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In general, correlations increased 
with scale (Fig. 2). At the largest scale 
(10-km), correlations between distribu- 
tion of harbor seals and primary prey 
CPUEs were often >0.8. The prey species 
with high correlations (>0.7) at the 10- 
km scale included plainfin midshipman 
(fall and winter), Pacific staghorn scul- 
pin (summer and fall), English sole (all 
seasons), and Pacific herring (winter) 
(Fig. 2). During spring, only English sole 
had a correlation of >0.7. For the forag- 
ing habitat map for spring, therefore, 
we included crab, which most closely ap- 
proached the 0.7 threshold value. Based 
on the threshold prey abundances identi- 
fied in the regression tree analysis, po- 
tential foraging habitat available in SFB 
ranged from 147 km 2 in spring to 238 
km 2 in fall (Table 3, Fig. 5). Foraging 
habitat available within 10 km of Castro 
Rocks ranged from 101 km 2 in spring to 
144 km 2 in fall. 
When assessing seasonal differences 
in harbor seals’ use of waters inside vs. 
outside SFB, the proportion of harbor 
seal locations on the outer coast was 
greater during the summer (0.33) and 
spring (0.21) than during the fall (0.01) 
or winter (0.08). Use of areas outside of 
SFB was not correlated with prey CPUE 
in SFB, number of harbor seals in SFB 
or at Castro Rocks, or upwelling indi- 
ces. In general, the proportion of harbor 
seal locations recorded outside of SFB 
decreased with greater levels of prey availabil- 
ity in SFB, increased with greater numbers of 
harbor seals in SFB as a whole, and increased 
with greater upwelling indices. In contrast, 
when the proportion of locations on the outer 
coast was compared with the average maximum 
count at Castro Rocks, the proportion on the 
outer coast tended to be lesser when numbers 
of harbor seals at Castro Rocks were greater. 
Discussion 
We used a simple approach for identifying harbor seal 
foraging areas, using satellite tracking, available infor- 
mation on harbor seal diets from previous studies, and 
a data set on prey distribution obtained from a local 
management agency. In many cases, the abundance and 
distribution of prey is the most important factor influ- 
encing the spatial distribution of predators (Davoren 
et ah, 2003). The primary motivation for a predator to 
move is to locate prey patches which offer a sufficient 
energetic “reward” (Charnov, 1976), i.e., provide suf- 
ficient energy gained from ingestion of prey, once the 
energetic costs of capture have been deducted. Not sur- 
122°30'0"W 
122°20'0"W 
122°10'0''W 
122°0'0"W 
Figure 4 
Spatial overlap of Pacific harbor seal ( Phoca vitulina richardii ) 
distribution with abundance of crab species (mixed species; pri- 
marily Dungeness crab. Cancer magister) in San Francisco Bay, 
CA, during the spring seal pupping seasons, 2001-05. 
prisingly, both terrestrial and marine mammals have 
been found to choose habitats that provide resources 
necessary for survival and reproduction, and for this 
reason, use of an area is often assumed to reflect quality 
or abundance of resources available in that area (Boyce 
and McDonald, 1999; Davoren et al., 2003). 
Despite some controversy about possible biases in 
fecal analyses used to estimate diets (Harvey, 1989; 
Tollit et ah, 1998), analysis of fecal samples remains 
an important method of identifying prey species of pin- 
nipeds. However, it is often difficult to identify where 
harbor seals are foraging, even when fecal analyses 
are supplemented by VHF telemetry tracking (Torok, 
1994). This problem has led some researchers to use 
techniques such as fatty acid signatures (Iverson et 
al., 1997) to identify harbor seal foraging areas. Our 
study presents an alternate approach to identifying 
these areas. 
