NOAA Technical Report NMFS 126 December 1996

Ecology ofJuvenile Walleye Pollock,

Theragra chalcogramma

Papersfrom the workshop

"The Importance of

Prerecruit Walleye

Pollock to the Bering

Sea and North Pacific

Ecosystems"

Seattle, Washington,

28-30 October 1993

Edited by

Richard D. Brodeur

Patricia A. Livingston

Thomas R. Loughlin

Anne B. Hollowed

U.S. Department of Commerce
u.s. Deparnnent

of Conunerce

William M. Daley

Secretary

National Oceanic and

Annospheric Acbninistration

D.James Baker

Under Secretary for Oceans and

Atmosphere

National Marine

Fisheries Service

Rolland A. Schmitten

Assistant Administrator for

Fisheries

The NOAA Technical Report NMFS

(ISSN 0892-8908) series is published by

the Scientific Publications Office, National

Marine Fisheries SeIVice, NOAA, 7600

Sand Point Way NE, Seattle, WA 98115-

0070.

Although the contents have not been

copyrighted and may be reprinted entirely,

reference to tlle source is appreciated.

The Secretary of Commerce has determined that the publication of this series is

necessary in the transaction of the public

business required by law of this Department. Use of funds for printing of this series has been approved by the Director of

tlle Office of Management and Budget.

For sale by the U.S. Department of

Commerce, National Technical Information Service, 5285 Port Royal Road,

Springfield, VA 22161.

NOAA

Technical

Reports NMFS

Technical Reports of the Fishery Bulletin

Scientific Editor

Dr. John B. Pearce

Northeast Fisheries Science Center

National Marine Fisheries Service, NOAA

166 Water St.

Woods Hole, Massachusetts 02543-1097

Editorial ColllDlittee

Dr. Andrew E. Dizon National Marine Fisheries Service

Dr. Linda L. Jones National Marine Fisheries Service

Dr. Richard D. Methot National Marine Fisheries Service

Dr. Theodore W. Pietsch University of Washington

Dr. Joseph E. Powers National Marine Fisheries Service

Dr. Tbn D. Smith National Marine Fisheries Service

National Marine Fisheries Service

Scientific Publications Office

7600 Sand Point Way NE, BIN C15700

Seattle, Washington 98115-0070

The NOAA Technical Report NMFS series of the Fishery Bulletin carries peer-reviewed, lengthy original research reports, taxonomic keys, species synopses, flora

and fauna studies, and data intensive reports on investigations in fishery science,

engineering, and economics. The series was established in 1983 to replace two

subcategories of the Technical Report series: "Special Scientific Report-Fisheries" and "Circular." Copies of the NOAA Technical Report NMFS are available free

in limited numbers to government agencies, both federal and state. They are also

available in exchange for other scientific and technical publications in the marine

sciences. Individual copies may be obtained from the U.S. Department of Commerce, National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161.
NOAA Technical Report NMFS 126

A Technical Report of the Fishery Bulletin

Ecology ofJuvenile Walleye Pollock,

Theragra chalcogramma

Papersfrom the workshop "The Importance ofPrerecruit Walltye Pollock

to the Bering Sea and North Pacific Ecosystems"

Seattle, Washington, 28-30 October 1993

Richard D. Brodeur

Patricia A. Livingston

Thomas R. Loughlin

Anne B. Hollowed (editors)

Sponsored by:

Alaska Fisheries Science Center

National Marine Fisheries Service

U.S. Dept. of Commerce, NOAA

Seattle, Washington

December 1996

u.s. Department of Commerce

Seattle, Washington

CONTENTS _

INTRODUCTION

R. D. BRODEUR

P. A. LIVINGSTON

T. R. LOUGHLIN

A. B. HOLLOWED

SESSION I: DISTRIBUTIONAND ABUNDANCE

1

H. MIYAKE

H. YOSHIDA

Y. UEDA

M. T. WILSON

R. D. BRODEUR

S. HINCKLEY

D.R.McKELVEY

Q. TANG

X.JIN

F. LI

J. CHEN

W.WANG

Y. CHEN

X. ZHAO

F. DAI

T. WYLLIE-ECHEVERRIA

J. TRAYNOR

D. SMITH

M. S. BUSBY

R. D. BRODEUR

M. T. WILSON

M. T. WILSON

M. SHIMA

M. SHIMA

A. B. HOLLOWED

G. VANBLARICOM

Distribution and abundance of age-O juvenile walleye pollock,

Theragra chalcogramma, along the Pacific coast of southeastern

Hokkaido,Japan 3

Distribution and abundance of age-O walleye pollock, Theragra chalcogramma,

in the western Gulf ofAlaska during September 1990 11

Juvenile walleye pollock, Theragra chalcogramma, distribution and abundance

in Shelikof Strait-what can we learn from acoustic survey results? 25

Summer distribution and abundance of age-O walleye pollock,

Theragra chalcogramma, in the Aleutian Basin 35

The relationship between the distribution of one-year-old walleye pollock,

Theragra chalcogramma, and sea-ice characteristics 47

Summer distribution and abundance of age-O walleye pollock,

Theragra chalcogramma, in the Bering Sea (abstract) 57

Distribution of early juvenile walleye pollock, Theragra chalcogramma,

and associated species in the Gulf of Alaska (abstract) 60

Size-related depth distribution ofjuvenile walleye pollock,

Theragra chalcogramma, in the western Gulf of Alaska during

1984-1990 (abstract) 61

Juvenile walleye pollock, Theragra chalcogramma, distribution and

association with other dominant species in the Gulf of Alaska (abstract) 62

iii
D. R. GUNDERSON Distribution and abundance ofjuvenile walleye pollock,

Theragra chalcogramma: summary and recommendations for future research 63

SESSION II: ECOLOGYAND GROWTH

N. MERATI

R. D. BRODEUR

A. NISHIMURA

K-I. MITO

T. YANAGIMOTO

J. F. MORADO

D. A. McFEE

A. L. BROWN

K M. BAILEY

O. S. TEMNYKH

V. I. RADCHENKO

B. L. OUA

M. W. DAVIS

C. H. RYER

S.M.SOGARD

B. L. NORCROSS

K P. SEVERIN

J. CARROLL

R. I. PERRY

Feeding habits and daily ration ofjuvenile walleye pollock,

Theragra chalcogramma, in the western Gulf of Alaska 65

Hatch date and growth estimation ofjuvenile walleye pollock,

Theragra chalcogramma, collected in the Bering Sea in 1989 and 1990 81

Diseases and parasites ofjuvenile walleye pollock, Theragra chalcogramma,

from the Gulf of Alaska, 1986-1988 89

Growth ofjuvenile walleye pollock, Theragra chalcogramma,

in the Gulf of Alaska (abstract) 104

The growth of walleye pollock, Theragra chalcogramma,

and structure ofscales during the first year of life (abstract) 105

Distribution, recruitment, and survival ofjuvenile walleye pollock,

Theragra chalcogramma: what role does behavior play? (abstract) 108

Electron microprobe analysis of otoliths ofjuvenile walleye pollock,

Theragra chalcogramma, from Alaska: a pilot stock separation study (abstract) 109

Ecology and growth ofjuvenile walleye pollock, Theragra chalcogramma:

summary and recommendations for future research III

SESSION III: PREDATOR-PREYRElATIONS

P. A. LIVINGSTON

G.M.LANG

G. L. HUNTJR.

A. S. KITAYSKY

M. B. DECKER

D. E. DRAGOO

A. M. SPRINGER

L. F. LOWRY

V. N. BURKANOV

KJ. FROST

R. L. MERRICK

D. G. CALKINS

Interdecadal comparisons of walleye pollock, Theragra chalcogramma,

cannibalism in the eastern Bering Sea

Changes in the distribution and size ofjuvenile walleye pollock,

Theragra chalcogramma, as indicated by seabird diets at the Pribilof Islands

and by bottom trawl surveys in the eastern Bering Sea, 1975 to 1993

Importance of walleye pollock, Theragra chalcogramma, in the diet of

phocid seals in the Bering Sea and northwestern Pacific Ocean

Importance ofjuvenile walleye pollock, Theragra chalcogramma,

in the diet of Gulf of Alaska Steller sea lions, Eumetopiasjubatus

iv

115

125

141

153
E. H. SINClAIR

G. A. ANTONELIS

B. W. ROBSO

R.R.REAM

T. R. LOUGHLI

L. W. FRITZ

V. P. SHUNTOV

E. P. DULEPOVA

M.-S. YANG

P. A. LIVINGSTON

R. D. BRODEUR

A. M. SPRINGER

M.B.DECKER

G. L. HUNTJR.

G. V. BYRD JR.

J. F. PIATT

S.A. HATCH

G. R. LILLY

Northern fur seal, Callorhinus ursinus, predation on

juvenile walleye pollock, Theragra chalcogramma

Juvenile walleye pollock, Theragra chalcogramma, bycatch in

commercial groundfish fisheries in Alaskan waters

Walleye pollock, Theragra chalcogramma, in the western Pacific Ocean

and Bering Sea ecosystems (abstract)

Juvenile walleye pollock, Theragra chalcogramma,

as food for teleosts in the Gulf of Alaska ecosystem (abstract)

Prerecruit walleye pollock, Theragra chalcogramma,

in seabird food webs of the Bering Sea (abstract)

Relationship between sea-surface temperature, abundance ofjuvenile

walleye pollock, Theragra chalcogramma, and the reproductive success and

diets ofseabirds at the Pribilof Islands, southeastern Bering Sea (abstract)

Seabird predation on juvenile walleye pollock, Theragra chalcogramma,

and other forage fishes in the Gulf of Alaska (abstract)

Predator-prey relationships ofjuvenile walleye pollock,

Theragra chalcogramma: summary and recommendations for future research

167

179

196

197

198

202

204

205

SESSIONW: POPULATION DYNAMICSAND MODEliNG

S. HINCKLEY

B. MEGREY

A.HERMANN

B. A. MEGREY

V. G. WESPESTAD

T.J. QUINN II

P. A. LIVINGSTON

R. D. METHOT

A. B. HOLLOWED

K. M. BAILEY

R. D. BRODEUR

M.ADKISON

M. PASCUAL

An individual-based model of the juvenile stage of walleye pollock,

Theragra chalcogramma, in the western Gulf of Alaska (abstract)

On the relationship ofjuvenile walleye pollock, Theragra chalcogramma,

abundance to adult recruitment and linkages with the environment

(abstract)

Importance of cannibalism in the population dynamics of

walleye pollock, Theragra chalcogramma (abstract)

Stock synthesis model of walleye pollock, Theragra chalcogramma,

including predation (abstract)

atural mortality estimates ofjuvenile walleye pollock,

Theragra chalcogramma, in the Gulf of Alaska (abstract)

Using mechanistic models to test hypothesized causes for Steller sea lion,

Eumetopiasjubatus, declines (abstract)

v

209

211

212

217

218

221
w.J. OVERHOLTZ Population dynamics and modeling ofjuvenile walleye pollock,

Theragra chalcogramma: summary and recommendations for future research 223

WORKSHOP PARTICIPANTS 225
Introduction

RICHARD D. BRODEUR, PATRICIA A. LIVINGSTON,

THOMAS R. LOUGHLIN, and ANNE B. HOLLOWED

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115

The Alaska Fisheries Science Center (AFSC) , National

Marine Fisheries Service (NMFS), hosted an international workshop, "The Importance of Prerecruit Walleye Pollock to the Bering Sea and North Pacific Ecosystems," from 28 to 30 October 1993. This workshop was

held in conjunction with the annual International North

Pacific Marine Science Organization (PICES) meeting

held in Seattle. Nearly 100 representatives from government agencies, universities, and the fishing industry in

Canada,Japan, the People's Republic of China, Russia,

and the United States took part in the workshop to

review and discuss current knowledge on juvenile pollock from the postlarval period to the time they recruit

to the fisheries. In addition to its importance to humans as a major commercial species, pollock also serves

as a major forage species for many marine fishes, birds,

and mammals in the North Pacific region.

This workshop originated in 1990, as we lamented

how little we knew about the juvenile phase of walleye

pollock relative to the larval and adult phases. Although

this generality may apply to almost any marine fish

species, we thought that the lack of knowledge was particularlyacute for pollock, considering their importance

to the North Pacific ecosystem. As an example of the

underemphasis on the juvenile phase of pollock life

history, the published proceedings of the last major

workshop on walleye pollock held in 1988 in Anchorage, Alaska, contained almost 800 pages, of which only

about 30 (less than 5%) pertained to juvenile pollock.

We were aware that scientists throughout the North

Pacific were interested, as we were, in juvenile pollock,

not only for its biology, but also for its importarice as

prey for a wide diversity of marine organisms. We

thought that much could be gained by gathering these

researchers together to discuss what is known, and alsoperhaps more important-what we still need to learn

about juvenile walleye pollock. We believed that the

timing of this workshop was particularly appropriate

because the North Pacific ecosystem has undergone

some extensive changes in the last two decades because of

both natural and human-induced perturbations.

The workshop was opened by William Aron, Director, AFSC, who challenged us to see beyond our own

research and to work together to understand pollock's

importance to the ecosystem. Warren Wooster, Chairman, PICES, next spoke on how this workshop fit into

the overall objectives of PICES. Michael Sissenwine,

Senior Scientist, NMFS, Silver Spring, Maryland, presented a keynote address on the general importance of

juvenile fishes to the productivity and management of

marine ecosystems. Sissenwine has long been a proponent of studying the juvenile phase as a way to understand recruitment of marine fishes, and he brought to

the workshop much experience with juvenile gadids.

The workshop continued with sessions on abundance

and distribution, ecology and growth, predator-prey

relations, and population dynamics and modeling, with

invited experts from each field serving as session moderators. Overall, 70 authors contributed to 29 oral and

7 poster presentations. All contributors were invited to

submit their papers for this publication, and 14 full

articles were submitted. Each manuscript was reviewed

by at least two anonymous referees consisting of both

workshop participants and "outside" reviewers. When

papers were to be published elsewhere, the authors

were given the option of publishing an extended abstract in these proceedings. These abstracts and the

comments of the session chairpersons did not receive

extensive peer review.

