*NOAA Technical Memorandum NMFS-AFSC-90

Food of Flathead Sole

Hippoglossoides elassodon

in the Eastern Bering Sea

by

R. E. Pacunski, P. A. Livingston, and B. S. Miller

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Alaska Fisheries Science Center

June 1998
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:

Pacunski, R. E., P. A. Livingston, and B. S. Miller. 1998. Food of the

flathead sole Hippoalossoides elassodon in the eastern Bering Sea.

U.S. Dep. Commer., NOAA Tech. Memo. NMFS-AFSC-90,27 p.

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

the National Marine Fisheries Service, NOAA.
NOAA Technical Memorandum NMFS-AFSC-90

Food of Flathead Sole

Hippoglossoides elassodon

in the Eastern Bering Sea

by

R. E. Pacunski1 P. A. Livingston: and B. S. Miller’

1

Fisheries Research Institute, University of Washington, WH-10

Seattle, WA 98195

2Alaska Fisheries Science Center

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

Seattle, WA 98115-0070

U.S. DEPARTMENT OF COMMERCE

William M. Daley, Secretary

National Oceanic and Atmospheric Administration

D. James Baker, Under Secretary and Administrator

National Marine Fisheries Service

Rolland A. Schmitten, Assistant Administrator for Fisheries

June 1998
This document is available to the public through:

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Notice to Users of this Document 

This document is being made available in .PDF format for the convenience of users; however, the 

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iii

ABSTRACT

The food habits of the flathead sole (Hippoglossoides elassodon) in the eastern Bering

Sea were investigated. Our analysis of 4,406 flathead sole stomachs containing food showed

that the diet was composed primarily of organisms living on the bottom (epibenthic) and

pelagic organisms in close association with the bottom (nektobenthic). Feeding shifted from a

crustacean-based diet to an ophiuroid-based diet with increasing fish size. Flathead sole less

than 30 cm total length (TL) consumed mainly mysids, gammarid amphipods, and decapod

shrimps, whereas flathead sole larger than 30 cm total length (TL) consumed mainly

ophiuroids, walleye pollock, and decapod shrimps. Diet composition varied with changes in

bottom depth. Feeding in shallow waters (<50 m) was almost exclusively on small

crustaceans and fish, but shifted to ophiuroids with increasing bottom depth for both size

groups. The diet of flathead sole is indicative of a generalist feeding strategy. Dietary

diversity ranged from moderately low to high as measured by Shannon-Weaver and Simpson’s

diversity indices, and the diet appears to be influenced mainly by prey availability and

abundance. Lower diversity values for flathead sole larger than 30 cm TL suggest that feeding

becomes more selective as fish grow. Competition with other flatfish species in the eastern

Bering Sea does not appear to be a major factor influencing the diet of flathead sole.
V

CONTENTS

Page

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

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..l

METHODS AND MATERIALS ........................................... 2

Description of the Diet ........................................ 3

RESULTS .......................................................... 6

Feeding and Bottom Depth ...................................... 7

Die1 Feeding ............................................... 8

DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..9

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

CITATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...17
INTRODUCTION

Flathead sole (Hippoglossoides elassodon) is a common pleuronectid fish occurring in

the northeast Pacific Ocean, Gulf of Alaska, and the Bering Sea (Hart 1973). The species

occurs throughout the eastern Bering Sea although abundance is greatest between bottom

depths of 100-250 m (Allen and Smith 1988). Between 1979 and 1996, the estimated biomass

of flathead sole2 in the eastern Bering Sea increased almost sixfold, from 104,900 t to

616,400 t (Walters and Wilderbuer 1996). In 1996, flathead sole comprised nearly 10% of the

total estimated flatfish biomass in the eastern Bering Sea. The large increase in population

size over the last decade suggests that flathead sole is a key and increasingly important

component of the benthic ecosystem of the region.

Throughout the past decade, the trend in fisheries management has been from a singlespecies management regime toward a more ecosystem-based approach. Livingston (1985)

demonstrated the importance of quantitative data from food habits studies for developing more

realistic multispecies population assessment and ecosystem models. Since the position of

flathead sole in the trophic structure of the eastern Bering Sea is poorly understood, defining

the trophic relationships between flathead sole and other fish and invertebrate species in the

eastern Bering Sea is an important step in understanding the dynamics of the regional

ecosystem. In addition, flathead sole are known consumers of several commercially

important species in the Bering Sea, including walleye pollock (Theragra chalcogramma),

snow crab (Chionoecetes opilio), and Tanner crab (Chionoecetes bairdi) (Livingston et al.

