Abstract— Diets of young-of-the-year 
(YOY) walleye pollock ( Theragra chal- 
cogi'amma) and Pacific herring (Clupea 
pallasi ) were compared between sea- 
sons (summer and autumn), years 
(autumn), and allopatric and sympatric 
fish aggregations (autumn) in Prince 
William Sound (PWS), Alaska. Fish 
were collected principally by mid-water 
trawl 20 July-12 August 1995, 5-14 
October 1995, and 7-13 November 1994. 
Prey fields were assessed from zoo- 
plankton samples in 1995. 
During the summer, the principal 
prey of allopatric pollock and herring 
was small calanoids and diet overlap 
was high (7? 0 >0.76). During the autumn, 
diets were composed of large calanoids, 
larvaceans, and euphausiids. Diet over- 
lap between sympatric species was 
greater in November 1994 (f? 0 <0.94) 
than in October 1995 (R 0 < 0.69). The 
seasonal diet shift to larger prey coin- 
cided with larger fish size and with 
decreased abundance and proportions 
of the principal zooplankter, small cal- 
anoids, and increased abundance and 
proportions of large calanoids and lar- 
vaceans in zooplankton tows. However, 
feeding decreased in autumn, compared 
with summer, especially for herring. 
Sympatric fish had higher rates of non- 
feeding than allopatric fish, and subtle 
differences in prey selection existed 
between the aggregations, but sampling 
variation could explain these feeding 
differences. 
The similarity in diets of YOY pollock 
and herring indicate the potential for 
competition. These species are impor- 
tant to commercial fisheries and as 
forage for marine birds and mammals. 
An understanding of their trophic inter- 
actions could help to explain shifts in 
fish community structure and bird pre- 
dation. If sympatry increases as prey 
resources decline, competition in au- 
tumn may be particularly important in 
regulating populations. 
Manuscript accepted 18 December 2000 
Fish. Bull. 99:482-501 (2001). 
Feeding habits, prey fields, and 
potential competition of young-of-the-year 
walleye pollock ( Theragra chalcogramma) 
and Pacific herring (Clupea pallasi ) 
in Prince William Sound, Alaska, 1994-1995 
Molly V. Sturdevant 
Auke Bay Laboratory 
Alaska Fisheries Science Center 
11305 Glacier Highway 
Juneau, Alaska 99801 
E-mail address: Molly.Sturdevant@noaa.gov 
Audra L. J. Brase 
Commercial Fisheries Division 
Alaska Department of Fish and Game 
PO Box 20 
Douglas, Alaska 99824 
Leland B. Hulbert 
Auke Bay Laboratory 
Alaska Fisheries Science Center 
11305 Glacier Highway 
Juneau, Alaska 99801 
Walleye pollock ( Theragra chalcogram- 
ma ) and Pacific herring (Clupea pallasi) 
are forage fish that inhabit the north- 
eastern Pacific Ocean rim. Both species 
are important components of marine 
bird, mammal, and fish diets, both sup- 
port important commercial fisheries in 
the Gulf of Alaska (GOA), and histori- 
cal data show dramatic variability in 
both their populations (Springer, 1992; 
Bechtol, 1997; Springer and Speckman, 
1997; Anderson and Piatt, 1999). Young- 
of-the-year (YOY) walleye pollock and 
YOY Pacific herring are found at the 
same locations and depths during at 
least part of the year (Brodeur and 
Wilson, 1996a; Willette et ah, 1997; 
Stokesbury et ah, 2000) and both con- 
sume zooplankton as their primary 
prey 1 ’ 2 (Willette et ah, 1997; Foy and 
Norcross, 1999a, 1999b). Because of 
these similarities and because the fre- 
quency and nature of their interactions 
may change as fish population struc- 
ture shifts, we investigated the poten- 
tial for feeding competition between 
these species. 
Several recent studies have found 
that the species composition of forage 
fish populations in the GOA and Prince 
William Sound (PWS) has changed dra- 
matically (Bechtol, 1997; Kuletz et ah, 
1997; Anderson and Piatt, 1999). Short- 
term population changes were attribut- 
ed to the Exxon Valdez oil spill in March 
1989 (Brown et ah, 1996a; Kuletz, 1996; 
Oakley and Kuletz, 1996; Kuletz et ah, 
1 Jewett, S., and E. Debevee. 1999. Diet 
composition, diet overlap and size of 14 
species of forage fish collected monthly 
in Prince William Sound, Alaska, 
1994-1996. In Forage fish diet overlap, 
1994-1996, p. 10-37. Exxon Valdez Oil 
Spill Restoration Project Final Report 
(Restoration Project 971630, Auke Bay 
Laboratory, National Marine Fisheries Ser- 
vice, 11305 Glacier Hwy., Juneau, AK 
99801-8626. 
2 Boldt, J. L. 1997. Condition and distri- 
bution of forage fish in Prince William 
Sound, Alaska. Unpubl. M.S. thesis, 
Juneau Center School of Fisheries and 
Ocean Science, Univ. Alaska Fairbanks, 
11120 Glacier Hwy., Juneau, AK 99801, 
155 p. 
