Grigg et al. : Spatial and seasonal relationships between Phoca vitulma richardii a nd their prey 
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Table 3 
Area (km 2 ) of Pacific harbor seal ( Phoca vitulina richardii) foraging habitat in San Francisco Bay, CA, based on areas where 
abundance of harbor seal prey species was above the threshold value calculated in the regression tree analyses. Area of foraging 
habitat throughout SFB is given, as well as area of foraging habitat within 10 km of Castro Rocks, the primary haul-out site used 
by harbor seals in this study. 
Season 
Prey species used in estimation of 
foraging habitat 
Area of foraging 
habitat (km 2 ) 
Area of foraging habitat 
within 10 km of 
Castro Rocks (km 2 ) 
Spring 
English sole ( Pleuronectes vetulus) 
Crab (mixed spp.; primarily Dungeness crab, Cancer magister ) 
146.8 
100.7 
Summer 
Pacific staghorn sculpin ( Leptocottus armatus) 
English sole 
222.9 
140.2 
Fall 
Plainfin midshipman ( Porichthys notatus) 
Pacific staghorn sculpin 
English sole 
238.4 
144.1 
Winter 
Pacific herring (Clupea pallasii) 
Plainfin midshipman 
English sole 
220.6 
130.8 
We assume that the spatial overlap between harbor 
seals and prey species reflects the tendency of harbor 
seals to frequent areas where the density of prey is 
greatest. Seasonal correlations between harbor seal loca- 
tions and prey density revealed the following patterns. 
Spring prey species 
The spatial correlation between harbor seals and Eng- 
lish sole during spring could reflect harbor seals’ use 
of shallower waters associated with breeding behavior 
because four of nine of the harbor seals tagged during 
this season were females of reproductive age, two of 
which were confirmed breeders (as determined from 
resightings with a pup). Harbor seal females wean 
their pups after four weeks and do not fast during 
the nursing period. Castro Rocks is submerged during 
high tides, requiring cows and pups to leave the site 
together. Pups have reduced diving capabilities (Bigg, 
1973), which may predispose the females with pups to 
forage in shallower waters. English sole are abundant 
in the waters of central SFB and San Pablo Bay during 
spring, mostly in shoal waters of San Pablo Bay. 
The consistent correlations between the distribution 
of harbor seal locations and crab may be due more to 
the foraging habits of harbor seal prey than to the 
harbor seals foraging directly on crab. Harbor seals 
consume crabs in California (Harvey et al., 1995), but 
whether crabs are important prey among harbor seals 
in SFB remains unclear. Pacific staghorn sculpin and 
English sole are major consumers of juvenile Dunge- 
ness crabs, and the vast majority of Dungeness crabs 
in SFB are juveniles (Reilly, 1983). In addition, crab 
numbers tend to be greatest in waters around Castro 
Rocks (particularly during spring), and therefore, the 
overlap of harbor seals and crab may also be related to 
proximity to Castro Rocks. 
During spring, greatest concentrations of Pacific 
staghorn sculpin were found in the extreme south 
bay, indicating that the abundance of this prey spe- 
cies was greater near a large south SFB pupping site, 
Mowry Slough. These south SFB aggregations of Pa- 
cific staghorn sculpin were apparently not used by 
Castro Rocks harbor seals during spring, and the lack 
of correlation between harbor seals and sculpin dur- 
ing this season likely reflects the tendency of harbor 
seals to remain closer to their primary haul-out site 
during pupping. 
Summer prey species 
English sole appear to be an important food source 
for SFB harbor seals. English sole use the SFB as a 
nursery area and remain in the bay for 6-18 months 
(Budd, 1940). Although there was a significant cor- 
relation between harbor seal locations and English 
sole abundances in all seasons (see also spring prey 
species, above), abundance of age-l+ sole was less in 
the summer, when the correlation between harbor 
seals and English sole was greatest. Because harbor 
seals are opportunistic foragers, they may exploit the 
abundant young English sole found near their haul- 
out site during the molting season. Large mature 
English sole (> 250 mm) were rarely caught by CDFG 
surveys, and Torok (1994) suggested that any forag- 
ing on flatfish by south SFB harbor seals took place 
outside of SFB; however, harbor seals were occasion- 
ally observed feeding on large flatfish near the Castro 
Rocks site. In addition, harbor seals often visited 
areas where Pacific staghorn sculpin were numer- 
ous, particularly in waters around the Castro Rocks 
haul-out site. Harbor seals spend a greater proportion 
of their time hauled out during the molting season 
(Thompson et al., 1989), and harbor seals in this 