As workshop conveners, we would like to thank all

those who contributed to the success ofthis meeting. In

particular, our Research Center and Division Directors-William Aron, Jim Balsiger, Gary Stauffer, Richard Marasco, and Howard Braham-provided financial

support to convene the meeting and edit the proceedings. The PICES Secretariat provided logistical support

throughout the workshop. We are indebted to the numerous AFSC personnel who helped with registration,

1
2 NOAA Technical Report NMFS 126

audiovisual presentations, and other technical aspects

ofthe workshop. We thank Ron Hardy and his editorial

staff for their assistance in the review process, Martha

Jackson and Gary Duker for assistance in editing the

proceedings, and also the many reviewers who helped

improve the manuscripts. Finally, we thank the session

moderators-Don Gunderson, Ian Perry, George Lilly,

and Bill Overholtz-for keeping the workshop on schedule and for offering insightful comments on their sessions both in their talks during the workshop and in

their written reports for these proceedings. Their generous giving of time contributed significantly to what

we consider a very successful workshop.
Distribution and Abundance of Age-O Juvenile Walleye Pollock,

Theragra chalcogramma, along the Pacific Coast

of Southeastern Hokkaido, Japan

HIROYA MIYAKE1

Hokkaido Kushiro Fisheries Experimental Station

2-6 Hama-cho

Kushiro, Hokkaido 085, Japan

HIDEO YOSHIDA

Hokkaido Central Fisheries Experimental Station

Hamanaka 238, Yoichi-cho,

Hokkaido 046,Japan

YOSHIYUKI VEDA

Muroran Branch, Hokkaido Hakodate Fisheries Experimental Station

Muroran, Hokkaido 051,Japan

ABSTRACT

Age-O juveniles of walleye pollock, Theragra chalcogramma, were studied along the coast

ofsoutheastern Hokkaido in autumn in order to describe interannual variations in distribution and abundance and to evaluate the feasibility of usingjuveniie abundance to measure

year-class strengths. The shishamo trawl net surveys for the assessment of shishamo smelt,

Spirinchus lanceolatus, were conducted in 1978-92, and bycatch data for walleye pollock were

collected. Age-O walleye pollockjuveniles were found mainly in the 50-m and the 60-m depth

strata, with the highest abundance offOhtsu. This indicates thatjuvenile pollock move from the

surface layer to the bottom layer, deeper than 40 m offsoutheastern Hokkaido, as they grow. An

age-O abundance index was calculated by summing the CPUE's of all 10-m depth intervals

for each year. There was no significant relationship between abundance indices and estimated numbers of age-O fish from VPA. Before 1987, however, abundance indices were

significantly correlated with estimated numbers from VPA (r=0.76).

Introduction

A key issue in fisheries biology is determining when the

relative strength of a year class is to be established. In a

management context, this information is useful for predictive purposes, revealing when prerecruit surveys

should be conducted. In order to give fisheries managers maximum lead time for establishing harvest levels,

surveys should be conducted as early as possible (Bailey

and Spring, 1992). Walleye pollock, Theragra chalcogramma, is important for commercial fisheries around

Japan. Its annual catch from the Pacific Ocean offJapan was 166,304 t in 1990, and 50,000-80,000 t from off

southeastern Hokkaido.

Recruitment appears to be the major factor that drives

change in the biomass of this stock. Bailey and Spring

(1992) found that age-O juvenile surveys can detect

relatively strong or weak year classes in the western Gulf

of Alaska. Many studies of the egg, larval, and early

juvenile stages of walleye pollock in the Pacific coast

population of Hokkaido have been undertaken (e.g.,

1 Current address: Hokkaido Wakkanai Fisheries Experimental Station, Wakkanai, Hokkaido 097,japan.

3
Figure 1

Survey area in southeastern Hokkaido showing the three

regions used to calculate local abundance indices.

in standard length, so we classified walleye pollock

smaller than 15 em as age-O juveniles. A Nansen bottle

sampler was used to gather information on the water

temperature of the bottom layer at each station.

We estimated the abundance index by summing up

the catch per unit of effort (CPUE) of each interval of

10-m depth in each region (Fig. 1) over all areas; 0

4 NOAA Technical Report NMFS 126

Kanamaru, 1985; Nakatani and Maeda, 1987; Nakatani,

1988), but little is known of the biology of age-O juveniles of walleye pollock around Japan in autumn (e.g.

Ishigaki et ai., 1960; Hayashi, 1976).

We describe the geographical distribution and abundance of the juvenile stage of walleye pollock that have

settled to the bottom layer. We then estimate an abundance index ofjuveniles from the results ofsurveys that

have been conducted in September and October since

1978. Finally, we evaluate the feasibility of using our

abundance index for estimating year-class strength.

Methods and Materials

Shishamo trawl net surveys for the assessment of shishamo smelt, Spirinchus lanceolatus, stock have been

conducted along the Pacific coast ofsoutheastern Hokkaido in September and October since 1978 (Fig. 1).

Captain Nahira modified the shishamo smelt beamtrawl net by taking the beam off, changing the net size,

making the upper net longer than the ground net, and

so on (Fig. 2). A total of 1,152 tows were made at 38

main stations and at some additional points, mainly off

Kushiro region. The No.5 Tensho-maru was used for all

surveys. Bycatch data for age-O walleye pollock were

recorded. We took samples of up to 100 individuals

each year for standard length measurements. We were

able to discriminate age-O walleye pollock from older

age classes by their length distribution in this season.

Almost all age-O walleye pollock ranged from 7 to 13 em

1400 1420

1440

440

420

h OHTSU

KUSHIRO

20mm mesh

Figure 2

Shishamo trawl net as modified by Captain Nahira.
__ Miyake et al.: Distribution and Abundance of Age-O Juvenile Walleye Pollock along the Pacific Coast ofJapan 5

catches were included. The equation used was

M N C

y = £..J

""

£..J

""

----!L..

;=1 j=l XI}

where Y is the abundance index, K is the number of

tows, G. is the total catch in the ii th area, i is the i th lJ :J

depth stratum, j is the j th region, N is the number of

depth strata, and M is the number of regions.

We compared abundance indices of age-O fish with

estimated numbers of age-O and age-4 fish derived from

virtual population analysis (VPA) for the Pacific stock

around Japan (Watanabe2).

each year by the three regions shown in Figure 1. In

1978,1979, and 1980, the highest index ofjuveniles was

in region I (off Kushiro); in 1981 and 1982 the highest

index was in region III (off Hiroo). Region II (off

Ohtsu) was dominant from 1983 through 1991. However

indices were very low in all three regions in 1992 (Fig. 5).

The bottom-water temperature ranged widely, from

6° to 16°C, in the areas where catches of age-O juveniles

were observed through all years (Fig. 6). We did not

catch any O-age juveniles in the areas where bottom

temperatures were lower than 6° or higher than 16°. The

highest catch (120 kg) was recorded at the station off

Ohtsu, where the bottom water temperature was 13°.

Abundance Indices

Results

Distributions ofJuvenile Pollock

The shishamo trawl net tows were made at depths from 5

m to 210m. After 1983, survey depths ranged from 5 m to

60 or 70 m in most years. The catches by haul of age-O

juveniles ranged from 0 kg to 120 kg. The largest catch was

observed in the 60-m depth stratum (Fig. 3).Juvenilesranged

from 5 cm to 15 cm standard length; average length was

9.4 cm from 1988 through 1991.Juveniles were found in

the 40-m to 100-m depth strata, mainly in the 50-m and 60-

m strata; very few were found shallower than 40 m in any

year (Table 1, Fig. 4). The highest CPUE was 100.0 kg/tow

in the 60-m depth stratum in region II in 1988.

To examine interannual changes in distribution along

the coast, we calculated the local abundance indices for

The changes in age-O abundance indices are shown in

Figure 7. Age-O juveniles were relatively abundant in

1978, 1981, 1986, and 1988-91. During 1988-91 the

abundance index was very high, but it decreased rapidly in 1992. The age-O abundance index of walleye

pollock ranged over two orders of magnitude, from

175.6 in 1989 to 1.2 in 1992.

The changes in estimated numbers of age-O juveniles

from VPA are also shown in Figure 7. The changes in

abundance indices and estimates based on VPA were

similar during 1978 to 1986. The relationship between

abundance indices and VPA estimates of age-O fish is

shown in Figure 8. The simple correlation coefficient

was not significant during 1978 to 1991 (r=-0.03, n=14);

however, it was highly significant from 1978 to 1986

(r=0.76, n=9, p=0.018).

2 Watanabe, K. Hokkaido National Fisheries Research Station,

Katsurakoi 116, Kushiro, Hokkaido, 085 Japan. Unpub!. data. Discussion

Abe (1968) reported that larval walleye pollock ranging

from 4 to 38 mm total length (TL) are distributed

mainly in the surface layer along the coast ofsoutheastern Hokkaido in June. When pollock reach 85 mm TL

in July, they move to the bottom layer in Funka Bay

(Nakatani and Maeda, 1987). These studies indicate

thatjuveniles migrate offshore and move to the bottom

layer as they grow. We found very few age-O juvenile

walleye pollock shallower than the 40-m depth stratum

in any year. A shishamo trawl net catches fish in the

bottom layer, and thus our results indicate that juveniles move from the surface layer to the bottom layer,

which is deeper than 40 m off southeastern Hokkaido

in autumn.

The highest age-O pollock catches were consistently

located in region II (off Ohtsu) from 1983 to 1991. In

other years, most fish were found in region I or III. Our

survey area was deficient in some strata, mainly those

-

T

-

T

I T/ \l

140

120

100

80

{j

1ij 60 u

40

20

o

o 20 40 60 80 100 120 140 160 180 200

Depth 1m)

Figure 3

Catches and depths. Dots show mean of catches and

bars show range of catches for each lo-m depth stratum.
Ql

Depth(m) 92 0

z

0

_____ I I ---......- I

>-l

l"l

I:l"

45 r--- \ ---------r ~I I I~I I --1---1 I I e.

l"l

e:.

40 r---- I --------r ~I I I~I I --r-----l I I--L I I~ I

'"0

0

35 ...

... .--.... z

30 "rl

'--" [JJ

w 25 - :::> ------ Ql

c.. 20

()

15

10

5

0

Figure 4

CPUE of age-O juvenile walleye pollock for each lO-m depth stratum from 1978 to 1992.
Table 1

CPUE of each IO-m depth stratum in the three regions, from 1978 to 1992.

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990

--

Depth (m) I II III I II III I II III I II III I II III I II III I II III I II III I II III I II III I II III I II III I II III

0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

10 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

20 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0

30 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.8 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.2 1.0 0.0 0.0 0.2 12.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 7.8 0.0 0.0 0.8

40 1.2 2.0 1.8 0.0 0.0 0.0 1.5 0.1 2.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 20 0.1 0.0 1.0 0.5 2.9 0.0 0.1 3.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.0 1.0 70.0 25.7

50 5.2 2.0 0.4 0.0 0.0 4.2 5.2 3.0 l.l 0.1 0.1 0.0 0.0 3.7 2.7 0.1 1.0 0.2 0.0 0.0 1.0 0.0 40.0 0.6 l.l 48 0.0 0.0 0.0 3.0 42.5 7.8 2.9 10.0 3.4

60 4.0 3.0 0.0 0.7 0.0 0.0 0.0 0.0 1.5 0.7 0.0 10.0 2.5 1.0 5.2 0.2 0.0 100.0 42.5 650 2.0 0.0 0.5 0.2

70 3.0 1.7 1.0 5.0 0.0 26.7 02 1.3 1.5 5.0 0.7 0.0 0.1

80 15.0 0.0 0.0 0.0 0.0

90 0.0 6.0 70.0 0.0 0.2 0.3

100 15.0 0.0 0.0 0.0 0.0

110 0.0 0.0

120 0.0 0.0

130

140

150 0.3

160 0.0

170 0.0

180 0.0

190

200 0.0 0.0

210 0.0

s::

i

"

t:i ; .

..

:!.

r::1'

:::

C".

0

= 1991 1992 I Q. =

>-

II III I II III r::1'

:::

=

Q.

0.0 0.0 = 0.0 0.0 0.0 0.0 0.0 0.0 '"

" 0.0 0.0 0.0 0.0 0.0 0.0 0

..., 0.1 0.0 0.0 0.1 0.0 0.0 >- CIll

17.6 0.0 0.3 0.1 0.0 0.0 "Q 6.2 60.0 1.5 0.6 0.3 0.0 '- 0.0 12.0 7.0 0.0 0.0 0.0 :::

<i

:."

;-

eo

;-

'<i

"'"I:l

g,

0'

'"ill:"

0'

CIll=..

::r

"'"I:l

~.

:l

'"C')

0

fIl

..

0

...,

'-

"0

=

..:r
o--t--""t-'--+

8 NOAA Technical Report NMFS 126

120

•I (off Kushiro)

100 III II (off Ohtsu)

III III (off Hiroo)

>< 80

Q)

\J

C

Q)

u

c 60 CO

\J

C

::J

..0

CO

CO 40

u

0

---1

20

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992

Year

Figure 5

Local abundance indices for the three regions, from 1978 to 1992.

- - - -- - - - - .. .a_I - ~. --- Ie (,_ l.)-_ -M~' ~_.. ... -.

140

120

100

OJ 80 6

.J::.

U

iii 60 0

40

20

o

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

Temperature (Oe)

Figure 6

Relation between catches and temperature of bottom layer, from 1978 to 1992.
___ Miyake et al.: Distribution and Abundance of Age-O Juvenile Walleye Pollock along the Pacific Coast ofJapan 9

180 4.0

160 .~ 3.5

140 9

3.0 Ol

0>

X cu_ Ol 120 .. - C "0 2.5 :: g .~ Ol .-

Ol 100 .. .o:e- () 85.6 c 20 if. cu

"0

C

80 ., ~> :::J '. 1.5 0 E .0 60 :;:; 0

« ~J::::

1.0 :::J

40 Q.

0

c..

20 0.5

0 00

78 79 80 81 82 83 84 85 86 87 88 89 90 91 92

Year

Figure 7

Change of age-O juvenile abundance index from 1978 to 1992 (solid line) and population

of age-O walleye pollock from VPA (dotted line). Numbers in figure are values of the

abundance index.

deeper than 70 m. In cases in which the highest CPUE

was in the edge stratum in the survey area, more fish

may have been distributed beyond that stratum. However, we observed such cases in only 5 years: 1980, 1983,

1987, 1988, and 1989. In the other 10 years our survey

probably covered the main distribution of juveniles

after their migration. The highest CPUE occurred most

frequently in the 50-m and 60-m depth strata in those

10 years (three times in each). Moreover, the largest

catch occurred in the 60-m stratum. Thus we concluded

that in autumn, O-age juveniles were located mainly in

the 50-m and the 60-m strata, with the highest abundance found off Ohtsu.

Kanamaru (1985) reported larvae under 10 mm long

distributed to the east of Kushiro. Our survey did not

cover this area, because this survey's main object was an

assessment ofshishamo smelt, and extensive surveys are

costly. However, survey there may yield further inforAbundance index

0.0

o w 100 1W1~1~1~ 50 100 150 200

Depth (m)

a

9.0

3.0

6,0

0.0

12.0

15.0

18.0

.90

89 •

.81 I r = -0.03

r = 0.76 ('78-'86)

.80

.82

• 87 .86 !J_9 tlT8

j33. 85 84

4.0

3.5 E

o

.l=

d; 3.0 0>

cu_

o § 2.5

m=-=

.0 :e- 2.0

E«

:::Jc..

> 1.5 o

10

:;

Q.

0.5

Figure 8

Relation between age-O juvenile abundance index and

population of age-O walleye pollock from VPA. Numbers in figure indicate years.

Figure 9

Relation between depth and bottom temperature at

survey points.
10 NOAA Technical Report NMFS 126

mation on interannual shifts in pollock distribution.

Thus we hope to extend the survey area eastward of

Kushiro.

Moreover, age-O juveniles were observed in bottom

water ranging from 6° to 16°C. alia and Davis (1990)

showed that juvenile pollock avoid cold water (3°C) in

the bottom of an experimental tank. This indicates that

cold water in the offshore bottom layer may be an

obstacle to age-O pollock. Our results show that cold

water under about 6°C forms an obstacle. This temperature was observed from 50 m to 110 m depth (Figure 9). Therefore, survey for age-O walleye pollock

should be extended to the 100-m depth stratum.

