20 
Fishery Bulletin 1 1 1 (1) 
mated to contribute most to harbor seal diets included 
Black and Yellowtail Rockfish, Chinook Salmon, adult 
Pacific Herring, and Shiner Perch (Fig. 2). Large differ- 
ences in fat mass among prey classes led to substantial 
differences in the 2 estimates. Most 
noticeably, the high fat content of 
mature salmon species (Table 2) 
reduced the contribution of adult 
Chinook Salmon in the estimates 
adjusted for fat mass, suggesting 
that few individual Chinook Salm- 
on need to be consumed for them 
to contribute significantly to the fat 
composition of harbor seals. 
Multivariate analysis of variance 
results revealed substantial hetero- 
geneity among estimated diets of 
individual seals by sampling loca- 
tion (PcO.OOl) and sex (P<0.001), 
although the interaction was not 
statistically significant (P=0.111). 
For that reason, the 49 seals were 
independently stratified by sam- 
pling location and sex and the mean 
diet composition, unadjusted for dif- 
ferential fat mass, was estimated 
for the seals in each stratum. Sea- 
son was eliminated from the model 
because it was not a statistically 
important covariate (see Discussion 
section). Seals sampled in the vicin- 
ity of Belle Chain and Bird Rocks, 
both of which are characterized by 
rocky, high-current habitat, had the 
most diverse diets, with important 
contributions from Black and Yel- 
lowtail Rockfish, adult salmon spe- 
cies, Pacific Herring, Shiner Perch, 
and Spiny Dogfish (Fig. 3). Con- 
versely, seals sampled from Padilla 
Bay, which consists of shallow estu- 
arine habitat, had diets that were, 
on average, dominated by Shiner 
Perch. Harbor seals sampled near 
Vendovi Island, which has rocky 
habitat with nearby access to sev- 
eral bays, appeared to have an in- 
termediate diet. 
Male harbor seals were esti- 
mated to consume larger quanti- 
ties of Black and Yellowtail Rock- 
fish, Pacific Herring, and Spiny 
Dogfish than females, for which 
Shiner Perch appeared to be more 
important (Fig. 4). Diet estimates 
for individual seals reflected ad- 
ditional between-seal heterogene- 
ity that was not explained by the 
covariates. For example, although 
Black and Yellowtail rockfish were estimated to be 
more important to males than females overall, males 
were not consistent in their reliance on rockfish spe- 
cies. Of the 24 males sampled, 10 had an estimated 
Prey group 
Figure 3 
Estimates of mean diet composition for harbor seals {Phoca uitulina) in the 
Salish Sea, unadjusted for differential fat mass among prey classes, by sam- 
pling location: (A) Belle Chain Islets, (B) Bird Rocks, (C) Padilla Bay, and 
(D) Vendovi Island. Error bars are ±1 standard error of the estimate. Prey 
classes are defined as B&YR (Black [Sebastes melanops] and Yellowtail [S. 
flavidus] Rockfish), CR (Copper Rockfish [S. caurinus]), PSR (Puget Sound 
Rockfish [S. emphaeus]), Chin (mature Chinook Salmon [Oncorhynchus 
tshawytscha 3), Chum (mature Chum Salmon [O. keta}), Coho (mature Coho 
Salmon [O. kisutch]), Sock (mature Sockeye Salmon [O. nerka ]), Pink (mature 
Pink Salmon [O. gorbuscha]), Sal-M (medium-size Chinook and Coho Salm- 
on), Sal-S (small Chinook, Chum, Sockeye, and Ppink Salmon), Pol (Walleye 
Pollock [Theragra chalcogramma]), Her (Pacific Herring [Clupea pallasii ] 
at least 2 years old), YH&SL (Pacific Herring less than 2 years old and 
Pacific Sand Lance [Ammodytes hexapterus]), NA (Northern Anchovy [En- 
grauiis mordax] ), SP (Shiner Perch [Cymatogaster aggregata ]), PM (Plainfm 
Midshipman [ Porichthys notatus]), SD (Spiny Dogfish [ Squalus acanthias]), 
OIS (Opalescent Inshore Squid [ Loligo opalescens]), G&S&F (Kelp Greenling 
[Hexagrammos decagrammus ], Pacific Staghorn Sculpin [Leptocottus arma- 
tus], and Starry Flounder [Platichthys stellatus}). 