1986). Thus, results from this study may have direct implications for the management of

some fisheries in the eastern Bering Sea.

Skalkin (1963) and Mineva (1964) examined the food habits of flathead sole in the

Bering Sea, however these studies were qualitative and based on small sample sizes.

Quantitative studies by Mito (1974), Livingston et al. (1986), and Lang and Livingston (1996)

provided a more complete description of the diet, but they were restricted to either a single

season, year, or geographic location. The objective of the current study is to provide a

comprehensive, quantitative description of the food habits of flathead sole in the eastern Bering

Sea based on data collected throughout the region across a 5-year time span. Spatial and die1

variations in feeding, predator size versus prey utilization, dietary diversity, the relationship

of diet to morphology, competition with other flatfish species, and the impact of flathead sole

predation on juvenile walleye pollock were all examined. Data from this study may be

incorporated into dynamic models of the Bering Sea ecosystem which may ultimately provide

better management of the region’s fisheries.

2 Biomass estimates from National Marine Fisheries Service trawl surveys combine flathead sole with the

morphologically similar Bering flounder (Hippoghsoides robustus). Thus, for the purposes of this study, H. robustus

is not considered as a separate species.
2

METHODS AND MATERIALS

Stomachs were collected from 6,167 flathead sole in the eastern Bering Sea from 1984

through 1988 (Table 1, Fig.1). Sampling was conducted by National Marine Fisheries Service

(NMFS) personnel during annual Alaska Fisheries Science Center research surveys and by

foreign fisheries observers trained by the NMFS Foreign Fishery Observer Program (FFOP)

working aboard foreign commercial trawlers. Samples from NMFS survey cruises were

collected during daylight hours only with the exception of one cruise (see Table 1). Samples

from FFOP cruises were collected throughout the 24-hour daily cycle, and all fish were

collected by bottom trawl gear.

Fish showing signs of regurgitation (digested food items in the mouth or gill cavity or a

flaccid or water-filled stomach) or net-feeding (freshly consumed prey items in the mouth or

throat) were excluded from sampling. Stomachs from sampled fish were excised and placed

into cloth bags with a specimen label containing sex, total length (TL), spawning condition

(spawning or non-spawning) and station information. Stomachs were preserved in a 10%

formalin solution buffered in seawater and transported to the Alaska Fisheries Science Center

(AFSC) Food Habits Lab in Seattle, Washington, for analysis. Individual fish weights were

not recorded at sea but were calculated using the formula:

W(g)=O.O040 x L(cm)3.

25g

derived from the eastern Bering Sea flathead sole database of the AFSC’s Resource

Assessment and Conservation Engineering (RACE) Division.

Stomach contents were sorted, identified to the lowest practical taxonomic level, and

counted. Prey digestion state and relative stomach fullness were recorded. The wet weight of

each prey taxon was recorded to the nearest 0.001 gm after removing excess moisture by

blotting on paper towels.

Description of the Diet

The detailed stomach content data were grouped into eight prey categories (i.e.,

euphausiids, decapods, mysids, miscellaneous crustaceans, ophiuroids, walleye pollock, other

fish, and miscellaneous prey) to elucidate general diet trends by predator size and bottom

depth. Preliminary analysis of the diet showed differences in prey utilization with ontogeny

(see Pacunski 1990). In the present study, size-related feeding differences were examined by

dividing samples into two size classes, (i.e., < 30 cm TL and 230 cm TL), referred to as

small and large flathead sole, respectively. Overall analysis of the diet was based on 4,406

stomachs containing food. Due to interannual differences in the geographic locations of
3

sampling from October to April, analysis of the diet by depth strata and die1 feeding were

conducted using only data collected from May to September (n = 3,862, excluding empty

stomachs). These months are reported as the main feeding period of flathead sole in the

eastern Bering Sea by Moiseev (1953), and correspond with peak sampling by AFSC and

FFOP observers.

The study area was divided into four strata for analysis of the diet by bottom depth:

O-50 m, 51-100 m, 101-200 m, and greater than 200 m, and are referred to as the inner,

middle, and outer continental shelves, and the continental slope, respectively (Fig. 1). These

areas have been widely utilized in studies of the production processes of the eastern Bering Sea

and correspond to approximate locations of major hydrographic fronts (Coachman 1986).