During 1988-91 this survey overestimated the abundance of age-O juveniles, compared with VPA estimates.

Therefore, we did not find a statistically significant

linear correlation between abundance indices and estimated numbers from VPA. However, before 1986 the

trends of abundance indices and VPA estimates were

very similar, and the correlation between them was

significant. Our survey did not cover all areas deeper

than 70 m after 1987, however we believe that our

survey covered the areas ofabundance, as discussed above.

Thus the low correlation coefficient was possibly caused

by some change around 1987. For example, the fisheries

environment has changed in Russian areas around the

South Kuril Islands since 1987; hence VPA may have been

problematic in recent years. In this case, VPA would have

underestimated the number of age-O walleye pollock.

In summary, our results will be useful in assessing

future recruitment not only ofshishamo smelt but also

walleye pollock. Further knowledge will be gained by

extending the survey area to east of Kushiro and to 100

m depth.

Acknowledgments _

Many scientists at Hokkaido Kushiro Fisheries Experimental Station (F.E.S.) took part in this survey, but we

particularly want to thank Koji Abe, now the director of

Hakodate F.E.S., and Mikio Koike, now a manager of

Hokkaido Central F.E.S., for starting this work at

Hokkaido Kushiro F.E.S. We also thank Isamu Nahira,

the captain of the No.5 Tensho-maru, for his assistance.

We thank Richard D. Brodeur, Alaska Fisheries Science

Center, and Hitoshi Uto, Hokkaido Wakkanai F.E.S.,

for reading and making suggestions on this manuscript,

and Tokio Wada and Kazutoshi Watanabe, Hokkaido

National Fisheries Research Institute, for making valuable comments on an earlier draft.

Literature Cited

Abe, K.

1968. Larvae ofsome species along the Pacific coast ofsoutheastern Hokkaido (l). Hokusuishi-geppo 25:352-368 [In

Jpn.].

Bailey, K. M., and S. M. Spring.

1992. Comparison of larval, age-O juvenile and age-2 recruit

abundance indices ofwalleye pollock, Theragra chalcogramma,

in the western Gulf of Alaska. ICES J. Mar. Sci. 49:

297-304.

Hayashi, K.

1976. Fisheries biological studies on the young Alaska pollock, Theragra chalcogramma (Pallas). Difference in growth

between year classes around the south-western coast of

Hokkaido in the northern Pacific Ocean. Sci. Rep.

Hokkaido Exp. Sta.18:1-8.

Ishigaki, T., T. Sakamoto, T. Sasaki, and M. Takano.

1960. Ecological studies of young Alaska pollack in the Pacific, east of Cape Erimo. Hokusuishi-geppo 17:66--75 [In

Jpn.].

Kanamaru, S.

1985. Characteristics of distribution for larvae and juveniles

around Hokkaido Island. Gyogyou Shigen Kenkyu Kaigi

Kita Nihon Sokouo Bukaiho 18:12-15. [InJpn.].

Nakatani, T.

1988. Studies on the early life history of walleye pollock

Theragra chalcogramma in Funka Bay and vicinity, Hokkaido. Mem. Fac. Fish. Hokkaido Univ. 35:1-46.

Nakatani, T., and T. Maeda.

1987. Distribution of copepod nauplii during the early life

stages of walleye pollock larvae Theragra chalcogramma in

Funka Bay and the adjacent waters, Hokkaido. Bull. Jpn.

Soc. Sci. Fish. 53:1585-1591.

Olla, B. L., and M. W. Davis.

1990. Behavioral responses ofjuvenile walleye pollock Theragra

chalcogramma Pallas to light, thermoclines and food: possible role in vertical distribution. J. Exp. Mar. BioI. Ecol.

135:59-68.
Distribution and Abundance of Age-O Walleye Pollock,

Theragra chalcogramma, in the Western Gulf of Alaska

during September 1990

MATTHEW T. WILSON, RICHARD D. BRODEUR, and SARAH HINCKLEY

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

ABSTRACT

A midwater trawl survey was conducted during September 1990 to examine the distribution patterns and to estimate the relative abundance of age-O walleye pollock in the

western Gulf ofAlaska. We first examined the effect that fishing strategy and gear type had

on age-O pollock catches. Oblique tows appeared to be more efficient than stepped tows in

catching age-O pollock; however, these methods did not differ with regard to the length of

fish caught (54-102 mm fork length). Gear type (anchovy trawl or shrimp trawl) did not

affect the catch per unit of effort or size composition of age-O pollock caught.

A subset of 67 hauls was used to examine the distribution and abundance of age-O

pollock. Numerically, age-o pollock dominated the catch, but older age groups of pollock and

gelatinous zooplankton were substantially more important by weight. The highest mean densities of age-O pollock occurred inshore of approximately the middle ofthe continental shelf and

were associated with temperatures "?7°C at 50 m. There was an alongshore trend of decreasing

density toward the southwest. The average catch per unit ofeffort in September-50,998 fish per

krn2 (133 kg/km2)-suggests that the 1990 year class was weak compared with previous years.

Overall, the fork length (FL) distribution of age-O pollock was unimodal, with an

average of 70.5 mm (range 44-102 mm). No significant differences in FL were found

among the six survey strata. The length-weight relationship for individuals 50-100 mm FL is

described.

Introduction

The juvenile stage of commercially important marine

fishes has become increasingly significant to those studying recruitment. The longer duration of the juvenile

stage relative to the egg and larval stages (Hjort, 1914;

Houde, 1987) has led some researchers to predict that

mortality during the juvenile stage can significantly

affect year-class strength (e.g., Sissenwine, 1984; Peterman et aI., 1988; Bradford, 1992). Bailey and Spring

(1992) used periodic estimates of abundance to suggest

that walleye pollock, Theragra chalcogramma, mortality

during the postlarval first year of life (age-O) may significantly reduce year-class strength.

Estimating the abundance of age-O pollock in the

Gulf of Alaska is difficult because their distribution is

widespread and complex. Ocean advection and diffusion expands what was once a fairly localized egg and

larval distribution and spreads it over much ofthe shelf

region in the western gulf (Kendall and Picquelle, 1990;

Hinckley et al., 1991). As nekton, age-O pollock have

been found at various depths from near-bottom to nearsurface. They exhibit diel vertical migration and aggregation behavior that may affect catchability (Traynor and

Williamson, 1983; Bailey, 1989), and have been found at

varying distances from shore in several surveys (Smith et

aI., 1984; Walters et aI., 1985; Hinckley et aI., 1991; Brodeur

et aI., 1995). This diversity of distribution and behavior

patterns makes it much more difficult to estimate the

abundance ofage-O pollock than the abundance oflarvae.

An effective survey of these fish requires that basic distribution patterns be determined (Koehler et aI., 1986) and

appropriate sampling strategies be developed.

As part of the Fisheries-Oceanography Coordinated

Investigations (FOCI) program (see Schumacher and

Kendall, 1991), we are studying the recruitment dyII
12 NOAA Technical Report NMFS 126

namics of walleye pollock, including the juvenile stage

up to the end of the first year of life. Since estimating

the abundance of age-O pollock is important to understanding recruitment mechanisms, an acoustic/trawl

survey was conducted during September 1990 to determine suitable sampling strategies and to examine the

large-scale abundance and distribution patterns ofthese

fish in the western Gulf of Alaska. We present an analysis of the trawl catches from this survey.

Study Area

Sampling was conducted on the continental shelf along

the Alaska Peninsula in water from 30 to 200 m deep

(Fig. 1). Most walleye pollock in the Gulf of Alaska

spawn in Shelikof Strait, and the prevailing currents,

principally the Alaska Coastal Current (ACe), carry the

larvae either along the Alaska Peninsula or into the

offshore waters of the Gulf of Alaska (Kim and

Nunnallee, 1990; Hinckley et aI., 1991). Historically,

high densities of age-0 juvenile pollock have been observed along the continental shelf and in nearshore

areas and bays along the Alaska Peninsula and around

Kodiak Island (Smith et aI., 1984; Walters et aI., 1985).

Surveys conducted in late summer from 1984 to 1988

(Spring and Bailey]) were used to establish the offshore

boundary of our study area.

Currents in the survey area are complex because they

flow over diverse topography punctuated by numerous

islands, reefs, and gullies. The most prominent topographic features are the Shumagin Islands, located in

the center of the study area, and, just to the northeast,

the Shumagin Gully, which appears as an indentation

of the 100-m isobath (Fig. 1). The ACC flows through

the study area with mean speeds of 10-20 cm/s, but

flow and salinity vary seasonally with changes in freshwater input and winds driving the current (Reed and

Schumacher, 1986). The oceanic Alaska Stream also flows

to the southwest offshore of the shelf break at speeds as

high as 100 cm/s (Reed and Schumacher, 1986).

I Spring, S., and K. Bailey. 1991. Distribution and abundance of

juvenile pollock from historical shrimp trawl surveys in the western

Gulf ofAlaska. AFSC Proc. Rep. 91-18, 66 p. Alaska Fish. Sci. Cent.,

Nat!. Mar. Fish. Serv., NOAA, 7600 Sand Point Way N.E., Seattle,

WA 98115-0070.

• Survey Stations (n=67)

o Gear Compo Stations (n= 11)

o Method Compo Stations (n=6)

58

57

Z

0

'--/

ill 56 u

:=J

-+--'

-+--'

0

--.J

55

54

3

2

...............

5

,yi t?c!i::'j~~/' ~""':/ ..../ .. ··-···_-········.._. ::t·

:·...?Oci"m : ....•..

....

....... .../ ..~" .. '

Pacific

Ocean

Shumagin Gully

Shumagin Islands

164 162 160 158 156

Longitude (OW)

154 152

Figure 1

Bathymetry and location of stations during the September 1990 survey in the Gulf of Alaska. Symbols indicate the

survey stations and where the paired-tow experiments were conducted. The strata boundaries are also shown.
_____ Wilson et al.: Distribution and Abundance of Age-O Walleye Pollock in the Western Gulf of Alaska 13

Methods _

Survey Design and Methodology

A sampling grid of 67 stations was designed along the

southern coast of the Alaska Peninsula on longitude

lines approximately 36 km apart (Fig. 1). Stations were

located 22 km apart along these lines from nearshore

out to the 200-m depth contour. The NOAA vessel

Miller Freeman trawled during all hours of the day between 6 and 23 September 1990. A total of 136 hauls

were made during this survey.

The standard sampling gear was an 18.6-m (footrope

and headrope lengths), high-{)pening shrimp trawl that

was fished with 1.5 x 2.1-m doors. This trawl is designed to

fish on bottom butwas used in midwater as in the 1984-88

surveys (Bailey and Spring, 1992). When the trawl was

fished this way, the height regulator and tickler chain were

removed. The stretched-netmesh in the body ofthis trawl was

3.2 cm, and the 3.8-cm-mesh codend was equipped with a 3-

mm knodess mesh liner. For the first half of our survey, the

shrimp trawl was deployed in a stepped manner, with an

average ship speed of 6 krn/h. The total number ofsteps

per tow was determined by water depth, with 10-min. steps

made at each 50-m depth interval.

We initially used the stepped method because we

thought that the net might not perform well when

towed obliquely and that concentrating on predetermined depth layers would not impair the quality of the

data. However, distinct layered patterns of sign at various depths were observed in acoustic transects between

stations (Brodeur and Wilson2), indicating that a stepped

net path at fixed depth layers was probably not sampling the water column adequately. Therefore, to

achieve a representative sample, the trawl was fished

along an oblique path for the rest of the survey.

A paired-tow experiment was conducted to examine

the relative efficiency of the oblique method of fishing

versus the stepped net path. This experiment consisted

ofsampling with each method at six stations where ageopollock were caught. The sequence of methods used

at each station was random. Both hauls of a pair were

done either during day, night, or twilight.3

We also compared the catch efficiency from the

shrimp trawl with a midwater anchovy trawl to evaluate

how well the shrimp trawl performed when fished

midwater. Unlike the shrimp trawl, the anchovy trawl was

designed to catch small pelagic fishes (Wyllie-Echeverria.

et aI., 1990). The anchovy trawl is a modified Cobb trawl

2 R. D. Brodeur and M. T. Wilson. Mesoscale acoustic patterns of

age-O walleye pollock (Theragra chalcogramma) in the western Gulf

of Alaska. Can.]. Fish. Aquat. Sci., in press, 1996.

3 Twilight was defined as the period 1 hour before to 1 hour after

sunrise or sunset at Sand Point, Shumagin Islands, Alaska, between

6 and 23 September 1990.

with an overall length of about 47 m and a square mouth

(26.2 m on a side). The mesh size (stretched) decreased

from 15.2 cm in the body to 3.8 cm in the codend. The

codend was also equipped with a 3-mm knodess liner.

To examine the relative efficiencies of the shrimp

and anchovy trawls, we used both gear types in alternate order at each of the last 11 stations of the survey.

Both hauls at each station were oblique and were done

either during day, night, or twilight.

At each trawl station, the catch was sorted, and the

taxonomic composition (by numbers and weight) was

determined. Walleye pollock were separated into

nonoverlapping length intervals to reflect age: age 0,

age 1, age 2, and 3 years and older (age 3+). Length

ranges in each age category are based on historical data

(Smith et aI., 1984) and were easily discernable in the

length distributions during this survey. A random length

sample (fork length [FL] to the nearest millimeter) was

taken of at least 100 age-O juvenile pollock. All older

pollock were measured, as were other selected species.

Individual wet weights (to the nearest gram) were taken

on whole age-O juvenile pollock at sea to examine variation in condition. Expendable bathythermograph

(XBT) casts were done at all trawl stations. A complete

listing of biological and physical sampling localities and

sequence is given in DeWitt and Clark (1992).

Data Analysis

Catch and length data were standardized to number or

weight per square kilometer of sea surface, or per

100,000 cubic meters filtered, with methods similar to

the area-swept method (Alverson and Pereyra, 1969).

For each tow, we estimated the volume filtered by multiplying mean net mouth area by distance fished (distance fished is the maximum wire out plus distance

travelled by the ship over the seafloor). We estimated

net mouth area by using a Scanmar net mensuration

system during several tows to measure height (distance

between headrope and footrope centers) or width (distance between wing tips) while the net was retrieved at

a constant rate (9 m/min). Least-squares linear and

nonlinear regressions were fit to the data so that the net

mouth area at a given length of wire out could be

estimated for all other tows (Fig. 2). An average mouth

area was then calculated for each tow. When fished in

steps, the mouth area was assumed to change instantaneously between steps. Catch per square kilometer of

sea surface was calculated as catch per volume multiplied by maximum net depth. Catch per unit area was

used to estimate abundance indices and to compare

geographic subareas, whereas density, or catch per unit

volume, was used to evaluate variations in survey strategy (e.g., paired-tow experiments). When catch is ex-
14 NOAA Technical Report NMFS 126

Shrimp Trawl

20

o

. (-0.02(Wire out+35.1)) Width = 15.1 1-e

Height 2.2-0.004(Wire out)

6

6

400

p

o

300

o 0

o 0

200

Anchovy Trawl

o 100

o

10

15

5

-+--'

...c

01

Q)

::r::

L 20

o

15 0

...c 10 -+--'

v

5

5 (

-0.01 (Wire out+43.S)) Width = 15.3 1-e

Height = 13.2-0.02(Wire out)

o

o 50 100 150 200

Wire Out (m)

Figure 2

Net mensuration data showing trawl geometry (height or spread of net)

with wire out for the shrimp trawl and the anchovy trawl (oblique tows

only). The best-fit equation describing the relation between the variables is

also given.

pressed per unit volume filtered, effort does not vary

with maximum depth fished; thus any confounding

effects of depth are not obscured.

