29 
Abstract— We estimate abundance and 
describe the depth distribution of harbor 
porpoise (Phocoena phocoena ) in north- 
ern California from a November 1995 
ship survey. This was the first ship sur- 
vey designed to systematically survey 
different depth strata out to 91 m (50 
fathoms) in this region. Harbor por- 
poise abundance in northern California 
is estimated at 5686 (CV=0.29, log-nor- 
mal 95% CI=3498-9242), based on 377 
km of transect effort and 153 porpoise 
sightings. Using a confidence interval 
of differences approach, we determined 
that our abundance estimate was sig- 
nificantly different (P=0.G8; a=0.10) 
from an aerial survey estimate obtained 
1 to 2 months earlier in the same 
region, where abundance was estimated 
at 13,145 (CV=0.39, log-normal 95% 
CI=6318-27,357). Possible explanations 
for differences in estimates include sea- 
sonal movement of porpoise to other 
areas or depths, insufficient transect 
effort during the ship survey, or under- 
estimates of the fraction of porpoise 
groups missed on the trackline due to 
large swells. Porpoise distribution in 
northern California was not random 
with respect to water depth; signif- 
icantly more porpoise than expected 
occurred at depths of 20 to 60 m and 
fewer porpoise than expected occurred 
at depths >60 m. 
Manuscript accepted 21 July 2000. 
Fish. Bull. 99:29-39 (2001). 
Abundance and depth distribution of 
harbor porpoise ( Phocoena phocoena) 
in northern California determined 
from a 1995 ship survey 
James V. Carretla 
Barbara L. Taylor 
Susan J. Olivers 
Southwest Fisheries Science Center 
National Marine Fisheries Serv/ice, NOAA 
P.O. Box 271 
La Jolla, California 92038 
E-mail address (for J. V. Caretta) Jim.Caretta@noaa.gov 
The abundance and distribution of har- 
bor porpoise (Phocoena phocoena ) in 
California waters has recently been 
described from a series of aerial surveys 
conducted by the U.S. National Marine 
Fisheries Service (NMFS) (Forney et 
al., 1991; Barlow and Forney, 1994; 
Forney 1995; Forney, 1999). The most 
recent abundance estimate was 5732 
(CV=0.28) for central California and 
11,066 (CV=G.39) for northern Califor- 
nia, based on aerial surveys conducted 
from 1993 to 1997. 1 Declining trends in 
porpoise abundance have recently been 
described in central California based 
on aerial surveys conducted from 1986 
to 1995 (Forney, 1995, 1999). This per- 
ceived decline is unexpected because 
the central California population was 
expected to recover following a sub- 
stantial reduction in fishery-related 
mortality beginning in the late 1980s 
(Barlow and Forney, 1994; Barlow 
and Hanan, 1995; Julian and Beeson, 
1998). Forney (1999) noted that harbor 
porpoise abundance was negatively 
correlated with positive sea surface 
temperature anomalies off central Cal- 
ifornia and suggested that perceived 
declines in porpoise abundance may 
be due to interannual movement of 
animals in and out of the study area 
in response to changing oceanographic 
conditions, rather than an actual popu- 
lation decline. Because the aerial survey 
study area was limited to nearshore 
waters <91 m (50 fathoms) in depth, 
one possible explanation was that por- 
poise might move into deeper waters 
in response to anomalous periods of 
warm water (Forney, 1999). Another 
explanation is that harbor porpoise 
moved north into northern California in 
response to warm-water episodes (For- 
ney’s trend data showed an increase in 
porpoise abundance in northern Cali- 
fornia during the same period of decline 
in central California, although sample 
sizes from northern California were 
too small to make unequivocal state- 
ments about trends in population size). 
Forney ( 1995, 1999) recommended that 
directed vessel surveys examine the 
influence of oceanographic and envi- 
ronmental variables on the distribution 
and abundance of harbor porpoise and 
focus on possible changes in the depth 
distribution of porpoise and the distri- 
bution of prey species (Forney 1995, 
1999). Current knowledge of harbor 
porpoise stock structure off California 
suggests that porpoise do not make 
long-distance movements; thus it is pos- 
sible that a perceived population decline 
in central California is the result of 
small-scale changes in porpoise distri- 
bution, given that aerial survey tran- 
sects have remained unchanged since 
1986 (Forney 1995, 1999). 
Today, NMFS recognizes (and man- 
ages separately) four stocks of harbor 
porpoise along the U.S. west coast; 1) 
1 Forney, K. A. 1999. The abundance of 
California harbor porpoise estimated from 
1993-97 aerial line-transect surveys. Ad- 
ministrative report LJ-99-02, National 
Marine Fisheries Service, Southwest Fish- 
eries Science Center, 16 p. [Available 
from Southwest Fisheries Science Center, 
P.O. Box 271, La Jolla, CA 92038.] 
