359 
Abstract — Knowing where pinnipeds 
forage is vital to managing and pro- 
tecting their populations, and for 
assessing potential interactions with 
fisheries. We assessed the spatial 
relationship between the seasonal 
distribution of Pacific harbor seals 
( Phoca vitulina richardii) outfitted 
with satellite transmitters and the 
seasonal distributions of potential 
harbor seal prey species in San 
Francisco Bay, California. Pearson’s 
correlation coefficients were calcu- 
lated between the number of harbor 
seal locations in an area of the San 
Francisco Bay and the abundance of 
specific prey species in the same area. 
The influence of scale on the analy- 
ses was assessed by varying the scale 
of analysis from 1 to 10 km. There 
was consistency in the prey species 
targeted by harbor seals year-round, 
although there were seasonal differ- 
ences between the most important 
prey species. The highest correlations 
between harbor seals and their prey 
were found for seasonally abundant 
benthic species, located within about 
10 km of the primary haul-out site. 
Probable foraging habitat for harbor 
seals was identified, based on areas 
with high abundances of prey spe- 
cies that were strongly correlated 
with harbor seal distribution. With 
comparable local data inputs, this 
approach has potential application to 
pinniped management in other areas, 
and to decisions about the location of 
marine reserves designed to protect 
these species. 
Manuscript submitted 30 May 2008. 
Manuscript accepted 21 April 2009. 
Fish. Bull. 107:359-372 (2009). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Spatial and seasonal relationships between 
Pacific harbor seals ( Phoca vitulina richardii) 
and their prey, at multiple scales 
Emma K. Grigg (contact author ) 1 
A. Peter Klimley 1 
Sarah G. Allen 2 
Deborah E. Green 3 
Deborah L. Elliott-Fisk 1 
Hal Markowitz 4 
Email address for contact author: ekgrigg@gmail.com 
1 Department of Wildlife, Fish 
and Conservation Biology 
University of California, Davis 
1 Shields Avenue 
Davis, California 95616 
* Present address: Division of Environmental 
Studies and Geology 
Alfred University 
1 Saxon Drive 
Alfred, New York 14802 
2 Point Reyes National Seashore 
1 Bear Valley Road 
Point Reyes, California 94956 
3 San Francisco Public Utilities Commission 
Bureau of Environmental Management 
1145 Market Street, Suite 500 
San Francisco, California 94103 
4 Department of Biology 
San Francisco State University 
1600 Holloway Avenue 
San Francisco, California 94132 
Identification of foraging habitat is 
essential to understanding the ecology 
of marine predators. This information 
is vital to managing and protecting 
populations, as well as assessing the 
potential effects of commercial and 
recreational fisheries on both the 
marine predator and the fisheries. 
We determined the spatial overlap 
of one marine predator, the Pacific 
harbor seal (Phoca vitulina richardii) 
and its prey in the San Francisco Bay 
estuary (SFB), California, in order 
to identify foraging areas and sea- 
sonal patterns of resource use by this 
coastal pinniped. 
The Pacific harbor seal (hereafter 
referred to as the harbor seal) is a 
small phocid seal common to wa- 
ters along the west coast of North 
America. Harbor seals are opportu- 
nistic predators, feeding primarily 
on benthic species and small, epi- 
benthic, schooling fishes, and occa- 
sionally foraging on pelagic species 
(Harkonen, 1987). A relatively small 
number of species tend to dominate 
the diet of harbor seals, but seasonal 
shifts in diet are seen in many ar- 
eas, associated with seasonal fluc- 
tuations in prey availability (Brown 
and Mate, 1983; Tollit et al., 1998). 
Fecal samples collected in SFB in- 
dicate that harbor seals in this re- 
gion feed on Pacific herring (Clupea 
pallasii), northern anchovy (Engrau- 
lis mordax), plainfin midshipman 
( Porichthys notatus), Pacific staghorn 
sculpin ( Leptocottus armatus), white 
croaker (Genyonemus lineatus ), yel- 
lowfin goby ( Acanthogobius flavima- 
nus), jacksmelt ( Atherinopsis californ- 
iensis ), and English sole (Pleuronectes 
vetulus) (Torok, 1994). Young harbor 
seals have a reduced diving capabil- 
ity, and eat benthic crustaceans — 
primarily shrimp (e.g., Crangon 
spp.) (Bigg, 1973). Based on VHF 
(very high frequency) radiotelemetry 
tracking, the foraging range of harbor 
seals in SFB is mainly within 1-5 
km of a haul-out site (Torok, 1994; 
Nickel, 2003), indicating that harbor 
seals in SFB feed on local prey. Abun- 
dance of prey and distance from the 
primary haul-out site are the stron- 
gest predictors of harbor seal use of 
an area in SFB (Grigg, 2008). 
