396 
Fishery Bulletin 108(4) 
identified for all dates and stations at which fish were 
collected. Identification of gelatinous animals was gen- 
erally poor because of their damage during collection. 
Abundances (densities) were calculated from the sub- 
sample counts and volumes (m 3 ) of water filtered (as 
measured by a flow meter). Data from different depth 
strata were weighted by the size (height, m) of the 
depth strata and then averages for the upper 50 m were 
computed. Zooplankton community composition was 
described by station by using the data from duplicate 
zooplankton tows to compute average abundances for 
each taxonomic group for each of four seasons (spring, 
early summer, late summer, and autumn) over the 
three years studied (1985-87). Dry weights (g/m 2 ) for 
zooplankton >216 pm were measured as described in 
Bollens et al. (1992b). 
For ease and clarity of presentation of the zooplank- 
ton compositional data, the category “other Copepoda” 
that is used in presenting juvenile salmon gut contents 
was partitioned into several subcategories, i.e., the gen- 
era Oithona, Metridia, and Pseudocalanus. In addition, 
several taxonomic groups that were common in salmon 
gut contents were rarely or never observed in the zoo- 
plankton samples (e.g., Insecta and Araneae, Cepha- 
lopoda, Teleostei, and Ctenophora) and were therefore 
placed into the “other” category for characterizing zoo- 
plankton community composition. 
Data analysis 
We estimated interspecific diet overlap between co-occur- 
ring juvenile salmon species on the basis of biomass of 
prey in common, using Schoener’s (1970) percent simi- 
larity index (PSI): 
PS/,, y = 10°(l-0.5(^|P xv -P Vii |)), (1) 
where P xi = percent biomass of food category ( i ) in the 
stomach of species x; and 
P yi = percent biomass of food category ( i ) in the 
stomach of species y. 
We first pooled the prey biomass data by size class 
for each salmon species. PSI calculations were made 
for the subset of samples with a minimum of three 
stomachs per species. We examined intraspecific spa- 
tial variation in diet by calculating the PSI of juvenile 
salmon between the two stations. PSI values >60% 
were considered significant (Brodeur and Pearcy, 1992; 
Landingham et al., 1998). 
Prey selectivity by juvenile salmon species was exam- 
ined by using Ivlev’s (1961) electivity index (E t ): 
E, = {r i -p i )Hr i + Pl ), (2) 
where E l = the electivity index; 
r- = the numerical proportion of the i th taxon in 
the stomachs; and 
p t = the proportion of the same taxon in the 
environment. 
The electivity values provide a species-specific mea- 
sure of prey selection by allowing a comparison of salm- 
on gut contents to available prey. Values for E t range 
from -1 to 1, where 1 indicates the highest selectivity 
(i.e., present in the diet, but never in the zooplankton 
samples), and -1 indicates lowest selectivity (i.e., never 
in the diet, but present in the zooplankton samples). 
We summarized these observations as average species- 
specific electivity scores for all size classes, seasons, 
and years combined to compare selection across salmon 
species. 
We compared juvenile salmon gut fullness (% body 
weight) and zooplankton abundance (dry weight) at each 
station, as well as each salmon species’ gut fullness be- 
tween the two stations, using Spearman’s rank correla- 
tions (Zar, 1999). The difference in mean zooplankton 
abundance (dry weight) between the two stations was 
tested by using a Mann-Whitney U test (Zar, 1999). 
Results 
Juvenile salmon diet 
Juvenile salmon in Dabob Bay showed species-specific 
patterns of occurrence throughout the April-October 
time period across years. In general, more and smaller 
juvenile salmon were caught at the nearshore shallow 
station during spring and early summer, and more and 
larger juvenile salmon were caught at the central-bay 
deep station during late summer and autumn. Chum 
salmon were most prominent during the spring and early 
summer, whereas Chinook salmon were caught more 
frequently later in the year, particularly at the deep sta- 
tion (190 m). All four salmon species were predominantly 
planktivorous, although there were some exceptions as 
described below. Because of the greater occurrence of 
chum and Chinook salmon diet samples, our analyses 
were focused on these two species. 
Juvenile chum salmon 
Chum salmon exhibited striking ontogenetic and spatial 
variation in diets, including a tendency for fish <100 mm 
FL to consume more insects and larvaceans and those 
>100 mm to consume more amphipods and decapod 
larvae. At the deep station in late spring, euphausiids 
were the dominant prey type by weight (Fig. 2). Small 
(<49 mm) fish also consumed insects, arachnids, and 
copepods other than Calanus pacificus, whereas larger 
fish (75-99 mm) consumed high percentages of teleosts 
(primarily unidentified species), hyperiid amphipods, 
and decapod larvae. The single fish of the 50-74 mm 
size range captured during early summer at the deep 
station consumed mostly insects and arachnids, whereas 
fish between 75 and 150 mm consumed predominantly 
larvaceans and hyperiids. The single large fish (>150 
mm) contained exclusively euphausiids. In late summer 
at the deep station, fish 100-149 mm consumed mostly 
hyperiid amphipods, whereas fish >150 mm consumed 
