NOAA Technical Memorandum NMFS-AFSC-67

Food Habits of Key Groundfish Species

in the Eastern Bering Sea Slope Region

by

G. M. Lang and P. A. Livingston

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Alaska Fisheries Science Center

November 1996
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:

Lang, G. M., and P. A. Livingston. 1996. Food habits of key groundfish

species in the eastern Bering Sea slope region. U.S. Dep. Commer.,

NOAA Tech. Memo. NMFS-AFSC-67, 111 p.

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

the National Marine Fisheries Service, NOAA.
NOAA Technical Memorandum NMFS-AFSC-67

Food Habits of Key Groundfish Species

in the Eastern Bering Sea Slope Region

by

G. M. Lang 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

Mickey Kantor, Secretary

National Oceanic and Atmospheric Administration

D. James Baker, Under Secretary and Administrator

National Marine Fisheries Service

Roland A. Schmitten, Assistant Administrator for Fisheries

November 1996
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iii

ABSTRACT

The eastern Bering Sea continental slope region contains many groundfish species that

are important to the Bering Sea ecosystem. Several studies have described the food habits

of these species in the eastern Bering Sea shelf region, however few studies have been

conducted for the slope region. Consequently, the food habits of the major groundfish in

this region are poorly understood. The focus of this study was to identify the trophic

interactions of walleye pollock (Theragra chalcogramma), Pacific cod (Gadus

macrocephalus), Greenland turbot (Reinhardtius hippoglossoides), arrowtooth flounder

(Atheresthes stomias), and flathead sole (Hippoglossoides elassodon) in the slope region

of the eastern Bering Sea. Stomach contents of 1,089 walleye pollock, 311 Pacific cod,

455 Greenland halibut, 282 arrowtooth flounder, and 366 flathead sole were examined.

The general diet of each species is described and dietary variation due to predator size,

depth, season, and latitude within the slope region are examined. Walleye pollock are

known to be the most important prey fish in the eastern Bering Sea shelf region. In the

slope region, species from the families Bathylagidae and Myctophidae were found, in

addition to pollock, to be important forage fish of groundfish predators
V

CONTENTS

ABSTRACT................................................................... iii

INTRODUCTION............................................................................................. 1

METHODS........................................................................ 5

RESULTS AND DISCUSSION. ...........................................................................11

Walleye Pollock

Result............................................................... ll

Discussion .. .................... . .. ................. ....... . ..31

Walleye Pollock Scan Data

Results............................................................................................ 36

Discussion......................................................................36

Pacific Cod

Results................................................................. 38

Discussion.....................................................................................................53

Greenland Turbot

Results............................................................56

Discussion............................................................................ 67

Flathead Sole

Results........................................................................ 71

Discussion....................................................................................................... 84

Arrowtooth Flounder

Results ..................................................... . 86

Discussion. .................................................... .95

DIETARY COMPARISONS. ..................................... 103

SUMMARY.................................................................................................... 106

CITATIONS..................................................................................................... 108
INTRODUCTION

The eastern Bering Sea supports one of the largest groundfish populations of the

world’s oceans, which in turn supports a multi-million dollar commercial fishery Species

such as walleye pollock (Theragra chalcogramma); Pacific cod (Gadus macrocephalus);

Pacific halibut (Hippoglossus stenolepis); and king (Paralithodes spp.); Tanner

(Chionoecetes bairdi); and snow crabs (Chionoecetes opilio) are important components of

the eastern Bering Sea ecosystem which is managed by state, federal, and international

fisheries management organizations. In recent years, managers have been moving toward a

multi-species management approach that requires a more thorough understanding of the

dynamics of ecosystems than had been previously known. One of the key parameters

needed for successful multi-species management is an accurate estimate of the natural

mortality pressure exerted on a particular species by other species in the system.

Understanding predator-prey relationships is a complex, yet crucial aspect of the natural

mortality rates of a population. Quite a body of work has been generated around the food

habits of key groundfish predators in the eastern Bering Sea continental shelf where the

majority of the groundfish populations are found, and researchers are beginning to look at

the multi-species interactions that occur between fish, marine mammals, and birds,

However, little work has been done to determine the food habits of these important

groundfish species over the eastern Bering Sea continental slope region,

The eastern Bering Sea continental slope region (Fig. 1) is the area which links the

eastern Bering Sea continental shelf (<200 m) to the Aleutian basin (>2,000 m); however,

most of the important groundfish populations are found between 200 and 500 m. The area

covered by the continental slope is small relative to the shelf region due to its steepness.

Groundfish populations in the eastern Bering Sea slope region are also small relative to

those on the eastern Bering Sea shelf but do represent an important component of the

ecosystem. Walleye pollock, Pacific cod, Greenland turbot (Reinhardtius

hippoglossoides), arrow-tooth flounder (Atheresthes stomias), and flathead sole

(Hippoglossoides elassodon) are important predators, prey and competitors in the eastern

Bering Sea ecosystem and are commonly found in slope waters.

Walleye pollock is an arctic-circumboreal Pacific Ocean species found at depths to

975 m (Allen and Smith 1988). They are the most abundant groundfish species in the

eastern Bering Sea ecosystem (7.7 million metric tons (t)) representing 64 % of the nearbottom biomass (North Pacific Fishery Management Council 1994). Approximately 2%

of the walleye pollock biomass and 1% of the population are found over the slope region.

The walleye pollock population in the slope region consists of larger; older individuals

than those found over the shelf (Bakkala et al. 1992, Goddard and Zimmermann 1993).

Most of the walleye pollock over the, slope are located within the 200-500 m depth zone

and are relatively evenly distributed north to south (Bakkala et al. .1992, Goddard and

Zimmermann 1993).

Pacific cod are a circumboreal Pacific Ocean species found at depths to 875 m (Allen

and Smith 1988). Like walleye pollock, the slope portion of the. Pacific cod population

represents a relatively small portion (<1% biomass and number) of the Bering Sea

population. Pacific cod in the slope region are also larger, older fish than are found over

the shelf (Bakkala et al. 1992, Goddard and Zimmermann 1993). Most of the cod found

in the slope region of the Bering Sea are found in the 200-500 m depth zone, with a

relatively even distribution north to south (Bakkala et al. 1992, Goddard and

Zimmermann 1993).

Flathead sole is a Kurile-Aleutian Islands, Bering Sea species found to depths of

1,050 m (Allen and Smith 1988). Flathead sole on the slope represents approximately

2-3% of the population size and biomass of the eastern Bering Sea stock (Bakkala et al.

1992, Goddard and Zimmermann 1993). Flathead sole over the slope are only slightly

larger than those over the shelf (Bakkala et al. 1992, Goddard and Zimmermann 1993).

Flathead sole over the slope are primarily found in the 200-500 m depth range, and they

3
are slightly more abundant in the southern portion of the slope (Bakkala et al. 1992,

Goddard and Zimmermann 1993).

Greenland turbot is an arctic-amphiboreal, deep-water species that is found to depths of

2,000 m (Allen and Smith 1988). In the eastern Bering Sea, they are found most

frequently between 50 and 100 m (Allen and Smith 1988). Juvenile Greenland turbot are

located primarily on the shelf while the larger adults of the species are found on the slope

and beyond (Bakkala et al. 1992, Goddard and Zimmermann 1993). In the 200-500 m

depth zone, Greenland turbot are more abundant in the southern strata than in the

northern strata. They are also more abundant in the southern region of the 500-800 m

depth zone (Bakkala et al. 1992, Goddard and Zimmermann 1993). The slope region of

the eastern Bering Sea contains approximately 75% the Greenland turbot biomass and

23% of the population (Bakkala et al. 1992, Goddard and Zimmerman l993).

Arrowtooth flounder is an eastern boreal Pacific Ocean species found down to 900 m.

In the eastern Bering Sea, they are primarily found between 50 and 400 m water depth

(Allen and Smith 1988). Over the slope region, arrowtooth flounder are most commonly

found in the 200-500 m depth zone and are evenly distributed north to south (Bakkala

et al. 1992, Goddard and Zimmermann 1993). Arrowtooth flounder on the slope are

larger on average than those found on the shelf. Approximately 8% of the biomass and

2% of the arrowtooth flounder population is found over the slope (Bakkala et al. 1992,

Goddard and Zimmermann 1993).

Walleye pollock, Pacific cod, Greenland turbot, flathead sole, and arrowtooth flounder

represent 5 of the 7 most abundant commercially exploited groundfish species in the

eastern Bering Sea; Pacific halibut and sablefish (Anoplopoma fimbria) make up the

other two species (North Pacific Fishery Management Council 1994), so knowledge of

their trophic ecology is therefore important. The goal of this study is to define the food

habits of the former five species in the eastern Bering Sea slope, compare and contrast

their diets, and explore spatial and temporal variation in their diets.

