Grigg et al.: Spatial and seasonal relationships between Phoca vitulma richardu and their prey 
363 
Table 2 
Harbor seals ( Phoca vitulina richardu) captured in San Francisco Bay, CA (2001-05) and tagged with satellite telemetry tags, 
with sample size information. Ages are abbreviated as A (adult), SA (subadult), and Y (yearling). Dates and duration (in days) 
of telemetry tag attachment are shown. Numbers of both raw seal location estimates and filtered location estimates are shown. 
Accuracy filtering methods were based on location quality rankings provided by Service Argos, and a spatial filtering mechanism 
to reduce improbable locations. Location filtering consisted of the removal of points that fell within 1 km of a haul-out site, or 
outside of the study area. 
Harbor seal 
ID 
Age 
Sex 
Dates of 
tag attachment 
Days 
tagged 
Number of raw 
location estimates 
Final sample size after 
accuracy and location filtering 
15345 
A 
M 
1/2001-6/2001 
153 
307 
141, 21 
15440 
SA 
F 
7/2001-8/2001 
34 
106 
65, 33 
15436 
SA 
F 
7/2001-8/2001 
31 
69 
42, 24 
19580 
A 
F 
7/2001-3/2002 
233 
947 
547, 365 
19582 
A 
M 
7/2001-8/2001 
22 
157 
81, 3 
15439 
A 
F 
1/2001-9/2001 
69 
204 
97, 27 
15437 
A 
M 
1/2002-5/2002 
126 
616 
188, 26 
10024 
A 
F 
8/2002-1/2003 
158 
561 
206, 87 
10278 
A 
M 
8/2002-11/2002 
85 
45 
22, 8 
10279 
A 
M 
8/2002-11/2002 
97 
353 
133, 63 
10280 
A 
M 
8/2002-3/2003 
229 
1013 
451, 218 
10297 
SA 
M 
8/2002-3/2003 
215 
1156 
480, 250 
10863 
SA 
F 
8/2002-2/2003 
190 
1269 
588, 336 
42526 
SA 
F 
8/2003-1/2004 
141 
896 
308, 111 
42527 
A 
F 
8/2003-3/2004 
204 
1369 
485, 274 
42529 
Y 
F 
8/2003-12/2003 
107 
986 
397, 270 
42530 
Y 
F 
8/2003-12/2003 
123 
1012 
372, 202 
21454 
A 
F 
1/2005-6/2005 
144 
577 
236, 53 
42528 
SA 
M 
1/2005-6/2005 
135 
524 
179, 49 
quently visited by harbor seals; lower abundances were 
found in cells repesenting areas less frequently visited 
by harbor seals. Grid cells with CPUE values above the 
threshold values identified in the regression tree were 
designated as potential harbor seal foraging habitat. 
Harbor seal habitat was mapped by using only the prey 
species with the highest correlations with harbor seal 
distribution (correlation coefficient >0.7) at the 10-km 
scale, because this coarser scale may be more appropri- 
ate for assessing behavior which influences lifetime fit- 
ness (Rettie and McLoughlin, 1999). The area (in km 2 ) 
of potential foraging habitat was then calculated, both 
for SFB as a whole, and within 10 km of the primary 
haul-out site, Castro Rocks. 
To examine potential reasons why some harbor seals 
were leaving the bay, correlation coefficients were cal- 
culated between the proportion of harbor seal loca- 
tions located outside of SFB, for each season, and the 
following three variables: 1) prey CPUE inside SFB; 
2) number of harbor seals inside SFB and at Castro 
Rocks; and 3) indices of upwelling measured along 
the adjacent outer coast. The number of harbor seals 
in SFB fluctuates seasonally (Fancher, 1979; Grigg et 
al., 2004; Grigg, 2008), and we hypothesized that an 
increased number of excursions outside SFB would 
be positively correlated with the increased number 
of harbor seals in SFB. To ascertain the numbers of 
harbor seals in SFB and at Castro Rocks, harbor seals 
were counted at the three largest haul-out sites in 
SFB and the numbers were averaged by season across 
years. For information on the strength of upwelling 
along the California coast, we used monthly upwelling 
indices compiled by the National Oceanographic and 
Atmospheric Administration Environmental Research 
Division. Statistical significance for all correlations was 
assessed with a=0.05. 
Results 
Harbor seal telemetry 
Nineteen harbor seals were captured between January 
2001 and January 2005 (Table 2). Data filtering for accu- 
racy reduced the number of location estimates for indi- 
vidual harbor seals by 39-69%, and additional location 
estimates were removed when within 1 km of a haul-out 
site, or outside of SFB. The mean number of locations 
per harbor seal after all filtering was 120 ±27 (standard 
error of the mean; SEM). The number of locations per 
season ranged from 134 during the spring season, to 
1139 during the fall. Locations were evenly dispersed 
between day and night (47% during the day, 6 a.m. to 6 
p.m., and 53% during the night, 6 p.m. to 6 a.m.). 
