Variations in Mean Stomach Content

Weights of Walleye Pollock, Theragra

chalcogramma, in the Eastern Bering Sea

by

M-S. Yang and P. A. Livingston

U.S. DEPARTMENTOF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Alaska Fisheries Science Center

April 1994
NOAA Technical Memorandum NMFS

The National Marine Fisheries Service’s Alaska Fisheries Science Center

uses the NOAA Technical Memorandum series to issue informal scientific

and technical publications when complete formal review and editorial

processing are not appropriate or feasible. Documents within this series

reflect sound professional work and may be referenced in the formal

scientific and technical literature.

The NMFS-AFSC Technical Memorandum series of the Alaska Fisheries

Science Center continues the NMFS-F/NWC series established in 1970 by

the Northwest Fisheries Center. The new NMFS-NWFSC series will be used

by the Northwest Fisheries Science Center.

This document should be cited as follows:

Yang, M-S., and P. A. Livingston. 1994. Variations in mean stomach

content weights of walleye pollock,Theraqra chalcogramma, in the

eastern Bering Sea. U.S. Dep. Commer., NOAA Tech. Memo. NMFSAFSC-36, 32 p.

Reference in this document to trade names does not imply endorsement by

the National Marine Fisheries Service, NOAA.
Variations in Mean Stomach

Content Weights of Walleye Pollock,

Theragra chalcogramma, in the

Eastern Bering Sea

by

M-S. Yang and P. A. Livingston

Alaska Fisheries Science Center

7600 Sand Point Way N.E., BIN C-15700

Seattle, WA 98115-0070

U.S. DEPARTMENT OF COMMERCE

Ronald H. Brown, Secretary

National Oceanicand Atmospheric Administration

D. James Baker, Under Secretary and Administrator

National Marine Fisheries Service

Rolland A. Schmitten, Assistant Administrator for Fisheries

April 1994
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iii

ABSTRAC T

Variations in mean stomach contentweights (as-percentage of

body weight) of walleye pollock, Theraqra chalcosramma, collected

in the eastern Bering Sea from 1981 to 1987 were analyzed. The,

results of a one-way analysis of variance (ANOVA) showed that

bottom depth and prey type explained the highest proportions of

the total variation in stomach content weights (5.7% and 4.6.%

respectively), followed by season and year with 3.8% and 2.8%. of

the total variation, respectively.. Time of day and predator

length were not important factors., They explained only 1% and

0.04% of the total variation, respectively.

 In general, the mean stomach content weights were low in the

continental slope area and high in the continental shelf area.,

Because this phenomenon does not reflect the abundance of

zooplankton, and because the percentage of empty stomachs

increases as the bottom depth increases, it is suggested that

undetected regurgitation played an important role in the

variation of the stomach content weights.

 Walleye pollock that ate both fish and invertebrates had

significantly higher stomach content weights than those that ate

only invertebrates. Prey type or size, therefore, is probably

one of the determinant-s of the gastric evacuation rate.
Abstract ..................................................

Introduction ..............................................

Methods ...................................................

Data Collection ......................................

Data Analysis ........................................

Results ...................................................

General Description........... .......................

Depth Effect .........................................

Prey Type Effect ......................................

Interannual Variations................. ..............

Discussion........... ......................................

Prey Type Effect............ ........................

Depth Effect................................ .......

Acknowledgments... .......................................

Citations .................................................

Appendix ..................................................

iii

1

1

1

3

6

6

7

7

16

16

22

23

27

28

31
INTRODUCTION

The weight of stomach contents for marine fish is an

important variable in the calculation of daily ration models.

Generally, these daily ration models use estimates of stomach

content weights from field-collected fish in conjunction with

laboratory-estimated gastric evacuation rates (Livingston et al.