4
METHODS

The eastern Bering Sea slope survey area included all water between 200 and 800 m

depth and between 54o and 60o N lat. (Fig. 1). This area is subdivided into four strata: 100,

southeast, 200-500 m; 110, northeast, 200-500 m; 120, southwest, 500-800 m; 130,

northwest, 500-800 m. Individual stomach samples were collected aboard research vessels

by National Marine Fisheries Service (NMFS) personnel in 1985, 1988, and 1991, and

aboard commercial ‘fishing vessels by fisheries observers from 1982 to 1992. Samples were

collected year-round and throughout the day. Stomachs were only collected from fish

which showed no signs of either feeding after capture or regurgitation, as determined by

the presence of prey in the buccal cavity, and were preserved in a 10% formalin-sea water

mixture. When the stomachs arrived at the laboratory for analysis, they were rinsed in

runnning water to remove the formalin and stored in 70% ethyl alcohol. In the laboratory,

stomach contents were removed, blotted to remove excess moisture, and weighed to the

nearest centigram. Individual prey taxa were then identified to the lowest practical taxa,

with emphasis on commercially important prey, enumerated, and weighed to the nearest

milligram. Length and sex were recorded for all fish and crab prey,

For the purposes of this diet study, data for arrowtooth flounder and Kamchatka

flounder (Atheresthes evermanni) were combined and considered as arrowtooth flounder

because of their previously demonstrated dietary similarity (Yang and Livingston 1986),

The diet data for flathead sole and Bering flounder (Hippoglossoides robustus) have also

been combined in previous studies (Pacunski 1990) due to their morphological and

ecological similarity and management status; the same was done in this current study

Qualitative walleye pollock stomach scans were performed in the field by observers

aboard commercial fishing vessels from 1982 to 1992. Walleye pollock were checked for

signs of feeding after capture and regurgitation and discarded if signs of either were

present. The stomach content volumes of selected walleye pollock were then estimated

using a graduated cylinder. Prey items were identified by the observers to general

5
taxonomic levels, enumerated, and the percentage of the total content represented by the

prey estimated. Length and sex were recorded when possible for fish and crab prey.

Examples of common prey identified by the observer were returned to the laboratory for

verification by NMFS personnel. While the observers were trained to identify prey to

general taxonomic levels, their varied backgrounds and experience resulted in different

levels of consistency and thoroughness in prey identifications.

Stomach contents of 1,089 walleye pollock, 282 arrowtooth flounder, 311 Pacific cod,

366 flathead sole, and 455 Greenland turbot were examined. Table 1 presents the number

of stomachs, average length, and length range of each predator for each stratum and

quarter combination sampled. The standard error of the length of each group was

relatively large, indicating that a large size range was sampled. However, the average

lengths of most groups were similar for each species.

General diet descriptions were compiled by combining all diet data for a given predator

over year, size, quarter, and area. Seasonal diet descriptions were calculated by pooling

diet data of a given predator by quarter of collection within each area. Spatial diet

comparisons by predator were compiled by area within a given season. Size categories

used in the general diet descriptions represent 10 cm size groups shown at the midpoint.

Prey size frequencies are derived from all diet data for a given predator. Combining data in

this manner is necessary to achieve adequate sample sizes for comparison and to help

offset the potentially patchy nature of predator-prey data. However, combined data may

exhibit increased variability or noise. All diet data are presented in terms of percentage by

weight unless otherwise noted.

To facilitate ease of analysis, predator diet data were pooled into the seven most

important, mutually exclusive prey categories (Table 2) and an eighth miscellaneous

category that included unidentified prey and prey that were not included in the seven other

categories, The deep-sea fish category was comprised of species from the families

Bathylagidae and Myctophidae. Offal was considered to be any prey that looked like a fish

6
Table l.--Average length (cm), standard error length (SE.), length range (cm), and

number of stomachs that contained food of each predator by quarter and

stratum.
Table 1. --(continued)
Table 2.--Pooled prey categories used to describe the diets of walleye pollock, Pacific cod, Greenland turbot,
processing discard from commercial fishing operations, primarily heads and tails of flatfish,

walleye pollock heads, and post-fillet remains of walleye pollock.

Schoener’s (1970) similarity index was calculated to compare the diets of each species

pair so that each species was compared with every other. For these calculations, all twelve

common prey (Table 2) were used. The miscellaneous prey category was not used.

Schoener’s (1970) index CXy was calculated as

where p represents the proportion of prey i in the diet of predator x and y, respectively

The result is then multiplied by 100 to represent the percentage dietary overlap.

10
RESULTS AND DISCUSSION

Walleye Pollock

Results

Walleye pollock were sampled in three of the four strata of the slope region (Fig. 2)

with the southwest stratum (120) being the only one from which no walleye pollock were

collected. Stomach samples of 1,272 walleye pollock were examined, 1,089 of which

contained food representing 119 prey categories (Table 3). Euphausiids were the most

important (26%) prey by weight in the diet of walleye pollock in the slope region of the

eastern Bering Sea (Fig. 3). Pandahd shrimp (14%) walleye pollock (16%) deep-sea fish

(myctophids and bathylagids) (12%) other fish (6%) squid (9%) and Pacific herring

(Clupea pallasi) (3%) were also common components of their diet. Fishery discards (offal)

were the primary component of the miscellaneous prey in addition to the unidentified prey

items. By frequency of occurrence the diet of walleye pollock followed the same trends,

with the exception of copepod and hyperiid amphipods being notably more important by

frequency of occurrence than by weight (Table 3).

The diet of walleye pollock varied with predator size in the slope region of the eastern

Bering Sea (Fig. 4). Euphausiids were the dominant prey (> 19%) for all sizes of pollock

but were a smaller component of the diet of larger (>50 cm) fish. Squid (3-7%) pandalids

(8-15%) and other fish (6-12%) were found in relatively similar proportions of the diet of

all sizes of pollock. Walleye pollock were least prevalent as prey of small (<40 cm)

pollock (4%) and were increasingly important to larger pollock (5% and 20%). Pacific

herring and deep-sea fish were also more prevalent in the diet of the larger (>50 cm) fish

The diet of walleye pollock varied seasonally when examined by strata (Fig. 5). The

greatest variation in the southeast stratum (100) was seen in the consumption of

euphausiids, which were more prevalent in the first quarter of the year, and decreased

consistently through the third and fourth quarters. Deep-sea fish were seen as prey in the

11
12
Table 3.--The diet of walleye pollock, Theragra chalcogramma, from the slope

region of the eastern Bering Sea, not including scan data.

13
Table 3 .--(continued).

14
Table 3 .--(continued).

15
Table 3.--(continued).

16
Figure 3.--The diet of walleye pollock by percent weight in the eastern Bering Sea

slope region based on quantitative laboratory analysis (n=number of

stomachs that contained food).

17
Figure 4.--The diet of walleye pollock by percent weight in the eastern Bering

Sea slope region by predator size based on quantitative laboratory

analysis (n=number of stomachs that contained food in each size

group).

18 
Figure 5.--Seasonal dietary variation of walleye pollock by percent weight

from strata 100, 110, and 130 in the eastern Bering Sea slope

region. N=number of stomachs that contained food.

D-s fish=deep-sea fish (Myctophidae and Bathylagidae).

O fish= other fish.

19
first and third quarters but were most prevalent in the third quarter. Pandalids were not

seen in the diet of walleye pollock in the first quarter and were most prevalent in the

fourth quarter. Seasonal variation was much greater in the northeast stratum (110).

Pollock as prey dominated the diet in the first quarter but they were a negligible portion of

the diet during the remaining quarters. The percentage by weight contribution of

euphausiids to the diet increased from the first through the third quarters and decreased in

the fourth quarter. During the first and third quarters pandalids were a relatively small

portion of the diet, while in the second and fourth quarters, they were the dominant prey

of pollock. The other prey groups represented a smaller portion of the diet. Only the first

and second quarters were adequately sampled in the northwest stratum (130) to allow diet

comparison. The primary difference between the two quarters was that other fish were the

dominant prey category in the first quarter, while euphausiid and miscellaneous prey made

up the bulk of the diet in the second quarter.

The diet of pollock varied somewhat with depth within a given quarter (Fig. 6). In the

first quarter, pollock were the primary prey in the shallower stations (stratum 110) while

other fish were the primary prey in the deeper water (stratum 130). Dietary variation with

depth was less pronounced in the second quarter.

The magnitude of latitudinal variation in the diet of pollock varied by quarter (Fig. 7).

In the first quarter, euphausiid prey dominated the diet in the southeastern stratum (100)

while pollock was the primary prey in the northeast (stratum 110). Most other prey were

found to be a small portion of the diet. Dietary variation was less pronounced in the third

quarter with most prey being found in similar proportions in both strata. In the fourth

quarter, pandalids were the primary prey in the northeast stratum whereas euphausiids,

pandalids, and miscellaneous prey were the most prevalent prey in the southeast stratum

Fish prey (pollock, deep-sea fish, and other fish) were much more common in the

southeastern stratum

20
Figure 6.--Depth-related variation of walleye pollock by percent weight in the

eastern Bering Sea slope region during the first and second quarters.

