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Fishery Bulletin 107(3) 
We did not attempt to identify average patch size of 
harbor seal prey in our study, choosing instead to dem- 
onstrate overlap with areas of abundant prey resources. 
As Dungan et al. (2002) noted, patches that are smaller 
than the size of the sampling unit cannot be detected. 
Average size of prey patches in SFB may be smaller 
than our minimum study scale of 1 km. Nickel (2003) 
suggested that the average maximum prey patch for 
harbor seals foraging in SFB was 200 m, based on frac- 
tal analyses of harbor seal positions obtained by VHF 
radiotracking. If true, our grid cells could potentially 
have contained no prey patches or more than one prey 
patch, and could illustrate relative quality of foraging 
areas rather than locations or sizes of individual prey 
patches. On the other hand, we would expect positive 
correlations between harbor seal locations and prey 
densities only at scales greater than the dimensions 
of aggregations of predator or prey. At scales less than 
the smallest aggregation of predator or prey, we would 
be more likely to see negative correlations, because of 
factors such as predator avoidance by prey (Rose and 
Leggett, 1990). The greater correlations seen at larger 
scales in our study may also reflect the tendency of large 
scales to reduce or average out the effects of stochastic 
processes affecting where prey patches are located (Corsi 
et al., 2001). Small but significant negative correlations 
were found only at smaller scales (1 to 4 km) in our 
study, primarily involving two species: jacksmelt (spring 
and summer) and yellowfin goby (spring). Jacksmelt are 
often found in large schools in SFB, and negative cor- 
relation of the jacksmelt and harbor seal distributions at 
smaller scales could be due to avoidance of foraging har- 
bor seals by jacksmelt schools. Catch per unit of effort 
of yellowfin goby is usually greatest in San Pablo Bay 
and Suisun Bay, and least in central SFB, which could 
explain the negative correlation with this species. 
Based on observations of foraging locations of indi- 
vidual SFB harbor seals seen in past studies (Fancher, 
1979; Torok, 1994), the amount of harbor seal habitat 
estimated by our methods may be conservative. Our 
estimates were based on only the species with the stron- 
gest correlations between harbor seal locations and prey 
abundance data during our study period. Had additional 
species’ distributions been included in the potential 
habitat maps, a greater proportion of SFB may have 
been designated as foraging habitat for harbor seals. 
In addition, our estimates were based on strong cor- 
relations between predator and prey for harbor seals 
captured at (and presumably using) a haul-out site in 
central SFB. Had harbor seals used in this study been 
captured at a site in the extreme southern or northern 
SFB, the relative strength of correlations between har- 
bor seals and individual prey species may have been 
different, if harbor seals that use these other sites focus 
on different prey species. However, both an earlier ra- 
diotracking study using harbor seals captured in south- 
ern SFB (Torok, 1994) and a 2000-2001 radiotracking 
study conducted using harbor seals captured at Castro 
Rocks (Nickel, 2003) identified large foraging areas in 
central SFB, a number of which fell within the same 
areas indicated by our habitat maps. In addition, as 
harbor seals will shift prey species with seasonal and 
annual changes in local prey abundance (Tollit and 
Thompson, 1996), significant changes in the abundance 
or distribution of prey species in SFB could cause har- 
bor seals to switch to foraging areas not identified in 
our maps. With new information on prey distribution, 
locations of potential foraging habitat could easily be 
updated in the GIS. 
Identifying factors that govern when harbor seals 
leave SFB to forage will require larger sample sizes, 
information on seasonal patterns of prey availability on 
the outer coast, and possibly more fine-grained data on 
individual harbor seal movements and behavior while at 
sea. In many areas, including SFB, harbor seals exhibit 
two foraging strategies (Thompson et al., 1998; Grigg, 
2008). In one strategy, harbor seals make shorter, daily 
trips to and from foraging areas near the haul-out site; 
in the alternative strategy, harbor seals make longer 
foraging trips to more distant foraging areas, often 
lasting for a number of days and followed by extended 
haul-out periods. Harbor seals often move to protected 
estuarine haul-out sites to breed and molt, and num- 
bers of harbor seals at some haul-out sites in SFB are 
greatest during these seasons (Grigg et al., 2004). The 
proportion of harbor seal locations on the outer coast in 
our study was greater during the spring (pupping) and 
summer (molting) seasons and may reflect the need for 
some individual harbor seals to forage in coastal waters 
when the density of harbor seals was high in SFB, in 
order to minimize intraspecific competition for prey 
(and therefore, animals would forage in coastal waters 
but return to SFB haul-out sites to rest between trips). 
Alternatively, this behavior could reflect the movement 
by some harbor seals to pupping or molting haul-out 
sites located outside of SFB, and use of coastal forag- 
ing areas closer to these haul-out sites. Inspection of 
individual harbor seal movements (not shown) indicated 
that both patterns were probably occurring during our 
study period (Grigg, 2008). 
Satellite telemetry enabled us to identify correlations 
between harbor seal distribution and the distribution 
of prey species known to be present in the diet of local 
harbor seals. Using the harbor seal locations and GIS 
overlay analyses, we identified species of fish likely to 
be important seasonal prey species of harbor seals in 
SFB, and areas likely to be important foraging areas 
(particularly for harbor seals using the Castro Rocks 
haul-out site). Use of Service Argos positions required 
substantial elimination of inaccurate positions, with 
resultant loss of sample size and resolution of harbor 
seal movements, and limited the scale at which the 
analyses could be conducted. Nonetheless, we feel that 
satellite telemetry provided a useful way to assess har- 
bor seal distribution in our coastal study area, allow- 
ing continual collection of information on harbor seal 
locations that would have been difficult using currently 
available VHF radiotelemetry tags, and eliminating 
the potential disturbance of an observer’s presence on 
foraging harbor seals. 