1986, Ney 1990). Gastric evacuation rates may be influenced by

many variables including temperature, predator size, and prey

type. Stomach content weights of field-collected fish may

reflect changes in these variables in addition to changes in food

abundance. For example, seasonal or interannual variation in

mean stomach content weight may be due to seasonal changes in

water temperature or prey availability. Geographic differences

in stomach weights could also reflect localized differences in

food abundance.' The analysis of the potential sources of

variation in stomach content weight may help us understand which

factors influence food intake of fishes and allow us to properly

stratify daily ration estimates by area, season, and year. The

objective of this study was to identify the main factors that

were associated with the observed variation in the mean stomach

content weights of walleye pollock, Therasra chalcoqramma, in the

eastern Bering Sea.

METHODS

Data Collection

For our analysis, we used the food habits database of the

Resource Ecology and Fisheries Management Division of the Alaska
2

Fisheries Science Center, which includes food habits data on

several commercially important groundfish species in the eastern

Bering Sea collected from 1981 to 1987. Stomach samples were

collected by the scientists on board research vessels or charter

boats, during the bottom trawl surveys conducted by the Alaska

Fisheries Science Center or were collected by the observers on

board the commercial fishing vessels. Before excising the

stomach, regurgitation and net feeding were checked. If a fish

had food in its mouth or around the gills, or if the stomach was

inverted or flaccid, it was categorized as a regurgitated fish,

and this specimen was discarded., If a predator had fresh food

(usually fish) sticking out of the mouth, or the throat, it was

categorized as a net-feeding fish, and was also discarded. When

a qualified stomach was excised from the fish, it was put in a

 cloth stomach bag. A field tag with the species name, fork

length of the fish; and haul data (vessel, cruise, haul number,.

specimen number) was also put in the bag. All of the samples

collected were then preserved in the buckets containing a l0%

formalin solution. When the samples arrived in the laboratory,

they were transferred to 76% ethanol alcohol before stomach

contents analysis. In the laboratory, the stomach contents were

first blotted with paper towel and the wet weight was then,

recorded to the nearest one tenth of a gram. After obtaining the

total stomach contents weight, the contents were placed on a

petri-dish and examined under the microscope. Details of walleye

pollock stomach collection in the field and the stomach content

analysis in the laboratory are described by Dwyer et al. (1987).
3

Data Analysis

An analysis of variance (ANOVA) procedure was used to

examine the variation in stomach content weights for pollock.

Theoretically, this procedure assumes that the samples are

normally distributed and that there is an homogeneity of

variances. These two assumptions were not met in this study,

even when data transformations (natural logarithms and arcsin)

were used. However, according to Zar (1974), the ANOVA is fairly

robust even with considerable heterogeneity of variances and

nonnormality of the distribution. Therefore, the ANOVA was

considered appropriate for the untransformed data set.

One-way ANOVAs of walleye pollock stomach content weight as

percentage of body weight (%BW) were performed separately on fish

with stomachs containing food for each of the following factors:

prey type; year, season, bottom depth, predator length, and time

of day. For analysis, the level of significance was set at

P > 0.05. A one-way ANOVA rather than a multiway ANOVA was used

(Zar 1974) because empty cells could not be included in the

multiway tests, and empty cells occurred in some years, seasons,

and bottom depths-. The prey type factor had two levels: Type 1

if the stomach contained some fish and Type 0 if the stomach

contained only invertebrates. The sampling area (Fig. 1) was

divided into three subareas based on bottom depth: Depth 1

(<l00 m), Depth 2 (100-200 m), and Depth 3 (>200 m). Five years

(1981, 1982, 1983, 1985; 1987) of data were included in this

analysis. The four seasons were assigned as: spring (15 March14 June), summer (15 June-14 September), fall (15 September-

5

14 December), and winter (15 December-14 March). Two predator

fork length (FL) groups (<40 cm FL and >40 cm FL) were used in

this study based on previous studies (Dwyer et al. 1987,

Livingston et al. 1986), which showed that walleye pollock

gradually shift their diet from zooplankton to fish beginning at

approximately. 40 cm FL. Time of day was separated into eight

time intervals of 3 hours each The proportion of the total

variance (the coefficient of determination, r) explained by each

of the main factors was calculated by dividing the sum of squares

of the between groups by the total sum of squares from the output

of the one-way ANOVA.

