38 
Fishery Bulletin 99(1 ) 
1998). We attribute these differences in depth distribu- 
tion between California and the San Juan Islands to habi- 
tat: porpoises near the San Juan Islands occupy an inland 
waterway characterized by numerous islands with steep 
bathymetry, wheras the open coastline of California has 
relatively gentler bathymetry. In both areas harbor por- 
poise do not occur far from land. Along the outer coasts 
of Oregon and Washington, Green et al. 12 reported approx- 
imately 25% of all harbor porpoise at depths of 100 to 
200 m; further, the depth distribution of harbor porpoise 
changed seasonally, in summer, 56% of porpoise were 
inshore of the 100-m isobath and during winter, this pro- 
portion increased to 86%. 
Much of what we know about harbor porpoise abun- 
dance and depth distribution in California comes from 
aerial and ship data collected in late summer and early 
autumn, largely because weather conditions are more fa- 
vorable at this time of year. For northern California, in 
particular, there is a bias towards collection of autumn 
data: the depth distribution model of Barlow (1988) con- 
tained only September data and past abundance surveys 
have relied on summer and autumn data (Barlow, 1988; 
Forney, 1995; 1999). The exception to this seasonal col- 
lection of data are year-round aerial surveys conducted 
by Dohl et al. 8 off central and northern California. How- 
ever, Dohl’s surveys were not ideally suited to estimate 
porpoise abundance because their transect lines were ori- 
ented perpendicular to the shoreline and most survey ef- 
fort occurred far offshore of known porpoise habitat. Ad- 
ditional winter and spring aerial surveys are needed in 
California to investigate seasonal differences in harbor 
porpoise distribution and abundance. To permit direct sea- 
sonal comparisons, we suggest that winter-spring surveys 
use existing transect lines from past summer-autumn aer- 
ial surveys. We recommend aerial surveys over ship sur- 
veys because they permit coverage of shallow water areas 
where larger research vessels cannot operate; are more 
cost-effective for surveying large areas, thus maximizing 
data collection during brief fair-weather periods; and re- 
duce the influence of large ocean swells that compromise 
the effectiveness of a ship hne-transect survey. 
Acknowledgments 
We thank the marine mammal observers who spent many 
hours collecting the sightings and effort data: W. Arm- 
strong, M. Donahue, B. Hanson, B. Odom, D. Outram, K. 
Raum-Suryan, R. Rowlett, (and S.J.C. and B.L.T. ). We also 
thank the officers and crew of the NOAA ship McArthur 
for all their support. Charles Stinchcomb assisted in iden- 
tifying shoreline-biased sightings. Karin Forney provided 
12 Green, G. A., J. J. Brueggeman, R. A. Grotefendt, C. E. 
Bowlby, M. L. Bonnell, and K. C. Balcomb III. 1992. Ceta- 
cean distribution and abundance off Oregon and Washington, 
1989-1990. Chapter 1 in Oregon and Washington marine 
mammal and seabird surveys (J, J. Brueggeman, ed.), p. 
1-140. Final report to OCS (Offshore Continental Shelf) 
Study MMS 91-0093. (Available from Ebasco Environmental, 
10900 NE 8 th St., Bellevue, WA 98004.] 
unpublished data on harbor porpoise encounter rates at 
depth from previous aerial surveys and patiently reviewed 
the CI rf method. Tim Gerrodette provided a FORTRAN 
subroutine used to correct sighting positions in relation 
to vessel positions. Rich Cosgrove provided NOS bathy- 
metric data used to determine the depth distribution of 
porpoise. This survey was funded by the NMFS Office of 
Protected Resources. Lastly we thank Jay Barlow, Debbie 
Palka, Kim Raum-Suryan, and two anonymous reviewers 
for their critiques of the manuscript. An earlier draft of 
this manuscript served as working paper 98-PSRG-5 at 
the November 1998 Pacific Scientific Review Group meet- 
ing in Seattle, WA. 
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