NOAA Technical Report NMFS 71

Greenland Turbot

Reinhardtius hippoglossoides

of the Eastern Bering Sea

and Aleutian Islands Region

Miles S. Alton

Richard G. Bakkala

Gary E. Walters

Peter T. Munro

December 1988

U.S. Department of Commerce
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NOAA Technical Report NMFS 71

Greenland Turbot

Reinhardtius hippoglossoides

of the Eastern Bering Sea

and Aleutian Islands Region

Miles S. Alton

Richard G. Bakkala

Gary E. Walters

Peter T. Munro

December 1988

u.s. DEPARTMENT OF COMMERCE

C. William Verity, Jr., Secretary

National Oceanic and Atmospheric Administration

William E. Evans, Under Secretary for Oceans and Atmosphere

National Marine Fisheries Service

James Brennan, Assistant Administrator for Fisheries
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this NMFS publication.

ii
CONTENTS

Purpose 1

Background 1

Distribution of species 1

General life history features 2

North Atlantic and North Pacific Greenland turbot fisheries 4

Management of eastern Bering Sea-Aleutian stock 4

Fisheries and research surveys 5

Data sources and analysis 5

Fisheries 5

Research survey data 7

Results 8

Analysis of Japanese fisheries information 8

Research vessel survey findings 20

Discussion 27

Distribution and migrations 27

Recruitment hypothesis 27

Current condition of eastern Bering Sea-Aleutians stock 28

Causes of recruitment failure 28

Acknowledgments 29

Citations 30

iii

Greenland Turbot

Reinhardtius hippoglossoides

of the Eastern Bering Sea

and Aleutian Islands Region

MILES S. ALlON

RICHARD G. BAKKALA

GARY E. WALTERS

PETER T. MUNRO

Northwest and Alaska Fisheries Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.E.

Bin C15700

Seattle, WA 98115

ABSTRACT

Greenland turbot (Reinhardtius hippoglossoides) is a commercially important

flounder in both the North Atlantic and North Pacific Oceans. In the latter, its

center of abundance is in the eastern Bering Sea and along the Aleutian Islands

chain where its population is managed as a single stock. Harvest levels in this

region ofthe North Pacific during the period 1970-81 were comparable with those

in the northwest and northeast Atlantic, with annual average catches of 53,000

metric tons (t). However, the catch in 1984 dropped sharply to 23,100 t, in part

because of reduced quotas arising from concern over continued poor recruitment

and declining catch-per-unit-effort.

Recruitment failure was manifested in 1) the sharp decline in the catch rate

of young flsh in annual research trawl surveys on the continental shelf of the

eastern Bering Sea and 2) an increasing proportion of older and larger fish in

the commercial catch from the continental slope of both the eastern Bering Sea

and Aleutian Islands. The cause ofthe decline in recruitment could not be clearly identifled.

Greenland turbot of the Bering Sea-Aleutian Islands share certain distributional features with the North Atlantic form. There is an apparent bathymetric

change in the size and age of fish, with younger animals occupying continental

shelf depths and the older individuals residing at depths ofthe continental slope.

At shallow depths the young are exposed to temperature fluctuations, whereas

older animals along the slope are exposed to relatively stable temperatures.

A hypothesis is proposed for describing the temporal and spatial paths by

which young animals reach the mature or spawning portion of the population.

Purpose _

This report provides a comprehensive treatment of our knowledge

of the Greenland turbot,l Reinhardtius hippoglossoides, stock of

the eastern Bering Sea and Aleutian Islands region (hereafter referred

to as the Aleutians). It covers the history of harvest and management ofthis stock and describes the characteristics of the fisheries,

such as nations and vessel types involved and the temporal and

spatial patterns of harvest and of catch-per-unit-effort. The report

also traces changes that have occurred in the abundance and composition of the stock as inferred from both fisheries and research

vessel survey data. A hypothesis is proposed for the eastern Bering Sea-Aleutian stock which describes the possible temporal and

spatial pathways by which young fish recruit to the adult population.

Background _

Distribution of species

Greenland turbot has an amphiboreal distribution, occurring in the

North Atlantic and North Pacific, but not in the intervening Arctic

Ocean (Fig. 1). Hubbs and Wilimovsky (1964) have suggested that

their range was probably continuous through the Arctic Ocean during the last interglacial period and possibly even during the

postglacial hypsithermal period when the Arctic Ocean waters were

warm. They found no significant morphological differences between

representative specimens from the two oceans. Fairbairn (1981),

however, using biochemical genetic techniques on muscle, heart,

and liver tissues, found genetic divergence approaching the subspecies level between samples from the northwest Atlantic Ocean

and the Bering Sea.

In the North Pacific, species abundance is centered in the eastern

Bering Sea and, secondly, in the Aleutians. On the Asian side, these

fish occur in the Gulf of Anadyr (about lat. 65°N) along the Bering Sea coast of the U.S.S.R., in the Okhotsk Sea, around the Kurile

Islands, and along the east coast ofJapan to northern Honshu Island

(about lat. 38°N) (Hubbs and Wilimovsky 1964, Mikawa 1963,

Shuntov 1965). Isolated occurrences have been recorded for the

Sea of Japan (Mikawa 1963).

Isolated occurrences have also been recorded along the North

American continent outside the distribution described above. In the

far north, Greenland turbot has been taken irregularly in Norton

Sound ofthe Bering Sea and in the southwest Chuckchi Sea (about

lat. 66°N). Species distribution is light and intermittent in the Gulf

of Alaska and southward to about lat. 45 ON. Isolated captures have

been recorded offBritish Columbia (Westrheim and Pletcher 1966),

Oregon (Niska and Magill 1967), northern and central California,

and as far south as northern Baja California in Mexico (about lat.

32°N) (Hubbs and Wilimovsky 1964).

Greenland turbot are distributed across the north Atlantic Ocean

(Fig. 1) and are most prevalent from Baffin Island and west

Greenland south to the northern Grand Bank (Bowering 1983), off

Iceland (Sigurdsson 1981), around Jan Mayen Island (Torheim

1979), and from Spitsbergen Island to the north coast of Norway

(Hognestad 1969). They are found as far north as Smith Sound

(about lat. 78°N) and as far south as Georges Bank (about lat. 40 0 N)

in the northwest Atlantic (Templeman 1973). In the northeast AtlanlThe common market name is used in this report rather than the scientifically accepted

common name, Greenland halibut.
Figure 1

Geographical distribution (shaded areas) of Greenland turbot, Reinhardtius hippoglossoides.

tic, they range from Spitsbergen Island in the north (about lat. 80 0N)

to southern Ireland (about 52°N) (Hubbs and Wilimovsky 1964)

and range eastward as far as the southwest coast of Novaya Zemlya

(Northland) in the Barents Sea (about long. 100 0

E) (Hognestad

1969).

General life history features

Most information on life history characteristics of the species has

come from studies in the Atlantic, and to a lesser extent from studies

in the Pacific Ocean. As will become evident, life history features

of turbot appear to be quite similar in the two oceans.

Spawning appears to occur in the eastern Bering Sea in winter

and may be protracted starting in September or October and continuing until March with an apparent peak period in November to

February (Shuntov 1970, Bulatov 1983). Spawning time in the

Atlantic varies by area: February-April off west Iceland (A. Sigurdsson, Mar. Res. Inst., P.O. Box 1390,121 Reykjavik, Iceland, pers.

cornmun., April 1986), November-January in the Barents Sea

(Fedorov 1968), and April-June off west Greenland (Jensen 1935)

and in the Norwegian Sea (Hognestad 1969). Spawning appears

to occur universally on the continental slope (Bulatov 1983;

Hognestad 1969; Sigurdsson 1979). Spawning Greenland turbot

have been caught at depths of 1,000 m both off Iceland (Sigurdsson 1979) and in Davis Strait in the northwest Atlantic (Smidt 1969).

Greenland turbot spawn relatively small numbers of eggs. In the

Bering sea, fecundity ranged from 23,900 to 149,300 eggs for

females 83.3 cm and smaller in length (D'yakov 1982). This range

2

is similar to that of turbot in the Atlantic from southern Labrador

and the southeastern Gulf of St. Lawrence (13,000-150,000 eggs;

Bowering 1980), southern Labrador and eastern Newfoundland

(l5,000-215,OOO eggs; Lear 1970), and the Barents Sea (6,000-

140,000 eggs; Sorokin 1967, cited by D'yakov 1982).

Eggs and early larval stages are bathypelagic in both the Pacific

(Musienko 1970) and Atlantic (Smidt 1969, Sigurdsson 1981) forms

of Greenland turbot. In the Atlantic, 10-18 rnm larvae have been

found in bathypelagic waters and were reported to gradually rise

to the pelagic zone as they grew and reached 16-24 rnm in length

(Atkinson et al. 1981). This rise to the pelagic zone appears to correspond to the absorption of the yolk sac which occurs at about

15-18 rnm (Pertseva-Ostroumova 1961, Smidt 1969) and the onset

of feeding. The period of larval development extends from April

or May (Bulatov 1983, Jensen 1935) to August or September (Jensen

1935). This prolonged larval stage results in an extensive period

of larval drift and broad dispersal from spawning areas (Haug and

Gulliksen 1982, Templeman 1973, Hubbs and Wilimovsky 1964).

In August or September, at about 70-80 mm in length, they settle

to the bottom by which time the left eye has migrated to the dorsal

side ofthe head (Jensen 1935, Pertseva-Ostroumova 1961, Andriyashev 1954, Hognestad 1969, Sigurdsson 1981, Sinidt 1969).

Unlike that of most flatfish, the migrating eye does not move completely to one side but stops at the top of the head which presumably

results in a greater field of vision (de Groot 1970, Atkinson et al.

1981). This morphological adaptation for vision, plus the equally

well-developed musculature of both sides of the body, helps to explain this species' tendency to feed off the sea bottom.
During their first few years as immature fish, Greenland turbot

inhabit relatively shallow water. In the eastern Bering Sea, they

inhabit continental shelf waters «200 m) until about age 4 or 5

years, and continental slope waters at older ages. In the Atlantic

Ocean, the young immatures tend to occupy deeper water than in

the eastern Bering Sea, mainly between 200 and 400 m (Atkinson

et al. 1981, Hognestad 1969, Templeman 1973), but even to depths

of 700 m (Bowering 1984). The immatures are quite tolerant of

cold temperatures, being frequently found in temperatures less than

O°C (Hognestad 1969, Shuntov 1965).

Adults occupy mainly continental slope waters from about 200

to 1,000 m or more. Off Newfoundland, turbot have been found

to depths of at least 1,400-1,600 m (Templeman 1973). Theyappear to undergo seasonal shifts in depth distribution, moving deeper

in winter and shallower in summer (Chumakov 1970, Shuntov

1970). Adults also frequently occupy water temperatures below O°C

(Chumakov 1970, Ernst 1974 cited by Haug and Gulliksen 1982,

Fedorov 1971, Sigurdsson 1981, Templeman 1973), but at times

are found at temperatures well over O°C (Fedorov 1971); in some

regions, such as the Labrador-eastern Newfoundland area (Bowering

1984) and the eastern Bering Sea, adults are most abundant in 3-5°C

water.

From spawning areas, the drift of eggs and larvae appears to be

mainly northward. In the Norwegian Sea, larvae are believed to

be carried from spawning areas between northern Norway and Bear

Island to off the western coast of Spitsbergen, which appears to

be a nursery area for young Greenland turbot (Haug and Gulliksen

1982). Immature and maturing fish must then migrate southward

to reach the spawning area (God¢ and Haug 1987). Tagging and

other studies have shown that spawning grounds exist off the west

coast of Iceland and that migrations take place between this area

and the feeding grounds off the northwest, north, and east coasts

of Iceland (Sigurdsson 1979). The drift of larvae is mainly westward

towards the east coast of Greenland, but to a minor extent to the

north coast of Iceland. Shallower areas offthe north and east coasts

of Iceland are frequented by immature fish. Tagging has also confirmed migration of Greenland turbot from Iceland to the coast of

Norway, thus indicating some mixing between the Iceland-Faroe

Islands and Norwegian-Barents Sea stocks (Sigurdsson 1981). A

northward drift of larvae takes place in the West Greenland Current of the northwest Atlantic, but some larvae are carried southward

by the Arctic Current (Bowering 1984). From this drift the young

become distributed along the west coast of Greenland and along

the coast of Baffin Island, Labrador, and eastern Newfoundland

(Atkinson et al. 1981, Bowering 1984). As will be discussed later,

the migratory cycle for eastern Bering Sea turbot may be most

similar to that of the Norwegian Sea stock, with a drift of larvae

from spawning grounds in the southern slope area of the eastern

Bering Sea northward to the northern shelf area, and with a gradual

shifting of immature and maturing fish to deeper and more southern

waters.

Greenland turbot may live more than 20 years and reach sizes

as great as 120 cm and 16-17 kg (Moiseev 1953, Vernidub and

Panin 1937, Smidt 1969). In the western Pacific, Mikawa (1963)

found the maximum life span of turbot to be 15 years for males

and 20 years for females. Off Greenland, maximum ages were found

to be 13 years for males and 19 years for females (Atkinson et al.