Major prey groups are described in terms of percent total stomach content weight

(% W) and frequency of occurrence (%FO) and were computed for each sample group using

the program ECO-INDEX (Vodopich and Hoover 1981). Both measures are used since it was

felt that using only percent weight tends to underestimate the importance of small, rapidly

digested organisms (e.g., mysids, euphausiids) or overestimate the importance of large, slowly

digested prey (e.g., fish).

Die1 feeding was examined by separating sample data from both size groups into eight

3-hour time periods. A one-way analysis of covariance (ANCOVA) was performed on each

size group to evaluate discontinuous feeding trends (Jenkins and Green 1977). Empty

stomachs were excluded from this analysis for two reasons: 1) we were trying to characterize

the behavior of fish that were actively feeding, and 2) to exclude any samples in which

undetected regurgitation had occurred, which would bias the results.

Dietary diversity was measured based on the weight of major prey groups in the diet

using the indices of Shannon-Weaver (1963) and Simpson (1949):

Shannon-Weaver

where Xi = the weight prey category i, and X = the total weight of all prey categories. The

Shannon-Weaver index measures species richness and species evenness and is most sensitive to

changes in rare categories. The maximum value of H’ = log 8 (where 8 = the number of

prey categories) and is attained when all categories are evenly represented in the diet (Peet

1974). Simpson’s index measures dominance and compliments the Shannon-Weaver index by

being sensitive to changes in the most common prey categories (Peet 1974). Values of s

approaching 1 indicate dominance of the diet by a single prey group, while values approaching

8 (the number of prey categories) indicate homogeneity of the diet. The calculated
4

Shannon-Weaver values were subjected to a t-test (Zar 1984) to compare dietary diversity

between size classes within depth strata as possible evidence of resource partitioning.

RESULTS

In total, 6,167 flathead sole stomachs were examined, of which 29% (n = 1,761) were

empty. Prey items identified from the remaining 4,406 stomachs containing food represented

177 taxa from 11 phyla (Appendix). In terms of the major prey groups consumed, ophiuroids

(brittlestars) were the principle component of the diet, comprising 47% of the diet by

frequency of occurrence and 42 % by weight (Table 2). Large crustaceans, mainly shrimp

(families Crangonidae and Pandalidae) and hermit crabs (family Paguridae), were the second

most common prey item (33 % FO) and accounted for 13 % of the diet by weight. Mysids and

miscellaneous crustaceans (mainly gammarid amphipods) were also common prey items (19 %

and 29% FO), but together comprised less than 3 % of the diet by weight. Juvenile walleye

pollock (ages 0 and 1) were uncommon (8 % FO) but comprised 20 % of the diet by weight.

Fish other than walleye pollock were slightly more common (11% FO) but less important in

terms of weight (11%). Polychaetes, bivalve clams, and fishery processing offal (fish remains

from commercial fishery operations) constituted the majority of the miscellaneous prey

category (33 % FO, 11% W). As a group, these foods appear to be significant in the overall

diet; however, polychaetes accounted for over one-half of the percent frequency of occurrence

of this group but made up less than 2% of the diet by weight. Conversely, bivalve clams and

fishery offal were responsible for 7% of the diet by weight but occurred in only 7% of the

stomachs examined.

Feeding and Bottom Depth

Diet composition varied between depth strata (Figs. 2 and 3) for the two size groups.

At bottom depths less than 50 m the diets of both size groups were composed almost

exclusively of crustaceans and fish. Mysids and decapods were the most common prey items,

whereas walleye pollock and decapods predominated by weight. Euphausiids were also

important to large flathead sole in shallow waters.

A distinct change in feeding occurred in the middle shelf area (bottom depths 51-

100 m) as marked by the appearance of ophiuroids in the diet (Figs. 2 and 3). No single prey

group dominated the diet of small fish at these depths, with miscellaneous crustaceans, mysids,

miscellaneous prey, decapods, and ophiuroids all being common prey items (25 % to 40 %

FO). Walleye pollock was the least common prey item (4 % FO), but it dominated the diet by

weight (29 % W), followed by decapods (17 %) and ophiuroids (14 %). Large flathead sole fed

mainly on ophiuroids (35 % FO, 21% W), decapods (32% FO, 11% W), and walleye pollock

(18% FO, 42% W).
5

In the outer shelf area (100-200 m), ophiuroids (54 % FO, 3 1% W) were the dominant

prey of small flathead sole, although decapods (37% FO, 29% W) and miscellaneous prey

(35% FO, 17% W) were also important. The diet of large flathead sole was dominated by

ophiuroids (73 % FO, 63 % W). Miscellaneous prey and decapods were the next most common

prey groups (32 % FO and 27 % FO) but together accounted for only 18 % of the diet by

weight. Walleye pollock were found in < 8% of stomachs of both size groups and made up

10% and 13 % of the diets of small and large flathead sole, respectively.