Age-O pollock density estimates from the paired trawl

hauls were examined for differences between shrimp

and anchovy trawls or fishing methods (stepped vs.

oblique tows). Standardized catches of age-O pollock

were compared by means of two-tailed paired-sample ttests. We also applied the nonparametric Wilcoxon

paired-sample test because the sample sizes were small

and we were uncertain whether the distribution of

pairwise differences was normal (Zar, 1984).

We compared length distributions from the paired

comparison hauls of age-O pollock between gears or

methods by using a multivariate analysis of variance

(MANOVA) similar to that used by Bailey and Spring

(1992) and Shima and Bailey (1994). The dependent

variable was standardized catches within each 10-mm

length interval, and the sampling pair was used as a

blocking factor. Pillai's trace and its F-distribution ap-
_____ Wilson et at.: Distribution and Abundance of Age-O Walleye Pollock in the Western Gulf of Alaska 15

proximation were used as the test statistics because they

have been shown to have the best characteristics in

terms ofpower and robustness ofthe available MANOVA

statistics (Olson, 1974).

A single shrimp trawl haul at each of 67 stations was

used to estimate the abundance and distribution of

age-O walleye pollock and other taxa of interest (Table

1). Where multiple hauls were made at a station, the

haul selected conformed to the northeast-to-southwest

sequence of occupation and, where possible, was fished

on an oblique net path. A limited number of hauls

targeting echo sign during this survey were not included

because standardized catches from these hauls were significantly greater than hauls fished at predetermined locations (P< 0.01, Kruskal-Wallis nonparametric ANOVA).

For complete spatial coverage of the survey area, we

included data collected during day and night; however,

among the 67 tows, the hypothesis of independence

between age-O pollock presence or absence and diel

period was rejected (X2=8.53, P=O.OI). Applying a presence/absence correction to catch data is problematic,

and therefore we looked for a diel effect on catch or

length. A Kruskal-Wallis mean rank test using all 67

hauls (X2=4.10, P=0.10), and an ANOVA using the naturallogs ofthe 35 hauls where age-O pollock were caught

(F2,32=1.72, P=0.20) indicated that the die1 effect on

catch was not significant. Nor was the diel effect significantly related to mean FL, based upon an ANOVA

where haul (n=35) was nested within the diel effect

(F2,32=0.009, P=0.99). Therefore, we did not apply a correction for die! variability in our catch or length data.

We examined alongshore and cross-shelf trends in

abundance (fish per km2) of age-O pollock by using

stratified means and geographic mapping. The survey

area was divided into six strata (three inshore and three

offshore) based on the expectation that alongshore and

cross-shelftrends in abundance exist (Fig. 1, Table 1) and

on our desire to equate sample sizes between each stratum. We also report stratified frequencies of occurrence

and means of log-transformed catches because these parameters describe distribution characteristics.

We obtained length-frequency composition for each

stratum by using a stratified estimator for age-O pollock

at I-mm intervals and for older pollock at l-cm intervals. When catch was subsampled, the observed length

frequencies were expanded to total catch. All frequencies were standardized by effort before we combined

data from multiple hauls.

Wet weight was measured individually on 401 age-O

pollock and Fulton's condition factor (K) (K = [WI

L3*l05], where W= wet weight in grams and L = FL in

millimeters) was calculated. These data were collected

from 28 tows in the survey area during 1990, but not all

were from the 67 gridded survey hauls. We compared

differences of age-O pollock mean length or mean condition among strata by analyzing the observed frequencies (not standardized) using a nested ANOVA design

where haul was nested within stratum.

Table 1

Characteristics of sampling effort, by stratum, expended during the 1990 juvenile walleye pollock survey aboard the RjV

Miller Freeman,

September Haulsl Mean

sampling

date Stepped Oblique Km Hour Wire Depth Surface

Area2 per per out temp.

Stratum (km2) First Last Total Sum D N T Sum D N T haul haul (m) Net Bot. (0C)

Onshore

NE (1) 12,526 9 12 10 10 7 1 2 0 0 0 0 2.2 0.40 204 120 144 11.7

Mid (2) 10,835 12 14 13 4 1 1 2 9 2 3 4 1.8 0.32 162 91 103 11.5

SW (3) 12,550 14 22 11 0 0 0 0 11 6 3 2 1.4 0.22 132 70 79 10.6

Combined 35,911 9 22 34 14 8 2 4 20 8 6 6 1.8 0.31 165 93 107 11.2

Offshore

NE (4) 12,210 10 11 10 10 1 6 3 0 0 0 0 1.7 0.32 156 89 124 11.8

Mid (5) 11,548 12 14 11 8 4 3 1 3 2 0 1 1.7 0.29 143 80 102 11.5

SW (6) 11,864 15 22 12 0 0 0 0 12 5 5 2 1.8 0.30 168 96 155 10.9

Combined 35,623 10 22 33 18 5 9 4 15 7 5 3 1.7 0.30 156 89 128 11.4

All Areas

combined 71,531 9 22 67 32 13 11 8 35 15 11 9 1.7 0.31 160 91 117 11.3

I The terms stepped and oblique refer to net path through the water column. D =day, N = night, T = twilight.

2 Figures for combined areas may differ from column sums due to rounding error.
16 NOAA Technical Report NMFS 126

Results

Paired-Tow Experiments

Comparison of Net Paths-Catch data from the 1990

survey were not adjusted to account for differences in

net path (stepped versus oblique). Six pairs of tows

indicated that the oblique-path method, which adds

net-retrieval rates to towing speed, was more efficient

than stepped tows (Table 2). But because of the variability in catch, six pairs were too few to enable a

suitably powerful statistical comparison or to allow for

the calculation of a reasonably precise correction algorithm (Munro and Hoff 4).

Distributions of age-O pollock length frequency did

not differ significantly (MANOVA; F} 6=1.56, P=0.55)

between the oblique (oX =71.5 mm FL) and stepped

( oX =73.4 mm FL) methods (Fig. 3), thus a correction

for size selection due to net path was not necessary.

Comparison of Gear-In terms of estimating age-O pollock density, the data collected by the shrimp trawl

were similar to the data from the anchovy trawl. The

mean CPUE from the 11 tows made with the shrimp

trawl was 72.43 x 105 fish per m3 (SE=40.25), and the

mean from the anchovy trawl was 82.46 x 105 fish per

m3 (SE=54.98). Differences in age-O pollock CPUE between the two trawls were not significant when examined parametrically (t=0.50, P=0.628) or nonparametrically (Wilcoxon Z= -0.25, p=o.799). Incidentally,

the anchovy net collected many taxa more frequently

4 Munro, P. T., and R. Z. Hoff. 1994. Two demersal trawl surveys in

the Gulf of Alaska: implications of survey design and methods.

Alaska Fish. Sci. Cent., Nat!. Mar. Fish. Serv., NOAA, 7600 Sand

Point Way N.E., Seattle, WA 98115-0070. Unpub!. manuscr.

Table 2

Catch per unit ofeffort (CPUE; no. per 100,000 m3 ofageopollock) for each haul from each of 6 pairs where the

shrimp trawl was fished obliquely and at stepped depths.

The difference in CPUE (stepped - oblique) is given.

CPUE

Pair Stepped Oblique Difference

1 3.49 18.48 -14.99

2 7.55 224.44 -216.89

3 14.05 83.39 -69.34

4 68.13 147.70 -79.57

5 105.98 226.82 -120.83

6 197.20 184.02 13.18

x 66.07 147.48 -81.41

SE 31.00 33.81 33.38

than did the shrimp trawl; this probably reflects the anchovy net's larger mouth area.

Distributions of age-O pollock length frequency did

not significantly differ between the anchovy trawl

(oX =81.37 mm) and the shrimp trawl (oX =79.23 mm)

(MANOVA; F} 9=29.43, P=0.14; Fig. 3). Thus the shrimp

trawl seemed comparable to the anchovy trawl in terms

of age-O pollock catch efficiency and size composition.

Abundance and Distribution

Numerically, walleye pollock was the dominant finfish

species collected over all six strata (Table 3). Age-O

individuals were taken at 35 of the 67 trawl stations and

had a mean CPUE of 50,998/km2• They ranked fourth

among finfish in terms of biomass because of their low

mean weight per individual (0.003 kg; Table 3). It is

noteworthy that the jellyfish biomass was greater than

that of all finfish combined.

The geographic distribution of age-O pollock abundance varied throughout the survey area (Fig. 4). Despite considerable small-scale patchiness, age-O pollock

appeared to be more closely associated with the coast

than with the shelf-slope break (200 m). Both the

frequency of occurrence and the number offish caught

tended to be lower offshore (Table 4). These data also

suggest that age-O pollock were more prevalent upstream,

rather than downstream, ofthe Shumagin Islands.

Temperature contours at 50 m, which roughly parallel the coastline, coincided with the overall geographic

pattern of age-O pollock abundance (Fig. 4). We used

temperature at 50 m as an index of midwater temperature and found that most large catches of age-O pollock

were in areas inshore of the 7°C isotherm. Incidentally,

ocean temperature at the surface and at the bottom did

not visually correspond as well with age-O catches; surface temperatures showed an alongshore gradient

(Table 1), whereas bottom temperatures largely reflected water depth. Thus, relatively warm water at middepths may be one feature that makes the nearshore a

good nursery area for age-O pollock.

Length and Condition Characteristics

The standardized length distributions of all pollock

from the 67 survey hauls showed several distinct modes

that could be attributed to different age groups (Fig.

5). The smallest length group, associated with age-O

pollock, was distinct from all other groupings and ranged

from 44 to 102 mm FL. Age-l (~185 mm FL) and age-2

(~280 mm FL) modes were readily apparent, but length

modes above 350 mm FL were not easily discernable.

We examined the length distributions by stratum
_____ Wilson et al.: Distribution and Abundance of Age-O Walleye Pollock in the Western Gulf of Alaska 17

Net Path Comparison Gear Comparison

6

5

4

3

2

>,

U

C

OJ

::J

CT

0

1.L

Stepped

Tows

Shrimp

Trawl

Tows

Anchovy

Trawl

Tows

Oblique

Tows

4

2

3

5

6-,-----------------, +-'

C

OJ

U

L

OJ

(L

o

50 60 70 80 90 100 110 120 50 60 70 80 90 100 110 120

Fork Length (mm)

Figure 3

Fork-length distributions of age-O walleye pollock from the net-path comparison (stepped YS. oblique net path) and the

gear comparison (shrimp YS. anchovy trawl).

(Fig. 6) and found no significant differences in length

among all strata (overall x=70.5) (ANOVA; F529=0.39;

P=0.85). Tests on larger subsets of the data '(i.e., inshore vs. offshore (Fj 33=0.030, P=0.864) and upstream

vs. middle vs. downst~eam (F232=0.607, P=0.55l) were

also not significant. '

Age-O pollock fork lengths accounted for 91 % of the

variation in weight (Fig. 7). The nonlinear equation fit

to untransformed data gave an exponent of 3.101. Although this is close to 3.0, variation in weight for a

given length makes it difficult to conclude whether the

increase in body size is isometric or allometric.

Although condition factor (K) was not significantly

related to stratum, some age-O pollock had low values.

All individuals with low condition factors were collected

from the northeastern stations in strata 1 and 4. evertheless, no significant differences in K were observed

among strata (nested ANOVA; F5,22=0.46, P=0.80). Thus,

Kdid not increase with fish size, nor was the variation of

Kassociated with stratum (overall x=0.68).
18 NOAA Technical Report NMFS 126

Table 3

Mean catch per unit of effort (CPUE) of fish and invertebrates from 67 stations sampled in the Gulf of Alaska along the

Alaska Peninsula during September 1990 by the RV Miller Freeman.

Number of CPUE per km2

hauls with Mean of

Number Weight (kg) individual

Catch Length weight

Species data data x S2 x S2 (kg)

Fish 52 76,470 3.105E+08 8,138 3.920E+07 0.106

Age-O Thcragra chalcogramma 35 35 50,998 2.475E+08 133 1.748E+03 0.003

Age-1 + Thcragra chalcogramma 13 12 10,906 8.420E+07 7,337 3.927E+07 0.673

Mesopelagic fishes 1 0 4,131 1.707E+07 23 5.277E+02 0.006

Mallotus villosus 17 4 2,956 8.359E+05 13 1.828E+Ol 0.004

Lumpenus maculatus 15 0 1,634 2.740E+05 4 2.744E+00 0.002

Athcresthes stomias 14 0 1,121 1.107E+05 278 1.161E+04 0.248

Thaleichthys pacificus 7 0 1,017 2.071E+05 19 7.280E+01 0.019

Conidae 4 0 953 7.439E+05 2 2.182E+00 0.002

Hippoglossoides elassodon 7 0 499 4.963E+04 1 3.117E-01 0.003

Salmonidae 8 0 306 1.306E+04 31 2.061E+02 0.103

Zaprora silenus 7 0 263 1.196E+04 7 1.735E+01 0.028

Scorpaenidae 5 0 125 3.014E+03 1 6. 198E-02 0.005

Cyclopteridae 3 0 112 4.202E+03 1 8.600E-02 0.005

Gadus macrocephalus 1 0 41 1.669E+03 287 8.251E+04 7.032

Agonidae I 0 25 6.460E+02 <1 1.344E-02 0.005

Invertebrates 67 66,842 7.675E+07 8,815 8.323E+05 0.132

Gelatinous zooplankton 67 54,4071 2.662E+07 8,777 8.243E+05 0.161

Decapoda (shrimps) 15 11,6791 3.265E+07 26 1.851E+02 0.002

Cephalopoda (squids) 4 653 1.220E+05 3 3.324E+00 0.004

Other invertebrates2 3 104 3.643E+03 9 6.781E+Ol 0.082

I Estimated as biomass x mean weight.

2 Includes a very small catch of benthic organisms.

Table 4

Abundance index (mean catch per unit of effort [CPUEj; no. per km2) of age-O walleye pollock for the six geographic

strata surveyed during September 1990. Frequency of occurrence and the average natural logarithm of positive CPUE

values are also given.

Occurrence

Number (per km2) Frequency

Mean

Stratum n x SE n (%) In (CPUE) SE

Nearshore

I. Northeast 10 146,808 87,680 7 (70) 10.7 0.875

2. Middle 13 47,283 24,033 8 (62) 10.1 0.703

3. Southwest 11 54,439 35,545 6 (55) 10.8 0.610

Offshore

4. Northeast 10 37,682 29,659 7 (70) 10.0 0.503

5. Middle 11 2,177 657 3 (27) 9.0 0.168

6. Southwest 12 10,826 15,342 4 (33) 9.1 0.936
_____ Wilson et at.: Distribution and Abundance of Age-O Walleye Pollock in the Western Gulf of Alaska 19

58

57

Z

0

'--../

Q) 56

v

:=i

-+---'

-+---'

0

----.J 55

54 D

l ~.············· ,- .

