Sturdevant et al.: Feeding habits, prey fields, and potential competition of Theragra chalcogramma and Clupea pallcisi 
489 
Table 3 
Seasonal fish size and feeding attributes of allopatric and sympatric YOY walleye pollock and Pacific herring from Prince William 
Sound, 1994-1995. Values are mean and standard error, SE, for fish pooled from stations shown in Table 1. 
Species and 
sampling period 
n 
FL 
Wet weight 
<g) 
% non- 
feeders 
Fullness 
(%) 
Content 
% BW 
Total 
number 
of prey 
Total 
weight of 
prey (mg) 
Walleye pollock 
Summer (allopatric) 
120 
58.0 (1.0) 
1.1 (0.1) 
19 
50 (5) 
1.4(04) 
294 (68) 
34.0 (5.6) 
Oct. 1995 (sympatric) 
20 
91.5 (2.4) 
5.0 (0.3) 
5 
75 (10) 
0.8 (0.3) 
63(11) 
39.0(8.9) 
Nov. 1994 
30 
107.2 (2.0) 
8.1 (0.3) 
23 
50 (10) 
0.9 (0.2) 
249 (80) 
34.8 (7.2) 
Allopatric 
10 
111.0(1.6) 
9.1 (0.4) 
0 
100 (5) 
1.8 (0.4) 
723 (156) 
71.6(11.4) 
Sympatric 
20 
105.4 (2.1) 
7.6 (0.5) 
35 
25 (10) 
0.4 (0.1) 
13(3) 
16.4 (5.8) 
Pacific herring 
Summer (allopatric) 
20 
52.7 (2.4) 
1.5 (0.3) 
20 
75 (10) 
1.9 (0.5) 
3011 (739) 
2714 (69.0) 
Oct. 1995 
30 
90.2 (2.0) 
6.6 (0.3) 
7 
50 (5) 
1.0 (0.1) 
528(119) 
82.2 (21.6) 
Allopatric 
10 
91.6 (1.4) 
6.8 (0.4) 
0 
50(10) 
1.4 (0.1) 
386 (90) 
103.4 (19.5) 
Sympatric 
20 
89.5 (3.0) 
6.5 (0.6) 
10 
75 (10) 
0.9(04) 
599 (173) 
71.6 (31.0) 
Nov. 1994 (sympatric) 
20 
94.6 (2.4) 
6.9 (0.3) 
55 
10 (10) 
0.3 (0.1) 
23(9) 
13.2 (6.8) 
Size of YOY pollock and herring 
Intraspecific patterns of size across seasons differed for 
YOY pollock and herring. Mean FLs for both species were 
approximately 60% longer in autumn than in summer. 
Herring size was similar in autumn of both years, but 
pollock were 14 mm longer in FL and 50% heavier in 
November 1994 than in October 1995 (Tables 2 and 3). 
Interspecific comparisons of size showed that YOY pollock 
were generally longer, but not heavier, than YOY herring 
in the same season. 
Feeding habits of YOY pollock and herring 
Prey composition of YOY pollock was similar to that of 
YOY herring in both summer and autumn (Figs. 3 and 
4), but prey compositions differed between seasons. Small 
prey predominated in summer and larger prey in autumn, 
especially in terms of biomass composition. 
In summer, small calanoid copepods (Pseudoealanus 
spp., Centropages abdominalis, and Acartia longiremis) 
dominated the diets, both numerically and in terms of 
prey biomass (Figs. 3 and 4). For pollock, small calanoids 
constituted 55% by number and 57% by weight of diet. 
By number, most of the remainder of summer pollock diet 
comprised minute invertebrate eggs (39%); by weight, the 
remainder was large calanoids (principally Calanus paci- 
ficus, C. marshallae, and Metridia pacifica), fish, hyperiid 
amphipods, and euphausiids (both larvae and older stag- 
es, including Thysannoessa sp.). Pollock also commonly 
consumed small amounts of other prey, such as larva- 
ceans, gastropods, and chaetognaths. For herring, small 
calanoids represented proportionally more of the diet than 
for pollock in summer. Small calanoids made up 77% by 
number and 88%' by weight of herring diet; they were the 
sole taxon consumed by the YOY herring at Eleanor Is- 
land station 110 (Table 2). Most of the rest of herring diet 
comprised other small prey (cladocerans, bivalve larvae, 
and minute invertebrate eggs), whereas decapod larvae, 
gastropods, hyperiids, and euphausiid larvae made minor 
contributions. 
In autumn of both years, pollock and herring again 
fed on the same prey categories. Compared with summer, 
small calanoids composed smaller percentages of the diet 
(only up to 37% of prey number and 9% of prey biomass). 
Larvaceans and large calanoid copepods numerically dom- 
inated their diets (57-91%; Fig. 3), whereas euphausiids 
and large calanoids generally dominated in terms of bio- 
mass percentages (49-89%; Fig. 4). Hyperiids occasional- 
ly contributed substantial dietary biomass. Fish in all of 
the autumn aggregations ate a variety of sizes of juvenile- 
adult euphausiids ( T. raschii, T. spinifera, and unidentifi- 
able species), and amphipods (Themisto pacifica , Primno 
macropa, and Hyperia sp.), but not the larval stages ob- 
served in summer diets. The large calanoids consumed in- 
cluded the same species present in summer diets, as well 
as M. okhotensis and Neocalanus spp.; the small calanoids 
included Pseudoealanus spp., Acartia longiremis , and Oi- 
thona similis. Some differences in autumn diet composi- 
tion existed between years. Invertebrate eggs (the ma- 
jority of “other”) were present less frequently in autumn 
diets than in summer diets. In general, the October 1995 
diets included proportionally more biomass from large cal- 
anoids, whereas the November 1994 diets included pro- 
portionally more biomass from euphausiids and propor- 
tionally more numbers from larvaceans. 
Diet overlap between YOY pollock and herring was 
high, particularly when prey species were grouped into 
principal taxa (Table 4). In summer, diets were very simi- 
lar (7? n >0.76) between allopatric species in terms of num- 
bers and weights of prey species or principal prey taxa. 
In autumn, interspecific diet overlap was also observed 
