360 
Fishery Bulletin 107(3) 
Given information on what harbor seals in the region 
were eating, our primary goal was to identify where 
these harbor seals were foraging within the study area. 
Identifying the preferred foraging habitat of pinnipeds 
can be difficult because of the logistical challenges as- 
sociated with locating and tracking these large, highly 
mobile animals while they are at sea. Harbor seals are 
considered “central place foragers” (Orians and Pear- 
son 1979) in that they return to a central place (the 
haul-out site) after foraging trips, they repeatedly visit 
specific foraging areas, and they tend to focus their for- 
aging effort in areas near the haul-out site (Thompson 
et ah, 1998). Radiotracking, both conventional VHF 
and satellite-linked (hereafter referred to as satellite 
tracking), is used to identify areas used by tagged 
animals, and to measure the frequency with which ani- 
mals return to these areas. For central-place foragers 
who feed primarily on a limited number of prey spe- 
cies and exhibit site fidelity to specific foraging areas, 
telemetry can be used in conjunction with information 
on prey distribution to identify foraging habitat (North 
and Reynolds, 1996; Robinson et al., 2007). The harbor 
seal was therefore an excellent subject with which to 
assess the usefulness of telemetry in identifying forag- 
ing habitat of pinnipeds. 
A second goal of this study was to assess the influ- 
ence of spatial scale on our analysis. A number of in- 
vestigators have assessed the spatial overlap between 
distributions of marine predators and their prey (e.g., 
Rose and Leggett, 1990; Fauchald et al., 2000; Davo- 
ren et al., 2003). These investigators noted that the 
results of such analyses vary depending on scale (Rose 
and Leggett, 1990; Fauchald et al., 2000). As a result, 
studies encompassing multiple scales are recommended 
for addressing questions related to habitat selection 
(Olivier and Wotherspoon, 2005). 
Although harbor seals in SFB appear to forage with- 
in the bay, harbor seals do periodically make trips to 
the outer coast (Grigg, 2008). A third goal of this study 
was to attempt to identify environmental factors asso- 
ciated with harbor seals leaving SFB to forage. Harbor 
seals could forage outside of SFB to exploit changes in 
availability of prey associated with coastal upwelling. 
Upwelling of cooler, nutrient-rich water is associated 
with increased productivity along the California coast 
and can influence the distribution of marine predators 
(Becker and Beissinger, 2003). Alternatively, harbor 
seals could forage outside of SFB when high numbers 
of harbor seals using haul-out sites within SFB re- 
sult in intraspecific competition for food resources in 
SFB. 
We examined the associations between harbor seals 
and potential prey species, using satellite tracking to 
identify patterns of harbor seal distribution in SFB and 
a database available from the California Department of 
Fish and Game (CDFG) on abundance and distribution 
of potential prey species in the area. The objectives of 
this study were 1) to identify spatial and seasonal pat- 
terns of association between harbor seals and their prey 
in an urbanized estuary; 2) to examine the influence 
of scale of analysis on the spatial relationship between 
predator and prey; 3) to relate shifts in prey abundance 
or environmental factors to seasonal differences in the 
use of waters inside and outside SFB; and 4) to assess 
the usefulness of satellite telemetry in identifying for- 
aging areas of harbor seals. 
Materials and methods 
Study area 
San Francisco Bay is a turbid estuary with mean depths 
ranging from 3-11 m, and is the largest coastal embay- 
ment on the Pacific coast of the United States (Conomos 
et al., 1985) (Fig. 1). Harbor seals use SFB year-round 
for foraging, pupping, and resting on terrestrial haul-out 
sites (Allen et al., 1993; Grigg et al., 2004). 
Harbor seal telemetry 
Between January 2001 and January 2005, harbor seals 
were captured at a primary haul-out site in SFB (Castro 
Rocks; Fig. 1). At high tide, “tangle nets” 20-40 m long 
and 5 m deep were set, and harbor seals were caught as 
they approached the site. Harbor seals were fitted with 
dorsal- or head-mounted satellite-linked Platform Ter- 
minal Transmitters (PTTs; model ST-18, Telonics, Mesa, 
AZ; models SDR-T16 and SPOT3, Wildlife Computers, 
Redmond, WA; depending on model, tag power ranged 
from 0.4 to 0.5 watt, repetition rate ranged from 45 to 
48 sec). Only harbor seals deemed large enough (>40 kg) 
to support the PTTs were tagged. The PTTs were glued 
to the harbor seal’s pelage with a quick-setting marine 
epoxy, and were shed by harbor seals before or during 
their annual molt. 
Service Argos (CLS America, Inc., Largo, MD) was 
the processing center for the satellite telemetry da- 
ta and provided the geographical coordinates of the 
tagged harbor seals. When a harbor seal is at the sur- 
face, PTTs send periodic radio transmissions which are 
detected by polar-orbiting satellites. These satellites 
relay the signals to processing centers, where animal 
location estimates are calculated on the basis of the 
Doppler effect. Based on the number of received trans- 
missions and other factors, all locations are grouped 
into location accuracy “classes,” ranging from zero to 
three, and two additional classes (A and B) for loca- 
tions that could not be assigned an accuracy estimate 
(Table 1). Marine mammals are considered to be good 
study animals for satellite tracking because they sur- 
face to breathe and this allows sufficient time for a 
position to be determined by the satellites. In addition, 
the elevation of the tag does not change while the tag 
is on the animal; changes in tag elevation have been 
cited as a primary cause of spatial inaccuracy (Keating 
et al., 1991). Recent studies have assessed the useful- 
ness of PTTs for studying movements and habitat use 
of marine animals (e.g., Vincent et al., 2002; White and 
Sjoberg, 2002). Location classes with lesser accuracy 