N=number of stomachs that contained food. D-S fish= deep-sea fish

(Myctophidae and Bathylagidae) O fish= Other fish.

21
Figure 7.--Latitudinal variation in the diet of walleye pollock by percent weight i

the eastern Bering Sea slope region during the first, third, and fourth

quarters. N=number of stomachs that contained food.

D-s fish =deep-sea fish (Myctophidae and Bathylagidae).

O fish=other fish.

n

22
Comprehensive stomach collections taken during scientific surveys of the slope region

in 1988 and 1991 allowed comparison of walleye pollock diets from those 2 years in

stratum 100 (southeast) and 110 (northeast) during the third quarter (Fig. 8). Euphausiids

were most important in 1991, northeast stratum, and were of lesser, but similar

importance in the other samples, Variation was seen in the importance of deep-sea fish,

squid, and to a lesser degree, pandalids by year and strata. All other prey were of limited

importance or did not vary between years and strata.

Pollock cannibalism occurred most extensively in the northeast stratum (110) and to a

lesser degree in the southeast stratum (100) (Fig. 9a). Pollock represented at least 50 %

of the diet of pollock at several stations in the northeast stratum. Pollock cannibalism was

concentrated in the northern section of the stratum. Squid were found in the diet of

pollock in all three strata that were sampled (Fig. 9b). Squid made up the highest

percentage of the diet and were found in the highest proportion of samples in the two

northern strata. Deep-sea fish were found in relatively high proportion in some stations in

all three strata that were sampled with little spatial pattern being evident (Fig. 9c). Pacific

herring were found in a high proportion of the diet of pollock in only one station (Fig. 9d).

Euphausiids were found in the stomachs of walleye pollock taken from every station

sampled (Fig. 9e). Euphausiids were a larger constituent of the pollock diet in the southern

stations and the northeastern stations and were less important in the northwestern stations,

Pollock primarily cannibalized juvenile pollock that were less than 120 mm,

representing age-0 fish, although a few larger age-1 pollock were also consumed (Fig. 10)

Larger adults consumed the larger juveniles, but some of the smallest juveniles were also

consumed by relatively large adults. There was positive correlation (r2=0.28, P<.00l)

between prey size with predator size. Walleye pollock length was poorly correlated

(r2=0,001, P=0.85) with the length of bathylagids consumed (Fig. 10). Myctophid length

was moderately correlated (r2=0.09, P=0.014) with walleye pollock length (Fig. 10).

23
Figure 8.--Dietary comparison of walleye pollock in the eastern Bering Sea

slope region during the third quarter of 1988 and 1991 from strata

100 and 110. N=number of stomachs that contained food. D-s

fish=Myctophidae and Bathylagidae.

24

Figure 9b.--The location and magnitude of walleye pollock predation on squid in the eastern Bering Sea slope region.

Figure 9d.--The location and magnitude of walleye pollock predation on Pacific herring in the eastern Bering Sea slope region.
Figure 9e--The location and magnitude of walleye pollock predation on euphausiids in the eastern Bering Sea slope region.
Figure 10--Walleye pollock (predator) size, prey size scatter plots

for walleye pollock, bathylagid, and myctophid prey in

the eastern Bering Sea slope region.

30
Discussion

The general diet of walleye pollock has been described for many areas of the Bering

Sea. On the eastern Bering Sea shelf the diet of walleye pollock has been shown to

primarily consist of walleye pollock, fish, euphausiids, and copepods (Dwyer et al. 1987,

Livingston et al. 1986, Mito 1974, Mito 1990, Yoshida 1994, Livingston et al. 1993,

Livingston 1991 a) in varying proportions depending on the season, year, or predator size.

In the deeper water of the Aleutian Basin, the diet of walleye pollock has been shown to

consist primarily of copepods, amphipods, euphausiids, squid, and larvaceans (Dwyer

et al. 1987, Yoshida 1994, Mito 1990). In the slope region, the diet of walleye pollock has

been described by Mito (1974) to consist of walleye pollock, euphausiids, and squid

during the fall. The diet of the walleye pollock sampled in our study had features similar

to both the shelf and basin, perhaps indicative of the close geographic proximity to these

areas. The greatest dietary differences between our study of the slope and those

conducted in other areas were in the presence of deep-sea fish, Pacific herring, and

pandalid shrimp in the diet of walleye pollock. A partial explanation is that these prey

may not have been specifically mentioned in the previous studies. Deep-sea fish

(myctophids and bathylagids) and Pacific herring have typically been grouped into

miscellaneous fish categories, as they were not recognized as important ecosystem

components. However, recent studies have shown them to be prey of marine mammals in

the eastern Bering Sea and therefore warrant individual consideration. Pandalid prey were

likely not prevalent enough in previous studies to be considered an important prey group.

However, in our study, pandalids represented a large enough portion of the diet to be

considered seperately. Copepods were not a significant portion of the diet by weight in

the eastern Bering Sea slope, which is in marked contrast to the diets of walleye pollock

in the Aleutian Basin and the shelf. On the eastern Bering Sea shelf, copepods are the

most important prey of smaller walleye pollock (Dwyer et al. 1987, Livingston 1991a,

Livingston et al. 1993, Mito 1974,). On the slope, however, the walleye pollock

3 1
population consists of larger fish (Goddard and Zimmermann, 1993): which is reflected in

our samples and, consequently, in the copepod component of the diet. In the deeper

Aleutian Basin area, copepods are very important in the diet of the primarily large

walleye pollock (Dwyer et al. 1987, Mito 1990, Yoshida 1994). In that area, other prey

such as juvenile walleye pollock and benthic prey are not as readily available. Squid were

not found to be important in the diet of walleye pollock on the eastern Bering Sea shelf

(Dwyer et al. 1987, Livingston 1991 a, Livingston et al. 1993); however, Mito (1974)

found them to be prevalent over the slope, as they were in our study. Gonatid squids,

those most commonly identified from walleye pollock stomachs, are primarily distributed

in deep water in the Bering Sea (Okutani et al. 1988), which accounts for their greater

prevalence in the diet of walleye pollock in deeper slope waters.

Previous researchers have shown that size-re1ated dietary variation exists for walleye

pollock; increased cannibalism concurrent with decreased invertebrate zooplankton

consumption with increasing predator size is the primary feature of the variation (Dwyer

et al. 1987, Mito 1974, Mito 1990, Livingston 1991 a, Livingston et al. 1993). The same

general trend was seen in this study, although the magnitude of the change in the diet

from the smallest to the largest was relatively small due to the narrow size range and

relatively large size of the walleye pollock sampled.

Cannibalism by larger walleye pollock has been shown to be higher in the autumn and

winter months on the eastern Bering Sea shelf (Dwyer et al. 1987, Mito 1990, Livingston

et al. 1986, Livingston, 1991 a). The results of the current study show the same trend

although the dietary differences are more notable in the northeast stratum where

cannibalism accounted for nearly 60% of the diet by weight than in either the northwest

or southeast strata. Cooney (198 1) showed that the average zooplankton biomass in the

slope region of the southeastern Bering Sea decreases through the fall and reaches its

lowest levels in winter. Decreasing zooplankton abundance likely results in higher

consumption of alternate prey, such as walleye pollock. Conversely, zooplankton such as

32
euphausiids and copepods have been shown to be most important in the diet of walleye

pollock in the spring and summer months on the shelf (Dwyer et al. 1987, Livingston

et al. 1986), and the same is generally true on the slope. Zooplankton are most abundant

during this time of year (Cooney 1981) due to the beginning ‘of the spring phytoplankton

bloom which occurs in April (Goering and Iverson 1981). The diet of walleye pollock in

the southeast stratum (100) did not follow these trends: euphausiids were most important

in the winter and fall (there were no spring samples). One possible factor in this trend

may be that walleye pollock in these samples were the smallest on average of all strata

sampled; smaller pollock tend to rely more heavily upon euphausiids in the winter than

do the larger ones (Livingston 1991 a).

Dwyer et al. (1987) showed that the primary winter forage fish of walleye pollock on

the eastern Bering Sea continental shelf was walleye pollock and that it was fish other

than walleye pollock in the Aleutian Basin area. More specifically, they showed that

walleye pollock cannibalism is non-existent in the Aleutian Basin, while other fish are

very prevalent in the diet. The results of the current study are in agreement with this

trend. Walleye pollock cannibalism accounted for nearly 60% of the diet by weight in the

shallow strata, while it was less than 5% by weight in the diet of walleye pollock in the

deep strata. The contribution of other fish to the diet of walleye pollock in deep strata

showed an opposite trend; other fish were the most important prey of walleye pollock in

the deep strata while cannibalism was less important. Walleye pollock in the shallower

strata fed more similarly to walleye pollock on the shelf, while walleye pollock in the

deeper strata fed more similarly to those in the Aleutian Basin. During spring quarter the

same notion holds true, although the similarities are not as striking. Decapods were

present in the diet of shelf walleye pollock (Dwyer et al. 1987, Livingston 1991a), but on

the slope, pandalids were the most important prey in the shallow strata; euphausiids were

the most important prey in the deeper strata.

