Grigg et al.: Spatial and seasonal relationships between Phoca vitulma richardii and their prey 
369 
feed (Fitch and Lavenberg, 1971). Harbor seals in SFB 
forage both during the night and day (Torok, 1994; 
Nickel, 2003), so harbor seals may be feeding on adult 
plainfin midshipman at night, in the same areas where 
greater numbers of juveniles were recorded during 
the day by CDFG trawls; spatial distribution of adult 
and juvenile plainfin midshipman is similar in SFB. 
Numbers of plainfin midshipman are greatest in cen- 
tral SFB during the fall. In addition, white croaker 
are abundant in SFB; age-l+ white croaker move into 
central SFB in the fall before migrating out of SFB 
in the late fall and winter. During fall, harbor seals 
occurred where there were greatest numbers of this 
species in central SFB and around Castro Rocks. The 
Pacific staghorn sculpin is one of the most abundant 
demersal fish in SFB, and is common in central SFB 
and San Pablo Bay. Numbers of adults are greatest 
during October through April. Harbor seals are more 
frequently located in areas with greatest abundance 
of this species, in both the fall and summer. 
Winter prey species 
The waters to the northeast of the Tiburon Peninsula 
and near the southeastern edge of Angel Island appear 
to represent important foraging areas for SFB harbor 
seals, particularly those using the Castro Rocks haul- 
out site. These areas were frequently visited by harbor 
seals year-round, and use of these areas has been noted 
in earlier studies of harbor seals in SFB (Torok, 1994, 
Nickel, 2003). During winter, abundance of plainfin 
midshipman was greatest in central SFB, most notably 
in waters to the northeast of the Tiburon Peninsula and 
the southeastern edge of Angel Island. Similarly, maxi- 
mum numbers of English sole were found in the central 
bay, around Tiburon Peninsula and Angel Island. 
Pacific herring is believed to be a preferred prey 
of harbor seals in SFB, and seasonal distribution of 
harbor seals in SFB reflects seasonal increases in 
abundance of Pacific herring, which spawn in SFB dur- 
ing the winter (Grigg, 2008). Correlations between the 
distributions of Pacific herring and harbor seals were 
greatest during winter, and harbor seals often were 
seen in SFB covered with herring eggs. Well-known 
Pacific herring spawning areas, such as Richardson 
Bay, around the Tiburon Peninsula, and along the 
eastern shoreline of SFB from Castro Rocks south- 
ward, were visited frequently by harbor seals during 
this season. 
Harbor seals do not appear to focus much foraging 
effort on Chinook salmon while they move through SFB. 
This part of the analysis may have been limited by the 
very small numbers of salmon caught by CDFG trawls. 
However, salmon were not reported in earlier studies of 
harbor seal diet in SFB based on fecal sample analyses 
(Torok, 1994). 
Overall, our findings agree with previously reported 
harbor seal diets based on fecal sample analyses in SFB 
(Torok, 1994), with the notable exception of yellowfin 
goby. Torok (1994) found that yellowfin goby was the 
most numerous species in harbor seal fecal samples, 
although his samples were primarily collected from 
harbor seals captured in the south SFB, whereas the 
tagged harbor seals used in this study were captured 
at a central SFB haul-out site. Yellowfin gobies are 
seasonally abundant in San Pablo Bay and Suisun Bay. 
Although both areas were visited by harbor seals in our 
study, these harbor seals may have foraged on gobies 
opportunistically, because they did not appear to fre- 
quently visit areas with abundant yellowfin goby. This 
may reflect partitioning of foraging habitat between 
harbor seals using primarily south SFB haul-out sites 
and using central and north SFB haul-out sites; separa- 
tion of harbor seals in northern and southern areas of 
SFB has been noted before (Allen et al., 1993; Grigg, 
2008). Use of haul-out specific foraging areas has been 
noted in other harbor seal populations (Iverson et al., 
1997; Thompson et al., 1998). 
The accuracy of our methods for identifying foraging 
habitat for harbor seals in this region is supported 
by the fact that our findings agree with earlier stud- 
ies, which demonstrate the tendency of harbor seals 
to forage on seasonally abundant, primarily benthic 
prey found near their primary haul-out site (e.g., Har- 
konen, 1987; Thompson et al., 1998). Spatial overlap 
between harbor seals and the prey species surveyed 
in this study was greatest in waters within approxi- 
mately 10 km of Castro Rocks and declined in waters 
beyond this range. This is consistent with central 
place foraging theory, where site use is expected to de- 
cline with distance from the central place (e.g., haul- 
out site; Orians and Pearson, 1979). There may also 
be a tendency for harbor seals to feed in areas with 
more predictable prey, e.g., the waters of central SFB, 
where abundance of fish caught in CDFG trawls tends 
to be high year-round. Greater correlations between 
predator and prey can be expected in these areas 
where prey is predictably present, and lesser correla- 
tions in other less predictable, “ephemeral” prey areas 
(Davoren et al., 2003), e.g., other parts of SFB. Simi- 
larly, harbor seals may select a larger-scale region 
(e.g., central SFB) where prey are found in greater 
abundance, a trend that is reflected in the greater 
correlations at larger scales in our study and in other 
studies of marine predators and their prey (Rose and 
Leggett, 1990; Mehlum et al., 1999; Fauchald et al., 
2000). Harbor seals’ tendency to return repeatedly 
to the same foraging areas indicates that they can 
track regions with predictably abundant prey over 
long time scales (e.g., seasonally), despite the likeli- 
hood that locations of individual prey patches may 
vary over much shorter time scales. In other systems 
and at smaller scales, prey patches are more variable 
in location, and correlations between predators and 
prey at smaller scales likely will be weaker (Mehlum 
et al., 1999). Harbor seals tend to choose haul-out 
sites located near abundant prey resources (Loughlin, 
1978), and the selection of the Castro Rocks location 
as a haul-out site almost certainly reflects availability 
and abundance of prey resources nearby. 
