30 
Fishery Bulletin 99(1) 
central California; 2) northern California; 3) Oregon-Wash- 
ington outer coast; and 4) inland Washington waters (Bar- 
low et al., 1997, 1998, Forney et al., 1999). Evidence 
for geographic subdivision of harbor porpoise populations 
along the U.S. west coast is apparent from pollutant stud- 
ies (Calambokidis and Barlow, 1991), which reveal latitu- 
dinal gradients in chlorinated hydrocarbon levels in wa- 
ters off Washington, Oregon, and California. Molecular 
genetic studies have also demonstrated larger-scale geo- 
graphic population subdivisions within the northeast Pa- 
cific, where four putative populations (Alaska, British Co- 
lumbia, Washington, and California) are recognized (Rosel 
et al., 1995). More recent genetic analyses, based on more 
sampling locations and different genetic markers, have 
provided evidence to support recognition of these four pop- 
ulations. Smaller-scale geographic subdivision within Cal- 
ifornia remains unresolved. 2 
We present the results of a 13-30 November 1995 line- 
transect ship survey in waters off California, which was 
designed to estimate abundance and describe the depth 
distribution of harbor porpoise. This was the first ship sur- 
vey to cover this region in late autumn and to survey sys- 
tematical lly different depth strata out to 91 m (50 fath- 
oms). The 1995 ship survey contrasts with previous NMFS 
ship surveys that were conducted earlier in the year along 
the 18-m isobath and that relied on models of porpoise 
density at different depths to calculate abundance (Bar- 
low, 1988). We estimate porpoise abundance for two re- 
gions within California (Fig. 1): Bodega Bay to the Cali- 
fornia-Oregon border, hereafter referred to as “northern 
California,” and Point Sur to San Francisco Bay in cen- 
tral California, hereafter referred to as “region 2,” as delin- 
eated by Barlow ( 1988). We emphasize results from north- 
ern California because relatively little survey effort was 
conducted in central California. We compare statistically 
aerial and ship survey abundance estimates in 1995 with 
those from recent aerial surveys in northern California 
and region 2 in central California, using a confidence in- 
terval of differences (CI tl ) method proposed by Lo (1994). 
We also describe the depth distribution of harbor porpoise 
in northern California from the 1995 ship survey sighting 
data. 
Methods 
Observers searched for marine mammals during daylight 
hours from the 53-m NOAA research vessel McArthur. 
Nine observers rotated through five duty stations on the 
flying bridge 10 m above sea level, spending 30 minutes at 
each station. Two observers at the periphery of the flying 
bridge scanned from the trackline to the ship’s beam (0 
to 90 degrees) with 7x binoculars and two inner observers 
scanned from the trackline to 45 degrees on each side. 
The middle observer recorded data into a laptop computer 
and searched for porpoise groups by naked eye. A sixth 
2 Chivers, S. J. 1999. Personal commun. NMFS, Southwest 
Fisheries Science Center, P.O. Box 271, La Jolla, CA. 92038. 
independent observer opportunistically searched from the 
ship’s bridge level (~7.5 m above sea level) to estimate the 
fraction of porpoise groups missed by the primary team of 
five observers. 
A series of predetermined saw-tooth transect lines be- 
gan at Point Conception, California (34°27'N) and ended at 
the California-Oregon border (42°00 / N). Transect lines ex- 
tended from approximately the 20-m isobath to the 91-m 
(50 fathom) isobath. Some depths greater than 91m were 
surveyed, especially where the axes of submarine canyons 
intersected our transects. The ship could not routinely op- 
erate in water depths shallower than 20 m. The ship pro- 
ceeded along the transect lines in passing mode, i.e. it did 
not deviate from course even when porpoise groups were 
sighted. The study area was divided into several a prioj-i 
analysis regions (Fig. 1), the same as those used by Barlow 
(1988) and Barlow and Forney (1994). The region referred 
to as “northern California” incorporates region 4 and that 
part of region 5 that is south of the California-Oregon bor- 
der (Fig. 1). Areas of each region were previously calculat- 
ed by Forney. 3 
An event-driven data acquisition program (PPCRUISE) 
was used to record all sighting and effort data. The pro- 
gram was run on a laptop computer linked to the ship’s 
GPS system to obtain geographic position data. For each 
harbor porpoise group sighted, bearing and distance from 
the ship were recorded. Observers obtained the bearing to 
harbor porpoise groups using mounted protractors on the 
flying bridge. Distance to harbor porpoise groups was read 
from calibrated reticle marks imprinted on the eyepiece of 
7x binoculars (Barlow and Lee, 1994) or estimated by eye 
for some close groups. Depth soundings were recorded sys- 
tematically every two minutes while the ship was under- 
way. The position of each porpoise sighting was calculated 
to correct for differences between the position of the ship 
and porpoise groups. Depths at which porpoise were sight- 
ed were determined from National Ocean Service (NOS) 
hydrographic survey data 4 by using a triangulated irregu- 
lar surface model in Arc Info GIS. 
To estimate abundance, we used transect data collected 
only during calm sea states, defined as Beaufort 0 through 
2 (wind speeds up to 6 knots, no whitecaps present). Por- 
poise abundance was estimated out to the 91-m isobath. 
Effort segments in deeper water were excluded from line- 
transect analysis to allow direct spatial comparison with 
recent aerial survey results (Barlow and Forney, 1994; 
Forney 1 ). Porpoise abundance in region i was estimated 
3 Forney, K. 1988. Contour mapping and the calculation of 
areas between 10m depth contours along the coasts of Califor- 
nia, Oregon and Washington. Administrative report LJ-88-23, 
National Marine Fisheries Service, Southwest Fisheries Center, 
18 p. [Available from Southwest Fisheries Science Center, P.O. 
Box 271, La Jolla, CA 92038.] 
4 National Oceanic and Atmospheric Administration (NOAA), 
National Geophysical Data Center (NGDC). 1999. National 
Ocean Service (NOS) hydrographic survey data, U.S. coastal 
waters, version 4.0, available on CD-ROM. Website: http:/ / 
www. ngdc. n oaa.gov / ngdc. h tm 1 