33
Fish, especially walleye pollock, were important prey of larger walleye pollock during

winter on the eastern Bering Sea shelf (Dwyer et al. 1987). Walleye pollock cannibalism

in the northeastern stratum of the slope during winter was important, however other fish

prey were the primary prey in the northwestern stratum. Euphausiids were the primary

prey in the diet of walleye pollock in the southeastern stratum during winter rather than

fish prey. However, euphausiids were more common in the diet of smaller walleye

pollock, even during the winter, on the shelf (Livingston 1991 a). Samples from our study

did come from somewhat smaller walleye pollock when compared with the other slope

strata sampled. During‘ summer, euphausiids were the dominant prey for walleye pollock

in both strata, which is more similar to the diet of walleye pollock on the shelf than in the

Aleutian Basin (Dwyer et al. 1987). During the fall, pandalids were more important in the

northeastern stratum, which is also consistent with the diet of walleye pollock in the

northwest shelf region of the Bering Sea (Dwyer et al. 1987).

Considerable dietary variability existed in the diet of walleye pollock in the two

shallow strata between the years 1988 and 1991. This variability is likely due to slight

changes in the timing or location of samples between years or inadequate sample sizes

rather than representing true changes in the diets of walleye pollock during these 2 years.

The presence of similar, relatively high values of some prey (i.e., squid and deep-sea fish)

from opposite strata between years is difficult to explain; however, it is likely that it is a

reflection of prey availability due to patchy distributions or movement of prey on a

diurnal basis.

Walleye pollock cannibalism was most prevalent in the northeast strata, which is

consistent with the distribution of juvenile walleye pollock in the eastern Bering Sea

(Fadeyev 1988, Kihara 1990). Squid are found throughout the deep waters of the Bering

Sea (Okutani et al. 1988) and were consumed by walleye pollock in relatively equal

amounts throughout all strata. Walleye pollock consumed deep-sea fishes equally in all

strata. However, it is difficult to associate this pattern with prey availability since little is

34
known about the distribution of deep-sea fishes in the Bering Sea. Pacific herring were

only consumed in the northeastern strata, which is likely due to their tendency to migrate

to this area in winter (Wespestad and Barton 1981). Euphausiids were consumed by

walleye pollock at every station sampled, reflecting their importance in the diet of

walleye pollock and their ubiquitous distribution (Cooney 1981).

The maximum size of prey consumed by walleye pollock appears to be a function of

predator size rather than prey availability. Most of the walleye pollock cannibalism was

focused on age-0 fish, although a few larger individuals were consumed. A significant

correlation between predator size and prey size indicates that larger predators are

consuming larger prey. Prey selection is not simply a matter of prey abundance, but also

of predator size preference. While the relationship between predator and prey size was not

as clear for the deep-sea fish prey, the maximum size consumed was consistent with the

range of the juvenile walleye pollock that were consumed. Livingston (1991 a) presented

similar results for walleye pollock cannibalism on the eastern Bering Sea shelf. Yoshida

(1994) presented similar results of fish prey size of walleye pollock in the Aleutian Basin.

Mito (1974) also demonstrated similar trends of walleye pollock cannibalism in the shelf

break and upper slope region of the eastern Bering Sea. This indicates that walleye

pollock have exhibited some prey size preferences that should be analyzed in more detail.

35
Walleye Pollock Scan Data

Results

Walleye pollock field scan data collected in the southeastern stratum (100) show that

euphausiids were the primary prey of walleye pollock during all quarters (Fig. 11), Other

fish were of some importance in the first, third, and fourth quarters. Squid were important

to walleye pollock in the third quarter and walleye pollock cannibalism was only seen in

the third quarter. The diet of walleye pollock collected in the northeastern stratum (110)

indicates that euphausiids were the primary prey in the third quarter (Fig. 11). Squid were

the most important prey in the first quarter; euphausiids and deep-sea fish were of

secondary importance. In the second quarter, walleye pollock was the most important prey

with other fish and miscellaneous prey constituting the remainder of the diet. Walleye

pollock collected in the fourth quarter from stratum 110 (northwest) had the most diverse

diet; squid, euphausiids, miscellaneous prey, other fish, and pollock were all relatively

important in the diet. The diets of walleye pollock collected in the northwestern stratum

(130) revealed very different diets for each of the three quarters sampled (Fig. 11). In the

second quarter, euphausiids were the primary prey, while miscellaneous prey and other

fish were of secondary importance. In the third quarter, other fish comprised 99% of the

diet by weight. In the fourth quarter, squid were the primary prey, with miscellaneous

prey, other fish, and euphausiids being of secondary importance.

Discussion

Walleye pollock field scan data shows somewhat greater seasonal and strata dietary

variation than seen in the results of the laboratory analysis presented here. Field scan data

should be considered qualitative in nature. Some of the sample sizes were relatively small,

and observers who performed these scans were given only limited training. Observers

were trained to identify, without the aid of magnification, only the most distinguishable

and most common prey such as walleye pollock, copepods, euphausiids, fish, and squid.,

36
“Pollock Herring Misc. Pandalid

D-s fish 0th. fish Euph. Squid

Figure 11 .--Seasonal variation in the diet of walleye pollock by percent weight in

the eastern Bering Sea region in strata 100, 110, and 130 as

determined from qualitative field scans. N=number of stomachs that

contained food. D-s=deep-sea fish (Myctophidae and Bathylagidae).

37
Some similarities between the field stomach scan data and lab stomach data do exist,

particularly for the data for the southeastern stratum (100) and northwestern stratum

(130). The data for the northeastern stratum (110) are different, primarily due to the large

presence of pandalids in the lab stomach data that might be difficult to identify in the field.

Pacific Cod

Results

Sampling locations for Pacific cod were fairly evenly distributed within each of the two

shallow strata (100 and 110) (Fig. 12). Stomach samples from 314 Pacific cod were

analyzed, of which 311 contained food representing 112 prey categories (Table 4).

Walleye pollock dominated the diet (52%) of Pacific cod in the slope region of the eastern

Bering Sea (Fig. 13). Other fish (13%) offal (10%) and pandalids (8%) were also

important components of the Pacific cod diet. Chionoecetes spp.(6%), Pacific herring

(4%) and squid (2%)o were of lesser importance in the cod diet. The miscellaneous prey

category was primarily made up of unidentified prey. Frequency of occurrence data

suggest the same trend in diet composition with the exception that gammarid amphipods

and polychaetes represented a greater portion of the diet than the percentage by weight

figures indicated (Table 4).

Walleye pollock became more important (19-59%) in the diet of Pacific cod with

increasing predator size (Fig. 14). Offal also became more important with size (8-12%)

although the overall importance of offal in the diet was less. The percentage by weight of

most other prey items remained relatively constant through all predator sizes, with the

exception of pandalids (25-l%) and miscellaneous prey (19-2%) which declined with size.

Pollock was the dominant prey for all sizes of Pacific cod.

38

Table 4.--The diet of Pacific cod, Gadus macrocephalus, from the slope region

of the eastern Bering Sea.
Table 4.--(continued).
Table 4.--(continued).

42
Table 4.--(continued).

Total prey weight: 38,731.14g

Total non-empty stomachs: 311 

‘Total empty stomachs: 3

Total prey categories: 112

43

Figure 14.--The diet of Pacific cod by percent weight in the eastern Bering

Sea slope region by predator size based on quantitative

laboratory analysis (n=number of stomachs that contained

food in each size group).

45
Seasonal data for Pacific cod were only available for third and fourth quarters (Fig.

15). In the southeastern stratum (l00), pollock was the primary prey of Pacific cod in

both quarters. Other fish were of secondary importance in the fourth quarter.

Chionoecetes spp. and pandalids were of secondary importance during the third quarter in

the southeastern stratum (100). Pollock were also the primary prey of cod of both quarters

in stratum 110. Herring were of secondary importance and only found in cod from the

fourth quarter. Offal, other fish, and pandalids were secondary prey in the third quarter.

Latitudinal variation in the diet of Pacific cod in the slope region of the eastern Bering

Sea was most prevalent in the fish prey categories (Fig. 16). Pollock were the primary

prey in both strata during the third quarter, although they were a larger proportion of the

diet in the southeastern stratum (100). Offal and other fish were more important in the

northeastern stratum. Little variation was seen between the two strata for the other prey.

During the fourth quarter, pollock were also the most important prey in both strata and

were equally important in both areas. Offal and Pacific herring were only seen from

stratum 110 and represented the second and third most important prey. Other fish were

the secondary prey in the southeastern stratum (100). The four remaining prey types were

of limited importance to Pacific cod during the fourth quarter.

