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Fishery Bulletin 99(3) 
sonal densities presented in our study were not directly 
comparable because of the different gear used, and our 
summer mesozooplankton densities were lower than those 
reported from other summer collections in PWS 8 9 (Cooney 
et al., 1981; Celewycz and Wertheimer, 1996). The food 
supply available to juvenile fish was likely more abun- 
dant in summer than in autumn, because other studies in 
the northeastern Pacific showed a steady decline in zoo- 
plankton biomass from summer to winter (Peterson and 
Miller, 1977; Cooney, 1986; Incze et al., 1997; Foy and 
Paul, 1999). Zooplankton taxonomic compositions also dif- 
fered seasonally, mainly in the larger percentages of alter- 
nate prey available in autumn. 
We found the surface-water feeding environment to be 
richer in numbers of prey than the deeper water in both 
seasons. Lower zooplankton density with depth has been 
reported by other authors in PWS, 10 Shelikof Strait (Napp 
et al., 1996), off the Oregon Coast (Marlowe and Miller, 
1975), and at Ocean Station “P” (Petersen and Miller, 
1977). Large calanoids were an exception in autumn, how- 
ever, when their densities and percentages were greater 
in the deeper tows, typical of subarctic locations (e.g. Mar- 
lowe and Miller, 1975; Nakatani, 1988; Napp et al., 1996). 
High abundances of Pseudocalanus spp. and other small 
calanoids and high biomass of large calanoids are char- 
acteristic of neritic locations in subarctic Pacific waters, 
such as those of PWS in summer (Springer et al., 1989; 
Coyle et al., 1990; Celewycz and Wertheimer, 1996; Incze 
et al., 1997). The time of collection for zooplankton is im- 
portant, however, because the location of peak abundance 
in the water column varies with their diel vertical migra- 
tion. Calanoid copepods are usually most abundant at the 
surface at night, migrating deeper during the day (e.g. 
Sekiguchi, 1975). Even though most of our samples were 
collected during the day, it is not surprising that abun- 
dances were greater in shallow tows than in deeper tows 
because Pseudocalanus was dominant. Of the species im- 
portant in our study, Pseudocalanus newmani was typi- 
cally the shallowest in depth distribution, Calanus pacifi- 
cus was intermediate and Metridia pacifica, the deepest 
(e.g. Bollens and Frost, 1989; Frost and Bollens, 1992; Bol- 
lens et al., 1993; Bollens et al., 1992b). However, the verti- 
cal distribution patterns of these species were influenced 
by the presence of predators and other factors (e.g. Bol- 
lens and Frost, 1989; Bollens et al., 1992b; Frost and Bol- 
8 ( continued ) Summer zooplankton density and composition esti- 
mates from 20-m vertical hauls using three net meshes. Alaska 
Fisheries Research Bull., Auke Bay Laboratory, National 
Marine Fisheries Service, 11305 Glacier Hwy., Juneau, AK 
99801. 
9 Sturdevant, M. V., and L. B. Hulbert. 1999. Diet overlap, prey 
selection, and potential food competition among allopatric and 
sympatric forage fish species in Prince William Sound, 1996. In 
Forage fish diet overlap, 1994-1996, p. 72-100. Exxon Valdez 
Oil Spill Restoration Project Final Report (Restoration Project 
97163C), Auke Bay Laboratory, National Marine Fisheries Ser- 
vice, 11305 Glacier Hwy., Juneau, Alaska 99801. 
10 Foy, R. J., and B. L. Norcross. In prep. Nearshore zooplank- 
ton community ecology in Prince William Sound, Alaska. J. 
Plankton Res., Institute of Marine Science, Univ. Alaska Fair- 
banks, P. O. Box 757220, Fairbanks, AK 99775-7220. 
lens, 1992; Bollens et al., 1993). The seasonal difference in 
depth distribution of large calanoids could have been due 
to seasonal vertical migrations (Sekiguchi, 1975; Mackas 
et al, 1993), differences in sampling time, or a response to 
the changing light regime in autumn. 
Feeding habits 
Summer and autumn diets of YOY allopatric and sympat- 
ric walleye pollock and Pacific herring in PWS were sim- 
ilar to those reported from other areas and to those of 
other pollock and herring caught during the study that 
did not meet our criteria (Sturdevant, unpubl. data; see 
“Materials and methods” section). In summer, both spe- 
cies consumed the abundant calanoid taxa as well as less 
abundant small prey. Calanoid copepods were likewise 
the predominant summer prey of YOY pollock in Japa- 
nese waters (Kamba, 1977; Nakatani, 1988), the region of 
the Kodiak Island-Alaska Peninsula, 11 the Gulf of Alaska 
and eastern Bering Sea (Grover, 1990, 1991; Brodeur et 
al., 1997), PWS (Willette et al., 1997; Foy and Norcross, 
1999a), and southeastern Alaska. 12 The small calanoid 
Acartia clausi was particularly important in southeastern 
Alaska diets from August to October, 12 but by late summer 
in other areas, euphausiids accounted for more prey bio- 
mass and calanoids continued to dominate numerically 
(Merati and Brodeur, 1996). The autumn prey composi- 
tion of pollock in our study was also similar to that of 
YOY pollock in the Gulf of Alaska (Merati and Brodeur, 
1996; Brodeur et al., 2000), eastern Kamchatka (Sobo- 
levskii and Senchenko, 1996), and southeastern Alaska. 12 
In those studies, increased fish size was correlated with 
decreased predation on small copepods and increased pre- 
dation on large copepods, larvaceans, and euphausiids. By 
winter, large copepods virtually disappeared from diets 
in some areas (Sobolevskii and Senchenko, 1996); chae- 
tognaths and epibenthic prey such as mysids, shrimps, 
caprellid amphipods, and cumaceans were incorporated in 
the diet as vertical distributions of the fish changed and 
pelagic prey became scarce 12 (Merati and Brodeur, 1996; 
Sobolevskii and Senchenko, 1996; Brodeur et al., 2000). 
Seasonal changes in prey have also been correlated with 
change in YOY pollock distribution and the use of different 
habitats (Nakatani, 1988). 
Like pollock, YOY Pacific herring depended on small 
calanoid prey in PWS and throughout their range. Addi- 
tional small prey taxa are commonly reported in Pacific 
herring and other species diets, including invertebrate 
eggs, barnacle larvae, cladocerans, oikopleurans, and ju- 
venile amphipods and euphausiids 9 (Wailes, 1936; Sher- 
man and Perkins, 1971; Last, 1989; Arrhenius and Hans- 
n Livingston, P. A. 1985. Summer food habits of young-of-the- 
year walleye pollock, Theragra chalcogramma, in the Kodiak 
area of the Gulf of Alaska during 1985. Unpubl. manuscr., 
Alaska Fish. Sci. Center, National Marine Fisheries Center, 
7600 Sand Point Way NE, Seattle, WA 981 15, 8 p. 
12 Krieger, K. J. 1985. Food habits and distribution of first- 
year walleye pollock, Theragra chalcogramma (Pallas), in Auke 
Bay, Southeastern Alaska. Unpubl. MS thesis, Univ. Alaska 
Juneau, 11305 Glacier Hwy., Juneau, AK 99801, 57 p. 