D

oj················

D

6

D :

eM) : 6

1

/8

:7

• >100,000

• 100,000 - 10,000

9,999 - 1,000

D No Catch

isotherms at o 50 m ( C)

164 162 160 158 156

Longitude (OW)

154 152

Figure 4

Geographic distribution of O-age pollock abundance (no./km2) superimposed on stratum boundaries (dotted lines) and

on isotherms measured at 50 m with an expendable bathythermograph (XBT).

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

Age-O Age-1 +

2

6

4

8

12

10

o -trrTTTTrffnrtrrrfffffrrn"'"

50 60 70 80 90 100 100 200 300 400 500 600 700

4

2

3

5

o --tnr'rTr'l'fffflff

40

-+-'

C

Q)

o

L

Q)

CL

>.

o

c

Q)

0-

Q)

L

LL

Fork Length (mm)

Figure 5

Length distribution, standardized to 1 km2, of O-age and older age classes of pollock from the

stratified survey sampling. For age-O pollock, the numbers of fish measured and their observed mean

length (nonstandardized) are given.
20 NOAA Technical Report NMFS 126 _

60 70 80 90 100

Stratum 5

- -

X,td = 70.5

- - Xobs = 69.2

n = 17

-

-

I I I 'I' 'I

2

8

Stratum 6

4

5

Xstd = 80.6

6 Xobs = 80.6

n = 61

15

o

10 ---,------------------,

10

20

10

Stratum 4

8

X,td = 68.7

6 Xobs = 68.6

n = 178

4

o ---tn-rTTTTTTTTTTTTTTrffrI'",,",-fn

80 90 100 40 50

o ---+nTTTTrrTTTrrr+r+m

40 50 60 70

10

Stratum 1

8

Xstd = 69.7

6 Xobs = 69.6

n = 710

4

2

0

10

>.,

() Stratum 2

c 8

Q)

X,td = 68.6 :J

0- 6 xobs Q) = 68.6 l.- n = 284 1.L

+-' 4

c

Q)

() 2 I.-

Q)

0.... 0

10

Stratum 3

8

Xstd = 73.5

6 Xobs = 73.4

n = 272

4

2

Fork Length (mm)

Figure 6

Length distribution ofage-O pollock by stratum, standardized to 1 km2. The numbers offish measured

and their observed mean lengths (nonstandardized) are given.

Discussion

Paired-tow Experiments

Available information indicates that variation in the

vertical distribution of age-O pollock is best handled by

choosing a survey strategy that distributes sampling

effort equally among all depths. Walleye pollock are

most abundant in the upper layers «SO m) during the

larval stage (Kendall et aI., 1987) and move deeper as

their size increases (Nakatani and Maeda, 1987). Postlarval age-O fish have been found in large numbers near

bottom over a broad range of water depths (40-120 m;

Smith et aI., 1984), and in midwater (Bailey and Spring,
_____ Wilson et al.: Distribution and Abundance of Age-O Walleye Pollock in the Western Gulf of Alaska 21

Figure 7

Length-weight relationship for age-O walleye pollock.

o

40 50 60 70 80 90 100 110

Fork Length (mm)

1992; Brodeur and Wilson2). In the Bering Sea, age-O

fish were most abundant in the upper 40 m over the

continental shelf (Lynde, 1984). Acoustic data indicate

that age-O pollock may be found at a substantial depth

range from near surface to within a few meters of the

bottom (Brodeur and Wilson2). Thus the depths for

age-O pollock are subject to substantial ontogenetic and

spatial variability.

Variation in environmental conditions has been associated with differences in the depth distribution of ageo pollock. Laboratory studies (alia and Davis, 1990;

Sogard and alia, 1993) indicate that the vertical distribution of age-O pollock in midwater results from an

interplay ofmany factors (e .g., temperature, light, food,

and predators). Unravelling the importance of these

factors in the field is difficult. Evidence suggests that

diel period and thermoclines in the Bering Sea may

affect the depth distribution of age-O pollock (Traynor,

1986; Bailey, 1989). Undoubtedly, age-O pollock occur

at a broad range of depths, and vertical positioning at

any specific time and place may depend on a suite of

environmental variables. Thus, targeting specific depths

to survey the abundance of age-O pollock would not be

an advisable sampling strategy, based on what we know

of the dynamics of their vertical distribution.

Performance of the sampling gear is another important consideration when choosing sampling strategies.

For instance, we initially assumed that the shrimp trawl

would perform best if fished stepwise, but we switched

to an oblique path for biological reasons, after finding

out that the performance of the trawl (as measured by

age-O pollock CPUE) was not compromised (Table 2).

In fact, this change in net path appeared to improve

trawl performance, perhaps because adding the wire

retrieval rate to the towing speed increased the speed

of the net through the water by 9 m/min (0.5 km/h).

If we had applied a correction factor to equate

stepped-tow and oblique-tow catches, the age-O abundance estimates, but not the distributional patterns,

would have changed substantially. The overall abundance estimates of age-O pollock would have substantially increased, since oblique tows generally caught

more fish (Table 2). Also, in areas of high abundance

all stations (strata 1 and 4), or some stations (stratum 2)

were fished with the stepped method. When integrating

the numbers and species composition ofoceanic micronekton over discrete depth layers, Pearcy et al. (1977)

used oblique rather than stepped net paths on the basis

that the former provide a more representative sample

because all depths within each stratum are fished. The

shrimp trawl performed well when fished obliquely,

and this should be the preferred method for estimating

age-O pollock abundance unless biological information

about the vertical distribution of these fish, perhaps

from echo sounders, suggests otherwise.

Although the shrimp trawl is designed to fish on

the bottom, it was as effective as the anchovy trawl

at sampling the age-O pollock that we collected in

midwater. But the anchovy trawl was designed to catch

small pelagic fish, and samples collected with it are

probably more representative of the overall midwater

nekton community. The anchovy trawl's high vertical

opening and large forward mesh allows more water to

be filtered per distance towed, thereby boosting sampling effort and catches of rarer or more evasive species. Nevertheless, for estimating the distribution and

size composition of age-O pollock of the size range that

we observed (53-110 mm), the two trawls appear to be

equivalent.

Abundance and Distribution

Although some walleye pollock seem to spawn in the

Shumagin Islands area and other areas of the western

Gulf of Alaska (Kendall and Picquelle, 1990), we assume that most ofthe age-O pollock caught were spawned

in Shelikof Strait, where large numbers of pollock historically spawn during early April and are transported

by southwesterly flow (Reed and Schumacher, 1986;

Kendall and Picquelle, 1990; Kim and Nunnallee, 1990).

The distribution center ofyoung pollock in 1987 moved

from the Shelikofspawning area in early April to downstream ofthe Shumagin Islands by September (Hinckley

et al., 1991). In 1990, satellite-tracked buoys deployed

n=401

-6 3.10

Wt=4.43*10 *L

2

r =091

./::.

1.11"·

A

···:.A~:'::·: ••1 •••••

..: .- :

9

8

7

6

5

4

3

2
22 NOAA Technical Report NMFS 126

into concentrations of larval pollock in Shelikof Strait

all passed the Shumagin Islands by mid-September

(Schumacher et al., 1993; Staben05). As pollock mature

they become more active in determining their distribution, which probably relates to the distribution of prey or

to areas ofsuitable temperature and other environmental

variables. It is likely that most of the age-O pollock collected had drifted from Shelikof Strait, and that their

broad distribution reflects both physical transport and

behavioral responses to their environment.

Given that most pollock in the western Gulf ofAlaska

spawn in Shelikof Strait and that their offspring pass

each year through our survey area as age-O fish, our

abundance estimates should reflect large-scale regional

abundance. Bailey and Spring (1992) estimated the

abundance of age-O pollock in this area for most years

from 1975 to 1988 (1975-84: random bottom trawls

during the day; 1985-88: midwater and bottom trawls

targeted on echo sign during the day). Because the

geographic extent of the data varied among years, we

converted the overall abundance index for each year to

number offish per square kilometer ofsea surface and

found that our abundance index for 1990 (50,998 fish/

km2) ranked ninth out of 12 years. Bailey and Spring

(1992) included results from around Kodiak Island,

which we did not survey extensively. Excluding the

results from this region did not change the conclusion

that 1990 is a fairly weak cohort (ranked 10 out of 12).

Assuming that these numbers are comparable and that

they indicate regionwide trends, the 1990 year class of

pollock in the western Gulf of Alaska appears to be

weak (similar to 1987), but not the weakest on record.

However, it is important to underscore that the sampling conducted in 1990 differed from earlier years

(effort not allocated based on acoustic sign), and this

may significantly affect age-O abundance estimates.

When combined with previous survey work, our findings indicate that a variety of areas are used by age-O

pollock as nursery habitat and that some interannual

variability may occur. Our survey indicated that most of

these fish were found throughout the middle and inner

continental shelf, and few fish were found near the

200-m isobath. However, Hinckley et al. (1991) show

that during August and September of 1987, high abundances of age-O pollock (>1,000/10 min) were found

offshore near the 200-m isobath. In fact, the distribution patterns that we observed in September 1990 are

more similar to those observed by Hinckley et al. (1991)

during June and July for late larval and early juvenile

pollock, than to those for late juveniles. Thus it appears

that movement of age-O pollock through the shelf waters off the Alaska Peninsula varies from year to year

5 P. Stabeno. Pac. Mar. Environ. Lab., NOAA, 7600 Sand Point Way

N.E., Seattle, WA 98115-0070. Unpub\. data.

and may be an important factor affecting survival. Perhaps this variation relates to conditions in the nursery

area, such as water temperature and prey availability,

which might influence age-O pollock distribution. Because these fish have recently become nektonic, variations in oceanic conditions during their planktonic

larval stage, such as patterns of advection (Kendall and

Picquelle, 1990), may also have important effects on

their distribution asjuveniles.

Neither our study nor that of Hinckley et al. (1991)

effectively sampled the full range of potential age-O

habitat, because little sampling was done in enclosed

bays. Smith et al. (1984) reported that mean catches of

age-O pollock in each of several bays located in our

survey area ranged as high as 555 x 104 fish per m2

(55,500 fish per km2). This upper range of mean abundance is similar to mean abundances found in our

stratum 3 (54,439 fish/km2; Table 4). The highest mean

abundance of age-O pollock found by Smith et al. (1984)

was 4,083 x 104 fish per m2 (408,300 fish per km2) for

Alitak Bay at the south end of Kodiak Island, which was

almost three times the highest mean abundance of any

stratum in our study. Sampling during September in

bays around Kodiak Island (M. Wilson, unpubl. data)

also indicated that age-O pollock occur, at all times of

the day, throughout the water column both inside and

outside of bays. Thus, while it is evident that substantial

numbers of age-O pollock inhabit much of the continental shelf along the Alaska Peninsula and Kodiak

Island, the importance of semienclosed areas as habitats still needs to be determined.

The association between high age-O pollock density

and warm water at intermediate depths over the inner

and middle shelf suggests that these habitats may be

more suitable for juveniles than colder offshore waters.

Brown and Bailey (1992) report an association between

growth rate, inferred from otolith microstructure, and

water temperature for age-O pollock collected from the

western Gulf ofAlaska during August and September of

1987. Smith et al. (1985) show in laboratory experiments that water temperature differences similar to

those observed in this study (50-9°C) affected pollock

growth rates. The increased growth rates could translate

into increased survival ifsize-selective mortality occurs.

Bottom topography may be another physical characteristic important to age-O pollock, because the alongshore distribution of these fish may relate to topographical differences along the continental shelf. In

particular, the high abundances of age-O pollock over

the Shumagin Gully may be associated with warm

nearshore water in close proximity to the greater depths

of the gully. This may facilitate an ontogenetic movement into deeper water, perhaps to satisfy changing

dietary requirements (Kamba, 1977; Nakatani and

Maeda, 1987). High densities of age-O pollock have
_____ Wilson et al.: Distribution and Abundance of Age-O Walleye Pollock in the Western Gulf of Alaska 23

been found in other gullies in the Gulf ofAlaska, which

extend from the continental slope (200 m) into bays

(Wilson, unpubl. data).

Although our sampling was conducted at all hours of

the day, future sampling of age-O pollock should consider diel differences in distribution as a means to reduce the variation associated with abundance estimates.

Diel variation in aggregation patterns has been observed amongjuvenile pollock (Traynor and Williamson,

1983; Traynor, 1986) and may explain our observation

that the presence or absence of these fish in the catch is

not independent of photoperiod. It is likely that these

fish disperse at night to feed (Merati and Brodeur,

1996) and are aggregated in schools during the day,

which could account for the day-night differences that

we observed. Age-O pollock were less frequently caught

during the day than at night, but when the fish were

caught, daytime catches were larger than nighttime

catches. We might have expected daytime catches to be

low and to contain mostly small individuals, since Bailey

(1989) found that large individuals sometimes move

close to the bottom during this time. This factor would

be particularly problematic in shallow water, where the

fish could move close to the bottom, below the depth

sampled by our midwater trawl. Thus, die1 sampling

may have introduced variation into our mean age-O

pollock CPUE estimates; its effect on large-scale distribution patterns, however, is probably negligible, because photoperiod was randomized with regard to abundance and area.

We did not attempt to correct our length data for

growth despite a collection interval of 17 d. Using

otoliths, Bailey et al.6 estimated a mean growth rate of

1.01 mm SLid for these fish. Therefore, we were not

surprised to find large mean sizes for fish collected in

the downstream strata, which were sampled last. Nevertheless, the lack of statistical differences among strata

suggests that, over the duration of our survey, increases

in fish size were small relative to other sources of variability. We tried to compare the results of our length

analysis to those ofWalters et al. (1985), but differences

in mesh size weaken the biological significance of this

comparison.

Some uncertainties encountered during this study

stemmed from a lack of understanding of the vertical

distribution of age-O pollock, catchability throughout

the photoperiod, schooling dynamics, and the time

that they spend in the nursery area. Resolving these

questions is fundamental to understanding age-O pollock ecology at a level sufficient for population-scale

6 Bailey, K. M., A. L. Brown, M. M. Yoklavich, and K. L. Mier. 1996.

Interannual variability in growth of larval and juvenile pollock

Theragra chalcogramma in the western Gulf ofAlaska, 1983-91. Fish.

Oceanogr.5 (suppl. 1):137-147.

assessments of their abundance and distribution, and,

ultimately, the source and magnitude of their mortality

during their first summer.

Acknowledgments _

We thank Susan Picquelle and Steve Syrjala for statistical advice and assistance. We also thank Trish Brown, Lisa

Britt, Leslie Lawrence, and Stella Spring for technical

assistance. Kevin Bailey, Art Kendall, Susan Picquelle, Dave

Somerton, Gary Walters, and two anonymous reviewers

offered much-appreciated suggestions on the manuscript.

This is FOCI Contribution No. FOCI-0204.

Literature Cited

Alverson, D. L., and W. T. Pereyra.

1969. Demersal fish explorations in the northeastern Pacific

Ocean-an evaluation of exploratory fishing methods and

analytical approaches to stock size and yield forecasts. ].