Pollock were found in the diet of Pacific cod at approximately one-half of the stations

sampled (Fig. 17a). Predation on pollock was more common in the northeastern stratum

than in the southeastern stratum and represented a larger portion of the diet. Pacific

herring were only found in the diet of Pacific cod in significant proportion at a relatively

small number of stations in the northeastern stratum (Fig. 17b). Chionoecetes spp. prey

were found in moderately large proportion of the diet of Pacific cod from stations in both

strata (Fig. 17c). Offal was found in a relatively small portion of the diet of Pacific cod in

less than one-half of the stations sampled (Fig. 17d). Offal represented >76% of the diet

by weight of Pacific cod at only one station.

46
Figure 15.--Seasonal dietary variation in the diet of Pacific cod by weight in the

eastern Bering Sea slope region from strata 100 and 110. N=number

of stomachs that contained food. Chion.=Chionoecetes spp.

47
Figure 16.--Latitudinal variation in the diet of Pacific cod by percent weight in the

eastern Bering Sea slope region from the third and fourth quarters.

N=number of stomachs that contained food. Chion=Chionoecetes spp.

48
Figure 17a--The location and magnitude of Pacific cod predation on walleye pollock in the eastern Bering Sea slope region.


Figure 17d.--The location and magnitude of Pacific cod predation on offal in the eastern Bering Sea slope region.
Cod consumed primarily larger pollock, representing age-l and older fish (Fig. 18).

Some small pollock were consumed, however there was no correlation between predator

size and prey size (r2=0.03, 0.l5>P>0. 10). Consumption of Chionoecetes bairdi and C.

opilio by Pacific cod was primarily focused on relatively small individuals (Fig. 18). In

both cases, prey size was positively correlated with predator size (r2=0.23, 0.05>P>0.01,

r

2

=0. 16 , P<0.00l, respectively).

Discussion

In the eastern Bering Sea, Pacific cod are rare in waters deeper than 300 m (Allen and

Smith 1988) and our sample locations reflect that distribution. The diet of Pacific cod in

western Bering Sea coastal areas has been described to consist primarily of walleye

pollock, Pacific sand lance (Ammodytes hexapterus), other fish, and crab (Tokranov and

Vinnikov 1991). In the Gulf of Alaska, the diet of Pacific cod consisted of walleye

pollock, capelin, other fish, and shrimp (Albers and Anderson 1985, Yang 1993). In the

eastern Bering Sea, walleye pollock, other fish, and crab were the primary prey of Pacific

cod (Livingston 1991b, Livingston et al 1993). Previous research in the slope region

during fall showed the-diet of Pacific cod to consist of polychaete worms, decapods, and

walleye pollock (Mito 1974). In the current study of the slope region of the eastern Bering

Sea, walleye pollock were the dominant component of the diet of Pacific cod. The diet of

Pacific cod over the slope is similar to that seen in other areas likely due to the relatively

shallow distribution of Pacific cod over the slope region. The large contribution of walleye

pollock seen in the slope region compared to previous studies is likely due to the larger

size of Pacific cod sampled in our study. In our study,, walleye pollock were increasingly

important in the diet with increasing predator size, which is consistent with, previous work

(Livingston 1991b, Livingston et al. 1993, Mito 1974, Yang 1993). These studies

primarily used somewhat smaller predators and showed the beginning of the relationship

53
Figure 18.--Pacific cod (predator) size, prey size scatter plot of

walleye pollock, C. bairdi, and C. opilio in the eastern

Bering Sea slope region.

54
of increased importance of walleye pollock in the diet of Pacific cod with increasing

predator size that is evident here. The relatively large presence of offal in the diet

compared to other areas is likely due to the fact that this prey category was separated out

in this study while it was lumped as miscellaneous prey or not counted as prey in some

previous studies. Yang (1993) and Livingston et al. (1993) show offal as being of minor

importance in the diet of Pacific cod in other areas, but that its importance increases with

predator size. Our results also show an increase in the importance of offal in the diet with

size, which confirms the size relationship seen in the previous works. Squid were more

common in the diet of Pacific cod over the slope due to their higher abundance in deeper

water (Okutani et al. 1988).

The presence of Pacific herring in the diet of Pacific cod in the northeastern stratum

during the fourth quarter is likely a reflection of the seasonal return of Pacific herring to

their over-wintering grounds located in the northwest section of the eastern Bering Sea

(Wespestad and Barton 1981). Spatial variation in the offal component in the diet is

difficult to explain based on these data. Offal would be expected to be highest in

prevelence in stomachs of predators collected near at-sea processing commercial fishing

operations, and most (64%) of the collections that contained offal were collected aboard

commercial vessels by observers. Variation in the other fish category can result from

variation in prey availability (i.e., changes in prey abundance and size) with season or area

However, data on these aspects are lacking for most of the prey considered here such as

juvenile walleye pollock and other small pelagic or mesopelagic fish. Changes in the

relative composition of the other fish category can also result from factors, such as level of

digestion, which prevent complete identification of prey fish and increases the proportion

of unidentified fish within this category.

Pacific cod have been shown to exhibit a significant relationship between predator size

and prey size in Alaska waters; bigger fish eat bigger prey (Livingston 1991b, Mito 1974,

Yang 1993). The results of this study show a positive, significant relationship between

55
Pacific cod standard length and carapace width of Chionoecetes bairdi and C. opilio, but

not for walleye pollock prey. Pacific cod over the slope region were relatively large, and

consequently consumed primarily large walleye pollock. The lack of small Pacific cod and

therefore small walleye pollock prey prohibited the development of the lower end of the

relationship

Greenland Turbot

Results

Greenland turbot samples were collected from each of the four strata in the slope region

(Fig. 19). Samples were collected from more stations in the outer, deeper strata than in the

inner, shallower two; however, spatial coverage was similar in all four strata. Stomach

samples from 858 Greenland turbot were analyzed, 45.5 of which contained prey items

representing 50 categories (Table 5). Walleye pollock (58%) dominated the general diet of

Greenland turbot by weight. Squid (19%) and offal (12%) were the second and third most

important prey by weight (Fig. 20). These three prey accounted for 89% of the diet of

Greenland turbot. Other fish (6%) snailfish (3%) Pacific herring (<l%) and deep-sea fish

(1%) made up the rest of the diet. By frequency of occurrence, squid (41%) other fish

(23%) and walleye pollock (23%) were the most important prey (Table 5).

Squid (up to 76%) were the most important, prey by weight of smaller (<70 cm)

Greenland turbot (Fig. 21); however, the importance of squid in the diet decreased with

size for fish larger than 70 cm. Squid became less important with increasing predator size

and walleye pollock became more important. Walleye pollock prey were most important

to the larger ( >70 cm) Greenland turbot and least important to the smaller sizes. Most of

the other prey were found in relatively constant proportion throughout the range of

predator sizes.

56

Figure 20. --The diet of Greenland turbot by percent weight in the eastern

Bering Sea slope region based on quantitative laboratory

analysis (n=number of stomachs that contained food).

58
Table 5.--The diet of Greenland turbot, Reinhardtius hippoglossoides, from

the slope region of the eastern Bering Sea.
Table 5.--(continued).

60
Figure 21 .--The diet of Greenland turbot by percent weight in the eastern

Bering Sea slope region by predator size based on

quantitative laboratory analysis (n=number of stomachs that

contained food in each size group).

61
Adequate sample sizes were collected to allow dietary comparisons in second, third,

and fourth quarters from strata 110 and 130 only (Fig. 22). In the shallower stratum (110),

seasonal variation in the diet was present, although not as prevalent as was seen in other

species. Walleye pollock were an important prey item in all seasons, although they were

more important in the third quarter than the others. Offal was most prevalent (40%) in the

diet of Greenland turbot in the fourth quarter and was not very important in the other two.

Squid were of equal importance (30%) in the second and fourth quarters and much less

important in the third. The other prey types were found in relatively equal proportions in

the other strata. In the deeper stratum (130) seasonal variation was somewhat less

pronounced. Walleye pollock were the most important portion of the diet of Greenland

turbot during all seasons. Squid were important in the fourth quarter but were not as

important in the other quarters. Offal was found in its highest contribution in the third

quarter, somewhat less in the fourth quarter and of negligible importance in the second

quarter. All other prey were found in relatively similar proportions in all quarters.

With the exception of walleye pollock and squid, depth-related variation in the diet of

Greenland turbot was relatively small in the second quarter (Fig. 23). Pollock were more

important in the diet from deeper water, while squid were more important in the shallower

water. In the third quarter, dietary variation was even more limited, and most prey were

of similar importance in both depth zones. Walleye pollock were more important in the

shallower depth, while offal was more important in the deeper water. In the fourth

quarter, offal was substantially more important in the shallow stratum while walleye

pollock, squid, and other fish were most important in the deeper water.