A multiway ANOVA was used to test differences of mean

-stomach content weights in 1982 by the following four factors:

bottom depth,

excluded as a

better sample

summer, fall)

areas and for

prey type, season, and predator length (time was

factor to eliminate empty cells and to provide a

size for each cell). Only three seasons (spring,

in 1982 contained data for all of the three depth

both of the two predator- size groups. Data from

the other years either contained less than three seasons data or

did not have a complete data set for all' three areas within one

season (see Appendix). Therefore, only comparable data sets with

no empty cells were used for the multiway ANOVA test to examine

the sources of variationand their interactions.

A multiway ANOVA was also used to test for variation in

stomach content during the same season in differant years. Year ,

prey type, and sampling depth were used as factors. When there

was insufficient data (i.e., the presence of empty cells) to
6

include all of the

factors were used.

multiway) revealed

factor having more

factors in a multiway ANOVA test, fewer

If the results of the ANOVA (one-way or

a significant difference between levels of a

than two levels, Tukey's multiple-range test

was used to determine differences between specific levels at

P = 0.05.

RESULTS

food

from

General Description

Data from 3,422 walleye pollock whose stomachs contained

were included in this study. Walleye pollock lengths ranged

16 cm to 78 cm FL (mean =.43 cm FL, S.E. = 0.182). The mean,

stomach content weight as a percentage of body weight (%BW) and

the sample sizes by sampling year, season, bottom depth, and prey

type are listed in the Appendix.

The results of a one-way ANOVA for the entire data set

(Table 1) indicated that depth and prey type described the

highest proportion of the total variation in stomach content

weights (5.7% and 4.6%, respectively), followed by season and

year (3.8.% and 2.8% of the total variation, respectively). Time

of day explained 1% of the-total variation. No specific trend in

%BW was observed for the different time periods (Fig. 2) except

during the time interval 0300-0600, which had the lowest %BW

value (0..49). The remaining seven time groups were not

significantly different from each other based on Tukey's

multiple-range test. Predator length explained only 0.04% of the

total variation and mean stomach- content weights between the two
7

size groups were not significantly different (P =0.2378).

Table 1. --Sources of variation in the mean stomach- content

weights as percent of body weight in walleye pollock,

Therasra chalcoqramma, collected in the eastern Bering

Sea from 1981-83, 1985, and 1987. (Total sample

size = 3,422)

Source of variation Percentage (%)

Between-cells

Depth 5.7

Prey type 4.6

Season 3.8

Year 2.8

Time of day 1.0

Predator length <O.1

Within cells 82.1

Depth Effect

Figure 3 shows the mean walleye pollock stomach content 

weights for each year and season by bottom depth. In general,

walleye, pollock in continental shelf areas (<200 m) had higher

stomach content weights than walleye pollock in continental slope

areas with bottom depths greater than 200 m. The mean stomach

content weights for walleye pollock collected in the continental

slope area were all less than 1% of the body weight. Also, most

walleye pollock from areas with bottom depths of less than 100 m

had higher mean stomach content weights than those from bottom

depths 100-200 m.

Prey Type Effect

The mean stomach content weights of the walleye pollock that

had fish (mainly juvenile pollock) in their stomachs were

generally higher than those that only ate invertebrates (Fig. 4).
Figure 2. Variations in mean stomach content weight (as percent

