486 
Fishery Bulletin 99(3) 
Feeding overlap between species and within species be- 
tween fish in allopatric and sympatric aggregations was 
described by using Horn’s overlap index (Horn, 1966; 
Smith and Zaret, 1982; Krebs, 1989). This index minimiz- 
es bias due to changing numbers of resource categories 
and resource evenness. Overlap was computed at two lev- 
els: prey resources were defined at the lowest level (spe- 
cies, stage, and size) and at a pooled level (principal taxa). 
Horn’s overlap index values, Rq, were expressed from 0 (no 
overlap) to 1 (total overlap) for predator species j and k: 
_ X (p " + p ' k ) x ln< p " + Pik ] - X p " x ln p ' _ X p ' k x ln p "> 
2 x In 2 
where p tj = proportion of z th resource in total prey re- 
sources utilized by j th species; and 
P,k = proportion of z th resource in total prey re- 
sources utilized by k th species. 
We considered R 0 values >0.60 to indicate similar use of 
resources and R 0 >0.75 to indicate very similar use of 
resources. 
In this report, we examined both intraspecific and inter- 
specific differences in YOY pollock and herring and quali- 
tatively compared data between seasons, between years 
(in autumn), between allopatric and sympatric aggrega- 
tion types, and between sympatric species. We also com- 
pared day-night feeding frequencies and prey condition to 
assess principal time of day of feeding. We compared zoo- 
plankton densities and percent densities of principal taxa 
between seasons and between depths (shallow and deep) 
sampled each season. Because our data were limited and 
the sampling design unbalanced, we present means and 
standard errors without statistical tests. 
Results 
Zooplankton prey fields 
Total zooplankton densities for all samples pooled were 
similar in summer and autumn of 1995, averaging 
approximately 1200 and 1400 organisms/m 3 (n = 37 and 8, 
mesh=303 pm and 243 pm, respectively; Table 1, Fig. 2). 
Approximately 3/4 of samples from each season were col- 
lected in daylight, between 10:00 and 20:00. 
Zooplankton taxa were less diverse in summer than in 
autumn, but small calanoids predominated in both seasons. 
Seasonal differences in the density and percentage contri- 
bution of a few taxa were apparent. Small calanoids made 
up a greater percentage of the total in summer compared 
with autumn (84% vs. 58%), but their density was similar 
in each season. Large calanoid density and percent density 
were both lower in summer than in autumn. Only a few 
other taxa contributed >5% to the total numbers of zoo- 
plankters in either season. In summer, these included cla- 
docera and gastropods (primarily the pteropod Limacina 
helicina); in autumn, they included large calanoids, bryo- 
zoan cyphonautes larvae, and gastropods (Table 1, Fig. 2). 
Calanoid species composition in the zooplankton also 
varied seasonally. Small calanoids in summer were pre- 
dominantly Pseudocalanus spp. (75%) and Acartia lon- 
giremis (5%); in autumn, Pseudocalanus spp. was 40%, 
Oithona similis, 15%, and A. longiremis, 6% of total zoo- 
plankton. Large calanoids in summer were principally 
Neocalanus and Calanus spp. (3% of total zooplankton) 
and in autumn, they were Metridia pacifica (10%) and 
Calanus spp. (1%). Among other taxa present in both sea- 
sons, density and percent density of gastropods were simi- 
lar between seasons, but larvaceans were less available in 
summer than in autumn. Cladocera were present only in 
summer and cyphonautes only in autumn. Hyperiid am- 
phipods, euphausiid larvae, chaetognaths, and barnacle 
and decapod larvae were rare in both seasons (Table 1). 
Zooplankton density per cubic meter varied with depth 
of sampling at all stations for both seasons. In summer 
(n= 7 stations), zooplankton density was more than twice 
as high in shallow hauls as in deep hauls, whereas in au- 
tumn (?z=l station), it was greater in the shallow tows 
than in the deep tows (Table 1). Depth-related differences 
in the abundance of principal taxa, but not in their per- 
centage composition, were also observed. Overall, in sum- 
mer, small calanoids were twice as abundant in the more 
shallow tows, but they comprised similar percentages at 
each depth. In autumn, both density and percentage of 
small calanoids were greater in shallow tows than in deep- 
er tows. Large calanoid density and percentage did not dif- 
fer between depths in summer, but in autumn they were 
lower in shallow tows compared with deep tows. Gastro- 
pod abundance in both seasons, and cladoceran abundance 
in summer, were greater in shallow tows than in deep 
tows, but percentages did not differ with depth. Larva- 
cean abundance and percentage abundance in either sea- 
son, and the abundance and percentage abundance of cy- 
phonautes in autumn, did not differ between depths. 
The macrozooplankters, larval euphausiids and young 
hyperiids, were present, but rare in our plankton tows. 
However, euphausiids were captured in 11% of summer 
trawls and in 43% of autumn trawls. In summer of 1995, 4 
juvenile-adult euphausiids were present in the northeast- 
ern and southwestern regions of the sound but were not 
caught in the central region or at any stations where fish 
were caught; in autumn ( 1994 7 and 1995 4 ), they were pres- 
ent in trawls from all three regions but were absent from 
the allopatric trawl in October 1995. We have no consistent 
data on the presence or absence of hyperiid amphipods. 
Fish catches 
Catches of YOY walleye pollock and Pacific herring sam- 
pled from PWS that met our allopatric-sympatric criteria 
(see “Materials and methods” section) were not evenly rep- 
7 Paul, A. J. 1995. Invertebrate forage species. In Forage fish 
study in Prince William Sound, Alaska, p. 43-54. Exxon Valdez 
Oil Spill Restoration Project 94163A Annual Report, Juneau 
Center School of Fisheries and Ocean Science, Univ. Alaska 
Fairbanks, 11120 Glacier Hwy., Juneau, AK 99801. 