Pollock were found to be a large portion (>25%) of the diet of Greenland turbot in

most stations sampled (Fig. 24a). Significant (>50%) amounts of pollock in the diet were

found in higher proportion of the stations in the northern strata than in the southern strata,

Squid were also found in relatively high proportions in the diet of Greenland turbot in

stations from all strata and were more prevalent in the inner strata (Fig. 24b).

62
Figure 22.--Seasonal variation in the diet of Greenland turbot by percent weight in

the eastern Bering Sea slope region from strata 110 and 130.

N=number of stomachs that contained food. D-s fish=deep-sea fish

(Myctophidae and Bathylagidae).

63
Figure 23.--Depth related variation in the diet of Greenland turbot by percent

weight in the eastern Bering Sea slope region from the second, third,

and fourth quarters. N=number of stomachs that contained food.

D-s fish=deep-sea fish (Myctophidae and Bathylagidae).

64
Figure 24a.--The location and magnitude of Greenland turbot predation on walleye pollock in the eastern Bering Sea slope region.

Consumption of offal by Greenland turbot was relatively sparse, although it was somewhat

more common in samples collected in the northern strata (Fig. 24c).

Greenland turbot consumed all size groups of pollock; however, the smaller (age-O) fish

were primarily consumed in the northern strata (Fig. 25). Smaller prey were also generally

consumed by the smaller predators and there was a significant correlation between

predator size and prey size (r2=0.41, P<0.00l). Myctophid predation was focused on

individuals less than 100 mm fork length with a positive correlation (r2= 0.22,

0.05>P>0.01) between predator size and prey size (Fig. 25). Bathylagid prey consumed by

Greenland turbot were primarily between 60 and 110 mm fork length (Fig. 25). There was

not ‘a significant relationship (r2=0.006, P=O.63) between predator length and prey length.

Discussion

Greenland turbot are primarily found in depths less than 600 m (Allen and Smith 1988).

Our sample locations reflect this distribution as Greenland turbot stomachs were sampled

in all strata in the slope region of the eastern Bering Sea. Yang and Livingston (1988)

described the diet of Greenland turbot in the eastern Bering Sea slope region to primarily

consist of gadids (primarily walleye pollock), cephalopods, and other fish. Mito (1974)

described the diet of Greenland turbot over the slope during the fall to consist primarily of

walleye pollock, with Pacific herring and arrowtooth flounder as secondary fish prey. On

the eastern Bering Sea shelf, the Greenland turbot diet has been described as consisting of

gadids, cephalopods, and euphausiids (at smaller predator sizes) as the primary prey, and

that bathylagids and myctophids (deep-sea fish in the current study) became increasingly

important with depth (Yang and Livingston 1988, Yang 1991a, Livingston et al. 1993).

The diet of Greenland turbot over the slope described here is in general agreement with

these studies although the importance of some fish (bathylagids and zoarcids) is less than

what might have been expected given the trends on the shelf,

67
Figure 24c.--The location and magnitude of Greenland turbot predation on offal in the eastern Bering Sea slope region.
Figure 25.--Greenland turbot (predator) size, prey size scatter

plots of walleye pollock, myctophid, and bathylagid

prey in the eastern Bering Sea slope region.

69
A higher dependence upon squid by smaller Greenland turbot in slope areas was also

seen by Yang and Livingston (1988) with a switch to walleye pollock at larger sizes. Other

work (Yang 1991a, Livingston et al. 1993) on the shelf, and Mito (1974) on the slope did

not show such a strong relationship. The reasons for this diet pattern and the differences

between the shelf and slope seen here are likely due to a lower abundance of squid and

higher abundance of juvenile walleye pollock in shallow water (Goddard and Zimmermann

1993, Okutani et al. 1988). Greenland turbot exhibited a positive, significant relationship

between predator size and walleye pollock prey size in previous work on the shelf (Yang

and Livingston 1988, Yang 1991a, Livingston et al. 1993) and in the current study

indicating the availability of small walleye pollock as prey. Therefore it is likely that the

relative lack of juvenile walleye pollock over the slope contributes to their decreased

importance in the diet of small Greenland turbot. In this study,. the majority of the smaller

walleye pollock were consumed in the northern strata, which is where juvenile walleye

pollock are thought to be most abundant (Fadeyev 1988, Kihara 1990). A significant

relationship did not exist between Greenland turbot size and bathylagid size. Although

there was a significant relationship between myctophid length and Greenland turbot

length, there was not a large variation in the size of myctophids consumed (40-100 mm)

and the number of myctophids measured was low.

Seasonal variation in the diet of Greenland turbot on the slope is consistent with that

seen on the shelf by Yang (1991a), although the proportion of walleye pollock in the diet

was generally larger on the slope due to larger predator sizes sampled. The most striking

variation lies in the importance of squid and offal in the diet. Differences in squid

predation may be a result of seasonal movement of squid in response to reproduction or

predation behavior. This variation may also be due to changes in the availability of

alternate prey; walleye pollock, in this case, varied inversely with squid in the diet,

Variation in the prevalence of offal in the diet likely lies in its availability to the predator.

Offal is an anthropogenic contribution to the prey pool; it can only be consumed within a

70
relatively close proximity to fishing operations. Offal consumption was primarily (85% of

the occurrences) seen in samples collected aboard commercial fishing vessels.

The location of stations where Greenland turbot consumed walleye pollock and squid

were fairly evenly distributed throughout all strata over the year. The relative importance

of these prey was also evenly distributed, indicating “a relative consistency of prey

availability in a strata. The results do not, however, shed light on the seasonal abundance

of prey. The location of stations where offal was important in the diet of Greenland turbot

primarily reflects areas where samples were collected aboard commercial processing

vessels.

Flathead Sole

Results

Flathead sole were primarily sampled in the two inner strata (100 and 110), although a

few stations were sampled in the outer strata (Fig. 26). Stomach samples from 528

flathead sole were analyzed, of which 366 contained prey representing 75 categories

(Table 6). Brittle stars were the most important prey by weight (50%) of flathead sole in

the slope region of the eastern Bering Sea (Fig. 27). Offal (24%) rockfish (10%) other

fish (4%) and walleye pollock (3%) also contributed to the diet of flathead sole over the

slope region of the eastern Bering Sea. The miscellaneous prey category (5% by weight)

was primarily unidentified material. By frequency of occurrence the diet of flathead sole is

very similar; brittle stars were the primary prey. However, small prey such as gammarid

amphipods and polychaetes which were not important by weight were more important by

frequency (Table 6).

Brittle stars were the dominant prey for all sizes of flathead sole collected in this study

(Fig. 28). Brittle stars and offal were of greater importance with increasing predator size:

Walleye pollock and rockfish prey were found to be less important in the diet of flathead

71
Figure 26 --The location of flathead sole stomach collections in the eastern Bering Sea slope region.

Table 6.--The diet of flathead sole, Hippoglossoides elassodon, from the slope

region of the eastern Bering sea.
Table 6.--(continued).
Table 6.--(continued).

Total prey weight: 1,641.14g

Total non-empty stomachs: 366

Total empty stomachs: 162

Total prey categories: 75

76
Figure 28.--The diet of flathead sole by percent weight in the eastern Bering

Sea slope region by predator size based on quantitative

laboratory analysis (n=number of stomachs that contained food

in each size group).

77
sole with increasing predator size. Pandalid shrimp, miscellaneous prey, and other fish

were found in similar proportions in the diet of all predator sizes.

For seasonal comparisons, adequate samples sizes were only obtained from strata 100

and 110 for the third and fourth quarters. In the southeastern stratum (100) variation

between the two quarters was pronounced (Fig. 29). In the third quarter, offal was the

predominant prey of flathead sole (almost SO,% by weight), while in the fourth quarter

brittle stars comprised over 80% of the diet by weight. With the exception of the one

dominant prey, all other prey were found to be a small proportion of the diet. In the

northeastern stratum (110), brittle stars were also the most prevalent prey in the diet of

flathead sole in the fourth quarter, representing over 90% of the diet by weight. All other

prey were found in negligible amounts. During the third quarter, rockfish were the

predominant prey by weight,, but brittle stars were also relatively important. Walleye

pollock, other fish, and offal were of minor importance as well.

Latitudinal variation within a given quarter was more pronounced in the third quarter

than in the fourth (Fig. 30). In the third quarter, offal was the dominant prey item in the

diet of flathead sole in the southeastern stratum (100); all other, prey were only of minor

importance. In the northeastern stratum (110), rockfish and brittle stars were the most

important prey, however walleye pollock and other fish were also of minor importance. In

the fourth quarter, there was little variation in the diet between strata, and brittle stars

were the most important prey of both areas with little else showing up in the diet.

Brittle stars were found in a high proportion of the diet of flathead sole in most of the

stations sampled; with little difference between the northern and southern strata (Fig. 3la)

Offal was only found at a few, primarily southern stations, but was very important in the

few stations at which it was found (Fig. 3 lb). Rockfish represented a moderate

proportion of the diet of flathead sole at a few stations in the northern stratum (Fig. 31 c).

