones National Marine Sanc- 
tuary, 57 p. Fort Mason Center Bldg. 201, San 
Francisco, CA 94123. 
8 Dohl, T. P., R. C. Guess, M. L. Duman, and R. C. 
Helm. 1983. Cetaceans of central and northern 
California, 1980-1983: status, abundance, and 
distribution. Contract rep. 14-12-0001-29090, 
p. 135-152. Pacific OCS Region, Minerals Man- 
agement Service, U.S. Department of the Interior, 
Washington D.C. 
