Carretta et al. Abundance and depth distribution of Phocoeno phocoena off northern California 
37 
Effort at depth ■ Porpoise at depth 
0.4 
B 
0.4 
0 3 
0.2 
0.1 
0 
sr <y 
Depth (meters) 
Figure 6 
Relative proportions of survey effort and porpoise sightings at depth for 
(A) calm sea states and ( B) rough sea states. Proportions were compared 
by using a Kolmogorov-Smirnov goodness-of-fit test. The proportion of 
porpoise at depth was significantly different from effort proportions for 
both calm (P<0.005) and rough (P<0.002) sea states. More porpoise than 
expected occurred at depths of 20-60 m in calm sea states and at depths 
of 0-40 m in rough sea states. 
effort. The most recent estimate of porpoise 
abundance for region 1 is 627 (CV=0.452) from 
a 1997 aerial survey (Forney 1 ) . 
Our estimate of porpoise abundance for re- 
gion 2 is 1041 (log-normal 95% Cl 559-1941, 
CV=0.44). This is less than the 1778 porpoise 
(95% log-normal CI=1096-2885, CV=0.25) es- 
timated by Barlow and Forney (1994) from 
1988-93 aerial surveys, and 1459 porpoise 
(CV=0.61) estimated from 197 km of ship sur- 
vey effort in 1984^85 (Barlow, 1988). Our esti- 
mate is statistically significantly different (90% 
CI (/ =-4084 to -162, P=0.07, a=0.10) from an 
aerial survey estimate made 1-2 months ear- 
lier (2861, 95% CI= 1353-6048, CV=0.39). 1 To 
obtain abundance CVs of 0.30, 0.20, and 0.10 
for the 1995 ship survey in region 2 would 
have required approximately 175, 400, and 1600 
km of transect effort respectively (we surveyed 
91 km). 
We surveyed 66 km in region 3 in the Gulf of 
the Farallones during calm sea states and did 
not detect any harbor porpoise. Harbor por- 
poise are known to be abundant in this area, 
where a total of 552 sightings were recorded 
during small-vessel studies conducted between 
1977 and 1985 (Szczepaniak and Webber 10 ). 
However, a majority of these sightings were 
10-20 km west of the mouth of San Francisco 
Bay and only a small number of sightings were 
recorded within 10 km of the Farallon Islands, 
where nearly all of our survey effort in region 
3 occurred (Fig. 2). Calambokidis et al. 7 con- 
ducted year-round surveys in the Gulf of the 
Farallones and found high porpoise densities 
within 10-20 km of San Francisco Bay but 
sighted no porpoise offshore near the Farallon Islands. 
Szczepaniak 11 found the same pattern during autumn sur- 
veys. No specimens were reported from the Farallon Is- 
lands in the 208 stranding records of harbor porpoise from 
1915 to 1985 summarized by Szczepaniak. 11 Further evi- 
dence of low porpoise densities in this area comes from 
studies at Southeast Farallon Island, where over the ten- 
year period 1982-92, observers stationed year-round at 
the island detected only 68 groups of harbor porpoise (Py- 
le and Gilbert, 1996). The collective evidence suggests 
that harbor porpoise are uncommon in the immediate vi- 
cinity of the Farallon Islands, where water depths general- 
ly range from 55 to 75 m. In California, this depth range is 
10 Szczepaniak, I. D., and M. A. Webber. 1985. Status of the 
harbor porpoise ( Phocoena phocoena ) in the eastern north 
Pacific, with an emphasis on California. Final report to 
Center for Environmental Education, 52 p. Washington, D.C. 
20004. 
11 Szczepaniak, I. D. 1987. Abundance and distribution of 
harbor porpoise ( Phocoena phocoena) in the Gulf of the Far- 
allones National Marine Sanctuary. Final report to National 
Park Service, 48 p. Point Reyes National Seashore, Point 
Reyes, CA 94956. 
characterized by relatively low porpoise densities, as dis- 
cussed in the next section. 
Depth distribution of harbor porpoise 
In northern California, we found that higher numbers of 
harbor porpoise than expected occurred at depths of 20 to 60 
m and fewer porpoise than expected in waters deeper than 
60 m (Fig. 6, A and B). Barlow (1988) found the abundance 
of harbor porpoise along the California, Oregon, and Wash- 
ington coasts to be roughly constant from shore to 55 m, 
declining linearly with depth, and he found that no harbor 
porpoises were detected in waters deeper than 110 m. 
We encountered 19 porpoise groups in waters deeper than 
91 m (range 101 to 215 m) over submarine canyons near 
Cape Mendocino, where deep canyons intersect shallow 
water habitat characterized by higher porpoise densities. 
Our observed depth distribution of harbor porpoise is sim- 
ilar to that of Calambokidis et al., 7 who found porpoise 
only at depths <70 m, but who surveyed to depths of 
120 m. In contrast, near the Northern San Juan Islands, 
Washington, porpoise occur more often than expected at 
depths >100 m and fewer than expected occur at depths 
<75 m (Raum-Suryan, 1995; Raum-Suryan and Harvey, 