All rockfish prey were 30-60 mm fork length and flathead sole that consumed them were

78
Rockfish Offal Chion. Brittle star

Figure 29.--Seasonal dietary variation of flathead sole by percent weight

in the eastern Bering Sea slope region from strata 100 and 110.

N=number of stomachs that contained food.

Chion.=Chionoecetes spp.

79
Figure 30.--Latitudinal dietary variation of flathead sole by percent weight in the

eastern Bering Sea slope region from strata 100 and 110. N=number

of stomachs that contained food. Chion.=Chionoecetes spp.

80

Figure 31b --The location and magnitude of flathead sole predation on offal in the eastern Bering Sea slope region.

> 22 cm standard length (Fig. 32). There was no correlation (r2

=0.002, P=O.665) between

predator size and prey size.

Discussion

Most of the flathead sole in the eastern Bering Sea are found in water depths less than

450 m (Allen and Smith 1988) which is reflected in our sample locations. The diet of

flathead sole on the shelf generally consists of brittle stars, walleye pollock, and decapod

crustaceans (Pacunski 1990, Pacunski 1991, Livingston et al. 1993). With increasing

depth and predator size, their diet tends towards brittle stars and decapod crustacea

(Pacunski 1990). Over the slope, the diet of flathead sole has been shown to consist of

brittle stars, decapods, and zooplankton (Mito 1974). The results of the present work

showed brittle stars to be the primary prey throughout the slope area. Decapod crustacea

were not important. Offal represented the second most abundant prey item and is again a

likely artifact of the sampling vessels. Rockfish were an important prey, but their overall

importance is probably overestimated by weight as they were only found in a few hauls,

but they were the dominant prey in those samples. Large, heavy prey can be overly

influential in the percentage by weight values when they occur in large quantities from a

small number of samples. The percentage frequency of occurrence of rockfish prey was

8%, which is another factor to consider in determining the importance of a particular prey

in the diet.

Size-related variation in prey consumption was slight due to the lack of small predators

in our study. Increased dependence upon brittle stars with size is consistent with previous

work (Mito 1974, Pacunski 1990). However, the inverse relationship in the amounts of 

rockfish and offal in the diet with increasing flathead sole size seen here is not likely due to

the decreased abundance of these prey categories. The offal found in flathead sole

stomachs (primarily walleye pollock parts) are likely too large for smaller predators to

84
Figure 32.--Flathead sole (predator) size, rockfish (prey) size

scatter plot from the eastern Bering Sea slope region.

85
consume, while the rockfish that were consumed were relatively small (< 60 mm standard

length) and could, therefore, be consumed by small predators with relative ease.

The dietary variation seen between the third and fourth quarters in the southeastern

stratum (100) could be a result of sampling location rather than changes in prey

availability. Most of the samples containing offal were collected in the southern portion of

the southeastern strata, an area where extensive walleye pollock fishing takes place.

However, the variation between these quarters in the northeastern stratum could be a true

reflection of opportunistic feeding on a seasonal abundance of juvenile rockfish.

The north-south comparison of the diet reflects the geographic distinction between an

area of higher commercial fishing operations and/or sampling aboard those vessels and the

area of seasonally abundant juvenile rockfish. The importance of offal in the diet may be

overestimated as the weight of offal in a few stomachs can be very high and may have an

exaggerated effect upon the percentage by weight calculations. In the fourth quarter, there

were no differences in the diet of flathead sole between strata indicating that the rockfish

had migrated, or grown to a size too large for flathead, sole to consume. Additionally, by

the fourth quarter, the walleye pollock fishery is generally over (Fritz et al. 1995).

There was no significant relationship between the size of rockfish prey and size of

flathead sole due to the limited size range of rockfish eaten. The size range of prey was

likely determined by availability rather than the ability of flathead sole to consume them.

Arrowtooth Flounder

Results

Arrow-tooth and Kamchatka flounder were sampled from all four strata in the slope

region; however, only one station was sampled in the southwest stratum (120) and the

inner, shallower strata were sampled more thoroughly than the outer strata (Fig. 33).

Stomach samples from 686 arrowtooth and Kamchatka flounder were analyzed, of which

86

285 contained food representing 48 prey categories (Table 7). For the purposes of this

study, arrow-tooth and Kamchatka flounder diet data were pooled. Walleye pollock was

the most important prey by weight (55%) in the diet of arrowtooth flounder. Pacific

herring (11%) other fish (16%) and squid (11%) were also relatively important (Fig. 34).

By frequency of occurrence, the same prey were important, although some prey (i.e.,

pandalids and other fish) were more important, while others (i.e., walleye pollock and

Pacific herring) appeared to be less important by this measure (Table 7).

With the exception of miscellaneous prey, all prey appeared in different proportions in

the diet of arrowtooth flounder depending upon predator size (Fig. 35). Offal was only

seen in the diet of 40-50 cm fish, and myctophids were only seen in smaller (<50 cm) fish.

The importance of walleye pollock in the diet increased with predator size while other fish

and pandalids became less important. Squid and herring were more important to fish in the

middle size range (40-60 cm) samples than to those at either end.

Seasonal variation was slight for most prey items in the diet of arrowtooth collected

from the northeastern stratum 110 during second, third, and fourth quarters (Fig. 36).

Walleye pollock was the primary prey during all quarters, but it was more important in the

diet during the third and fourth quarters than in the second quarter. Squid were important

in the second quarter, but they were negligible in the others. Pacific herring were only

found in the diet of arrowtooth flounder in the fourth quarter, In the northwestern stratum

(130), there was less dietary variability (Fig. 36). Squid were the primary prey during both

quarters sampled and were found in relatively similar proportions of the diet. Other fish

and miscellaneous prey were also found in relatively similar proportions of the diet in both

quarters. Walleye pollock were substantially more prevalent in the second quarter diet

while pandalids represented a larger portion of the diet in the fourth quarter.

During the third quarter, samples were collected from strata 100 and 110, allowing a

latitudinal comparison of the diet of arrowtooth flounder. Walleye pollock was the primary

prey of arrowtooth in both strata, representing more than 60% of the diet (Fig. 37).

88
Table 7.--The diet of arrowtooth flounder, Atheresthes stomias, and Kamchatka

flounder, A. evermanni, from the slope region of the eastern Bering Sea.
Table 7.--(continued).

Figure 35.--The diet of arrowtooth flounder by percent weight in the eastern

Bering Sea slope region by predator size based on quantitative

laboratory analysis (n=number of stomachs that contained food in

each size group).

92
Figure 36.--Seasonal diets of arrowtooth flounder by percent weight in the

eastern Bering Sea slope region from strata 110 and 130.

N=number of stomachs that contained food. Myct.=Myctophidae.

93
Figure 37.--Latitudinal variation in the diet of arrowtooth flounder by percent

weight in the eastern Bering Sea slope region during the third quarter.

N=number of stomachs that contained food. Myct.=Myctophidae.

94
However, Pacific herring was the second most important prey in the southeastern stratum

(l00), while other fish and pandalids were of secondary importance in the northeastern

stratum (110).

In comparing the diet of arrow-tooth flounder at differing depths within a given season,

it is apparent that the magnitude of dietary variability depends upon the season (Fig. 38).

In the second quarter, the diets of arrow-tooth flounder in the two strata were similar.

Walleye pollock and other fish were more important in the shallower stratum (110), while

squid were somewhat important in the deeper stratum (130). Walleye pollock, squid, and

other fish were the three most important prey in both regions. In the fourth quarter,

however, walleye pollock were of little importance to arrowtooth flounder in deeper

water, while they were the primary prey of those in the shallower water. Pacific herring

were only found in the diet of arrow-tooth flounder in shallow water. Other fish were

relatively important to arrowtooth flounder in both strata. Squid and pandalid shrimp were

substantially more important in the diet of arrowtooth flounder collected from the deeper

water of stratum 130.

Pollock were found in relatively high proportion in the diet of arrowtooth flounder in

stations from all strata sampled, although they were more prevalent in the diet of

arrowtooth flounder in the inner strata (Fig. 39a). Squid were only found as a major

dietary component of arrowtooth flounder at most stations in the two northern strata and

were relatively rare in the southern strata (Fig. 39b). Pacific herring were important in the

diet of arrow-tooth flounder primarily in the two northern strata (Fig. 39c). There was a

significant, positive relationship (r2=0.58, P<O.OOl) between arrowtooth flounder size and

walleye pollock size (Fig. 40).

Discussion

Arrow-tooth flounder are found to depths of 900 m, however most are found in water

depths less than 400 m (Allen and Smith 1988). Our sample locations reflect this

95
Figure 38.--Depth-related variation in the diet of arrowtooth flounder by percent

weight in the eastern Bering Sea slope region. N=number of stomachs

that contained food. Myct=Myctophidae.