In the continental slope zone ( > 200 m), ophiuroids remained the dominant prey item

of small flathead sole (62% FO, 62% W). Decapods occurred in 25 % of the stomachs and

comprised 25 % of the diet by weight. Miscellaneous prey and miscellaneous crustaceans were

common prey items (28 % and 25 % FO) but made up < 5 % of the diet by weight. No walleye

pollock were found as prey of small flathead sole at these depths. Consumption of ophiuroids

by large flathead sole increased to 86% FO and 75 % W from the outer shelf area.

Miscellaneous prey (55 % FO, 12 % W) and decapods (20% FO, 9% W) were the next most

important prey, and walleye pollock ( < 1% FO, 2 % W) were virtually absent from the diet of

large fish at these depths.

Die1 Feeding

The mean percent body weight (%BWT) of stomach contents of small flathead sole

decreased from post-midnight (0000-0259 h) to late morning (0900-l 159 h), then increased

from morning to midnight, suggesting a possible diurnal feeding pattern (Fig. 4). No

discernable pattern was seen for large flathead sole; the mean %BWT of stomach contents

ranged between 0.8% and 0.95% except for the time periods 0300-0559 h and 1200-1459 h

when the mean %BWT was 0.4% and 0.7%, respectively. Results of the ANCOVA on each

size group showed no significant differences in stomach content weight between time periods

after adjusting for fish weight.

Diet Diversity

Shannon-Weaver diversity indices for the overall diet based on percent weight were

0.804 for small flathead sole and 0.700 for larger fish. T-tests on Shannon-Weaver diversity

indices showed significant differences (a = 0.05) between the diets of small and large flathead

sole in the inner, middle, and outer continental shelf zones, but not in the continental slope

zone (Table 3). At bottom depths greater than 50 m, the diet of small flathead sole was less

diverse than large flathead sole, which can be attributed to the dominance of walleye pollock

and decapods as prey. At bottom depths greater than 50 m, small flathead sole showed higher

dietary diversity than larger fish due to the more even distribution of prey groups in the diet.

Dietary diversity was highest for small flathead sole at bottom depths of 51-100 m, and for

large fish at bottom depths less than 100 m. Both size groups showed lowest dietary diversity

along the continental slope ( > 200 m), where ophiuroids were the dominant prey consumed.
6

DISCUSSION

Flathead sole in the eastern Bering Sea consumed mainly invertebrates living on the

bottom (epibenthic) and pelagic invertebrates in close association with the bottom

(nektobenthic) invertebrates, and some demersally occurring fishes. Ophiuroids, shrimp,

mysids, and gammarid amphipods were the most important invertebrate prey items identified

in the present study. The majority of fish prey were juvenile walleye pollock, although

rockfish (family Scorpaenidae), gunnels (family Stichaeidae), and a few flatfish (family

Pleuronectidae) were consumed. Clams, polychaetes , and fish processing offal supplemented

the diet but were not staple foods. Previous studies of flathead sole food habits in the eastern

Bering Sea have shown similar results. By frequency of occurrence, Mineva (1964) found

ophiuroids, shrimp, amphipods, fish remains (assumed to be fishery offal by the authors), and

bivalves (Yoldia spp.) to be the main foods of flathead sole in the Bering Sea. We did not find

high frequencies of bivalves in the diet, which may be due to the broader geographic

distribution of sampling of our study. In terms of weight, Mito (1974) reported Ophiura spp.

and Pandalus borealis to be the primary foods of flathead sole along the eastern Bering Sea

continental slope. Livingston et al. (1986) found that in summer, flathead sole less than 25 cm

consumed mainly crustaceans and walleye pollock (by weight), while the diet of larger fish

was composed mainly of brittlestars (ophiuroids) and walleye pollock.