1981). Similar maximum ages (14 years for males and 18 years

for females) were derived for turbot off Baffin Island (Bowering

1983). Otoliths have been the structure most frequently used for

ageing, although some investigations have had success with vertebrae and scales.

3

Maturity is reached at 5-10 years in the eastern Bering Sea

(D'yakov 1982). In the northwest Atlantic, 50% maturity in females

was found to vary from 7.8 years (58 cm) in the Gulf of St.

Lawrence to 12 years (81 cm) in more northern areas (Bowering

1983); for males, 50% maturity was reached at 5 years (40 cm)

in more southern regions and 7.9 years (56 cm) in the north. Smidt

(1969) reported for the west Greenland area that the smallest mature

male was 55 cm (about age 7) and the smallest maturing female

66-70 cm (8-10 years), while most males matured at about 60 cm

and most females at 70-80 cm. In the Norwegian-Barents Sea, heavy

exploitation of Greenland turbot in the 1970's reduced abundance

ofthe stock to such an extent that growth rates increased and maturity was reached at smaller lengths and earlier ages (Kovtsova and

Nizovtsev 1985). In the later 1970's and 1980's, large numbers

of males matured at 41-55 cm (ages 5-9 years) and large numbers

offemales at 61-70 cm (ages 10-12 years), while in the 1960's and

early 1970's maturity was reached by males at ages 9-10 and by

females at ages 12-13 years.

Food consumed by Greenland turbot corresponds to the more

semipelagic life ofthe species and often consists ofplankton, nekton,

and nektoplankton, while true benthos are of negligible importance

(Smidt 1969, de Groot 1970, Livingston et al. 1986, Bowering and

Lilly 1985). Off Newfoundland and west Greenland, evidence indicates that turbot ascend 100m or more off the bottom to feed

on capelin, Mallotus villosus, and other species (Templeman 1973).

Feeding habits vary with water depth and fish length. In continental shelf waters, shrimp are often relatively important prey, along

with pelagic or semipelagic species of fish such as capelin in the

Atlantic and walleye pollock, Theragra chalcogramma, in the

Pacific. Squid and bathypelagic or demersal fish become more important in continental slope waters (Smidt 1969, Livingston et al.

1986, Bowering and Lilly 1985). In the eastern Bering Sea the importance of walleye pollock in the diet increased from 3% by total

weight of stomach contents in turbot less than 50 cm in length, to

57% in turbot greater than 70 cm, while squid showed a corresponding decrease (Livingston et al. 1986). In samples from offsouthern

Labrador and northeastern Newfoundland, small turbot «20 cm)

preyed mainly on small crustaceans and squids, intermediate size

fish (20-69 cm) on capelin, and large size fish (>70 cm) on a variety

of demersal fishes, particularly redfish (Sebastes spp.) and other

Greenland turbot (Bowering and Lilly 1985). Feeding appears to

be more intense in summer than winter (Shuntov 1970, Mikawa

1963).

Pelagic and bathypelagic fishes are the main prey of Greenland

turbot in the North Pacific, with walleye pollock often a major

species in the diet (Mikawa 1963, Shuntov 1970, Livingston et al.

1986). This may result from the availability of pollock in many

regions of the North Pacific. Mikawa (1963) found a variety of

species (mainly fish) predominating in the diet of Greenland turbot

along the Asian coast and suggested that these geographical differences were due to the distribution of prey animals. Squid are

also an important component, with euphausiids and shrimp of lesser

importance.

In the Atlantic Ocean, primary prey items are capelin (Mallotus

villosus), Arctic cod (Boreogadus saida), rockfishes (Sebastes spp.),

grenadiers (Macrouridae), shrimp, euphausiids, and squid (Andriyashev 1954, Chumakov 1970, de Groot 1970, Smidt 1969, Haug

and Gulliksen 1982, Bowering 1982, Bowering and Lilly 1985).

Turbot are also cannibalistic to some extent (Bowering and Lilly

1985).

Observed predators of Greenland turbot are northern fur seals

(Callorhinus ursinus) in the Pacific (Kajimura 1984) and Greenland
Table 1

Commercial catches (t) of Greenland turbot from the northeast and northwest Atlantic Ocean and from the eastern Bering Sea and Aleutian Islands region of the

Pacific Ocean. (Catch statistics from Int. Counc. Explor. Sea (ICES) 1972-83; Northwest Atl. Fish. Organ. 1983; Bakkala et aI. 1985).

Accumulative

Area 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 total

Northeast Atlantic

Barents Sea 4,011 5,399 8,548 5,667 5,251 6,495 2,479 2,273 1,591 945 602 1,230 44,491

Norwegian Sea 12,819 10,504 14,564 8,190 7,852 3,166 3,985 10,396 12,892 10,287 5,473 5,284 105,412

Spitzbergen-Bear Island 53,925 60,182 18,310 16,081 24,660 28,511 29,610 16,221 10,134 6,080 7,209 8,504 279,427

North Sea and

English Channel 5 30 49 34 17 21 12 33 21 216 7 445

Iceland 15,043 11,732 10,507 7,386 7,866 3,308 5,448 15,679 11,588 16,976 27,927 15,780 149,240

Faroes, NW Scotland,

N. Ireland, and

N. of Azores 11 498 403 334 586 333 661 731 566 1,342 580 6,045

East Greenland 841 10,422 8,061 12,719 28,089 19,627 273 241 2,166 6,231 2,148 2,893 93,711

Total 86,644 98,250 60,518 50,495 74,086 61,710 42,149 45,483 39,135 41,106 44,917 34,278 678,771

Northwest Atlantic

West Greenland 2,000 3,000 4,000 7,000 13,000 23,000 11,000 9,000 11,000 18,000 7,000 6,000 114,000

Baffin Island + 1,000 10,000 2,000 1,000 2,000 5,000 4,000 1,000 1,000 2,000 + 29,000

Labrador 11,000 11,000 13,000 15,000 16,000 12,000 9,000 11,000 7,000 6,000 2,000 5,000 118,000

Newfoundland 26,000 14,000 17,000 15,000 12,000 17,000 16,000 21,000 32,000 29,000 31,000 26,000 256,000

Gulf of SI. Lawrence and

Nova Scotian shelf 1,000 1,000 1,000 1,000 1,000 2,000 2,000 4,000 7,000 9,000 7,000 3,000 39,000

Total 40,000 30,000 45,000 40,000 43,000 56,000 43,000 49,000 58,000 63,000 49,000 40,000 556,000

Eastern Bering Sea & Aleutian Islands area

19,976 42,214 77,384 63,946 78,442 67,789 62,590 30,161 42,189 41,409 52,552 57,321 635,973

shark (Somniosus microcephalus), white whale (Delphinapterus

leucas), narwhal (Monodon monoceros), and Atlantic cod (Gadus

morhua) in the Atlantic (Smidt 1969). There are undoubtedly other

predators, particularly on early life history stages.

Smidt (1969) noted a decline in turbot abundance in west Greenland waters at the same time cod stocks were increasing. He

attributed this to predation on larval and early bottom stages of

Greenland turbot. Conversely, W.R. Bowering (1984; pers. commun., March 1986, Can. Dep. Fish. Oceans, Fish. Res. Branch,

P.O. Box 5667, St. John's, Newfoundland, AIC 5X1, Canada) has

observed exceptionally good year-classes of Greenland turbot in

the Labrador-eastern Newfoundland area coincident with a dramatic

decline in abundance of cod off west Greenland. Recently, a substantial decline in recruitment of Greenland turbot in the eastern Bering Sea has coincided with a period of high abundance of Pacific

cod (Gadus macrocephalus).

North Atlantic and North Pacific

Greenland turbot fisheries

The stimulus for development of intensive directed fisheries for

Greenland turbot in both the Atlantic and Pacific Oceans appears

to have been their substantial presence during the 1960's in bycatches

of fisheries targeting on redfishes and rockfishes (Sebastes spp.)

and other deepwater species. Target fisheries for Greenland turbot

existed as early as the 1850's off Newfoundland and later in the

fjords of west Greenland, but these were always small scale efforts

with catches usually less than 1,000 metric tons (t) (Templeman

1973). Following the development of target fishing, catches in the

northeast and northwest Atlantic and eastern Bering Sea-Aleutians

area (the only known target fishing in the North Pacific) have been

of a similar magnitude (Table 1). Cumulative catchesin the period

1970-81 were 679,000 t in the northeast Atlantic, 556,000 t in the

northwest Atlantic, and 636,000 t in the eastern Bering Sea4

Aleutians. Peak annual catches occurred in the northeast Atlantic

(98,000 t) and Bering Sea (78,000 t) in the early 1970's and in the

northwest Atlantic (63,000 t) in the late 1970's.

As the target fisheries developed, Greenland turbot became important commercially in regions of the Atlantic Ocean such as the

Norwegian Sea and Spitsbergen area, off Iceland, and in the Canadian northwest Atlantic (Haug and Gulliksen 1982, Bowering 1983,

Sigurdsson 1979, Templeman 1973). In the northeast Atlantic,

largest cumulative catches in 1970-81 came from the SpitsbergenBear Island grounds (279,400 t), off Iceland (149,200 t), and from

the Norwegian Sea (105,000 t). In the northwest Atlantic, largest

cumulative catches in this period came from Newfoundland (256,000

t), Labrador (118,000 t), and west Greenland (114,000 t).

Depths oftarget fisheries for Greenland turbot in both the Atlantic

and Pacific Ocean are usually greater than 300 m (Bakkala 1985,

Haug and Gulliksen 1982, Hognestad 1969, Templemen 1973).

Fishing gear used in the Atlantic has included trawls, longlines,

and gillnets. For many years catches were salted, but now fresh

and frozen fillets are produced. Trawls have been the primary fishing

gear used in the eastern Bering Sea, but longlines have also been

used occasionally; the fish are headed, gutted, and frozen.

Management of eastern Bering Sea-Aleutians stock

Meaningful regulation of the Greenland turbot fishery was not

established in the eastern Bering Sea until after implementation in

1977 ofthe Magnuson Fisheries Conservation and Management Act

(MFCMA) of 1976 which established the 200-mile fisheries conservation zone (FCZ) offthe United States. Some time-area restrictions and catch quotas were established prior to 1977 through

bilateral agreements and the International North Pacific Fisheries

Commission (INPFC), involving the United States and nations

harvesting groundfish in the eastern Bering Sea and Aleutians.

However, catch quotas which applied to broad groups of species,
such as all flatfish or all species other than walleye pollock, probably had little influence on any target fisheries for Greenland turbot.

Following implementation of the MFCMA and establishment of

the fishery conservation zone (FCZ), Greenland turbot ofthe eastern

Bering Sea and Aleutians were managed as part of a group ofspecies

for which an annual allowable catch or optimum yield (OY)2 was

set. Greenland turbot was included in a group designated as "other

flounders" for the years 1977-79. Other members of this group

were arrowtooth flounder (Atheresthes stomias), rock sole

(Lepidopsetta bilineata), flathead sole (Hippoglossoides elassodon),

and other miscellaneous flatfish species. The OY for this "other

flounder" group was 100,000 t in 1977 and 159,000 t for the years

1978 and 1979. Because of similarities in their life histories and

distribution, Greenland turbot and arrowtooth flounder were placed in a separate management unit called "turbot" beginning in 1980.

The OY ofthe "turbot" group was 90,000 t for the years 1980-83,

then reduced to 59,610 t in 1984, and further reduced to 42,000

t in 1985 because ofindications of population decline in Greenland

turbot. Since this decline was accompanied by increases in abundance of arrowtooth flounder, each species began to be managed

as a separate unit in 1986.

In establishing OY for Greenland turbot, the condition ofthe stock

has been determined from trends in catch-per-unit-effort (CPUE)

and young fish abundance and from recent estimates of adult abundance. Stock abundance is estimated to have been reduced from

about one million t in the 1960's to about 400-500 thousand t in

1984-85 (Bakkala et al. 1986).

Fisheries and

research surveys _

Data sources and analysis

Fisheries

Catch and effon-Fisheries from Japan and the U.S.S.R. were the

first to harvest substantial amounts of bottomfish from the eastern

Bering Sea and Aleutians. Japanese trawl fisheries operated in the

eastern Bering Sea as early as the 1930's, but these fisheries were

interrupted because of World War II and postwar restrictions. When

Japan's fisheries resumed operations in the eastern Bering Sea during

1954, they expanded rapidly in geographic and species coverage

as well as in level of harvest, diversifying into a number of distinct

fisheries. By 1960 the total catch ofbottomfish by Japanese fisheries

reached some 449,000 t, of which 36,800 t were identified as

"turbot" (Greenland turbot and arrowtooth flounder). This was

the first year that Japan had reported turbot as a separate category.

As a member ofthe INPFC, Japan began in 1964 to provide catchand-effort data for its fisheries operating in U.S. contiguous waters.

Such data are given in a manner prescribed by the INPFC (i.e.,

by species or higher taxa for statistical blocks of 10

longitude by

0.5 0latitude, by month, fishery, and vessel class). The catch of

turbot was not reported by species until 1977. However, Wakabayashi and Bakkala (1978) using knowledge of how Greenland turbot were classified in catch statistics from the various fisheries,

estimated the Japanese turbot catch by species for the years 1970-76.