96
Figure 39a.--The location and magnitude of arrowtooth Hounder predation on walleye pollock in the eastern Bering Sea slope region.
Figure 39b.--The location and magnitude of arrowtooth flounder predation on squid in the eastern Bering Sea slope region.
Figure 39c.--The location and magnitude of arrowtooth flounder predation on Pacific herring in the eastern Bering Sea slope region.
Figure 40.--Arrowtooth flounder (predator) size, walleye pollock

(prey) size scatter plot for the eastern Bering Sea

slope region
distribution with most samples coming from the shallower strata. In Alaska waters, the

diet of arrowtooth flounder consists primarily of walleye pollock, other fish (especially

Pacific herring and capelin), and crustaceans (Mito 1974, Yang 1991b, Yang and

Livingston 1986, Livingston et al. 1993, Yang 1993). In these studies, the importance of

fish prey increased with increasing predator size. The present study shows that walleye

pollock was the main prey of arrow-tooth flounder in the slope region of the eastern Bering

Sea; Pacific herring, squid, and other fish were of secondary importance. The differences

seen in the present study are likely a reflection of differences in prey composition and

larger predator size composition on the slope.

Seasonal variation in the diet of arrow-tooth flounder has been shown to consist of

changes in the percentage composition of some of the secondary prey (euphausiids,

shrimp, and cephalopods [Yang, 1991b]) but walleye pollock remained as the primary

prey. Our study found the same results for the northeastern stratum. Pacific herring are

found in higher abundance in the fourth quarter, probably reflecting their return to over;

wintering areas (Wespestad and Barton 1981). The seasonal variation in squid is also

likely a reflection of their seasonal migrations associated with spawning and growth

(Okutani 1988). In the northwest stratum, walleye pollock and pandalids in the diet were

inversely related.

Latitudinal variation in the diet of arrow-tooth flounder during the third quarter in the

shallow strata indicates that the primary difference in the diets is the large presence of

Pacific herring in the southeastern stratum while not being consumed in the northeastern

stratum. Other fish were more common in the diet in the northeastern stratum. It is

possible that Pacific herring were consumed in the northeastern strata, but they were not

identified as such (due to digestion) and were consequently recorded as unidentified fish.

It is also possible that Pacific herring were more common in the southeastern stratum

because they were more available as prey in that stratum. Pacific herring are generally

inshore for spawning during the spring and summer months, but begin migrating toward

101
their overwintering grounds in late summer via the Alaska Peninsula and Aleutian Basin

(Wespestad and Barton 1981, Wespestad 1994) and therefore become available only to

predators in the southeastern stratum during the third quarter..

Depth-related variation in the second quarter was slight; however, during the fourth

quarter, it was substantial. The variation seen in the diet of arrow-tooth flounder in the

fourth quarter by depth is inconsistent with dietary variation seen for the same prey,

quarter, and stratum for Greenland turbot. It is most likely that this variation is an artifact

of the sampling scheme. There were relatively few. samples from the deep stratum, which

likely came from only a few hauls, representing a smaller spatial scale. This leads to a

higher likelihood of patchy prey distributions being reflected in the diet.

The spatial distribution in the consumption of walleye pollock and Pacific herring by

arrowtooth flounder reflects their distribution throughout the year in the slope region of

the eastern Bering Sea. Walleye pollock are found throughout the region and were

consumed by arrowtooth flounder throughout the region as well. Pacific herring are

seasonally variable in their availability and are most common over the slope in the northern

section (Wespestad and Barton 1981) which is where they were most commonly

consumed by arrowtooth flounder. Gonatid squids, those most commonly identified from

arrow-tooth flounder stomachs, are thought to be distributed throughout the deeper waters

of the eastern Bering Sea (Okutani et al. 1988). However, arrow-tooth flounder preyed

more heavily upon squid in the northern strata. The cause of this relationship is not clear

due to our poor understanding of the seasonal variation in abundance of squid along the

north-south gradient on the eastern Bering Sea.

The significance of the relationship of predator length with prey length for arrow-tooth

flounder and walleye pollock is consistent with previous work (Yang and Livingston 1986,

Yang 1991 b, Yang 1993) in Alaska waters, Our study, however, predicts a much steeper

regression line due to the lack of smaller predators, and the presence of relatively small

walleye pollock in predator stomachs, despite the larger predator size.

102
Dietary Comparisons

The overall diets of all five predator species are summarized in Figure 41. Fish,

especially walleye pollock, were the primary prey of Greenland turbot, arrowtooth

flounder, and Pacific cod. The diet of walleye pollock was the most general in terms of

number of significant prey types. The diet of flathead sole was restricted to a few prey

types: brittle stars, fish, and offal. Dietary similarity was greatest between the piscivorous

predators Greenland turbot, arrowtooth flounder, and Pacific cod (Table 8). Walleye

pollock and flathead sole had relatively low dietary similarity with the other predators

studied.

The high dietary overlap values calculated for Greenland turbot, arrowtooth flounder,

and Pacific cod are primarily the result of the significance of walleye pollock in their diets.

The low dietary overlap of flathead sole with all other predators was the result of their

dependence upon brittle stars as their primary prey, while the other species rarely

consumed brittle stars. Walleye pollock did-not have high dietary overlap with any other

predator despite the presence of common prey due to the relatively equal importance of

several prey in their diet. Mito (1974) presented results of dietary similarity during the fall

over the slope region which also indicated the prevalence of walleye pollock in the diet

increased the extent of dietary similarity,

103
Figure 41 .--Diets of walleye pollock, Pacific cod, arrowtooth flounder,

Greenland halibut, and flathead sole in the eastern Bering Sea

slope region.

104
Table 8.--Schoener’s (1970) index of dietary overlap of Pacific cod, walleye pollock,

arrow-tooth flounder, flathead sole, and Greenland turbot in the eastern Bering

Sea slope region. Low overlap = O-33, moderate overlap = 34-66,

high overlap = 67-100.

105
SUMMARY

As seen in previous studies of groundfish diets on the eastern Bering Sea shelf, walleye

pollock were generally the primary prey of most piscivorous fish in the eastern Bering Sea

slope region. In the slope region, however, myctophids, bathylagids, and rockfish were

also found to be important. The importance of rockfish, found primarily in the diet of

flathead sole, is likely overestimated by the percentage by weight contribution in the diet.

Nonetheless, rockfish were found in 8% of the flathead sole stomachs that contained food

and could still be considered an important prey. The prevalence of deep-sea fish

(myctophids and bathylagids) in the diet of walleye pollock is indicative of a different

species composition in the pelagic food web than is seen over the shelf region. Walleye

pollock over the slope are generally larger, and therefore more piscivorous. Juvenile

walleye pollock are less abundant, leading to a greater dependence upon other fish prey.

Myctophids, bathylagids, and squid are generally not available over the shelf region, and

therefore are not a significant part of the shelf food web. The predators studied in the

slope region of the eastern Bering Sea were larger on average, which resulted in higher

piscivory in the diets.

Walleye pollock, myctophids, bathylagids, and squid are not only important prey of fish

in the eastern Bering Sea, but they also are important prey to marine mammals and birds.

Lowry et al. (1988) report that walleye pollock are consumed by 15 of the 31 species of

marine mammals that occur in the eastern Bering Sea. ‘Sinclair (1988) also presented

information detailing the importance of bathylagids and gonatid squids in the diet of

northern fur seals (Callorhinus ursinus) in the eastern Bering Sea. Hunt et al. (198 1)

reported myctophids as an important prey of red-legged kittiwakes (Rissa brevirostris) in

the eastern Bering Sea. Population level estimates of the impact of predation by key fish in

the region were not calculated in the current study due to the relatively small portion of

-

106
the groundfish populations that are found over the slope, Nevertheless the complexity of

the eastern Bering Sea slope food web warrants further investigation,

Dietary differences betweeen predators on the slope and shelf region of the eastern

Bering Sea increased with distance from the shelf, Diets of fish from the outer strata were

generally less similar to those in the shallow strata, depicting a transition from the shelf

environment to the bathypelagic environment. A lack of adequate sample sizes in our

study precluded a thorough analysis of the seasonal variation of the diets of most

predators. The paucity of detailed life history information for many of the invertebrate

prey seen in the diets of these fish limited the interpretation of some dietary variation.

Further food habits studies in this area should focus on more detailed dietary analyses

achieved through larger sample sizes with greater spatial resolution, food web analyses,

and possible dietary variation associated with canyons.

107
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111
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KINOSHITA, R. K., and J. M. TERRY. 1996. Oregon, Washington, and Alaska exports of edible fishery

products, 1995,48 p. NTIS No. PB96-214663.

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between 82o - 115o E: Cruise report from World Ocean Circulation Experiment (WOCE) 18s and l9S,

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YANG, M-S. 1996. Diets of the important groundfishes in the Aleutian Islands in summer 1991, 105 p.

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SMALL, R. J., and D. P. DEMASTER. 1995. Alaska marine mammal stock assessments 1995-57, 93 p,

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