Sturdevant et al.: Feeding habits, prey fields, and potential competition of Theragra chalcogramma and Clupea pallasi 
485 
Table 1 
Mean zooplankton density and percent density (standard errors, SE, in parentheses) from pooled vertical hauls in summer (77=37 
hauls, 303-pm mesh) and autumn (n= 8 hauls, 243-pm mesh), and from shallow (< 25 m) and deep (50-100 m) hauls in Prince Wil- 
liam Sound, 1995. 
Prey category 
Mean density (number of zooplankton/m 3 
) 
Summer 
Autumn 
Overall 
Shallow 
Deep 
Overall 
Shallow 
Deep 
Barnacle larvae 
<1 
<1 
<1 
<1 
<1 
<1 
Calanoids, large 
29 (4) 
37 (7) 
28 (6) 
204 (60) 
104(3) 
176(3) 
Calanoids, small 
1018 (133) 
1175(181) 
550 (87) 
828 (130) 
685 (48) 
426 (40) 
Chaetognaths 
4 (<1) 
3(1) 
4 (1) 
3 (<1) 
1 (<1) 
5 (<1) 
Cladocera 
33(9) 
25 (9) 
6(1) 
0 
0 
0 
Cyphonautes larvae 
0 
0 
0 
205 (50) 
297 (7) 
333(8) 
Decapod larvae 
1 (<1) 
2 (<1) 
<1 
0 
<1 
0 
Euphausiid larvae 
3(1) 
3(1) 
2 (<1) 
2 (<1) 
1 (<1) 
2(1) 
Gastropods 
60 (10) 
99(23) 
36(5) 
96 (19) 
141 (4) 
84(5) 
Hyperiid amphipods 
2 (<1) 
2 (<1) 
2 (<1) 
1 (<1) 
1 (<1) 
1 (<1) 
Larvaceans 
14 ( 4) 
6(2) 
8(3) 
45(11) 
50 (12) 
22(7) 
Other 
17 (2) 
17 (3) 
8(1) 
14 (2) 
18(6) 
9(1) 
TotaP 
1184(138) 
1371 (191) 
645 (91) 
1414 (185) 
1299 (64) 
1064 (56) 
Percent composition 
Summer 
Autumn 
Prey category 
Overall 
Shallow 
Deep 
Overall 
Shallow 
Deep 
Barnacle larvae 
0 
0 
0 
0 
0 
0 
Calanoids, large 
3 
3 
5 
13 
8 
17 
Calanoids, small 
84 
84 
84 
58 
53 
40 
Chaetognaths 
<1 
<1 
<1 
<1 
<1 
<1 
Cladocera 
5 
2 
1 
0 
0 
0 
Cyphonautes larvae 
0 
0 
0 
16 
23 
14 
Decapod larvae 
0 
0 
0 
0 
0 
0 
Euphausiid larvae 
<1 
<1 
<1 
<1 
<1 
<1 
Gastropods 
6 
7 
6 
7 
11 
8 
Hyperiid amphipods 
<1 
<1 
<1 
<1 
<1 
<1 
Larvaceans 
<1 
<1 
<1 
2 
4 
2 
Other 
2 
1 
1 
1 
1 
1 
1 Numbers do not add to column totals because of rounding. 
principal prey taxa, and total organisms in each vertical 
tow by using the expanded organism count divided by the 
water volume of the tow; mean values of replicate tows 
were used to represent each station. 
Feeding selectivity of allopatric and sympatric aggrega- 
tions of pollock and herring was calculated for summer and 
early autumn, 1995, when zooplankton were collected at 
the fish sampling stations. Occasionally, in summer, zoo- 
plankton samples from adjacent stations were substituted 
for those fishing stations without prey samples (Table 1). 
At stations where zooplankton was collected at two depths, 
the selection values presented were based on zooplankton 
from the depth where fish diet samples were collected. We 
used Strauss’ linear selection index, L 0 (Strauss, 1979), a 
measure varying from -1 to +1, where negative values in- 
dicate no preference for the prey taxon and positive values 
indicate preference for the prey taxon: 
L o = r , ~Pi> 
where r ; = percentage of 7 th prey resource in the diet; and 
p ( = percentage of 7 th prey resource in the environ- 
ment. 
Prey resources for selection were defined as the species, 
stages, and sizes of prey pooled into principal taxa. 