'Optimum yield refers to the annual catch which will provide the greatest overall

benefit to the U.S., and which is prescribed on the basis ofsustainable yield as modified

by any relevant economic, social, or ecological factors. For Greenland turbot, OY

is set by the North Pacific Fishery Management Council.

5

U.S.S.R. fishing fleets entered the eastern Bering Sea in 1959

and, like the Japanese fisheries, subsequently grew in scope. Reports

of the Food and Agricultural Organization of the United Nations

began to include Soviet catch statistics in 1965. The Soviet catch

was given by a very broad geographical area that encompassed the

eastern Aleutian Islands, eastern Bering Sea, and the Gulfof Anadyr.

Beginning in 1972, the U.S.S.R. provided statistics on their fisheries

in U.S. offshore waters under terms of bilateral agreements. These

statistics were reported by species (including turbot) and by broad

INPFC statistical areas (Fig. 2). The 1970 and 1971 U.S.S.R.

catches of Greenland turbot were estimated by Wakabayashi and

Bakkala (1978) by prorating the Soviet statistics reported to FAO

based on Japanese catch data.

With implementation ofthe MFCMA, all nations were required

annually to report catch and effort of their fisheries operating in

the FCZ ofthe United States according to INPFC standards. Nations

whose catch of turbot was less than 15% of their total groimdfish

catch were not required to report that catch by species.

In 1977, the U. S. Foreign Fisheries Observer Program instituted

procedures for obtaining an independent estimate of the foreign

catch in the FCZ. It is called "best blend," since the estimating

procedure uses both observer estimates of catch and data reported

by the foreign fishing vessels themselves (Nelson et al. 1981).

Best blend catch of Greenland turbot is given by INPFC statistical

areas.

Fisheries statistics for Greenland turbot can thus be placed into

three reporting periods: The first, years 1960-69 (Table 2), gives

an incomplete picture of the actual removals and does not provide

catches by individual species consistently; the second, 1970-76

(Table 3), is one in which the catch estimate for Greenland turbot

is given but by statistical areas that differ among nations; and the

third, beginning in 1977 (Table 3), is one in which all nations follow

the same reporting format and the U.S. Observer Program provides

best blend catch estimates.

In this report, major emphasis is placed on the catch-and-effort

data from those fisheries (namely Japanese) which have targeted

Greenland turbot or have taken large amounts of Greenland turbot

as bycatch. Targeted fisheries include the landbased dragnet fishery,

the North Pacific trawl fishery (hereafter, called North Pacific small

trawlers), and vessels using longline gear. The nontargeted fishery

is comprised of mothership operations where catcher vessels land

their catches aboard the processing or mothership vessel.

The landbased dragnet fishery has only operated west of long.

170 0 W in the Bering Sea and Aleutians region because of licensing restrictions imposed in Japan. Vessels in this fishery cannot

transship their catch to Japan, but must offload their processed catch

in Japan themselves. Neither the North Pacific small trawlers nor

longline vessels have these restrictions.

The fisheries targeting on Greenland turbot operate mainly on

the continental slope, whereas the mothership fishery operates mainlyon the continental shelf targeting on pollock for the production

of surimi.

U.S. Foreign Fisheries Observer Program data have been used

almost exclusively to calculate CPUE of targeted fisheries and the

incidence rate in the mothership fishery. The exception is the use

of Japanese reported statistics for CPUE estimates of their landbased dragnet fishery.

CPUE was not calculated for U.S.S.R. vessels because of the

difficulty of identifying targeted effort and the corresponding catch

of Greenland turbot, and because ofthe limited time series ofsignificant annual catches of Greenland turbot (Table 3). Catches of

Greenland turbot by fisheries other than those of Japan and the
....:.

\

------..--- ---------

--- ./

",-.-/"-

65 OON

60 OON

55 OON

50 OON

-1~~--"--"---"'---"'---"--"--- 165 OOE '75 OOE 175 OOW 165...---.---"'-.:5 OOW 155 OOW OON

Figure 2

International North Pacific Fisheries Commission statistical areas used in the reporting of catches by the U.S.

Foreign Fisheries Observer Program.

Table 2

Estimates of turbot catch (1,000 t) from the eastern Bering Sea and Aleutians' by Japanese and U.S.S.R. fisheries, 1960-69.

Nalion Species 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969

Japan2 Greenland turbol and 36.8 57.3 58.2 31.6 33.7 8.0 10.8 21.2 20.0 19.2

Arrowlooth flounder combined

USSR3 Greenland turbot NA NA NA NA NA 1.2 1.5 6.1 17.8 15.0

Arrowlooth flounder NA NA NA NA NA 0.6 0.7 3.2 9.1 7.8

All All NA NA NA NA NA 9.8 13.0 30.5 46.9 42.0

IReported annual calches for the A1eulians ranged from 63 10 504 t.

2From Wakabayashi and Bakkala (1978).

3From FAO 1975, 1976.

NA = not available.

Table 3

Annual catches (1,000 t) of Greenland turbot in the eastern Bering Sea and Aleutian Islands region as reported by Bakkala et at. (1986) for 1970-76 and made

available by the U.S. Foreign Fisheries Observer Program for 1977-86. (tr = catch of SO t or less.)

Nation 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986

Eastern Bering Sea

Japan 14.7 30.2 49.8 43.4 58.8 52.6 51.6 33.6 54.7 42.4 46.9 50.5 44.0 42.2 21.1 14.6 6.8

USSR 5.0 10.3 14.7 11.9 10.8 12.2 8.9 2.7 4.9 0.5 Ir

Other 0.1 0.2 2.0 2.4 1.8 1.2 0.2 tr 0.9

Total 19.7 40.5 64.5 55.3 69.6 64.8 60.5 36.3 59.7 43.1 48.9 52.9 45.8 43.4 21.3 14.6 7.7

Aleutian Islands region

Japan 0.3 1.7 12.7 8.3 8.8 3.0 2.0 1.7 8.2 9.9 3.7 4.4 6.3 4.1 1.8 Ir 0.1

USSR 0.2 0.3 Ir 0.1 Ir Ir Ir

Others Ir Ir If Ir If Ir If tr 2.0

Total 0.3 1.7 12.9 8.6 8.8 3.0 2.1 1.7 8.2 9.9 3.7 4.4 6.3 4.1 1.8 If 2.1

Grand total 20.0 42.2 77.4 63.9 78.4 67.8 62.6 38.0 67.9 53.0 52.6 57.3 52.1 47.5 23.1 14.7 9.8

6
,./200 m,_

,./ ('--'; \.,') •••••" •••: 'Ot _', «

(." , .....• '-.

NORTHERN

SLOPE i '. \

". "...... ----.,-'\

.............,. \

2000 m ; ,

600 N

550

1750 E 1800 1750 W 1700 1650 159'

Figure 3

Location of the three eastern Bering Sea continental slope areas referred to in the presentation of fISheries

and research survey fmdings.

U.S.S.R. have been small for most years; hence no attempt has

been made to estimate CPUE for such fisheries.

Biological data-All biological data used in this report from the

fisheries were collected by U. S. observers aboard Japanese vessels.

These data are part of the observer program fisheries database

(designated NORPAC) which is stored in the Northwest and Alaska

Fisheries Center's (NWAFC) mainframe computer. The data consist, for the most part, offish lengths by sex for the years 1978-84.

Otoliths were collected during these years but were not read because

of other priorities placed on the NWAFC Ageing Unit.

Standard procedures established by the U.S. Observer Program

(Nelson et al. 1981) were followed in the collection oflength data.

When Greenland turbot was a vessel's targeted species, the

observers obtained a random sample of some 150 fish from each

day's catch ofthis species. The total length of each fish and its sex

were determined. In addition to random samples, a sample stratified

by length-groups (5 fish of each sex for each I-cm interval) was

collected in which each fish was weighed to the nearest hundredth

of a kilogram and its otolith removed for later age determination.

Data from these stratified-by-length collections provided the means

of converting catch-in-weight to catch-in-numbers by using lengthweight relationships.

Since individual trawl catches were sampled in the targeted

fisheries for length information, the observer was able to record

important ancillary information so that length data could be related

to catch magnitude, fishing effort, area, and depth. Whenever length

information was combined into large area-time-depth cells, lengths

from individual hauls were weighted by the catch of these hauls

and then expanded to the total catch in the cell.

7

The majority of length data came from the catch ofthe landbased

dragnet fishery or the North Pacific small trawlers. The U.S.

Foreign Fisheries Observer Program database does not distinguish

between these fisheries, but places vessels from both fisheries into

the standard category of small freezer trawlers. Length data were

also available from the Japanese longline and mothership fisheries.

Through trial and error, an area-season-depth stratification was

constructed, based on similarities and differences in length compositions offish sampled from small freezer trawlers. The stratification that best described the changes in length composition between

area, season, depth, and year was one based on three areas of the

eastern Bering Sea continental slope (Fig. 3), three depths (184-450

m, 451-730 m, >730 m), and quarterly periods. For the Aleutians,

length data were examined by the same depth intervals as above

but by year rather than by quarter because of the limited seasonal

data from the region, and for two areas, the eastern and western

Aleutians divided at 180 0

•

Research survey data

In examining temporal and geographic features of the distribution

and abundance of Greenland turbot, we have used data gathered

from extensive research trawl surveys of the eastern Bering Sea

and Aleutians. Annual trawl surveys of crab and bottomfish have

taken place beginning as early as 1971, but it was not until 1973

that the surveys began to annually sample a standardized and significant area of the eastern Bering Sea shelf. However, the 1973 and

1974 surveys covered only the southeastern part of the eastern Bering Sea shelf. The first survey to adequately cover much of the

eastern Bering Sea shelf occurred in 1975. This extensive survey

was done by the NWAFC in response to the need for baseline in-
Table 4

Annual catch (1,000 t) by the Japanese fISheries of the eastern Bering Sea and Aleutian Islands region. Statistics

provided by the U.S. Foreign Fisheries Observer Program. (tr = catch of SO t or less.)

Fishery or vessel class 1978 1979 1980 1981 1982 1983 1984 1985

Eastern Bering Sea

Small freezer IrawlersI 38.4 30.1 33.3 41.2 38.1 38.2 19.9 14.2

Mothership 11.9 9.8 9.8 5.4 2.8 1.7 0.7 0.3

Other Irawler types 3.1 0.7 0.3 0.5 0.2 0.1 0.1 Ir

Longline 1.3 1.8 3.3 3.4 2.9 2.2 0.4 0.1

Total 54.7 42.4 46.7 50.5 44.0 42.2 21.1 14.6

Aleutian Islands region

Small freezer trawler' 6.7 8.9 3.2 4.1 6.0 3.9 1.6

Other Irawler types 0.4 0.1 tr Ir tr Ir Ir

Longline 1.1 0.9 0.5 0.3 0.3 0.2 0.2 Ir

Total 8.2 9.9 3.7 4.4 6.3 4.1 1.8 Ir

'Combined calches of Iwo major Japanese fisheries: the North Pacific Irawl fishery and the landbased dragnel fishery.

formation to evaluate the potential effects of oil exploration and

development on the fishery resources of the region (Pereyra et al.

1976). Surveys of much lesser area coverage occurred in 1976,

1977, and 1978. Another extensive shelfsurvey took place in 1979

and, for the first time, continental slope waters were comprehensively sampled through a joint effort by the NWAFC and the Fishery

Agency of Japan (Bakkala et al. 1985). Following a triennial

schedule, Japanese and U.S researchers conducted similar surveys

ofthe shelf and slope in 1982 and 1985. In intermediate years (1980,

1981,1983, 1984), extensive areas of the shelf were sampled, but

the slope was not sampled by the Japanese except in 1981. Thus

we have 4 years ofslope survey data (1979, 1981, 1982, and 1985),

8 years of extensive shelf coverage (1975, 1979-85), and 13 years

(1973-85) of coverage that included the southeastern part of the

shelf. We have treated the results ofthe Japanese survey ofthe slope

separate from that of the shelf area and have not attempted to

interrelate data from the two regions. As discussed earlier, trawls

and trawl rigging used in the two regions differed, and comparative fishing experiments have not been adequate to measure the

relative efficiencies of these trawls. There are the additional

problems of marked changes in species composition and bottom

topography in the two regions which further complicates interrelating these data.

There have also been changes in trawls and trawl rigging during

the NWAFC shelf survey which may have created some bias in

the time series ofsurvey results. For example, the standard survey

trawl was changed in 1982, and evidence suggests that the newer

trawl was more affective in capturing small flatfish than the trawls

used prior to 1982. Therefore, the catch efficiency for young

Greenland turbot on the shelf has possibly been higher since 1982.

Extensive trawl surveys ofthe Aleutians, which covered the areas

both north and south of the Aleutian chain, began in 1980 and

followed a triennial schedule so that a second survey was conducted

in the Aleutians in 1983. These surveys have also been a joint effort by the United States and Japan. Because of the relatively small

shelf area in the Aleutians, most of the survey effort took place

at depths greater than 183 m (Wilderbuer et al. 1985).