Fish. Res. Board Can. 26:1985-2001.

Bailey, K. M.

1989. Interaction between the vertical distribution of juvenile walleye pollock Theragra chalcogramma in the eastern Bering

Sea, and cannibalism. Mar. Ecol. Prog. Ser. 53:205-213.

Bailey, K. M., and S. M. Spring.

1992. Comparison of larval, age-O juvenile and age-2 recruit

abundance indices of walleye pollock, Theragra chalcogramma,

in the western Gulf ofAlaska. ICES]. Mar. Sci. 49:297-304.

Bradford, M.].

1992. Precision of recruitment predictions from early life

stages of marine fishes. Fish. Bull. 90:439-453.

Brodeur, R. D., M. S. Busby, and M. T. Wilson.

1995. Summer distribution of early life stages of walleye pollock (Theragra chalcogramma) and associated species in the

western Gulf of Alaska. Fish. Bull. 93:603-618.

Brown, A. L., and K. M. Bailey.

1992. Otolith analysis of juvenile walleye pollock (Theragra

chalcogramma) from the western Gulf of Alaska. Mar. BioI.

112:23-30.

DeWitt, C., and]. Clark.

1992. Fisheries-Oceanography Coordinated Investigations

ShelikofStrait: 1990 field operations. U.S. Dep. Commer.,

NOAA Data Rep. ERL PMEL-39, 98 p.

Hinckley, S., K. M. Bailey, S.]. Picquelle,]. D. Schumacher, and

P.]. Stabeno.

1991. Transport, distribution, and abundance of larval and

juvenile walleye pollock (Theragra chalcogramma) in the western Gulf of Alaska. Can.]. Fish. Aquat. Sci. 48:91-98.

Hjort,].

1914. Fluctuations in the great fisheries of northern Europe

reviewed in light of biological research. ICES Rapp. Proc.

Verb. 20:1-228.

Houde, E. D.

1987. Fish early life dynamics and recruitment variability. In

R. D. Hoyt (ed.), 10th Annual Larval Fish Conference, p.

17-29. Amer. Fish. Soc. Symp. 2, Bethesda, MD.

Kamba, M.

1977. Feeding habits and vertical distribution of walleye pollock Theragra chalcogramma (Pallas), in early life stage in
24 NOAA Technical Report NMFS 126

Uchiura Bay, Hokkaido. Res. Inst. North Pac. Fish.,

Hokkaido Univ., Spec. vol., 175-197.

Kendall, A. W.,Jr., M. E. Clarke, M. M. Yoklavich, and

G. W. Boehlert.

1987. Distribution, feeding, and growth of larval walleye pollock, Theragra chalcogramma, from Shelikof Strait, Gulf of

Alaska. Fish. Bull. 85(3) :499-521.

Kendall, A W.,Jr., and S.]. Picquelle.

1990. Egg and larval distributions ofwalleye pollock Theragra

chalcogramma in Shelikof Strait, Gulf of Alaska. Fish. Bull.

88:133-154.

Kim, S., and E. P. Nunnallee.

1990. Distribution of walleye pollock, Theragra chalcogramma,

spawning in Shelikof Strait, Gulf of Alaska, based on acoustic and ichthyoplankton surveys 1981, 1984, and 1985. Bull.

Korean Fish. Soc. 23:425-438.

Koeller, O. A, P. C. F. Hurley, P. Perley, and]. D. Neilson.

1986. Juvenile fish surveys on the Scotian Shelf: implications

for year-class size assessments.]. Cons. Int. Explor. Mer

43:59-76.

Lynde, C. M.

1984. Juvenile and adult walleye pollock of the eastern Bering

Sea: literature review and results of ecosystem workshop. In D.

Ito (ed.), Proceedings ofthe workshop on walleye pollock and

its ecosystem in the eastern Bering Sea, p. 43--108. U.S. Dep.

Commer., NOAA Tech. Memo. NMFS F/NWC-62.

Merati, N., and R. Brodeur.

1996. Feeding habits and daily ration ofjuvenile walleye pollock, Theragra chalcogramma, in the western Gulf of Alaska.

In R. D. Brodeur et al. (eds.), Ecology of juvenile walleye

pollock, Theragra chalcogramma, p. 65-79. U.S. Dep.

Commer., NOAA Tech. Rep. 126.

Nakatani, T., and T. Maeda.

1987. Distribution and movemen t of larval and juvenile walleye pollock (Theragra chalcogramma) in Funka Bay and adjacent waters. Bull.Jpn. Soc. Sci. Fish. 53:1585-1591.

Olla, B. L., and M. W. Davis.

1990. Behavioral responses ofjuvenile walleye pollock Theragra

chalcogramma Pallas to light, thermoclines and food: possible role in vertical distribution. ]. Exp. Mar. BioI. Ecol.

135:59-68.

Olson, C. L.

1974. Comparative robustness ofsix tests in multivariate analysis of variance. ]. Amer. Stat. Assoc. 69:894-908.

Pearcy, W. G., E. E. Krygier, R. Mesecar, and F. Ramsey.

1977. Vertical distribution and migration of oceanic

micronekton off Oregon. Deep-Sea Res. 24:223--245.

Peterman, R. M., M.]. Bradford, N. C. Lo, and R. D. Methot.

1988. Contribution of early life stages to interannual variability in recruitment of northern anchovy (Engraulis mardax).

Can.]. Fish. Aquat. Sci. 45:8-16.

Reed, R. K., and]. D. Schumacher.

1986. Physical oceanography. In D. W. Hood and S. T.

Zimmerman (eds.), The Gulf of Alaska: physical environment and biological resources, p. 57-75. U.S. Min. Manage. Servo Publ. 86--0095.

Schumacher,]. D., and A. W. KendallJr.

1991. Interactions between young walleye pollock and their

environment in the western Gulf of Alaska. Calif. Coop.

Oceanic Fish. Invest. Rep. 32:22-40.

Schumacher,]. D., P.]. Stabeno, and S.]. Bograd.

1993. Characteristics of an eddy over a continental shelf:

Shelikof Strait, Alaska. ]. Geophys. Res. 98:8395-8404.

Shima, M., and K. M. Bailey.

1994. Comparative analysis ofichthyoplankton sampling gear

for early life stages ofwalleye pollock (Theragra chalcogramma).

Fish. Oceanogr. 3:50-59.

Sissenwine, M. P.

1984. Why do fish populations vary? In R. M. May (ed.),

Exploitation of marine communities, p. 59-94. SpringerVerlag, Heidelberg.

Smith, R. L., A.]. Paul, and]. M. Paul.

1986. Effect of food intake and temperature on growth and

conversion efficiency of juvenile walleye pollock (Theragra

chalcogramma (Pallas)): a laboratory study. ]. Cons. Int.

Explor. Mer 42:241-253.

Smith, G. B., G. E. Walters, P. A. RaymoreJr., and

W. A. Hirschberger.

1984. Studies of the distribution and abundance ofjuvenile

groundfish in the northwestern Gulf of Alaska, 1980-82:

Part I, three-year comparisons. U.S. Dep. Commer., NOAA

Tech. Memo. NMFS F/NWC-59, 100 p.

Sogard, S. M., and B. L. Olla.

1993. Effects of light, thermoclines and predator presence

on vertical distribution and behavioral interactions ofjuvenile walleye pollock, Theragra chalcogramma Pallas. ]. Exp.

Mar. BioI. Ecol. 167:179-195.

Traynor,].].

1986. Midwater abundance of walleye pollock in the eastern

Bering Sea, 1979 and 1982. Int. North Pac. Fish. Comm.

Bull. 45:121-135.

Traynor,].]., and N.]. Williamson.

1983. Target strength measurements of walleye pollock

(Theragra chalcogramma) and a simulation study of the dual

beam method. In O. Nakken and S. C. Venema (eds.),

Symposium on fisheries acoustics. Selected papers of the

ICES/FAO Symposium on Fisheries Acoustics, Bergen, Norway, 21-24.June 1982, p. 112-124. FAO Fish. Rep. 300.

Walters, G. E., G. B. Smith, P. A. Raymore.Jr., and

W. A. Hirschberger.

1985. Studies of the distribution and abundance ofjuvenile

groundfish in the northwestern Gulf of Alaska, 1980-82: Part

II, biological characteristics in the extended region. U.S.

Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-77, 95 p.

Wyllie-Echeverria, T., W. H. Lenarz, and C. A Reilly.

1990. Survey of the abubance and distribution of pelagic

young-of-the-year rockfishes off central California. U.S. Dep.

Commer., NOAA Tech. Memo. NMFS-SWFC-147, 125 p.

Zar,]. H.

1984. Biostatistical analysis, 2nd ed. Prentice-Hall, Inc.,

Englewood Cliffs, NJ, 718 p.
Juvenile Walleye Pollock, Theragra chalcogramma,

Distribution and Abundance in Shelikof StraitWhat Can We Learn from Acoustic Survey Results?

DENISE R. McKELVEY

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

ABSTRACT

Since 1980, winter and spring echo-integration trawl surveys have been conducted by

the Alaska Fisheries Science Center aimually (except in 1982) in the Gulf ofAlaska to assess

the distribution and abundance of walleye pollock, Theragra chalcogramma. Survey data are

used to describe vertical and geographical distribution of age-1 walleye pollock in Shelikof

Strait, as well as relationships between age-1 abundance and distribution. Age-1 pollock

were consistently found in the southern region of Shelikof Strait, and relatively strong year

classes formed a distinct scattering layer extending from the central part of Shelikof Strait

south to near the Semidi Islands. A significant correlation was found between the abundance of age-1 pollock in Shelikof Strait and estimates for the same year class 2 and 3 years

later in the Gulf of Alaska. This finding supports the use of acoustic estimates to forecast

year-class strength of walleye pollock for management purposes.

Introduction

Fishery managers and biologists focus considerable effort on understanding the population dynamics ofGulf

of Alaska walleye pollock, Theragra chalcogramma

(Megrey, 1991; Schumacher and Kendall, 1991). Research on recruitment variability is motivated by its

effect on pollock stock biomass (Alton et al. 1987;

Megrey, 1989; Hollowed and Megreyl) and in estimating resource productivity for fisheries management purposes (Sissenwine, 1984). Further motivation is provided by the desire to predict properties of the upcoming

year classes (Rothschild, 1986; Wooster and Bailey, 1989).

Since 1980, the Alaska Fisheries Science Cen ter

(AFSC) has annually (except 1982) conducted an echointegration trawl survey in the Shelikof Strait region of

the Gulf of Alaska to assess the distribution and aoundance ofwalleye pollock. These surveys were conducted

between late February and early April, when the abundance ofspawners was believed to be at its peak (Nelson

and Nunnallee, 1985a). Although the primary goals

and design of the surveys focused on the adult spawning population, the surveys also provide data on juvenile (age-I) pollock. The goals of this paper are to

describe the juvenile pollock data that are available

from these historical acoustic surveys (1981, 1983-93)

and to evaluate the time series of abundance estimates

for age-l pollock as an index of year-class strength.

Materials and Methods

Vessel, Acoustic Equipment, and Trawl Gear

All survey data were collected aboard the National Oceanic and Atmospheric Administration (NOAA) research

vessel Miller Freeman, a 66-m stern trawler. In 1983, the

NOAA research vessel Chapman, a 39-m stern trawler,

also participated in the survey.

In all surveys, acoustic data were collected with a 38-

kHz echo-sounder system interfaced to computer systems designed to implement echo-integration data analyses. The acoustic systems were improved over the study

I Hollowed, A. B., and B. A. Megrey. 1990. Gulf of Alaska walleye

pollock: population assessment and status of the resource in 1991.

Alaska Fish. Sci. Cent., 'at!. Mar. Fish. Serv., NOAA, 7600 Sand

Point Way N.E., Seattle, WA 98115. Sect. 1,68 p. Unpub!. manuscr.

25
26 NOAA Technical Report NMFS 126 _

period; acoustic data were collected with a Biosonics2

dual beam system during 1981-85 (Nelson and

Nunnallee, 1985a, b), a Biosonics split beam/dual beam

system during 1986-90 (Traynor and Ehrenberg, 1990),

and a split beam Simrad EK500 system during 1991-93

(Bodholt et al. 1989). From 1981 to 1990, the transducer was housed in a towed body deployed behind the

vessel approximately 17 m deep. From 1991 to 1993 the

transducer was mounted on the bottom of the vessel's

centerboard, about 9 m below the water surface (ana

and Traynor, 1990).

The acoustic equipment was calibrated to estimate

the transmitting and receiving characteristics of the

system. From 1981 to 1987 a standard hydrophone was

used to measure the acoustic parameters (Blue, 1984)

before and after each survey. Since 1988, the more

accurate standard sphere calibration procedure (Foote

et al. 1987) was used, allowing calibrations to be made

anytime during or after a survey.

Three different midwater trawls and three different

bottom trawls were used between 1981 and 1993 (Table

1). Midwater echo sign was primarily sampled with a

Diamond 1000 midwater trawl in 1981 and 1983-87,

and with modified Northern Gold 1200 midwater rope

trawls in 1988-93. Demersal echo sign was sampled

with an Eastern 83/112 bottom trawl except in 1990,

when a Poly-Nor'eastern bottom trawl was used, and in

1992, when a Nor'eastern bottom trawl was used. A

codend liner with 3.2-cm (1.25-in.) mesh was used for

all trawls. Vertical net dimensions were determined for

2 Reference to trade names or commercial firms does not imply

endorsement by the National Marine Fisheries Service, NOAA.

all surveys with a netsounder mounted on the headrope;

horizontal net measurements were made on some surveys with a SCANMAR net mensuration system.

Survey Methods

Survey operations were conducted 24 hours per day

and consisted of two or more passes through Shelikof

Strait along zigzag (1981-87) or parallel (1988-93)

tracklines. Generally these tracklines traversed the strait

between the 91-m isobaths and extended from near the

Semidi Islands, around 56°00'N latitude, northward to

about 58°30'N, depending on the extent of echo sign

(Fig. 1). Tracklines were uniformly spaced at intervals

of3.5-10 n.mi., depending on the survey pass and year.

Often additional passes were made to sample the densest pollock aggregations observed on previous passes.

Vessel speed varied between 5 and 12 knots, depending

on weather conditions.

Trawl hauls were made in an opportunistic manner

during each survey to identify echo sign and to gather

data on its biological composition. The duration of

each haul varied, depending on the estimated time

needed to provide an adequate sample size as indicated

by the netsounder echo trace. Average trawling speed

was about 3 knots. Standard AFSC catch-sorting and

biological sampling procedures (Hughes, 1976) were

used to determine weight and number by species for

each haul. Pollock were further sampled to determine

the fork length (FL), weight, sex, age, and maturity

composition of the haul. Catch data were not used to

generate abundance estimates.

Table I

Trawl gear used during the winter and spring echo-integration trawl surveys in Shelikof Strait, 1981 and 1983-1993.