of body weight, %BW) of walleye pollock, by 3-hour

time intervals, in the eastern Bering Sea. Each data

point is the. %BW + S.E. The number above each data

point is the sample size.
9

Figure 3. Variations in mean stomach content weight (as percent

of body weight, %BW) of walleye pollock in the eastern

Bering Sea. Each bar represents the mean of the

sample collected from the same year, season (SP=

spring, S = summer, F = fall, W = winter) and samping

depth.
10

Figure 4: Differences in the mean stomach content weight (as

percent of body-weight, %BW) of walleye pollock that

had. fish in their diet versus those that only ate

invertebrates. Each data point, is the sample from

same year, season (SP = spring, S = summer, F = fall,

W = winter:, and bottom depth (Dl =<lOO m,

D2 = l00-200 m, D3 = >200 m); for example, 81-S D1

means sample collected from depth <l00 m in summer

1981.
11

(Each data point is the mean stomach content weight from a sample

collected from the same year, same season, same depth). When a

one-way ANOVA was performed separately by year, season, depth,

and predator size, pollock that ate fish in conjunction with

invertebrates showed higher mean stomach content weights than

those that only ate invertebrates (Fig. 5). The data points

(fish + invertebrates versus invertebrates) were all

significantly different (P < 0.01) from each other except for the

spring data (they were barely non-significant, P = 0.0553).

Multiway Analysis of Variance of the 1982 Samples

The results of the multiway ANOVA test on the 1982 samples

were similar to those of the one-way ANOVA tests on the entire

sample. Bottom depth,-prey type, and season described ll%, 6%,

and 5% of the total variation, respectively (Table 2). Even

though predator fork length was .a significant factor (P = 0.042,

Table 2), it only explained 0.3% of the total variation.

Interactions between different factors are presented in Table 2.

Within the two-way interactions, only the interaction of "season

by depth" (9% of the total variation ) played animportant role

in explaining variation of pollock stomach content weights. Each

of the remaining interaction effects (two-way, three-way, and

four-way) described less than 2% of the total variation, even

though some of these interactions are significant.

Tukey's multiple-range test was applied to examine the

differences between specific levels of the depth and season

factors (Tables 3-8). During spring and fall in areas less than

100 m in depth, pollock contained more food (2.15% and 1.98 %BW,
12

Figure 5. Differences in the mean stomach content weight '(as

percent of body weight, %BW) of walleye pollock

between those that ate fish plus invertebrates from

those that ate invertebrates only by using different

factors to group the data: Season/Depth, Year, and

Predator Size.
Table 2.--Test of significance for stomach content weights as

percent of body weights (%BW) of walleye pollock,

Therasra chalcoqramma, collected in spring, summer, and

fall in 1982 in the eastern Bering Sea.
14

Table 3.--Tukey's multiple-range test on the variations of the.

mean stomach. content weights of pollock,, Therasra

chalcoqramma, from samples collected in three depths

area in spring 1982. (*) denotes pairs of means that

are significantly different at the '0.05 level.
15

Table 6.--Tukey's multiple-range test on the variations of the

mean stomach content weights of pollock, Therasra

chalcosramma, from samples collected in Depth 1 (<l00 m)

in spring, summer, fall of 1982. (*) denotes pairs of

means that are significantly different at the 0.05

level.

Summer Fall Spring

Count Mean(%BW) Group

173 1.29 Summer

101 1.98 Fall *

96 2.15 Spring *

Table 7.--Tukey's multiple-range test on the variations of the

mean stomach content weights of pollock, Therasra

chalcoqramma, from samples collected in Depth 2

(100-200 m) in spring, summer, fall of 1982. (,f) denotes

pairs of means that are significantly different at the

0.05 level.

Spring Fall Summer

Count Mean(%BW) Group

234 0.43 Spring

61 0.45 Fall

263 1.63 Summer, * *

Table 8.--Tukey's multiple-range test on the variations of the

mean stomach content weights of pollock, Therasra

chalcoqramma, from samples collected in Depth 3

(>200 m) in spring, summer, fall of 1982. (*)-denotes

pairs of means that are significantly different at the

6.05 level.

Spring Fall Summer

Count Mean(%BW) Group

132 0.12 Spring

215 0.56 Fall *

82 0.79 Summer *
16

respectively) than fish from the two deeper zones. In summer,

walleye pollock in the two areas less than 200 m deep contained

more food (1.63 and 1.29 %BW, respectively) than fish in the

areas greater than 200 m (0.79 %BW).