Trawling locations were preselected for the eastern Bering Sea

and Aleutian surveys so as to cover both depth and area. Locations

are given for the eastern Bering Sea by Pereyra et a1. (1976),

Wakabayashi et al. (1985), Umeda and Bakkala (1983), Sample

et al. (1985), Bakkala and Wakabayashi (1985), Hirschberger

(1985), and Halliday and Umeda (1986); and for the Aleutians by

8

Wilderbuer et al. (1985). Haul duration was 0.5 hours for U.S.

vessels and 1.0 hours for Japanese vessels.

Commercial-type fishing trawls were used. Those fished from

U.S. vessels had a small mesh web (32-mm stretched mesh) lining

the codend so that small animals such as juvenile fish could be retained. Japanese trawls had no liners but had triple layers of 90-100

mm stretched mesh in the codend. All trawls used in the Aleutians

and in the slope area ofthe eastern Bering Sea were equipped with

roller gear to reduce damage and hangups on the sea bottom. Roller

gear was not used during shelf surveys in the eastern Bering Sea.

Survey sampling methods are described by Wakabayashi et al.

(1985). In general, total catches of less than about 1,150 kg were

sorted by species and then weighed and counted. Catches over 1,150

kg were subsampled before sorting. Random samples of Greenland

turbot were separated by sex and measured to the nearest centimeter.

Otoliths were removed for ageing from at least five fish of each

sex per centimeter length-interval during the survey. U.S. age

samples through 1982 were read by NWAFC age readers. Due to

reservations about methodology and results from these readings,

we did not use these data. Samples collected since 1982 have not

been read due to higher priorities for other species. However, age

and related length data provided by the Fisheries Agency of Japan

were used 1) as an age-length key to convert length data collected

during the NWAFC 1979 survey to age, and 2) to approximate the

age of young fish during NWAFC surveys of the eastern Bering

Sea shelf in 1975 and 1980-85. Japanese age data were based on

otoliths collected by a Japanese research vessel operating mthe shelf

and slope region of the eastern Bering Sea in 1979. Approximation ofthe age of young fish was possible because of the high growth

rate ofjuvenile Greenland turbot, which results in very little overlap

in the length ranges of successive ages of young fish.

Results

Analysis of Japanese fisheries information

As mentioned earlier, actual removals of Greenland turbot by

foreign fisheries prior to 1970 cannot be adequately estimated.

Although estimates of the annual catch of this species became

available in 1970, it was not until 1977 that catch of Greenland

turbot could be related to specific fisheries and to depth. Infonnation on the length and sex offish also began to be collected in 1977.

However, because oflimited observer coverage in 1977, these data
Table 5

Annual Japanese catcbes (1,000 t)1 of Greenland turbot by tbe Iandbased dragnet flSbery and small trawlers of tbe Nortb Pacific trawl flSbery in tbe eastern Bering

Sea and Aleutian Islands region, 1970-84.

Fishery or

vessel class 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984

Eastern Bering Sea

Landbased 9.6 7.1 25.1 14.2 21.2 20,6 17.6 13.3 19.2 13.7 13.1 16.2 \7.9 17.2 14.9

N. Pacific

trawl fishery2 3.0 3.8 6.7 8.1 10.0 10.0 11.5 8.2

Total 16.3 23.0 20.4 21.2 26.2 27.9 28.7 23.\

Aleutian Islands region

Landbased 0.3 0.5 12.0 7.3 7.8 2.2 1.I 2.0 4.1 5.6 4.8 2.5 3.3 2.6 1.6

N. Pacific

traw1 fishery2 0.4 0.2 0.2 0.1 0.2 0.1 0.1 0.0

Total 2.4 4.3 5.8 4.9 2.7 3.4 2.7 1.6

lAs reported by Japan through INPFC.

2Fonnat of reported statistics prevents identification of catch by this fishery for the years 1970-76.

Table 6

Catcb in numbers of animals and average weigbt of Greenland turbot by tbe Japanese motbersbip fisberies, 1977-84.

1977 1978 1979 1980 1981 1982 1983 1984

Catch (millions of fish) 36.6 49.6 40.5 38.9 9.1 2.0 1.8 0.7

Average individual

weight (kg) 0.235 0.240 0.242 0.252 0.593 1.421 0.921 0.937

were not considered in some ofthe analyses concerned with changes

in availability and length of fish by area and depth. This section,

therefore, emphasizes findings from the Japanese fisheries for the

years 1978-84. Other nations' catches ofGreenland turbot have been

inconsequential compared with those taken by the Japanese fisheries

(Table 3), and very little biological information was collected for

other fisheries.

600--.---------------__...,

400

Figure 4

Annual catcb and incidence (percent by weight oftbe annual catches of all species)

of Greenland turbot by the Japanese surimi mothership fIShery in the eastern

Bering Sea, 1978-84. (Information from U.S. Foreign Fisheries Observer Program database.)

200

15+------------------4

1984

GREENLAND

TURBOT

CATCH

1982

Year

1978 1980

2

5

TOTAL CATCH

OF ALL SPECIES

-

c

Q)

c..

a

a

a

_ 10

a

a

a

Annual catch by region and fishery-The all-nation annual catch

of Greenland turbot in the combined regions of the eastern Bering

Sea and Aleutians has ranged from 9,800 t to 78,400 t during the

period 1970-84 (Table 3). Most of the catch has come from the

eastern Bering Sea, where annual catch reached levels of 60 to 70

thousand t from 1972 through 1978. This was followed by a general

decline, so that in 1986 only 7,700 t were taken. Annual catch in

the Aleutians has fluctuated between less than 50 t to as much as

13,000 t (Table 3).

Japanese fisheries have accounted for the majority of the

Greenland turbot catch from both the eastern Bering Sea and Aleutians. Among these fisheries, the landbased dragnet trawlers, North

Pacific small trawlers, the surimi mothership fishery, and the

longline fishery have been the most important in terms of harvest

magnitude or targeting on Greenland turbot (Table 4). Estimates

of catch for the landbased dragnet fishery and that of the North

Pacific small trawlers are not distinguished, but have been combined under the designation Japanese "small freezer trawlers"

(Table 4).

The small freezer trawlers' annual catch does not correspond very

closely with the Japanese reported combined catch of landbased

dragnet and North Pacific small trawlers (Table 5) for either the

eastern Bering Sea or Aleutians. The U.S. observer best blend

estimate of catches by small freezer trawlers almost consistently

exceeds that reported by the Japanese. For the eastern Bering Sea,

the correspondence between best blend and Japanese reported catch

9
EASTERN BERING SEA (west of 170· W longitude)

Species and Group 0-183 m 184-450 m

% 25 50 75 % 25 50 75

451-730 m

% 25 50 75

> 730 m

% 25 50 75 100

Soles

Pollock

Cod

Arrowtooth flounder

Rockfish

Sablefish

Greenland turbot

Rattails

Total catch sampled

EASTERN BERING SEA (east of 170· W longitude) 'I I I I, I I I I I I I I I

oles t t

ollock t t

od

rrowtooth flounder t S t

ockfish t t

ablefish t t

Greenland turbot t

Rattails t t P

Total catch sampled 44 x 103t tJ 8 x 103

t tJ 9x 103

t P7 x 103t

S

P

C

A

R

S

ALEUTIAN ISLANDS

Soles

Pollock

Cod

Arrowtooth flounder t

Rockfish

Sablefish

Greenland turbot

Rattails

Total catch sampled

Figure 5

Importance of Greenland turbot relative to other species in the catch by Japanese small trawlers in the eastern Bering Sea and

Aleutians, 1980-84. Total catch refers to that taken by trawlers with U.S. observers aboard. (Information from U.S. Foreign Fisheries

Observers Program database.)

improves with time, so that by 1984 the best blend catch approximates that of the Japanese reported catch:

Ratio (Best blend/Reponed catch)

78 79 80 81 82 83 84

1.7 1.5 1.6 1.6 1.4 1.3 p.9

Annual catch of the landbased fishery, as reported by Japan for

the eastern Bering Sea, rose sharply in 1972 to 25,100 t and then

fluctuated between 13 and 21 thousand t in subsequent years. In

the Aleutians, the 1972-74 period was particularly productive for

these fisheries, with annual catch ranging from 7.3 to 12.0 thousand t. However, since then there has been a downward trend in

the level of catch, so that by 1984 the catch was only 1,600 t

(Table 5).

Prior to 1977, we could not determine the Greenland turbot catch

for the North Pacific small trawlers, the surimi mothership fishery,

and the longline fishery. Since 1977 the catch of this species by

North Pacific small trawlers in the Aleutians has been insignificant, but in the eastern Bering Sea such catches have risen, peaking at 11,500 tin 1983 and then dropping to 8,200 t in 1984 (Table

5). The mothership fishery operates only in the eastern B~ring Sea

and mainly on the continental shelf. Although there is no known

targeting of Greenland turbot by this fishery, the incidental catch

ofthis species was as high as 11,900 t in 1978 (Table 4). The catch

dropped to 9,800 t in 1979 and 1980 and then rapidly declined in

subsequent years, so that by 1984 less than 1,000 t were taken.

The number of turbot estimated to have been caught by the mothership fisheries during the years of high catches (1978-80) ranged

from 36.6 to 49.6 million (Table 6). In contrast to the mothership

fishery, the Japanese longline fishery mainly operates in the slope

region and does target Greenland turbot in both the eastern Bering

Sea and the Aleutians. Its catch in the eastern Bering Sea has consistently been higher than that in the Aleutians. In the former region,

10
BOTTOM DEPTH

Percent of total catch

Pacific cod

Sablefish

Greenland turbot

Rattails

Pacific halibut

Arrowtooth flounder

TOTAL CATCH

25

184-450m

50

3

4 x 10 t

75 25

451-730m

50 75

Trace

25

> 730m

50 75

Figure 6

Importance of Greenland turbot relative to other species in the catch by Japanese longline vessels in the eastern Bering Sea. Total catch

refers to the amount landed on vessels with U.S. observers aboard. (Information from U.S. Foreign Fisheries Observer Program database.)

the catch reached high levels (2,900 to 3,400 t) in the 1980-82 period

and then declined to as low as 100 t in 1985. For the Aleutians,

the catch by the longline fishery has steadily declined since 1978,

so that less than 100 t were taken in 1985 (Table 4).

Re1lJtive imporlance of Greenland turbot in the Japanese fisheries-The importance of Greenland turbot relative to other species

in the fisheries of the eastern Bering Sea and Aleutians increases

with increasing bottom depth. On the shelf, the proportion of

Greenland turbot in the catch is insignificant in comparison with

that of targeted species such as pollock, cod, and the various species

of sole. In the Japanese mothership fishery, the incidence of

Greenland turbot was slightly more than 2 % in 1978, but since then

the incidence has declined to less than 0.3% in 1984 (Fig. 4).

Catches by the Japanese small freezer trawlers and longline vessels

are inconsequential at depths less than 183 m in both the eastern

Bering Sea and Aleutians. In the slope region of the eastern Bering

Sea, however, the importance of this species in the catch of these

fisheries increases with depth, so that at the deepest depths (>450

m) it is the predominant species caught (Figs. 5 and 6). This is not

the situation in the slope region of the Aleutians, where pollock

predominates in the catch ofsmall freezer trawlers from upper slope

depths (183-450 m), and where rattails comprise most of the catch

from lower slope depths (>450 m).

Since 1980 the importance of Greenland turbot relative to sablefish

in the longline fishery has been declining, so that by 1984 it was

no longer the dominant species in the annual catch.

28.0

20.0

12.0

4.0

12.0

4.0

-'= 12.0 ;;:

0 4.0

20.0

~.

0 12.0 0

0

::: 4.0

u 20.0

12.0

4.0

20.0

12.0

4.0

12.0

4.0

N

WEIGHT

c

1978

S N

Slope area

NUMBERS

c s

Catch patterns ofGreenland turbot by area, season, and depth-In

this section we rely considerably on information provided by the

u.s. Observer Program best blend estimate of catch. We place most

of our emphasis on findings for the eastern Bering Sea because of

the greater sampling coverage by time, depth, and area. For the

Aleutians, the desultory nature of the fishing by time and area and

the paucity of observer coverage prevent an adequate identification of area-time-depth catch patterns for this region.

In the eastern Bering Sea, Japanese small freezer trawlers have

consistently taken a greater tonnage of Greenland turbot from the

northern slope area than from either the central or southern slope

area (Fig. 7). In turn, removals from the central slope area have

been greater than from the southern slope area except in 1984 when

II

Figure 7

Annual Greenland turbot catch by Japanese small trawlers in the continental slope

region of the eastern Bering Sea, 1978-84 (N =northern slope area, C =central

slope area, and S = southern slope area).

the reverse occurred. By 1984 the catch was more evenly distributed

among the three areas.