Approximate

Head-rope Foot-rope Mesh size! Spread Height opening

Trawl type Years used (m) (m) (cm) (m) (m) (m2)

Midwater

Diamond 1000 1981,1983-87,1993 54 54 40.6, 81.2, 8.9 No data 16

Northern Gold 1200

rope with bridle 1988 91 85 163,8.9 45 27.5 1237.5

with no bridle 1989-93 95 50 163,8.9 No data 19

3/4 scale Norse 1984 63 58 81.2,8.9 No data 7.5

Bottom

Eastern 83/112 1981,1983,1986,1991,1993 26 34 10.2,8.9 18 2 36

Poly-Nor' eastern 1990 27 37 12.7,8.9 14 7.6 108

Nor'eastern 1992 27 32 12.7,8.9 18 6 106

I First number is mesh size at wings. Last number is mesh size at codend. Where three numbers are given, second is size of a section of

mesh between wings and codend. Codend liner was ofsame mesh size in all cases (see text).
___________ McKelvey: Juvenile Walleye Pollock Distribution and Abundance in Shelikof Strait 27

59 OON

Figure 1

Survey trackline for the win ter/ spring 1992 pollock

echo-integration trawl survey of Shelikof Strait, showing one pass through the strait area and a second pass

of more closely spaced transects through the region of

high spawner densities.

158 OOW

a

sernidi Is.

155 OOW

58 OON

57 OON

56 OON

55 OON

152 OOW

1986, when they were selected as 9-15 cm. Fish from

this size range formed a clearly discernible size mode

and were believed to have hatched 11-12 months before the survey. The proportion of age-1 pollock in

each catch was determined from the length-frequency

sample and extrapolated to the total catch.

Trawl hauls where age-1 pollock dominated the catch

by number were identified. The corresponding extent

and depth of the scattering layers for those fish were

determined. Geographic and vertical distribution plots

of age-1 pollock were created for each survey. Hauls

with unsatisfactory gear performance were not included.

Catch rates (no/h) were used to describe age-1 pollock

relative abundance (e.g., high, medium, low) throughout the strait but were not used to determine actual

abundance estimates. The difference in gear types probably affected catch rates (e.g., mouth area ofthe largest

midwater net is 11-34 times larger than the mouth area

of the bottom trawl; Table 1). Although no corrections

were made to account for these differences, catch rates

from the midwater trawls should be roughly comparable to those from other midwater trawls, and rates

from the bottom trawls should be comparable to those

from other bottom trawls.

Results

Juvenile Pollock Indices

Echo-integration data were used to obtain estimates of

fish abundance along each transect. Transects were partitioned into strata by depth and time/distance intervals.

Strata where echo sign was determined to represent pollock were then selected, and the echo integration values

within each stratum were summed over all chosen depth

strata to provide estimates of biomass per unit area. Surface echo-integration values were scaled to estimates of

absolute density (kg/m2) by means of weighted pollock

size compositions, a length-weight relationship derived

from the trawl catches, and a previously-derived relationship between target strength and fish length (TS = 20

10g(FL) -66; Foote and Traynor, 1988). Biomass was calculated as the product of the average surface density and

the area surveyed. Length- and age-specific biomass and

population estimates were calculated with the biomass

estimate, length-eomposition data, a length-weight relationship, and an age-length key. Because of equipment

difficulties, pollock abundance was not estimated in 1987.

Analytical Approach

Age-1 pollock were selected from length-frequency

records as the size range 9-16 cm FL in all years except

Estimates of age-1 pollock abundance are presented in

Table 2. Age-1 juvenile pollock were most abundant

during the survey years 1985, 1986, and 1989, corresponding to the relatively strong 1984, 1985, and 1988

year classes.

The ShelikofStrait age-1 acoustic estimates were compared to the Gulf of Alaska age-3 and age-4 model

estimates to see if Shelikof Strait age-1 abundance can

be used to indicate year-class strength of Gulf of Alaska

pollock. The GulfofAlaska estimates were derived from

an age-structured stock synthesis model described by

Hollowed et al, 3 which incorporated fisheries, survey,

and egg production data to generate a pollock abundance estimate that represents the entire GulfofAlaska.

Age-1 acoustic data were not included in this model.

There was a significant relation (R2=0.61; Fisher's ztransformation, p=O.Ol) between acoustic estimates of

age-1 abundance and model estimates ofrecruited age3 pollock (Fig. 2), particularly for the stronger year

3 Hollowed, A. B., C. Wilson, E. Brown, and B. A. Megrey. 1994.

Walleye pollock. In Stock assessment and fishery evaluation report

for the groundfish resources of the Gulf of Alaska as projected for

1995. N. Pac. Fish. Manage. Council, P.O. Box 103136, Anchorage,

AK 99510. Sect. 1,66 p.
28 NOAA Technical Report NMFS 126

2000

1800

;;; 1600 88

c: -84

1400

:! 1200

'" 85

.~ 1000

M" 800 "'"

Qj '" 600 83 R' = .61 " - 0

:; 80 P = .01 400 1,89 - 200 82

0 1- 90

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Acoustic age-l estimate (millionsl

Figure 2

Scatterplot of Gulf of Alaska stock synthesis mode! age-3 pollock estimates

and Shelikof Strait winter acoustic age-I pollock estimates for year classes

1980-90. The fitted linear regression line indicates a positive relationship

and is not intended for predictive purposes.

classes (Fig. 3). The same relation was found between

acoustic age-I and the model age-4 pollock. It would

seem that the 1984 year-class data point drives this

correlation, but when it was removed, the relationship

remained strong (R2=0.74; p<O.OI).

Age-I abundance estimates were also compared to

acoustic estimates of ages 2-4 from the same year class.

All comparisons showed a significant relationship. However, the strongest relationship was between the age-I

and age-3 acoustic estimates (R2=0.89; p<O.OI).

The Shelikof Strait age-2 acoustic abundance estimates were then compared to the Gulf of Alaska age-3

model estimates to see if the age-2 acoustic estimates

could be used to indicate year-class strength. This relationship was also significant (R2=0.77; p<O.OI).

Juvenile Pollock Distribution

Age-I pollock were present in the southern strait region (south of 57°00'N) in all survey years; in several

years they were also found in the northern strait (Fig.

4). In "low" abundance years (Table 2), age-l pollock

were found dispersed throughout the strait and mixed

together in scattering layers of predominately larger

pollock, usually over a depth range of 225-250 m, within

50 m of bottom. However, patches of predominately

age-I pollock echo sign were observed in 1984 at the

southern end of the strait and in 1990 at the northeastern end. In contrast, during the "medium" and "high"

abundance years, age-I pollock formed a discrete scattering layer in the southern region of Shelikof Strait; in

1985 and 1989, the layers extended up to the central

part of the strait.

Age-I pollock scattering layers were observed at various depths (Table 2). In 1985, 1987, and 1989, layers

were generally at 140-215 m, above a deeper scattering

layer of older pollock. In 1985 and 1989 there were also

occurrences of a "sandwich" effect, where the age-I

scattering layer was observed between two layers of

older pollock. On two occasions the 1985 scattering

layer was observed by itself within 15 m of bottom. In

con trast, the 1986 age-I pollock layer was found at 195-

245 m, within 15 m of bottom, without the deeper or

shallower layers of older pollock (Fig. 5). Although

abundance and distribution information for age-I pollock in 1992 is not mentioned in Table 2, Figure 4

indicates relatively high bottom trawl catches of age-l

pollock. Further investigation of how vertical distribution has changed over the years is warranted.

Discussion

Echo integration has become a well-established technique

for measuring fish abundance and can provide quick

information about pelagic fish distribution (MacLennan

and Simmonds, 1992). The annual Shelikof Strait acoustic surveys provide information that describes the winter

distribution and relative abundance of age-I pollock.

For walleye pollock and many other marine fish species, the magnitude of recruitment may be established
McKelvey: Juvenile Walleye Pollock Distribution and Abundance in Shelikof Strait 29

Acoustic age-1

2500

2000

1500

1000

500 >

0

0 Z

BO B' B2 B3 B' B5 B. B7 BB B9 gO

Model age-3

1800

1600

1400

.

c 1200

! 1000 .

0

800

c

... 600 «

400

200

0

BO B' B2 B3 B' B5 B. B7 BB B9 gO

Year class

Figure 3

Shelikof Strait winter acoustic age-l pollock estimates and Gulf of

Alaska stock synthesis model age-3 pollock estimates.

Table 2

Echo-integration trawl survey abundance estimates (millions) and echo sign distribution of age-l pollock from Shelikof

Strait, Alaska, for the years 1981, 1983-86, and 1988-91.

Survey Abundance Relative abundance

Year (millions) categoryl Age-1 pollock echo sign

1981 77.7 Low None

1982 No survey

1983 1.2 Low None

1984 61.7 Low Light patch in south

1985 2,091.7 High Moderate, south to central

1986 575.4 Medium Moderate in south

1987 No estimate Light in south

1988 17.4 Low No echograms available

1989 399.5 Medium Moderate, south to central

1990 49.1 Low Light patch in north

1991 22.0 Low one

Depth ofjuvenile

layer (m)

180-200

140-215

195-245

175-205

175-215

223-228

1 Abundance <250 million was considered "low." Abundance ~250 and <1250 million was considered "medium." Abundance ~1,250

million was considered "high."
1.>0

<:> '-3

1981 «10' C?:> ti I l\:J ~.j .... flJ..... - C?:> rI I 1984

58 f-{ Ad}-C &

Z qt~Jl-+-l< 0 ,'I< + + ,.. + o-l +

O~ n + c:r

e. °0° :1= n 57 k 5'S"' +:j: + +

+~ e!. + + 0 := 0 + -0 -0 + ++ -0 + + +

~Cb c?Cb "0 c?Cb + 0

0 ., + ...

Z

8 x + 0-10 a x + 0-10 8 0 + 0-10 'oj

56 f- <l {:, 0 11-99 <l {:, 0 11-99 <l 0 11-99 rJJ

+ f:::, o 100-999 cG cG - f:::, o 100-999 cG o 100-999

6 0 ~1000 6 0 ~1000 o ~1000 Q)

+

1985

58

57 k5'S"' c9~:

0 9.. 0 -0 0 -0 -0

~Cb OV c?Cb 0 c?Cb + 0-10

6 0 o 11-99 6 0 x + 0-10 8 + 0-10

56 f- <l

0

o 100-999 <l {:, o 11-99 <l o 11-99

cG o 1000-10,000 C0 cG f:::, 0 100-999 cG o 100-999

0 ..2. 10,000 60>1000 o ~1000

157 156 155 154 153 157 156 155 154 153 157 156 155 154 153

Figure 4

ShelikofStraitwinter age-l pollock distribution plots for survey years 1981, 1983-87. Catch rates (no/h) are indicated for midwater trawls (+,0) and bottom

trawls (x, 1\).
1989 1990 1988

58

4' 0

57 k~ 3 ©~o ~v.r 0

S

0 f.,J a:: "'" "'" "'" /")

0+ t:?Q::::> t:?Q::::> + ~Q::::>

0 <' + 0-10 +0

d 0 + 0-10 d 0 11-99 d x + 0-10 ':'i

c.....

56 I- <l 0 11-99 <l o 100-999 <l '" 0 11 -99 c

-<

tG o 100-999 tG 81000-1 0,000 tG 6 o 100-999 = o c..1OOO 6 0 c..1OOO ~.

2. 10,000 {;"

III

if

'<

1993 ~. 0i d "t:I _fl, 0

g:

58 k ~",'F ~nv T

+<.:1 r--.c~ [---( .",,,,~ ~A T ~~ [---( ~",'*' ~':+.- v ~~ /")

il':"

[ii'

...

::I.

0 c:r

c

.6. 0 ... Q o' + = 57 k~ e:yvO~ l.-J - 0 III

=

P Co

0 0

+ "'" "'" "'" t:?Q::::> + t:?Q::::> l() ~Q::::> c

0 +

Co= !Z). + 0 x + 0-10 III

= d x + 0-10 d 6 x + 0-10 d

0 '" 0 11-99 /")

56 I- <l '" 0 11-99 <l '" 0 11-99 <l 6 o 100-999 S'

tG 6 o 100-999 tG 6 o 100-999 '" tG 601000-10,000 'J)

6 0 c..1OOO 6 0 c..1OOO t:r 60 2. 10,000 -

0

..., 157 156 155 154 153 157 156 155 154 153 157 156 155 154 153 'J)

...

'1

~.

Figure 4 (continued)

Shelikof Strait winter age-1 pollock distribution plots for survey years 1988-93. Catch rates (no/h) are indicated for midwater trawls (+,0) and bottom trawls

(x, /\). \,>0

-
32 NOAA Technical Report NMFS 126

1985

---·1·· ----------------.------.-------.--------..-- .---------.- --.---- -.--.----.. -- ..------.-- ----.---.--..... -. -------.-- - .....---.-.------

---- -TeT-----·-·- ----.------..-

203 . - i

1986

-.--.-----.-.~._-------_.----..---------._---..~-~--.-----1

, "-,---....--0--.-----.--.----.--_-0.-.---.---- ..----.-..-.----.------.---.~----- ..-.---..1

_._--_.. _----j .------·------1

---I

..:....__.._ .._---:

"," :t. ~\h':·~~';':!~··' .~'p" ..•.; ~.:I ,;o:~. '.~~:

j

I

I :.X1".j;,.Hti,~:\;.,:':l:'>{+· ".,,:~':' ;,:\;-,\-::.', ':':!<, ...•:.-I.~j ,;':"'. ,- .... bottom i

I

216 m

Figure 5

Echograms of age-l pollock layers found during the 1985 and 1986 winter acoustic surveys. In 1985, the upper layer was

predominately age-l pollock and the lower layer was primarily older pollock.
____________ McKelvey: Juvenile Walleye Pollock Distribution and Abundance in Shelikof Strait 33

in the early life stages, suggesting that prerecruit surveys may be a valuable tool for previewing the upcoming year class (Wooster, 1983; Bailey and Spring, 1992;

Hinckley et aI., 1993). Although the primary objective

of the Shelikof Strait acoustic surveys was to assess the

distribution and abundance ofthe adult spawning population, results suggest that Shelikof Strait age-l acoustic

estimates can be used to indicate relative year-class

strength for the Gulf ofAlaska, particularly as an indicator of high recruitment. Results also showed a strong

relationship between age-2 acoustic and model estimates. Since, however, the goal was to find an indicator

of year-class strength as early in the life history of pollock as possible, age-l acoustic estimates are emphasized. To use the abundance estimates as a relative

index of recruitment, one must be willing to assume

that the same percentage of the Gulf of Alaska age-l

pollock inhabit Shelikof Strait each year.

Sources of Bias

Accuracy of an index depends on the errors incurred.

Various sources of error are discussed by Traynor et aI.

(1990) and MacLennan and Simmonds (1992) regarding abundance estimates derived from echo-integration surveys.

Target strength is a particularly difficult parameter

to estimate but is necessary for converting echo-integration values to fish density. The TS-Iength function (TS

= 20 10g(FL) - 66) was used to estimate mean target

strengths. However, this function was derived primarily

from larger pollock.

Another important source of error affecting acoustic

estimates of age-l pollock abundance is thresholding.