Interannual Variations

Since the one-way ANOVA results indicated that time and

predator fork length described only a small amount of the

variation in stomach content weight (1.0% and 0.04%,

respectively), the data used in the following comparisons were

not subdivided into size and time groups for analysis.

The results of a three-way ANOVA (Table 9, Fig. 6) show that

stomach content weights were significantly different between

winter 1983 and winter 1987. They also show significant

differences between the spring-samples of 1982 and 1983 and the

summer samples of 1982 and 1985. A three-way ANOVA was not

applied to the fall data of 1981 and 1982 since we did not -have

samples collected from the Depth 3 area in 1981 (Appendix).

Even though the three-way ANOVA tests demonstrate that year

was a significant factor explaining variation in stomach content

weights, its contribution to the total variation was not a's,

important, as the depth or prey type factors (Table 9).

DISCUSSION

Feeding behavior is affected by both biotic and abiotic

factors. Biotic. factors include availability of food,

competition with other predators, metabolic rate, spawning

condition, and migratory behavior (vertical and seasonal).
17

Table 9.--Three-way ANOVA of the mean stomach content weights

(%BW) of walleye pollock, Therasra chalcocramma, of

different seasons by the factors of year, prey-type,

and depth.
Figure 7. Mean weight of invertebrates (mainly zooplankton) per

fish in the stomachs of walleye pollock in the eastern

Bering Sea. Depth l (<l00 m); Depth 2 (100-200 m);

Depth 3 (>200 m).
19

Abiotic factors include water temperature, currents, habitat, and

other hydrographic and climatic factors. Because the diet of a

fish is influenced by both these biotic and abiotic factors, it

is not possible to quantitatively separate the effects of these

factors without simultaneous measurement of each factor. T h e

purpose of this study is not to explain how all these biotic and

abiotic factors influence the feeding of pollock but to find some

broad factors that might be used-to describe the variation in

feeding behavior.

In this study, bottom depth and prey type were the main

explained sources of variation in stomach content weights. The

importance of other factors (predator fork length, sampling time,

season, year) and the interactions of different factors were

minor (even though some interactions were statistically

significant) since they explained only a small portion of the

total variation. However, even. the explained sources of

variation only accounted for less than 40% of the variance, the

within-cell variations (unexplained variations), were high due to

the large range of stomach content weights (from 0.01 to 9.47

%BW). Bromley (1987) found that the last traces of- food

(approximately 0.1% of body weight) are evacuated very slowly in

turbot (Scophthalmus maximus L.) and he classified those stomachs

with O.l% of body weight of food as empty. In our study, 1,121

out of 3,422 stomachs had content weights less than 0.1% of body

weight. If these stomachs are excluded from the one-way ANOVA in

this study, however, the unexplained variation is still very high

(89%) . Amundsen and Klemetsen (1986) studied the stomach content
20

weights of Arctic char (Salvelinus aloinus L.) and found that,

due to the large range in stomach content weights within each

sample (3-hour time intervals), there were usually no significant

differences between consecutive samples taken throughout each 24-

hour period. Because of this, they believed that Elliott and

Persson's (1978) model (based on the differences between

consecutive samples) was not appropriate for. estimating the

consumption rate of fish with no diel feeding pattern, and that

the modified Bajkov (1935) model (based on the average stomach

content weight for each 24-hour period) by Eggers (1979) should

be regarded as the most appropriate method. They also noted that

large variations in stomach content weights are common in

published studies and that, at least in their study, these

variations were not caused by small sample sizes but seemed to be

biological. Though the within-haul variations were not included

in our statistical analysis, they can be large (Table 10).

Within-sample (haul) variations can sometimes be as high as 126-

fold (6.30:0.05); This would seem to support Amundsen and

Klemetsen's findings of large within-sample variations in fish

stomach content weights.