When the catch of Greenland turbot by the small freezer trawlers

is examined in terms of numbers of fish, the importance ofthe northern slope area becomes even more pronounced because of the

smaller weight of individual Greenland turbot in the northern slope

relative to the other slope areas (Fig. 8). In 1978, the estimated
1984

0--0451-730 m

......... 183-450m

6.--6.> 730 m

1980 1982

Year

1978

NORTHERN

SLOPE

CENTRAL

SLOPE /\

0_°

~O

20

20

40

60IT------------;:========:;--,

c O--t---r---¥==-=-=-~'-==>f_ll:..=.=="rk-=-=-~Il:..=.-=-=>Tl_-____1

.,

a1984

LEGEND

0--0 Northern slope

l:r -6 Central slope

"'-'Southern slope

1982

Year

1978 1980

300

200 w

:::J

aU

100

0

6.0

en

.r::

C> 4.0 'Q;

i:

.,

C>

.,

> 2.0 <l:

Figure 8

Annual changes in CPUE and average weight of Greenland turbot by Japanese

small trawlers in the slope areas of the eastern Bering Sea, 1978-84.

.0 -+-,----,---r---.----,--r---...-----.

number of fish taken by these trawlers from the northern slope was

in excess of 20 million, and average annual removals for the years

1978-82 were about 17 million fish. In contrast, the average annual removals for the same period for the central slope were about

5 million fish and for the southern slope about 2 million fish.

In the northern and central slope areas most of the catch has come

from shallow (184-450 m) and intermediate (451-730 m) bottom

depths, with intermediate depths having the highest catch (Fig. 9).

Figure 9

Distribution of annual Greenland turbot catch by Japanese small trawlers by depth

and area in the continental slope area of the eastern Bering Sea, 1978-84.

NORTHERN

I I I i I I I I I I I

2 4 2 4 2 4 2 4 2 4 2 4

1979 1980 1981 1982 1983 1984

Year and quarter

12

2 4

1978

: SOUTHERN

~;:=A

8

6

4

2

a

a 8 a CENTRAL

.r::

B 6

U 4

2

Figure 10

Quarterly catch of Greenland turbot by Japanese

small trawlers in the continental slope region of

the eastern Bering Sea, 1978-84.
1981

1984

L:J"...6451-999m

(). oQl84-450m

1982

Year

1980

LEGEND

4.0,---------------------,

3.0

0

0

.s;;;

0

0

0

6 2.0

0.

.s;;;

-

u 1.0

Figure 12

Catch-per-unit-effort of Greenland turbot by Japanese longline vessels operating

in the eastern Bering Sea, 1978-84.

1982

tr

tr

tr

1978

5.0

1979

5.0

tr

1980

5.0

e5

0

0 5.0 :::

.<:

B

u'"

5.0

1983

5.0

15

tr

1984 tr = trace

50

N C

Shelf area

S

10

Japanese Mothership Fishery

<184 m

1,350 len9ths

Mean = 27.0 em

Continental Shelf, 1980

5

Figure 11

Annual catch (1978-84) of Greenland turbot by the Japanese

surimi mothership fIShery by area of the eastern Bering Sea shelf

(N = north of 58°, C=55°-58°N lat., and S=south of 55°N lat.). 0

'"

u

W0-

Figure 13

Length composition of Greenland turbot taken by various Japanese fISheries in

the eastern Bering Sea, 1980.

80 100

Japanese Small Freezer Trawlers

184-999 m

23,015 lengths

Mean.: 50.3 em

Cenlral Slope Area, 1980

60

Length (em)

20 40

Japanese longline Fishery

184-999 m

4,398 lengths

Mean = 78.4 em

Cenlral Slope Area, 1980

5

5

However, in 1984 the proportion of catch from intermediate depths

sharply declined, and, for the fIrst time, the proportion from shallow

depths exceeded that of intermediate depths in the northern slope

area. For the southern slope, the distribution of catch has tended

to be more evenly distributed among the three depth zones.

A seasonal catch pattern was apparent for the small freezer

trawlers in both the central and southern slope areas which was

most pronounced after 1980 (Fig. 10). In the central slope area the

catch for most years was lowest in the first quarter and then increased, usually reaching its highest level in the fourth quarter. In

the southern slope area no catch was recorded for the first quarter;

fishing began in the second quarter, with the catch increasing in

the third quarter, and then either leveling off or declining in the

fourth quarter. For the northern slope area the highest catch tended

to be taken in the second quarter.

The Japanese surimi mothership fishery is active mainly during

the summer and early fall months and operates for the most part on

the continental shelf. Since 1980 there has been a inarked decline in

magnitude ofthe catch from the northern shelf (Fig. 11). By 1984

very few Greenland turbot were taken anywhere on the shelf.

13
NORTHERN SLOPE CENTRAL SLOPE SOUTHERN SLOPE

10 1978 lOJ 1978 10 1978

5 5 5

0 0 0

10 1979 10 1979 10 1979

5 5 5

0 0 ~.,;........... 0

10 1980 10 1980 10 1980

5 5 5

.~ ... ,..' ........

0 0 0

10 1981 10 1981 10 1981

5 5 5

~.'\ 0

,/~." _..... ' \~"" OJ I~i ,'-., 0

10 1982 10 1982 10 1982

5 5 5

0 0 0

1983 1983 1983

10 10 10

5 5 ",

:'

" "\,

0 0 " 0

1984 1984 1984 l\ 10 10 10 ,I'.

..: \

f \ 5 5 5 . '

,I \ 0 0

20 40 60 80 100 20 40 60 80 100 20 40 60 80 100

Length (em)

Figure 14

Length composition by sex of Greenland turbot taken by Japanese small trawlers in the continental slope region of the eastern

Bering Sea, 1978-84.

The Japanese longline fishery has operated mainly in the central

and southern slope region. Its main harvest of Greenland turbot

and highest catch rates have come from depths greater than 450

m (Fig. 12).

In the Aleutians, Greenland turbot is also taken mainly at depths

greater than 450 m by both the small freezer trawlers and longliners.

Since the catch of Greenland turbot in this region has not been of

much significance, we have not given any breakdown of the annual catch by area.

Length and sex composition-There are marked differences among

the Japanese fisheries in terms of the length of fish captured (Fig.

13). On the continental shelf of the eastern Bering sea where the

motherships operate, the catcher vessels take only small, immature

fish. The larger juveniles and older fish tend to occur in the slope

region, where they are subject to harvest by the Japanese small

freezer trawlers and longline vessels. The small freezer trawlers

capture a broader size range of fish than the longline vessels in the

same region of the slope. The latter vessel type takes only the large

fish, which are predominantly females. This difference in gear selectivity also occurs in the Aleutians.

Information from the small freezer trawlers provides the best indication of patterns in the length composition of Greenland turbot

catches by area, depth, and season. Length data from the longline

and mothership fisheries are incomplete. For the eastern Bering

Sea slope there were consistent differences by area in the length

of fish captured by small freezer trawlers (Fig. 14). The length of

fish increased from north to south and exhibited distinctive modal

distributions by sex. Annual changes in length composition were

also evident. Obvious changes over time were 1) decline in the pro14
CENTRAL SLOPE

30

20 1978

10

0

60 1979

50

40

30

20

10

0

50 1980

40

30

c 20

10

'0

5 1981 l! 50

c

0 40

30

D 20 E

c 10 c

"' 0 u

40 1982 u

30

20

10

0

30 1983

20

10

0

30 1984

20

10

0

25 35 45 55 65 75 85 95 105

Length (five em intervals)

40

30

20

10

0

50 1980

40

c 30

20 £

'0 10

0 0

"' 1981 c 50 0

0. 40

.0 30 E 20 c

.£ 10 "'

u

0 u

40 1982

30

20

10

0

30 1983

20

10

0

30 1984

20

10

30 1978

20

10

o+r--'-~'+-""-9f-.lL4.-"+IhP"tx'P-f'""-t--'--r'--r"""'-T""""l

60 1979

50

NORTHERN SLOPE

25 35 45 55 65 75 85 95 105

Length (five em intervals)

Figure 15

Catch rate by Japanese small trawlers or Greenland turbot by

sex and S-cm length intervals in the northern slope area or the

eastern Bering Sea, t978-84.

Figure 16

Catch rate by Japanese small trawlers or Greenland turbot by sex

and S-em length intervals in the central slope area or the eastern

Bering Sea, t978-84.

portion of small fish in the length range of 30-50 cm in all areas,

but which was most pronounced in the northern slope, and 2)

emergence of sharply defined modal groups by sex in the central

and southern slope areas.

Not only had there been a decline in the proportion of small fish

in all areas of the slope, but the availability of fish of most length

groups, as indicated by annual catch rates of freezer trawlers, also

declined (Figs. 15-17). A decline in the proportion of smaller fish

(in this instance, less than 60 cm) was also observed in the Aleutians in 1983 and 1984 (Fig. 18).

To examine seasonal patterns in the catch by length, we partitioned

both the males and females into two length groups, with the dividing

line at 55 cm and 70 cm, respectively. This grouping simplified

the presentation and was based on the annual length distributions

(Fig. 14) and the suggestion from length-at-maturity information

(D'yakov 1982, Smidt 1969) that the larger groups are mostly

mature fish and the smaller length groups are mostly immatures.

The availability of fish in the smaller length groups was examined

for both the northern and central slopes, though not for the southern

area because of the small number of animals of these lengths taken

there. The CPUE ofthese small length groups was consistently much

higher in the northern than in the central area, particularly in the

shallowest depth interval (Fig. 19). The importance ofsmaller length

groups in both the northern and central areas and at all depths

declined markedly in 1983 and 1984.

In both the northern and central slope areas the availability of

the smaller length groups showed a pattern of high catch rates in

the first two quarters. with the lowest catch rates usually occurring

in the fourth quarter of the year. Females showed a tendency to

be more available than males, particularly on the northern slope.

Some general tendencies were observed in the availability of fish

in the larger length groups. In all slope areas at the shallowest depth

interval (184450 m), the availability of both sexes tended to decline

in the fourth quarter, whereas in the deepest depth interval (>730

IS
80 100

1983

ss = 12.676

L =13.6 em

80 100 20 40 60

Length (em)

1984

55 =6.252

L = 65.8 em

20 40 60

0-'----"1'--

5

5

1700 _1760 west longitude west of 1760 longitude

1978 1978

5 55 =8.280 No data

I =61.8 em

0

1979 1979

= 5.891 55 = 2.604

5 L =58.2 em 1: =69.0 em

0

1980

No data 5

0

1981

c: 55 = 294 Ol 5 u L = 75.9 em

Q;

<l.

0

1982 1982

No data 55 = 28.022

I = 68.5 em

SOUTHERN SLOPE

30 1978

20

10

60 1979

50

40

30

20

10

a

50 1980

'"

c 40

g 30

'0 20

10

c

0 a

0. 1981 50

., 40

30 .S

.c 20 u

8 10

a

40 1982

30

20

10

a

30 1983

20

10

a

30 1984

20

10

25 35 45 55 65 75 85 95 105

Length (five em intervals)

Figure 17

Catch rate by Japanese small trawlers of Greenland turbot by sex

and S-cm length intervals in the southern slope area of the eastern

Bering Sea, 1978-84.

Figure 18

Length composition ofGreenland turbot taken by Japanese small trawlers in the

Aleutians, 1978-84.

16
NORTHERN SLOPE

6001

400

300

184-450 m

en 200 .S

0 100

:J

0

.<::

'"

c

0 0

Ii;

0. 400 -£i

;;:::

'0

Ii;

.0 300 E

:J

Z

200

100

CENTRAL SLOPE

LEGEND

0- 0 Females

__ Males

184-450 m

2 4 2 4 2 4

1981 1982 1983

2 4 2

1984 1981

Year and quarter

4 2 4

1982

2 4

1983

2 4

1984

Figure 19

Catch-per-unit-effort of Greenland turbot by Japanese smaU trawlers by sex, depth, quarter, and area of the continental slope region of the eastern Bering Sea, 1981-84 (length groups: males <55 em, females <70 em).

m) their availabilities tended to increase in the fourth quarter (Fig.

20). Another tendency was the decrease in the availability of males

from the northern to southern slope area, with the converse occurring among females.

Changes in CPUE and incidence rate-Bakkala et al. (1986)

showed that the annual Greenland turbot CPUE of the landbased

dragnet fishery has generally declined in the eastern Bering Sea

since the early 1970's (Fig. 21). They assumed that CPUE is a

relative measure of biomass and viewed these annual CPUE changes

as reflecting a decline in Greenland turbot biomass in the eastern

Bering Sea.

The CPUE ofsmall freezer trawlers having U.S. observers aboard

showed a trend similar to that of the landbased fishery from 1978

to 1984 (Fig. 21). However, CPUE of the small freezer trawlers

was much higher than that of the landbased dragnet vessels during

the earlier years of that period (1978-81). The match in CPUE

between the small freezer trawlers and landbased vessels became

quite good in the years 1982-84.

Regression analysis using dummy variables was performed to

determine effects due to differences in area, season, and depth in

CPUE (tons per hour trawled). A regression model with no interaction terms was used as a basis for the analysis (Table 7). Though

interactions may well exist, they were not included in the model

since there were enough empty cells to cause the hat matrix to ap17

pear nonsingular due to rounding error; tolerances for interaction

terms were too low to allow their inclusion in the model (Draper

and Smith 1981, Weisberg 1985). Results of the regression analysis

suggest that all effects were both meaningful and significant (Table

8). Statistical significance of a factor depends on the significance

ofits representative dummy variables as a group, not the significance

of individual coefficients representing separate levels of a factor.