Miller (1970) and Flora (1980) reported that flathead sole feed benthopelagically in

East Sound, Orcas Island, Washington, and Auke Bay, Alaska, respectively. Mysids were

predominate in the diet in both areas, although shrimp and fish were also important. In

contrast to our results, clams and polychaetes were important foods while ophiuroids were

almost completely absent from the diet in both areas. Differences in prey assemblages

between study areas probably account for these differences, although detailed information on

the faunal communities of East Sound and Auke Bay are unavailable to support this

hypothesis.

One size-dependent feeding pattern was apparent from our results; feeding on

crustaceans was greater by small flathead sole, but shifted to ophiuroids as fish size increased.

Mito (1974) observed the same trend in one group of flathead sole studied along the eastern

Bering Sea continental slope. Miller (1970) reported size-dependent feeding by flathead sole

in East Sound, Orcas Island, Washington; mysids were the main food of fish 40-179 mm TL,

but as predator size increased, shrimps, then fishes (mainly herring) and clams became the

most important foods, although mysids remained important to all size groups.

American plaice, Hippoglossoides platessoides, the ecological counterpart of flathead

sole in the Atlantic Ocean (Allen 1984), undergoes a size-related feeding transition similar to

that of flathead sole. Powles (1965) and Scott (1973) reported that the diet of American plaice

in the Magdalen Shallows and Gulf of St. Lawrence, respectively, shifted from crustaceans to

echinoderms with increasing fish size. Scott (1973) points out that this transition involves a
7

change from highly nutritious to relatively poor-quality food groups, and hypothesized that the

ease of capture and relative abundance of echinoderms compensates for their low nutritive

value and thus supports the change in food habits. Ophiura sp. and 0. sarsi, the dominant

ophiuroids consumed by flathead sole in this study, are widespread along the outer shelf and

continental slope regions of the Bering Sea, often occurring in enormous numbers (D’yakanov

1954). Consequently, Scott’s (1973) hypothesis for American plaice may be applicable to

flathead sole in the eastern Bering Sea.

The shift from crustaceans to ophiuroids in the eastern Bering Sea may also be a

function of energy cost. One or a few small crustaceans may be sufficient to sustain small

flathead sole; however, as fish grow, the energy expended to capture sufficient quantities of

these organisms may exceed the energy obtained from their consumption, resulting in a

transition to a more abundant and easily captured prey (i.e., ophiuroids).

Size-related changes in foraging behavior may result from the shift from highly mobile

nektobenthic prey (crustaceans) to slow-moving epibenthic prey (ophiuroids) . At smaller

sizes, flathead sole appear to utilize a pursuit-oriented strategy, whereas larger fish are

probably searcher-grazers. Our results support Allen’s (1984) characterization of flathead sole

as an active ambusher/searcher/pursuer; however, this study is the first to propose differences

in foraging behaviors between size groups.

Differences in diet composition between depth zones appear to be related to prey

distribution. Quantitative results from this study confirm observations made by Skalkin (1963)

that ophiuroids disappeared from the diet as flathead sole migrated to shallower waters.

Ophiuroids do not occur in large numbers within the inner shelf of the eastern Bering Sea, but

are dominant members of the faunal community at bottom depths greater than 50 m

(D’yakanov 1954, Zenkevitch 1963, Ivanov 1964, Haflinger 1981, Stoker 1981). This

distribution pattern, coupled with a decreasing availability of pelagic fish and crustaceans with

depth, probably accounts for the absence of ophiuroids in the diet of flathead sole collected

from the inner shelf region and the increasing utilization of these organisms by flathead sole in

deeper waters. Conversely, the majority of mysids consumed by flathead sole in this study are

described by Mauchline (1980) as inshore species, thus the predominance of these organisms

in the diet at depths less than 50 m can be attributed to their greater abundance in shallow

waters.

Euphausiids were generally of little importance as prey, which is likely due to their

mainly pelagic nature (Mauchline 1980). However, at bottom depths less than 50 m, these

organisms (mainly Thysanoessa raschii) made up a substantial portion of the diet of a few

large flathead sole (n= 13) near the Alaska Peninsula. Smith et al. (1978) documented a high

incidence of T. raschii in the diet of flathead sole collected from shallow waters near the

Alaska Peninsula in the Gulf of Alaska. Similarly, Skalkin (1963) reported that yellowfin sole

fed heavily on euphausiids in some areas of the eastern Bering Sea inner shelf, Due to the

small sample size in our study, we cannot conclude with any confidence whether euphausiids
8

are a major food of flathead sole in this area of the Bering Sea, or whether feeding on these

organisms was an opportunistic response to a locally abundant resource.