These results point out that estimated mean stomach content

weights of walleye pollock from field collected samples are

probably biased downwards by undetected regurgitation. This

suggests that walleye pollock daily ration values calculated from

mean stomach content weights of field collected stomachs in

conjunction with laboratory-estimated gastric evacuation rates

are also biased downwards. This bias would be more pronounced
21

Table l0.--Example of within-sample (haul) variations of the

stomach content weights of pollock, Therasra

chalcoqramma, collected in the area >200 m deep in

winter 1983.

Total stomach, content Total body weight Stomach weight as

weight (g) (g) % body weight

when using samples collected

is the main depth where most

et al. (1986) compared daily

from waters deeper than 100 m, which

pollock can be found. Livingston

rations of Bering Sea groundfish

estimated in -this fashion with daily rations calculated using a

bioenergetic approach. They found that groundfish daily rations

using field-collected stomach content weights were not only lower

than those from the bioenergetic approach but were also

insufficient to account for observed growth. This implies that

daily rations should be calculated using a bioenergetic approach,

especially for fish such as pollock that may have high undetected

regurgitation rates. Because of the relative importance of the

effect of bottom depth and prey type, the two factors are

discussed separately.
22

Prey Type Effect

Prey type was one of the two mainfactors explaining

variation in the stomach-content weights (4.6% for the entire

sample and 5.7% for 1982 sample). Walleye pollock that ate fish

in addition to. invertebrates had significantly higher stomach

content weights than those that ate only invertebrates. Man y

studies have shown. that prey type or size is one of the

determinants of the gastric evacuation rate; Jobling (1987)

suggested that differences in surface-to-volume ratios between

large and small food particles, and the friabilities of different

food types are important in determining the pattern of emptying.

He also noted that dietary energy content appeared to be an

important factor. The surface-to-volume ratio of prey juvenile

pollock is smaller than that of zooplankton, thereby increasing

the effort required to complete digestion. Therefore, the

evacuation rate for prey fish might be slower than for

invertebrates. Thus, the observed mean stomach weights of

pollock that ate fish would tend to be higher than-those that 

only ate invertebrates. DOS Santos and Jobling (1988) did

experiments on Atlantic cod Gadus morhua) that consumed Atlantic

herring, (Clupea harensus) and found that whole herring were

digested and emptied from cod, stomachs much more slowly than

minced herring, indicating that particle size is also an

important factor controlling the emptying rate and thus the

observed differences in stomach weight due to prey type.
23

Depth Effect

Geographic differences (i.e., bottom depth) explained the

highest proportion (5.6% for the entire sample and 11.0% for 1982

sample) of variation in stomach content weight. The depth effect

might be attributed to two factors. The first reflects the

potential differences in food abundance across areas. The second

is undetected regurgitation that might be expected to increase

with capture in increasing water depth.

The zooplankton biomass in the continental slope and outer

shelf area is much greater than that in the middle shelf area

(Cooney 1981, Vidal and Smith 1986). According to Cooney (1981),

the zooplankton production was about 13 gC/m /year in the oceanic

region (>200 m), 8 gC/m /year in the mixed region (100-200 m),

and between 2 and 6 gC/m /year in the middle shelf and coastal

areas (cl00 m). The community over the outer shelf and slope is

dominated by large copepods (mainly Neocalanus spp. and Eucalanus

bunsii bunsii). The middle shelf and coastal community is

dominated by the small copepods Acartia lonqiremis and

Pseudocalanus spp. and the euphausiid Thvsanoessa raschii. Smith

and Vidal (1984) and Vidal and Smith (1986) described the

relationship between temperature and the zooplankton communities

in the Bering Sea.. They found that the sea surface temperatures

on the slope and outer shelf area were higher than 3°C from late

March until December; however, in the middle shelf, sea surface

temperature was substantially lower until late May when sea

surface temperature rose above 3°C. They noted that temperature

was not, a good predictor of copepod abundance over the outer
24

shelf and slope areas, but it was a relatively good predictor of

their abundance over the middle shelf., We calculated the mean

weight of invertebrates (mainly zooplankton) eaten by pollock to

compare it with the zooplankton biomass in the Bering Sea.