For the regression model results in Table 8, all factors were

statistically significant at ex: = 0.05; the p-value for each group effect was always less than 0.025. By area, the highest regression

coefficient was found for the southern slope area (Fig. 22), where

CPUE for most years was greater than that of other areas for comparable years. By quarter, the greatest deviation of the regression

coefficient from the standard (third quarter) was estimated for the

first quarter when the CPUE tended to be consistently higher annually than in the other quarters. By depth the greatest deviation

of the regression coefficient was estimated for the shallowest interval fished (184-450 m), where the CPUE was almost invariably

less than at the greater depths for each of the slope areas. Whether

we look at CPUE by area, quarter, or depth, it is obvious that CPUE

tended to peak in 1980 and then reach a low point in 1982 or 1983

(Fig. 23). The regression coefficient for annual effect reflects this

tendency by bt:ing the highest in 1980 and lowest in 1982 and 1983.

When the CPUE in weight was converted to CPUE in numbers,

a somewhat different annual trend was found. For the northern and
NORTHERN SLOPE CENTRAL SLOPE SOUTHERN SLOPE

100

184--450 m 184--450 m 184--450 m

75

.!:'"

t j 50 \ X ..

:: t- o

0

:; 25 \- A / o,o~ /~o \ .s:; /0-0 0

" 0

<: 0 0 0

a;

Co

.8 451-730 m 451-730m 451-730 m

E 75

"

~~Y\ 0, <:

// .!: -l 0,

.s:; 50 0

s.. J 0 ...0 0

u

/ / ,,0 25 0 0

cI ............ J

2 4 2 4 2 4 2 4

1981 1982 1983 1984 0

75 I > 730 m >730m

0

50 .j)\ LEGEND 7 o-oFemales

25 -Males V \

2 4 2 4 2 4 2 4 2 4 2 4 2 4 2 4

1981 1982 1983 1984 1981 1982 1983 1984

Year and Quarter

Figure 20

Catch-per-unit-effort of Greenland turbot by Japanese small trawlers by sex, depth, quarter, and area of the continental slope region of the

eastern Bering Sea, 1981-84 (length groups: males ~55 cm, females ~70 cm).

s

o

C

Area

o

---0---- ---

N

o

184 451 < 730

Depth

o --0--- ---

o

4

o

Year

2 3

Quarter

o

o

o

o

0.30.,.------------,

0.24

018

0.12

0.06

0.12

0.06

o

-0.06

-0.12 -"--,--.,.---,---,----'

o 0------- - -0---

5 -006 .;;;

-0.12 -'-.,.--,-.,.--,,-,--,-,..-1

1978 1980 1982 1984

'0

'u

;;::

] 0.18.,.-------------, u

Figure 22

Results of regression analysis of Greenland turbot catch-per-unit-effort, 1978-84, showing

regression coefficients by main effects (year, area,

quarter, and depth).

:0- 60

"

l 40 .s:;

0

0

20 w

::J

Cl.

U

0

1970 1972 1974 1976 1978 1980 1982 1984

Year

Figure 21

Annual catch-per-unit-effort of Greenland turbot by the Japanese landbased

dragnet fIShery and that ofJapanese small trawlers operating in the eastern Bering

Sea, 1970-84.

18
Table 7

Regression model (no interactions) applied to Greenland turbot catch-perunit-effort of the eastern Bering Sea.

Model Effeets

Table 8

Results of analysis of variance and regression analysis of Greenland turbot

catch-per-unit-effort by Japanese small freezer trawlers operating in the

eastern Bering Sea, 1978-84. (Data from U.S. Foreign Fisheries Observer

Program.)

'Zero by definition, because the cell represents the standard.

**Significant at a = 0.05

Range Estimate SE of estimate

0.195

184-450 m 0.103 0.022

451-730 m Cf

>730 0.058 0.026

1978 0.096 0.034

1979 0.121 0.035

1980 0.254 0.038

1981 0.214 0.036

1982 0.040 0.033

1983 0'

1984 0.140 0.036

Jan.-March 0.130 0.032

April-Jan. 0.014 0.026

July-Sept. 0'

Oct.-Dee. 0.011 0.026

Northern Cf

Central 0.045 0.023

Southern 0.134 0.029

CPUE = Bo*

+ BIOI + B,D2

+ B3Yl + B4Y2 + BsY3 + B6Y4 + B7Y5 + BsY6

+B9QI + BlOQ2 + BIIQ3

+ Bl2AI + B13A2

Definitions of dummy variables

Depth 01 D2

451-730 0 0

184-450 I 0

>730 0 1

Year YI Y2 Y3 Y4 Y5 Y6

83 0 0 0 0 0 0

78 I 0 0 0 0 0

79 0 1 0 0 0 0

80 0 0 I 0 0 0

81 0 0 0 1 0 0

82 0 0 0 0 1 0

84 0 0 0 0 0 I

Quaner Ql Q2 Q3

july-Sept. 0 0 0

Jan.-March I 0 0

Apr.-June 0 I 0

Oct.-Dee. 0 0 I

Area Al A2

Nonhern 0 0

Central I 0

Southern 0 I

Depth

Year

Quaner

Area

Source

Depth

Year

Quaner

Area

Error

Parameter

Intercept

Depth

Year

Quaner

Area

df

2

6

3

2

437

Partial sum of squares

1.7824

2.9486

0.8793

0.9534

18.8572

Panial F-value

20.63**

11.38**

6.78**

11.03**

bb

1977

b~79 81 83 1977 79 81 83

*Bo is the intercept or standard, and b; (not included in the model) is the deparlure from the standard because of a specific main effect.

Figure 23

Annual changes in Greenland turbot catch-perunit-effort by Japanese small trawlers given by

depth and area of the continental slope of the

eastern Bering Sea, 1978-84.

19

NORTHERN

1.4°b

1.20 184 m - 450 m

.c. 1.00

0.80

0.60

0.40 ~_

0.20 -,

o

,

1'40~ .20 451 m - 730 m

-;, 1.00

:;; 080

;i'0.60

u 0.40

0.20

o

1.4°li :E 1.20 > 730 m

-;, 1.00

:;;0.80 0.60

u 0.40 ....-

0.20

o

1.40

1.20 All depths

1.00 combined

0.80 w 0.60 ;i' 0.40 u 0.20

0

1977 79 81 83

Years

CENTRAL

Years

SOUTHERN

Years
20 r------------------------,

300

NUMBERS OF FISH PER HECTARE

6 r----------.----------------,

1983 1984 1985

NORTHERN

o

CENTRAL

-

1981 1982

SOUTHERN

-

19"19 1980

15

Mature adults " ........ .-.-- .-.

.....

:J

o

..c

--- ..c

(Il

;;::::

.....

o 100 .....

Q)

.0

E

:J

Z

Cl

C

'"

.....

....

'0 200

•

O............------..l-_......._.l.-_............. MEAN WEIGHT (kg)

Figure 24

Annual changes in catch-per-unit-i!tTort ofmature and immature Greenland turoot

by Japanese small trawlers in the eastern Bering Sea, 1978-84.

Research vessel survey finding!>

Continental slope region of the eastern Bering Sea-Findings

presented here are based on data provided by the Fisheries Agency

ofJapan on their research trawling surveys for the summers of 1979,

1981, 1982, and 1985. They are given by the three areas (northern,

1979 1980 1981 1982 1983 1984 1985

central, and southern slope) similar to those used in presenting the

findings from the fisheries.

Relative density in numbers of fish per hectare declined sharply

between the 1979 and 1981 surveys in all slope areas, while mean

weight of the fish increased (Fig. 25). Density continued to decline

in both the northern and central slope areas from 1981 to 1985,

but for the southern slope area no marked change in density could

be discerned between 1981 to 1985. As for mean weight ofthe fish,

no well-defined trend is apparent from 1981 to 1985.

The above changes in density and mean weight can be accounted

for in part by the decline in small fish less than 55 cm in length

in all slope areas (Fig. 26). By 1985 length samples showed two

distinct modal groups in both the central and southern areas, with

the lesser length modal group (55-69 cm) being comprised mainly

of mature males and the large length modal group (75 cm and

greater) ofmain1y mature females. For the northern slope area only

one distinct modal group was found in 1985 that corresponded to

the lesser length modal groups observed from samples of fish from

the other two slope areas in 1985. These survey findings for 1985

correspond with the length compositions obtained from sampling

of the fisheries in 1984 (Fig. 14).

Age data were available for only 1979. Individuals 4-7 years in

age were dominant and comprised a considerable proportion offish

caught in the northern slope at depths of 184-450 m and in the cenFigure 25

Catch rates and average weight of Greenland turbot taken by Japanese research

trawlers in the continental slope region of the eastern Bering Sea in 1979, 1981,

1982, and 1985.

80 84

central slope areas CPUE in numbers peaked in 1979, but for the

southern slope areas CPUE in numbers peaked in 1980 (Fig. 8).

Following these peaks, the CPUE in numbers fell to low levels in

1983 and 1984.

Corresponding to this general decline in CPUE by weight and

number since 1979 or 1980, there has been an increase in the average

weight of the fish caught (Fig. 8). These corresponding but opposite trends in CPUE and average fish weight may reflect an aging

and declining population, since the CPUE of smaller size fish has

been decreasing, whereas that oflarger fish shows little change since

1978 (Fig. 24).

The CPUE (by weight) of Japanese longline vessels also shows

a decline in recent years. At depths greater than 450 m where most

of the Greenland turbot catch is taken by these vessels and where

CPUE is the highest, CPUE peaked in 1981 and then sharply

declined in subsequent years (Fig. 12). At shallower depths of the

upper slope, CPUE peaked in 1982 and then declined.

Since the Japanese mothership fishery does not target Greenland

turbot, we did not examine CPUE trends but instead looked at the

incidence of this fish in the fishery by year. The declining trend

in incidence (Fig. 4) cannot be attributed entirely to the decreasing

number of animals on the shelf as inferred from the results ot

research trawl surveys. The Japanese modified the trawls used in

their mothership fisheries in 1980 to reduce the catch of crab and

halibut. This gear change, which raised the footrope of the net off

the sea bottom, may have contributed to the catch reduction of

Greenland turbot.

20
...

c

CIl

CIl

c..

NORTHERN

25,--------------,

1979

15

5

oL.--'--'--'--'--'--'--'--'-.LL.LL--.J

25,-------------,

1981

15

5

olL.-LLJUJ--'--L-'--.l..-LLJLo-J

25,--------------, 1982

15

5

OL.--'--'--'--'--'--'--'--'--'--'--'--'----..J

CENTRAL

1979

1982

SOUTHERN

1979

1981

1982

250-------.-------,

15

5

100 25

1985

50

Length (em)

25

1985

100

Figure 26

Length composition of Greenland turbot by 5-i:m length groups from sampling by Japanese research trawlers in the continental slope region of the eastern Bering Sea in 1979, 1981, 1982, and 1985.

tral and southern slope at intermediate depths (451-730 m). In the

deepest zone, females of age 10 and older were most frequently

captured.

Temperatures near the sea bottom along the slope generally were

3-5°C, with a slight tendency to decrease from the shallowest to

the deepest depth zone (Fig. 27). The widest temperature ranges

tended to occur in the shallowest depth zone because of the occasional influence of warm or cold water near the interface of shelf

and slope. Mean water temperatures in the two deepest zones almost

always fell within a very narrow range of 1°C (3-4°C).

Continental shelfregion ofthe eastern Bering Sea-Results given

here are from two sets of survey data: the longer time series

(1973-85) that consistently covered the southeastern part of the shelf,

and the time series (1975, 1979-86) of survey data which covered

a major part of the shelf.

The availability distribution of Greenland turbot by survey year

shows some obvious patterns by time and area. For those surveys

that covered much ofthe eastern Bering Sea shelf, the highest concentration of animals was found in the northwestern part ofthe shelf

west and south of SI. Matthew Island (Fig. 28). In the early years

of the shelf surveys Greenland turbot were caught throughout the

region, though mainly at intermediate (50-100 m) and outer shelf

depths (101-200 m) (Table 9). However, with time the geographical

catch distribution shrank so that by 1985 and 1986 only the area

west and south of SI. Matthew Island was yielding any amount of

21

fish (Fig. 28), but the magnitude of catches within this area has

decreased over time. Survey fmdings from the 1973-86 southeastern

portion of the shelfsuggest that young fish were abundant as early

as 1973 and that their decline began noticeably in 1980 (Fig. 29

and Table 10).

With the decline in number of fish on the shelf, the average age

and size ofthe fish increased. This can readily be seen when tracking changes in abundance by age (Table 10) and size groups (Fig.

30). In 1979, when the greatest number of fish was found on the

shelf during the surveys, fish of ages 1 through 4 were abundant,

but by 1981 their numbers had greatly declined. The decline continued in subsequent years so that by 1985 mainly age-5 and older

animals were dominant on the shelf, though present only in low

numbers. By size, there has been a noticeable shift to large size

groups over the period of the surveys, so that in recent years

(1984-85) fish greater than 30 cm have become the most important

component of survey catches.

