Carretta et al Abundance and depth distribution of Phocoena phocoena off northern California 
31 
Figure 1 
Study regions referred to in text. “Northern California” is defined as region 4 as well 
as part of region 5 south of the California-Oregon border. Central California includes 
regions 1-3. The thin line just offshore from Point Conception to Cape Blanco, Oregon, 
delineates the 91-m (50-fathom) depth contour. 
by using standard line-transect methods (Buckland et al., 
1993): 
^ A,n,S/( 0) 
2L i g(0) 
where A ( = size of the study area (km 2 ) in region i\ 
n i - number of sightings in region i; 
S' = mean group size in region i; 
fl 0) = sighting probability density (/km) at zero per- 
pendicular distance 
L ( = length of transect line (in km) surveyed in 
region i, and 
g(0) - probability of seeing a porpoise group directly 
on the trackline. 
The histogram of radial sighting distances revealed 
spikes at distances corresponding to whole and half-ret- 
icle binocular readings, which suggests that observers 
tended to round-off radial sighting distances. In the ab- 
sence of rounding error, the distance interval between 
any two adjacent radial sighting distances should have 
been relatively equal for both near and distant sight- 
ings, but these intervals usually increase with distance 
because of reduced precision in reading reticles at great- 
er distances. Theoretically, a linear regression of dis- 