Juvenile walleye pollock were most commonly consumed in the inner and middle shelf

areas at bottom depths less than 100 m. Hydroacoustic surveys of juvenile walleye pollock in

the eastern Bering Sea conducted in 1982, 1984, and 1985 showed that densities of age-0

walleye pollock were highest over the inner and middle shelf areas (Walters et al. 1988), and

only small quantities of age-0 pollock were observed at depths greater than 100 m in 1979 and

1982 (Traynor 1986). Thus, the results of the present study should be expected assuming the

spatial distribution patterns of age-0 pollock remains consistent on an annual basis. In

contrast to our results, Mito (1974) reported that age-0 pollock made up 15 % (by weight) of

the diet of 15l-250 mm flathead sole collected on the continental slope. However, his sample

size was small, and, as seen from our results, a high weight percentage does not necessarily

imply a high frequency of occurrence, making comparisons with the present study difficult.

Walleye pollock support the largest commercial fishery in the eastern Bering Sea,

consequently, predation impacts on this species are an important concern of fishery managers.

Juvenile walleye pollock were found in over 30% of the flathead sole stomachs collected from

the inner and middle shelf regions. Since 35 % of the population of flathead sole occurs at

bottom depths less than 100 m (Allen and Smith 1988), the impact of flathead sole predation

on this species might be significant, and was therefore investigated (see Pacunski 1991).

Estimates of the number of juvenile pollock consumed annually ranged from 7.1 billion to

9.9 billion fish between 1984 and 1986. In contrast; estimates of adult walleye pollock

consumption of juvenile walleye pollock ranged from 118 billion to 624 billion for the same

time period (Livingston 1991), about 20 to 90 times the number eaten by flathead sole. From

these results, it was concluded that flathead sole predation does not significantly impact

populations of juvenile walleye pollock in the eastern Bering Sea.

The use of diversity indices provided a quantitative measure of changes in flathead sole

diet between depth strata. Diversity values were highest for small flathead sole between 51 m

and 200 m bottom depth, corresponding to a more heterogeneous distribution of prey in the

diet at these depths. Lower values in the shallowest and deepest zones are directly related to

the two main prey groups in the diet; walleye pollock and decapods in the shallowest zone,

and ophiuroids and decapods in the deepest zone (Figs. 2a-3b). Large flathead sole did not

show the same pattern as small fish. Increasing bottom depth was accompanied by a decline in

diversity values, which can be attributed to the increasing proportion of ophiuroids in the diet.

Results of t-tests on calculated Shannon-Weaver indices indicated that, in terms of weight, the

diet diversity values of small and large flathead sole were significantly different in all depth

strata except the continental slope strata.

While most fish are specialized in their feeding to some extent, their actual diet is

influenced by the availability of both common and uncommon prey items (Ivlev 1961).

Feeding in generalist fishes is mainly a response to available food resources, whereas
9

specialists feed primarily on a specific prey type (or types) regardless of abundance. Flathead

sole in this study consumed a wide variety of prey ranging from clams and polychaetes to

crustaceans, ophiuroids, and fish. As seen in the depth strata analysis, the diet of flathead sole

corresponds well with the distribution of the major prey types consumed in this study. Our

results generally support Allen’s (1984) conclusion that flathead sole are generalist feeders,

although results of diversity analyses indicate that large flathead sole are more specialized

feeders than small flathead sole, especially between bottom depths of 50 to 200 m.

Ivlev (1961) suggests that competition with other fishes may play a significant role in

influencing a fish’s diet. Yellowfin sole Pleuronectes asper, rock sole P. bilineata, and Alaska

plaice P. quadrituberculatus are the most abundant pleuronectids in the eastern Bering Sea.

These species are similar in size to flathead sole and occur sympatrically in portions of their

range. However, the diets of yellowfin sole, rock sole, and Alaska plaice are dominated by

polychaetes and clams (Livingston et al. 1986, Lang 1992), prey rarely utilized by flathead

sole. These three species have much greater spatial overlap with each other in the inner and

middle shelf areas of the eastern Bering Sea than they do with flathead sole, whose distribution

is greater over the middle and outer shelf areas.