In contrast to the findings from research surveys and samplings

of the fisheries catch in the slope region, where most ofthe animals

were greater than 35 cm and of ages 4 and older, the shelf surveys

encountered relatively small, young fish. Fish on the shelf are not

substantially recruited to the survey gear at sizes below 10 cm or

at age-O. Age-O fish have never been prominent in survey catches

because of their small size which allows them to pass through the

mesh of the net and because many of them have probably yet to

settle to the sea bottom from their pelagic stage. The sharp decline
SOUTHERN SLOPE CENTRAL SLOPE NORTHERN SLOPE

, 1211

1191

..L (261

I I

--L.. 151

f- ......L..- 161

r- .... 1151

I I

f- ..L 13)

.I- 110)

f- ....L- IllI

r- 123)

-'- (81

.... (31)

1 I

.I 12)

...L. (71

..J... (26)

I (2)

f- .L (7)

f- ..L (10)

I

...L. (24)

......J.... (41)

I 1621

..L (71

.... (12)

--L (28)

r- ...L.. 14)

r- --L.. (81

f- ..L (9)

1979

184--450 m 1982

1985

1979

451-730 m 1982

1985

1979

731-1000 m 1982

1985

2 3 4 5 6 7 2 3 4 5 6 7 2 3 4 5 6 7

Bottom water temperature 0 c

Figure 27

Water temperatures near the sea bottom as observed during Japanese research trawl surveys of the continental slope region of the eastern Bering

Sea (1979, 1982, and 1985). Numbers of observations are given in parentheses.

56°00

'<;.-

1986

1985 "-00

, ~r< -, CATCH RATES

INlJmberpefnaulocalmileJ

L~;;;;;;;;;;;;;;;;;;;;;;;;;;;., 154000

r • .64°00N

• • .6

4

"00N

58°00

< 11

, 11-50

51-150

CATCH RATES

INurnberpelnaulic;allf\llel

180·00 116"00 W

Figure 28

Estimated density distribution of Greenland turbot during summer bottom-trawl surveys by the National Marine Fisheries Service in the shelf region of the eastern

Bering Sea in 1979, 1982, 1985, and 1986.

22
Table 9

Catch rate of Greenland turbot in weight (kg) and numbers (parentheses) per hectare trawled by area and depth in the shelf region

of the eastern Bering Sea during annual bollom trawl surveys by the Northwest and Alaska Fisheries Center, 1979 and 1981-85.

Area Depth(m) 1979 1981 1982 1983 1984 1985

Northern shelf I-50 0.8 (40.5) 0.0 (0.2) 0.0(0.1)

(>61 ° N. lat.) 51-100 8.9(131.0) 1.1 (6.4) 0.2(0.7)

101-200 13.9(137.3) 4.1(26.5) 6.7(19.4)

Central shelf 1-50 0.3 (5.0) 0.2 (0.6) 0.0 (0.1) 0.0(0.0) 0 (0) 0.0(0.0)

(58°-61 ° N. lat.) 51-100 7.2 (52.3) 2.2 (9.0) 0.8 (3.9) 0.5(1.4) 0.1(0.2) 0.0(0.2)

101-200 6.5 (44.2) 7.2(26.2) 2.7 (5.6) 2.4(4.5) 2.0(2.6) 0.8(1.3)

Southern shelf I-50 0.0 (0.4) 0.0 (0.0)

«58° N. lat.) 51-100 1.6 (9.7) 0.3 (0.8) 0.3 (0.4) 0.4(0.9) 0.0(0.0) 0.0(0.0)

101-200 1.4 (14.7) 0.6 (0.4) 0.3 (0.2) 0.5(0.4) 0.2(0.1) 0.2(0.4)

Millions

300,------------------------,

250 ,- ----------------.--.-.-...-.----

Table 10

Number of Greenland turbot (million rtsh) by age groups (estimated from

boltom trawl surveys) in two areas of the eastern Bering Sea shelf with consistently good survey coverage and relatively high abundance. Area N lies

between lat. 58°N and 61°N; area S is south of 58°N. Both areas are within

bollom depths of SO to 184 m.

Ages

Year Area o 2 3 4 5+ AU

200 -_. -----------------.-----.

73 74 75 76 77 78 79 80 81 82 83 84 85

YEAR

Figure 29

Annual estimates of the relative population numbers of Greenland turbot in an

area of the southeastern Bering Sea shelf which has been consistently surveyed

since 1973.

150 -

100 r----

50

o

1----------------

---

1975

1979

1980

1981

1982

1983

1984

1985

N

S

Both

N

S

Both

N

S

Both

N

S

Both

N

S

Both

N

S

Both

N

S

Both

N

S

Both

0.05 34.54

o 19.46

0.05 54.00

0.52 257.27

o 33.01

0.13 290.28

o 70.57

o 4.74

o 75.31

0.04 16.40

o 0.67

0.04 17.07

o 8.59

o 0.67

o 9.26

0.09 4.70

o 0

0.09 4.70

o 0.34

0.15 2.17

o 0.84

0.15 3.01

275.58

14.03

289.61

173.14

29.38

202.52

229.01

18.58

247.59

35.90

0.61

36.51

9.00

0.67

9.67

1.91

0.13

2.04

0.68

0.86

1.70

2.56

210.15

50.74

260.89

205.03

41.97

247.00

183.75

8.76

192.5l

99.41

1.69

101.10

11.90

1.12

13.02

8.49

1.75

10.24

1.21

1.34

0.85

2.19

44.13

14.05

58.18

114.02

23.67

137.69

183.60

11.45

195.05

101.90

2.67

104.57

29.05

0.93

29.98

17.23

0.59

17.82

7.12

1.54

o

1.54

2.72

2.05

4.77

18.61

9.34

27.95

12.56

2.67

15.23

17.90

3.14

21.04

16.06

3.12

19.18

13.15

0.83

13.98

11.96

5.18

0.95

6.13

567.12

100.33

667.45

768.59

137.37

905.57

679.49

46.20

725.69

271.55

8.78

280.33

74.60

6.51

81.11

45.57

3.30

48.87

21.31

0.67

21.98

11.24

4.34

15.58

23

'No length data available for conversion to age.
4

3

2

~.2 1979

in numbers of fish older than age-4 on the shelf may be accounted

for by movements of these animals into the slope region.

Using the estimated abundance of age-I fish as a measure ofyearclass strength (Table 10), we can conclude that there has been a

continuing decline in year-class abundance since the strong showing of the 1978 year-class as age-1 fish in 1979. The 1979 through

1984 year-classes are therefore considered weak relative to the 1978

year-class.

In the shelf region, trawl catches of Greenland turbot were

associated with a wide range of bottom water temperatures, but there

was generally a low incidence of turbot where water temperatures

were below DoC.

1984

1983

Aleutian Islands-In both 1980 and 1983 the highest catch rates

of Greenland turbot occurred in the northeast area of the Aleutians,

whereas the lowest rates were observed in the southeast and

southwest areas (Fig. 31). There was a definite declining trend in

magnitude of the catch rates from east to west along the Aleutian

Island chain. The most pronounced difference between the two

surveys in terms of catch rates was observed in the northeast area,

where the 1983 average catch of 17.0 kg/ha was twice that of 1980

(8.4 kg/ha).

Relative biomass was greater in 1983 than in 1980, but relative

population numbers were greater in 1980 (Fig. 31). Although the

number of animals declined, their average weight increased (Fig.

31) so that biomass actually increased as well. The increase in

average weight of fish occurred in all but the northwest area. The

increase in average weight of fish in 1983 corresponds to an increase in the average length of fish (Fig. 32).

Highest catch rates or apparent densities occurred in the slope

region at depths greater than 300 m (Fig. 33).

Bottom water temperatures recorded during the Aleutian surveys

tended to decrease by depth zone and to exhibit the widest ranges

in the shallowest depth zones «200 m and 201-300 m) (Fig. 34).

Where Greenland turbot was most available by depth (>300 m),

mean temperatures of the bottom waters fell within the range of

3 to 4°C.

4

3

2

m

---'"0

..s

QJ

...

E 2

<..>

...

u'"

1982

1985

1986

o L-..L.....Ilrza::ll~_...L.----'-_.L..-.....J....----l----tU{4..

5 10 15 20 25 30 35 40 >40

Length (5 cm intervals)

Figure 30

Catch rate ofGreenland turbot by 5-an length groups during annual swnmer bottom-trawl surveys by the National Marine Fisheries Service in

the shelf region of the eastern Bering Sea, 1979-86.

24
BOW~

1»<1"

% Attu (;)

IT'

G;;:> ... NW 900m NE

«f I .0

I A~ P

.,f .. 0 I Adak 0

... I 0 ?J>p!)f>:P"

"'- I . I O' SW Amchitka SE

i

I

I

I

I

172°E 1740

1

1760

I

1780 1800

I I

178°W 176 0

I

1740

I

1720 1700

Abundance

Aleutians

+5Kg

+5Kg

o

NE

SE

Bowers

Ridge

Average weight of fish

NW

SW

Numbers

(1.106 fishl

14

11

33

Biomass

(1000'1

48

01980

~1983

LEGEND

NE

15

'" 10 J::

--OJ

5

0

SW

5

Figure 31

Relative density, abundance, and average weight of Greenland turbot as found during the 1980 and 1983 U.S.-Japan cooperative trawl surveys

in the Aleutian Islands region.

25
Females Males

16 16

1980 1980 14 14

12 Average length =69.0 em 12 Average length =53.6 em

10 10

8 8

6

IZ

W

(,) 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

a::

W 16 16 l:l. 1983 1983 14 14

12 Average length =81.5 em 12 Average length =64.1 em

10 10

8 8

6 6

o 10 20 30 40 50 60 70 80 90 100 o 10 20 30 40 50 60 70 80 90 100

LENGTH (em)

Figure 32

Length composition of Greenland turbot taken during U.S.-Japan cooperative bottom-trawl surveys in the Aleutian Islands region in 1980

and 1983.

LEGEND

0---01983

• .1980

10

5

.~

'"o 6

4

2

1-200 201-300 301-500 501-900

Depth (meters)

Figure 33

Catch rate of Greenland turbot by depth during U.S.-Japan trawl surveys

in the Aleutian Islands region in 1980 and 1983.

26
WESTERN

(West of 180· W Longitude)

EASTERN

(East of 180· E Longitude)

1980

301-500 m

12311

I I

5 6

, (42)

1661 ,

, (221

I (431

I I I I

-L. lSI

---1...- (201

I I I 1

..L (101

I

--J...

I I I I

5 6 7 2 3 4

Bottom water temperature· C

32 4

- , (231

, (591

I

......L.- 12S)

I (361

...J... 1241

.....L..... (25) i

I

I

- ....L- (121 I

..I- 1401 I

r I I .~

1980

1983

1980

1983

1983

1980

1983

<200 m

501-900 m

201-300 m

Figure 34

Water temperatures near the sea bottom as observed during U.S.-Japan research trawl surveys in the

Aleutian Islands region, 1980 and 1983. Numbers of observations are given ill parentheses.

Discussion _

Distribution and migrations

Greenland turbot has certain distributional features throughout its

range which are also manifested in our findings for this species in

the eastern Bering Sea and Aleutians. There is an apparent

bathymetric gradient involving fish size and age with the younger

animals tending to occupy shallower depths, and the older animals

being more likely to reside at depths of the continental slope. At

shallow depths the young are exposed to a wide range oftemperature

conditions, whereas older animals along the slope live where

temperatures are relatively stable. Seasonal movements are also suggested from our fmdings and the observations of other investigators.

For the eastern Bering Sea, Shuntov (1970) discussed seasonal

depth movements of both young and older animals. Movement of

young fish is confined to the continental shelf. In spring the movement is from deep parts of the shelf to shallower parts, followed

by another movement to even shallower depths in summer. By fall

the fish have returned to deeper waters of the shelf. The spring

and fall movements are believed to occur in response to changes

in water temperatures, whereas the summer movement is believed

to be a response to feeding. For adults, Shuntov (1970) showed

two concentrations of animals along the slope-one west and

southwest ofSt. Matthew Island, and the other farther south between

the Pribilof Islands and the Unimak Pass. He found that both concentrations of fish exhibited a spring to summer, feeding-related

movement from deeper parts of the slope to shallow parts, followed

by a return movement to deeper waters in October.

For animals occupying the slope, our findings suggest seasonal

depth changes. Ifwe interpret seasonal shifts in availability by depth

as indicative of movements, then there is some suggestion, particularly in the southern slope area, that movements of large fish

into deeper water occur in the fall (Fig. 20).

We were not able to discern any long-distance migrations related

to spawning and feeding as reported for the northwestern Atlantic

stock (Bowering 1984) and the Icelandic stock (Sigurdsson 1979).

However, our findings for the eastern Bering Sea do suggest a longterm movement of maturing fish from the northwest slope towards

the predominantly mature population in the central and southern

slope regions, where the majority of spawning is believed to take

place. Similar movements of maturing animals towards the spawning

region have been alluded to for the Greenland turbot stock of the

northwestern Atlantic and the stock off Norway. Bowering (1984)

has presented evidence that maturing fish move from the Newfoundland-Labrador area to the spawning grounds in Davis Strait,

and he raises the possibility that these fish may remain in that vicinity, namely off West Greenland. Based on north-south changes in

fish size for the stock off Norway, God~ and Haug (1987) hypothesize a movement of animals from the vicinity ofthe nursery grounds

off Spitsbergen south to the spawning grounds along the continental slope between Spitsbergen and northern Norway.

