Sturdevant et al.: Feeding habits, prey fields, and potential competition of Theragra chalcogramma and Clupea pallasi 
493 
10 
8 
20 
G In prey numbers 
I In prey biomass 
10 10 20 
pollock herring herring pollock pollock herring pollock herring 
July 1995 Oct 95 Nov 94 Oct 1995 Nov 1994 
allopatric allopatric sympatric sympatric 
Figure 6 
Total number and total biomass of prey (ln-transformed means) consumed 
by allopatric and sympatric YOY walleye pollock and YOY Pacific herring 
from Prince William Sound, Alaska, in July-August 1995, October 1995, 
and November 1994. Number of individuals is indicated. 
by time of day was different for the two species. Pollock 
had greater percentages of mostly digested contents dur- 
ing the day than during the night (before and after 22:00). 
Conversely, herring had greater percentages of mostly di- 
gested contents during the night than during the day. The 
day-night percentages of feeding and nonfeeding fish were 
similar for both, however. 
Comparisons between allopatric and sympatric fish 
aggregations 
The allopatric-sympatric size pattern was different for 
pollock and herring. We did not pool sympatric fish from 
October 1995 and November 1994 for comparison with 
allopatric groups because of the interannual differences in 
size and feeding measures. For pollock in November 1994, 
FLs of allopatric and sympatric fish were similar, but fish 
were approximately 1.5 g lighter in sympatric aggrega- 
tions than fish in allopatric aggregations. For herring in 
October 1995, the sizes of allopatric and sympatric fish 
were similar (Table 3). 
The allopatric-sympatric feeding pattern was somewhat 
different for pollock than for herring in autumn. Among 
November 1994 pollock, the allopatric fish consistently 
had the highest feeding measures and the sympatric fish 
consistently had the lowest feeding measures (Fig. 6, Ta- 
ble 3). This finding coincided with slightly increased nu- 
merical percentages of larvaceans, increased gravimetric 
percentages of euphausiids, and increased frequencies of 
euphausiids, amphipods, and large calanoids in the diet of 
allopatric pollock compared with the diet of sympatric pol- 
lock (Figs. 3 and 4). For October 1995 herring, allopatric 
fish consumed the greatest prey biomass and %BW, but 
their prey numbers and fullness index were lower than 
those of sympatric fish (Fig. 6, Table 3). This finding coin- 
cided with decreased numerical percentages of larvaceans, 
increased numerical and gravimetric percentages of large 
calanoids and euphausiids, and increased frequencies of 
occurrence of hyperiids and euphausiids in the diet of al- 
lopatric herring compared with diet of sympatric herring 
(Figs. 3 and 4). Allopatric herring were more selective of 
large calanoids, but selection for larvaceans was similar 
for both groups of herring (Fig. 5). For both species, intra- 
specific diet overlap between allopatric and sympatric fish 
was extensive at the principal taxon level. In October 1995 
intraspecific overlap in terms of prey biomass was 0.93 for 
herring, and in November 1994, intraspecific overlap in 
terms of prey number was 0.91 for pollock. 
Discussion 
Zooplankton prey fields 
The prey fields available to planktivorous YOY walleye 
pollock and Pacific herring were different in summer and 
autumn 1995. Although we report similar total zooplank- 
ton densities for the two seasons, we believe that summer 
densities of small calanoids were underestimated with the 
303-pm net. This conclusion is based on a 1995 compan- 
ion study that showed that a 243-pm mesh net retained 
small calanoids better than a 303-pm mesh net. 8 The sea- 
Sturdevant, M. V. In review. 
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