Previous studies of pleuronectids have shown that alimentary tract morphology and

food habits are highly interrelated. Moiseev (1953), DeGroot (1971), and Allen (1984)

concluded that piscivorous flatfish generally have large, symmetrically opening mouths, large

stomachs, and very short intestines, with benthic feeders being characterized by a small,

downwardly protractile mouth, a medium-sized stomach, and an intestinal loop of moderate

length and complication. Flathead sole fall somewhere in the middle of these two

morphological types, possessing a large, almost symmetrically opening mouth, relatively large

stomach and medium length intestine, and would be predicted to have a diet somewhere

between piscivores and benthic feeders. With a diet comprised mainly of epibenthic

invertebrates but including some fish, the food habits of flathead sole are consistent with the

species’ morphology. Yellowfin sole, rock sole and Alaska plaice conform to the morphology

of benthic feeders (Allen 1984), thus the predominance of buried, benthic prey in their diets is

predicted by DeGroot (1971).

Based on differences in prey utilization and morphological differences in feeding

structures, we conclude that competition with sympatric flatfishes in the eastern Bering Sea is

minimal and has little influence on the flathead sole diet. Food competition might occur more

between flathead sole and some of the lesser-studied groundfish of the middle and outer shelf

areas of the eastern Bering Sea such as zoarcids and cottids. More detailed dietary analysis of

these species is warranted to better understand their role in benthic energy transfer in the

eastern Bering Sea.
10

ACKNOWLEDGMENTS

The authors would like to acknowledge the funding support provided by NOAA

(contract 50ABNF-200078) to the University of Washington for this study. We also

acknowledge the efforts of the many NMFS and FFOP observers who collected and processed

the samples. Finally, we extend our grateful appreciation to Geoff Lang, Tom Wilderbuer,

Gary Duker and James Lee who provided many helpful comments on the manuscript.
11

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15

Table l.-- List of cruises in the eastern Bering Sea where flathead sole stomachs were collected, 1984 -88.

Vessel Cruise* Collection

Type Period

Total

Stomachs

* AFSC = research surveys conducted by the Alaska Fisheries

Science Center

FFOP = foreign commercial fishing monitored by the

Foreign Fisheries Observer Program

**24-hour sampling period
16

Table 2 .--Percent by weight (% W) and frequency of occurrence (% FO) of major

taxonomic groups consumed by flathead sole Hippoglossoides elassodon in the

eastern Bering Sea from 1984 to 1988.

Prey Category %W %FO
17

Table 3.--Diversity indices for two size groups (based on total length) of Hippoglossoides

elassodon, by bottom depth, based on percent weight of prey in diet.

Shannon-Weaver Simpson

Bottom

Depth

<30 cm 230 cm < 30 cm 230 cm

<50m 0540 0665 ** 2.951 4.319

51-100 m 0:826 0:665 *** 5.804 3.725

101-200 m 0.736 0.506 *** 4.473 2.220

>200 m 0.446 0.379 n.s. 2.106 1.720

** P< 0.01, *** P< 0.001, n.s. = not significant
18
Figure 2.-- A. Percent frequency of occurrence (%FO) and B. Diet composition by percent

weight (% W) for < 30cm flathead sole, by bottom depth. (Mys =mysids,

Euph = euphausiids, Dee = decapods, Oph = ophiuroids, WP = walleye pollock,

OF = all other fish prey, MC = miscellaneous crustaceans, MP = all other prey

including unidentified material).
Figure 3 .-- A. Percent frequency of occurrence (%FO) and B. Diet composition by percent

weight (%W) for >30cm flathead sole, by bottom depth. (Mys=mysids,

Euph = euphausiids, Dee = decapods, Oph = ophiuroids, WP = walleye pollock,

OF = all other fish prey, MC = miscellaneous crustaceans, MP = all other prey

including unidentified material).
Figure 4.--Die1 changes in mean stomach content weight (%BW + 1 SE) in the stomachs of two

size classes of flathead sole (numbers above bar are sample size).
22

Appendix.-- Prey items identified during analysis of 6,167 flathead sole stomachs (9%

of stomachs were empty) collected from the eastern Bering Sea from June 1984 to

September 1988.

%FO = percent frequency of occurrence

%N = percent of total prey numbers

%W = percent of total prey weight
23

Appendix -- (cont.).

Prey Taxa %FO %N %W
24

Appendix -- (cont.).

Prey Taxa %FO %N %W
25

Appendix -- (cont.).

Prey Taxa %FO %N %W
26

Appendix -- (cont.).

Prey Taxa %FO %N %W
27

Appendix -- (cont.).

Prey Taxa %FO %N %W
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