Recruitment hypothesis

We propose for the eastern Bering Sea-Aleutian Islands population of Greenland turbot the following hypothesis for describing

the temporal and spatial paths by which young animals reach the

mature portion of the population. We begin with the assumption

that most spawning occurs in the deep part of the slope. region

between Unimak Pass and the vicinity of Pribilof Islands in the late

fall and early winter. Several Soviet investigations (PertseyaOstroumova 1961, Musienko 1970, Shuntov 1970, Bulatov 1983)

have indicated a spawning period in the Bering Sea that begins as

early as September and ends as late as March, with peak spawning

in November-December. As for location, Bulatov (1983) places

spawning on the slope off Unimak Pass, whereas others (Musienko

1970, Pertseva-Ostroumova 1961) refer to a more northern location in the Bering Sea from off St. Matthew Island to the Gulf of

Anadyr. It is likely that spawning occurs throughout the range of

the adult fish, an area inclUding the Aleutians, but we argue that

the heaviest spawning occurs between Unimak Pass and the

27
Pribilofs. This is based on 1) Waldron (1981), who shows the highest

occurrences of larvae in this region, and 2) our findings, which

indicate that most adult fish were found in the central and southern

slope regions.

Eggs and early larvae are bathypelagic, but with growth the larvae become epipelagic (Musienko 1970, Jensen 1935 cited by Atkinson et al. 1981). Larvae are in rnidwater from early winter to early

fall before they settle to the sea bottom. Waldron (1981) shows

highest catches of larvae in plankton surveys during the springsummer period in the vicinity of the shelf's edge and slope, with

small catches scattered in the shelf region and a few occurrences

over deep water; larvae were found as far north as the vicinity of

St. Matthew Island. A fall settling of the larvae is suggested from

the reports of Hognestad (1969), Smidt (1969), and Atkinson et

al. (1981) for the North Atlantic form of the species. However,

in the eastern Bering Sea, a few O-age fish were found during July

and August surveys of the shelf, implying that some larvae apparently settle earlier than the fall in this region.

Young-of-the-year were found mainly in the central and northern

areas ofthe shelf, occurring most densely in the northern area. They

apparently spend their early years at shelf depths, but by age 4 and

particularly by age 5, many of them have moved into the slope

region. The fish on the shelf were consistently found at highest densities in an area west and southwest of St. Matthew Island. We

believe that this high-density area is an important pathway by which

young animals move from the shelf into the slope region. The area

lies adjacent to the northern and central slope regions where we

find the highest catch rates as well as the highest proportions of

older immatures. We assume these fish move south as they age,

toward the southern slope region where mature fish are predominant.

For the Aleutian Islands region, we hypothesize that spawning

takes place there because of the presence of large, adult fish yearround. However, due to the lack of any extensive shelf, and perhaps

because of currents, there is little settling of the larvae in this region

as suggested from the infrequent encounters of small fish «35 or

40 cm in length) in this region by research surveys and fisheries.

The eastward current on the northern side of the Aleutians (Kitani

1983) could be a means by which eggs and larvae are transported

towards the broad shelf area of the southern part ofthe eastern Bering Sea. We, therefore, assume that most adult fish in the Aleutians spend their juvenile stages in the shelf region of the eastern

Bering Sea.

In summary, we believe that the main spawning area lies in the

slope region between Unimak Pass and the Pribilof Islands. Larvae produced from spawning in this area tend to be transported

towards the central and northern portions of the continental shelf

where they settle on the sea bottom in the summer and fall. The

young begin to move from the shelfto the deeper water of the slope

by ages 4 and 5. As they mature they move towards the spawning

area. Most fish in the Aleutians are believed to have spent their

early years on the eastern Bering Sea shelf. We propose this recruitment hypothesis as a framework for future investigations. In 1986

a tagging program was begun to test this hypothesis.

Current condition of eastern Bering Sea-Aleutians stock

Our fmdings suggest that since 1981 the population has been declining and ageing. The number of small fish «40 cm) on the shelf

and intermediate-size fish (40-55 cm) on the slope has decreased.

This is evident from the changes in catch rates by size on the shelf

(Fig. 30) and in CPUE by size in the slope region of the eastern

Bering Sea (Figs. 15-17). Fish in the intermediate size range of

28

40-55 cm were much more available to the Japanese small freezer

trawlers in 1979 and 1980 than in subsequent years. By 1982 their

availability declined; these fish were either dying (natural and fishing

mortality) or growing into the next size category with insufficient

younger fish to replace their numbers. This decline in intermediatesize fish was also evident in the Aleutians by their decreasing

proportion in the total size composition of animals taken in the commercial (Fig. 18) and research (Fig. 32) catches. Our findings suggest that this decline was brought about by a succession of weak

year-classes spawned between 1979 and 1984.

Causes of recruitment failure

We assume that recruitment failure is real based on research survey

and fisheries findings. Our reservations to this assumption arise

because of the absence of 1) research trawl surveys in the northern

part ofthe eastern Bering Sea shelf and 2) data on recruitment prior

to 1973 because of no sampling. The decline in young fish abundance may in actuality be the result of a distributional shift of the

young fish towards the north and beyond the research trawl survey

area. We raise this as a possibility, although the decline of intermediate-length fish (30 to 55 cm) on the slope would argue against

this. We have been assuming that the abundance of young fish on

the shelf during the years 1973-79 was typical, when, in fact, it

may be the result ofsome unusual conditions bringing about strong

year-classes. What we perceive as a decline in recruitment may in

fact be a retreat to normal recruitment levels.

Assuming that recruitment failure is real, several possibilities may

account for it. Recruitment failure may be the result of an insufficient number of spawners. The spawning stock has been declining, but we have no understanding as to the relationship ofspawners

to the number of recruits to give us any guidelines for considering

this possibility.

An obvious potential source of mortality of the young would be

the trawl fisheries on the shelf. In the years 1977-79 the Japanese

mothership fishery took a considerable number of Greenland turbot as a bycatch in their pollock fisheries. This bycatch amounted

to an average ofsome 41 million animals a year. Yet these removals

(Table 6) were only a small fraction «5%) of the animals estimated

to be in the shelf area by National Marine Fisheries Service surveys

in 1979 and 1980 (Table 10) and should have had no serious impact on the abundance of young fish in those years.

One source of mortality that could potentially account for the

decline of young fish would be increases in predator populations

or a predator shift toward Greenland turbot. Here we are faced with

a wide choice of potential predators from carnivorous zooplankters,

to fish, to marine mammals. Unfortunately, we can say nothing

about zooplankters except that they could do damage at the egg and

early larval stages.

Observations of Greenland turbot in stomachs of fish are rare.

The Trophic Interactions Program at the NWAFC has examined

over 15,000 stomachs offish from the eastern Bering Sea, including

Pacific cod, walleye pollock, Greenland turbot, arrowtooth flounder,

and others. Yet the only instances of predation on Greenland turbot were by adult yellowfin sole (limanda aspera) and flathead sole

on young-of-the-year fish. In the five occurrences of turbot in

yellowfin sole stomachs, the prey ranged in length from 22 to 33

mm, a size which suggest that the turbot had not yet settled to the

sea bottom. The single occurrence of turbot predation by a flathead

sole was on a young fish whose length was not recorded. Potentially, yellowfin sole could have an impact on young-of-the-year

Greenland turbot, since yellowfin sole is one of the most abundant
flatfish of the eastern Bering Sea and is distributed at shelf depths.

The biomass of yellowfin sole as estimated from NMFS trawl

surveys almost doubled between 1975 and 1979 (from 1.0 to 1.9

million t), and peaked at over 2.0 million t in 1983 (Bakkala et al.

1985).

There has not only been an increase in the abundance ofyellowfm

sole, but Pacific cod and other fish populations have also increased

during the period of Greenland turbot year-class decline.

Atlantic cod is known to prey on young Greenland turbot in the

northwest Atlantic (Jensen 1935, Hansen 1949, Dunbar and

Grainger 1952, Lear and Pitt 1975). Smidt (1969) has even suggested that the decline of the Greenland stock ofturbot in the 1930's

may have been due to cod, which were abundant in those years.

Although we have no evidence that Pacific cod is an important

predator ofturbot, we know that cod biomass in the eastern Bering

Sea increased four- to fivefold between 1976 and 1979 and then

continued to increase, reaching 1.13 million t in 1986 (Thompson

and Shimada 1986). In regions of the shelf where Greenland turbot has historically been abundant, cod abundance, as assessed by

the NWAFC's annual surveys, increased dramatically between 1975

and 1979, and exceeded Greenland turbot abundance after 1981.

Pacific cod is therefore suggested as possibly being an important

predator of Greenland turbot based on overlapping distributions,

an inverse relational change in abundance, and the inference that

if Atlantic cod preys on young turbot its congener may also have

the potential to feed on them. However, fmdings by Shimada et al.

(1986) based on the examination of over 1,850 cod stomachs collected in 1981 from the eastern Bering Sea, suggest that Greenland

turbot is of little consequence in the cod's diet. This may be a reflection of the narrow time and area window of their sampling. There

were no samples collected from the fall period when most settling

of young turbot is believed to occur.

Arrowtooth flounder and sablefish (Anoplopomafimbria) are other

species in the eastern Bering Sea whose abundance increased during the period of turbot decline. As arrowtooth flounder abundance

on the shelf rose beginning in 1980, Greenland turbot abundance

declined so that by 1983-85 arrowtooth flounder abundance exceeded that of Greenland turbot (Bakkala et al. 1986). High-density

areas of arrowtooth flounder lie at outer shelf depths (100-200 m)

and on the continental slope. In 1978 sablefish were encountered

in unusual numbers during an NWAFC survey on the shelf (Umeda

et al. 1983). These fish, belonging to the 1977 year-class, shifted

from the shelf to the slope region in 1979, and by 1981 their abundance on the slope was exceptionally high compared with earlier

surveys. Both arrowtooth flounder and sab1efish are potential

predators of young Greenland turbot in its pelagic stage or after

it settles on the sea bottom.

Among marine mammals, Kajimura (1984) has found that juvenile

Greenland turbot was the fourth-ranking finfish (in volume of food)

in stomachs of fur seals captured near the Pribilof Islands. One

animal had as many as 500 Greenland turbots in its stomach.

Samples of these juveniles were 45-60 mm in length and of a size

which suggested that they had either settled to the sea bottom or

were approaching that stage. Presumably, the fur seal had captured

them off the sea bottom, since fur seals tend to feed on nektonic

fishes and invertebrates. In the 3 years (1964, 1973, 1974) for which

detailed information on the feeding of fur seals in the eastern Bering Sea is available, Greenland turbot comprised from 1.1 to 2.1 %

ofthe total volume offur seal stomach contents (Fiscus and Kajimura

1965, Kajimura et al. 1974, Kajimura and Sanger 1975). This could

translate to a substantial number of turbot, considering the energy

needs of the fur seals and assuming that fur seals consume mainly

29

juvenile turbot. The population of female seals alone has been

estimated at 570 x 103, (Briggs and Fowler 1984). They forage

and feed their young during the period when the turbot larvae are

in the water colunm or settling to the sea bottom (June to

November).

Fur seals have been in the Bering Sea in large numbers for a long

time, but why should they suddenly begin to seriously reduce the

abundance of young Greenland turbot? Perhaps predation by fur

seals, along with that of increased predation by other species, could

be accounting for the year-class decline.

As in many attempts to explain changes in the year-class strength

of a species, there is no concrete evidence here for a particular

causative factor or factors. Several possibilities have been discussed

for the purpose of evaluating either their plausibility or their promise

as avenues for further study. We have emphasized predation by

fish and fur seals in our discussion, but that is because most of our

data on abundance changes, and predation of eastern Bering Sea

biota deal with these animals. Ifwe knew more about the predatory

habits of zooplankon, squid, or bottom invertebrates along with their

temporal-spatial changes in abundance, our discussion would have

been more extensive. The same could be said about parasitism,

disease, and changes in the physical environment. Therefore, we

are left with a considerable degree of uncertainty as to the causes

of the continuing decline in year-class abundance of Greenland turbot. There is also the possibility alluded to earlier that the decline

may be a return to more normal levels of recruitment from high

levels during the 1970's and not the result of a recruitment failure.

Acknowledgments _

The authors thank the following individuals for their review of certain parts of the manuscript and their valuable comments: William

R. Bowering of the Northwest Atlantic Fisheries Center in St.

John's, Newfoundland; Olav R. God~, Institute of Marine Research

in Bergen, Norway; Hiroshi Kajimura of the National Marine Mammal Laboratory in Seattle, Washington; Adalsteinn Sigurdsson,

Marine Research Institute in Reykjavik, Iceland; and Hirotsune

Yamaguchi, Tohoku Regional Fisheries Research Laboratory in

Hachinohe, Japan. Thanks is also expressed to Hiroshi Kajimura

for providing unpublished data on Greenland turbot found in the

stomachs of fur seals, and to the Fisheries Agency of Japan for

age data.
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