Within each depth area, pollock ate fewer invertebrates in winter

and fall than in summer (Fig. 7). Figure 7 also shows large

variations in the amount of invertebrates eaten by pollock in

spring .Cooney (1981) found that the biomass of oceanic

zooplankton is greatest in May and June and lowest in November in

the outer shelf area. His observations concur with our findings

that large amounts of zooplankton were available to be eaten by

pollock in the outer shelf (Depth 2) -area in spring. These

observations point out that abundance of available zooplankton,

may explain, the variation in stomach content weights of pollock

in different depth, areas;

The second possible explanation for decreasing stomach

content weight with depth is undetected regurgitation. Some 

pollock stomachs may have been empty due to complete

regurgitation and others with low stomach content weights might

be the result of partial regurgitation. Bowman, (1981) noted that

regurgitation is commonly observed in fish caught when bottom

trawling in water deeper than 100 m. Bowman (1985) also noted

that the percentage of detectable regurgitation for some fish

species increases considerably with increasing trawl depths. He

found that lower stomach content weights were often observed in

the deeper areas even in those fish showing no signs of

regurgitation. He attributed the low stomach content weights to-
Figure 6. Interannual

(as percent

the eastern

variations in mean stomach content weight

of body weight, %BW) of walleye pollock in

Bering Sea in winter, spring, and summer.
2 6

Figure 8. Percent of empty stomachs of walleye pollock collected

in the eastern Bering Sea from 1981 to 1987.

(SP = spring, S = summer, F = fall, W = winter)
27

undetected regurgitation. In our study, the high percentage of

empty stomachs (Fig. 8) and the low stomach content weights in

the deep area greater than 200 m (Fig. 3) strongly suggest that

undetected regurgitation probably played an important role in the

variation of the stomach content weights of pollock. Figure 8

shows the percent of empty stomachs of each subset sample by

year, season, and depth. Except for the subset sample of Depth 1

in spring 1983 (with small sample size 27), the percentage of

empty stomachs increases with depth, especially in the slope

areas (>200 m).

ACKNOWLEDGMENTS

We would like to thank Russ Kappenman and Vidar Wespestad

for reviewing the manuscript. Thanks also extend to John Kershaw

(Center for Quantitative Science, University of Washington,

Seattle, Washington) for statistical consulting assistance.
28

CITATIONS

Amundsen, P-A., and A. Klemetsen. 1986. Within-sampl e

variabilities in stomach contents weight of

fish--Implications for field studies of consumption rate.

In C. A. Simenstad and G. A. Cailliet (editors),

Contemporary studies on fish feeding, p.307-314. Dr. W. Junk

Publishers, Dordrecht.

Bajkov, A. D. 1935. How to estimate the daily food consumption

of fish under natural conditions. Trans.

65:288-289.

Bowman, R. E. 1981.' Examination of known-and potential causes

Am. Fish. Soc.

of variation in fish feeding studies. Natl. Mar. Fish.

Serv., Northeast Fish.. Ctr., Woods Hole, MA 02543. woods

Hole Lab. Ref. Doc. No. 81-23; 29 p.

Bowman, R. E. 1985. Effect of' regurgitation on stomach content

data of marine fishes. In c. A. Simenstad and G. A.

Cailliet (editors), Contemporary studies on fish feeding,

p.171-181. 'Dr. W. Junk Publishers, Dordrecht.

Bromley, P. J.. 1987. The effects of food type, meal size and

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29

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31

Appendix . List of the mean stomach content weights as percent

of body weight (%BW) of walleye pollock, Theragra

chalcoqramma, in the eastern Bering Sea.

Year Season Bottom Prey %BW Number of Standard error

depth(m) typ e stomachs of the mean
32

Appendix. Continued.

Year Season Bottom Prey. %BW Number of Standard erro r

depth(m) type stomachs of the mean
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