The noise threshold setting on the echo integrator

must be low enough to accept signals from valid fish

targets but high enough that the integration of noise

(unwanted signals) does not contribute significantly to

echo integration. The scientist may also introduce error during post-processing of the echo-integrator data

by incorrectly excluding data from analysis. If age-l

pollock are lightly dispersed, it is possible that they will

be excluded from the echo-integration values by either

the mechanical or judgement-based thresholding process. If, on the other hand, age-l pollock are in a dense

aggregation, as in a layer, their likelihood of contributing to echo integration will be much higher and they

will have a greater chance of being sampled. Thus the

index will be more accurate in "high" abundance years.

Other possible sources of bias include differences in

vulnerability to the trawls and net selectivity (MacLennan

and Simmonds, 1992; Gunderson, 1993). These sources

of bias would hamper obtaining a biological sample

representative of the corresponding echo trace. This

would affect the size-composition (and therefore agecomposition) estimates as well as the target strength

estimates for those years when target strength was estimated by means of size composition and a TS-Iength

relationship.

The index described in this paper could be improved

by more intensive sampling in the southern strait (known

area for age-l abundance) to better define the southern extent ofjuvenile echo sign and to sample in lowdensity situations. Also, the survey design did not necessarily include areas where juvenile pollock were found

in the past (e.g., inside bays)(Hinckley et aI. 1991; Bailey

and Spring, 1992). A survey designed to cover these areas

could be used to develop a more accurate index.

Future Work

Future work should address the "selection" and maintenance of a particular depth stratum by age-l pollock as

well as how vertical distribution has changed over the

years. Several possible hypotheses explaining habitat

choice are related to predator avoidance, prey availability, density-dependent habitatselection (MacCall, 1990),

and environmental cues.

Acknowledgments _

I thank Taina Honkalehto and Chris Wilson for their

critical review and Ed Nunnallee for his guidance.

Literature Cited _

Alton, M. S., M. O. Nelson, and B. A. Megrey.

1987. Changes in the abundance and distribution of walleye

pollock (Theragra chalcogramma) in the western Gulf ofAlaska

(1961-1984). Fish. Res. 5:185-198.

Bailey, K. M., and S. M. Spring.

1992. Comparison of huval, age-1 juvenile and age-2 recruit

abundance indices of walleye pollock, Theragra chalcogramma,

in the western Gulf of Alaska. ICES]. Mar. Sci. 49:297-304.

Blue,]. E.

1984. Physical calibration. Rapp. P.-V. Reun. Cons. Int.

Explor. Mer 184:19-24.

Bodholt, H., H. Nes, and H. Solli.

1989. A new echo-sounder system. Proe. Inst. Acoust.

I 1:(Part 3) 123-130.

Foote, K. G., H. P. Knudsen, G. Vestnes, D. N. MacLennan, and

E.]. Simmonds.

1987. Calibration of acoustic instruments for fish density estimation: a practical guide. 1nt. Coune. Explor. Sea Coop.

Res. Rep. 144,57 p.

Foote, K. G., and].]. Traynor.

1988. Comparison ofwalleye pollock targetstrength estimates

determined from in situ measurements and calculations

based on swimbladder form. ]. Acoust. Soc. Am. 83:9-17.
34 NOAA Technical Report NMFS 126

Gunderson, D. R

1993. Surveys of fisheries resources. John Wiley & Sons,

New York, 248 p.

Hinckley, S., K. M. Bailey, S.]. Picquelle,]. D. Schumacher, and

P.]. Stabeno.

1991. Transport, distribution, and abundance of larval and

juvenile walleye pollock (Theragra chalcogramma) in the western Gulf of Alaska. Can.]. Fish. Aquat. Sci. 48:91-98.

Hinckley, S., K. M. Bailey, S.]. Picquelle, M. Yoklavich, and P.]. Stabeno.

1993. Age-specific mortality and transport of larval walleye

pollock Theragra chalcogramma in the western Gulf of

Alaska. Mar. Eco!. Prog. Ser. 98:17-29.

Hughes, S. E.

1976. System for sampling large trawl catches of research

vessels. ]. Fish. Res. Board Can. 33:833-839.

MacCall, A. D.

1990. Dynamic geography of marine fish populations. Univ.

Washington Press, Seattle, 153 p.

MacLennan, D. N., and E.]. Simmonds.

1992. Fisheries acoustics. Chapman and Hall, Lond., 325 p.

Megrey, B. A.

1989. Population dynamics of walleye pollock (Theragra

chalcogramma) in the Gulf of Alaska. Ph.D. dissert., Univ.

Washington, Seattle, 385 p.

1991. Population dynamics and management of walleye pollock (Theragra chalcogramma) in the Gulf of Alaska, 1976-

1986. Fish. Res. 11:321-354.

Nelson, M. 0., and E. P. Nunnallee.

1985a. Acoustic-midwater trawl surveys of spawning walleye

pollock in the ShelikofStrait region, 1980-81 and 1983-84.

In R L. Major (ed.), Condition ofgroundfish resources of the

Gulf of Alaska region as assessed in 1984, p. 179-206. U.S.

Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-80.

1985b. Results of acoustic-midwater trawl surveys for walleye

pollock in the Shelikof Strait, 1985. In R L. Major (ed.),

Condition of groundfish resources of the Gulf of Alaska

region as assessed in 1985, p. 23-49. U.S. Dep. Commer.,

NOAA Tech. Memo. NMFS F/NWC-106.

Ona, E., and]. Traynor.

1990. Hull mounted, protruding transducer for improving

echo integration in bad weather. ICES CM 1990/B:31.

Rothschild, B.].

1986. Dynamics of marine fish populations. Harvard Univ.

Press, Cambridge, MA. 277 p.

Schumacher,]. D., and A. W. KendallJr.

1991. Some interactions between young walleye pollock and

their environment in the western Gulf of Alaska. Calif.

Coop. Oceanic Fish. Invest. Rep. 32:22-40.

Sissenwine, M. P.

1984. Why do fish populations vary? In R. May (ed.), Exploitation of marine communities, p. 59-94. SpringerVerlag, Berl.

Traynor,].]., and]. E. Ehrenberg.

1990. Fish and standard-sphere target-strength measurements

obtained with a dual-beam and split-beam echo-sounding

system. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 189:

325-335.

Traynor,].]., N.]. Williamson, and W. A. Karp.

1990. A consideration of the accuracy and precision of fishabundance estimates derived from echo-integration survey. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 189:

101-111.

Wooster, W. S.

1983. On the determinants of stock abundance. In W. S.

Wooster (ed.), From year to year, p. 1-10. Univ. Washington Press, Seattle.

Wooster, W. S., and K. M. Bailey.

1989. Recruitment ofmarine fishes revisited. In R]. Beamish

and G. A. McFarlane (eds.), Effects of ocean variability on

recruitment and an evaluation of parameters used in stock

assessment models. Can. Spec. Pub!. Fish. Aquat. Sci.

108:153-159.
Summer Distribution and Abundance of Age-O Walleye Pollock,

Theragra chalcogramma, in the Aleutian Basin

QISHENG TANG, XIANSHIJIN, FUGUO LI,JUFA CHEN, WEIXIANG WANG,

YUZHEN CHEN, XIANYONG ZHAO, and FANGQUN DAr

Yellow Sea Fisheries Research Institute

Qingdao 266071

People's Republic ofChina

ABSTRACT

An echo-integration and midwater trawl survey of the Bering Sea for walleye pollock,

Theragra chalcogramma, was conducted from June to August 1993. Large concentrations of

age-O walleye pollock were observed in the Aleutian Basin. Age-O juveniles were mainly

distributed in the northeastern part of the basin, extending from northwest to southeast,

with the highest concentrations between the eastern continental slope and the international waters of the central Bering Sea. For the most part, age-O juveniles inhabited waters

80-120 m deep, but a distinct diurnal vertical migration was observed. During July, age-O

pollock in the northeastern part of the Aleutian Basin ranged from 29 to 47 mm fork length

(FL), while juveniles in the Bogoslof Island area ranged from 11 to 17 mm FL. This

difference may indicate two separate spawning stocks. The temperature of the water inhabited by juveniles was 3°_5°C; the salinity was about 33%0. A relationship exists between

concentrations of age-O pollock and the continental slope environment.

Introduction

Walleye pollock, Theragra chalcogramma, is one of the

most biologically and economically important species

in the Bering Sea. Many recent studies have investigated the biology and the juvenile stages of this species

to gain a better understanding of Bering Sea pollock

population dynamics (Traynor, 1986; Bulatov, 1989; Mulligan et al., 1989; Traynor and Smith, 1996; NRIFSF1, 2, 3;

Yoshimura4). This paper presents new information on

geographic distribution, vertical migration, growth, and

relative density of age-O walleye pollock, and on how

these juveniles relate to the environment and juveniles

ofother species in the Aleutian Basin. This information

was gathered by an echo-integration and midwater trawl

survey conducted by the Yellow Sea Fisheries Research

Institute during the summer of 1993.

Materials and Methods

Survey Design

An acoustic/midwater trawl survey ofthe abundance of

walleye pollock in the Aleutian Basin during the period

of 28 June-2 August 1993 was conducted by the Chinese R/V Bei Dou, a 56.2-m ship with fisheries and

oceanographic capabilities. The transects and stations

occupied for pelagic trawling and environmental observations are shown in Figure 1. The distance between

transects was 22 n.mi. near Bogoslof Island, and 44

n.mi. in other areas.

I National Research Institute of Far Seas Fisheries (NRIFSF).

1994. Preliminary report of biological information obtained from

1990 summer pollock stock research in the Bering Sea by Daian

Maru No. 128. Document for Bering Sea pollock survey working

group.January 1994, Tokyo,Japan.

2 National Institute of Far Seas Fisheries (NRlFSF). 1994. Preliminary report of biological information obtained from 1991 summer

pollock research in the Bering Sea by Shoyo Maru. Document for

Bering Sea pollock survey working group. January 1994, Tokyo,

Japan.

3 National Institute of Far Seas Fisheries (NRlFSF). 1994. Preliminary results of larval pollock survey conducted by the Kaiyo Maru in

1993. Document for Bering Sea pollock survey working group.

January 1994, Tokyo,Japan.

4 Yoshimura, T. 1990. Biological information on pelagic pollock in

the Aleutian Basin during the summer of 1988. Compilation of

papers presented at the International Symposium on Bering Sea

Fisheries, April 2-5, 1990, Khabarovsk, USSR, p. 261-275.

35
36 NOAA Technical Report NMFS 126 _

The acoustic data were collected with a scientific

echo sounding-integrating system (SIMRAD5 EK400j

5 Mention of trade names or commercial firms does not imply endorsement by the National Marine Fisheries Service, NOAA.

38 KHz echo sounder with a hull-mounted SIMRAD

ES38 transducer and a SIMRAD QD digital echo integrator). All data were logged onto a personal computer. The SIMRAD EK400 (120 kHz) echo sounder

was also used to help identify different echo signs in the

o

LD

to-

.....

wo

o

to-

.....

z

o

.....

LD

o

M

L..'?

o

Lt)

Lt)

o

C>

Lt)

-.',\

)J

o

......

co
______ Tang et aI.: Summer Distribution and Abundance of Age-O Walleye Pollock in the Aleutian Basin 37

upper 100-m layer. The integration value (SA' area backscattering strength) was given for each 5-n.mi. section

ofthe transect for a set ofsuccessive depth intervals and

was regarded as a relative abundance index for the

detected fish. The overall acoustic system was calibrated

with a standard target (60-mm-diameter copper sphere

with known target strength of-33.6 dB) at a frequency of

38 kHz (Foote et aI., 1987; Zhu and Iversen, 1990) at the

beginning and end of the cruise while anchored outside

Qingdao Harbor. No significant differences in the system

performance were observed between the calibrations. The

main instrumental parameters are shown in Table 1.

trawl was towed from the stern of the vessel and had a

vertical opening of 38-40 m; the distance between two

otter boards was 121-123 m. Thirty-minute trawls provided adequate catches of age-O juveniles. Other species caught in the trawls helped us identify the various

acoustic targets. Some juvenile specimens were also

collected with an 80-cm surface plankton net.

Water temperatures, salinity, and dissolved oxygen

(DO) at the surface and at 25, 50, 100, 150, 200, 300,

and 500 m were measured at predetermined stations.

In addition, water temperature at 5-m depth was continuously monitored with a CHINO EA2POO temperature recorder.

Biological Sampling and Environmental

Observations Results

The acoustic survey covered about 710 X 103 km2 of the

Aleutian Basin of the eastern Bering Sea (Fig. 1). A

pelagic trawl with 400-cm wing mesh and 4-cm-mesh

codend was used to collect juvenile specimens. The

Geographic Distribution

Large numbers of age-O pollock were discovered in the

northern and eastern parts of the Aleutian Basin (Fig. 2),

51 oN

sr

55<1

_ 59°

Donut hole

r·.

'..)

. .

(

I

\ : ../

r""--""?--~I""-----~:::-------r-------r------~-,61<1 ../

......

170 0E 175° 180° 175° 170 0

165° W

Figure 2

Distribution and relative density (integration value) ofjuvenile walleye pollock in the Aleutian Basin (June 28-]uly 24,

1993). Circled numbers indicate sampling stations for juvenile pollock.
38 NOAA Technical Report NMFS 126

Table I

Technical specifications and main settings of acoustic equipment during R/V BeiDou survey from June 28 to July 24, 1993.

Echo sounders

Frequency

Recorder gain

TVG and gain

Pulse duration

Bandwidth

Transmitting power (dummy 60 ohm)

Basic range

Receiver gain

Transducer

Beamwidth (-3 dB) LP/TP

Equivalent beam angle (10 log)

Source level + voltage response

Calculated instrument constant C1 (ref. -20 dB)

Integrator

In tegrator threshold

In tegrator gain

Integration intervals

SIMRAD EK 400

38 kHz

9

20 log R-20

1.0 ms

3.3 kHz

3500W

0-300 m

84.5 dB

SIMRADES38

7.90 x 8.40

-20.1 dB

139.8 dB

SIMRAD QD/38 kHz

13.8 mV

-22.49 dB

5-100/100-150/150-175/175-200/

200-225/225-250/250-300/300-581 m

SIMRAD EK 400

120 kHz

5

20 log R

1.0 ms

3.3 kHz

500W

0-300 m

84.0 dB

SIMRAD 68BA

100 x 100

-17.6 dB

113.2 dB

extending from the edge of the eastern Bering Sea

shelf to the basin. The zero-distribution isoline was

approximately parallel to the 200-m isobath, and about

100 n.mi. from the 200-m-depth contour line. Areas

with densest concentrations were located at the northeastern part ofthe surveyed area (north of 56°30'N and

east of 1800 E); the highest integration value was 498.

An area of approximately 30 x 103 km2 had an integration value of more than 100. Low concentrations were

observed in the southeastern corner of the basin (east

of 168°W, around Bogoslof Island), where integration

values were mostly below 100. No juvenile pollock were

found within a broad area ofthe western Aleutian Basin

(Fig. 2), where mainly adults (4-20 years old) were

distributed (Fig. 3). Adult distributions were centered

in the central to northern parts of the international

waters and the southeastern basin. Thus there was some

overlap between the distributional areas ofjuveniles and

adults, but no overlap between the areas with the densest

juvenile and adult concentrations. No immature (1-3

years old) pollock were found in the Aleutian Basin.

Vertical Migration

Although juvenile pollock were centered around the

100-m water layer in summer, a distinct diurnal vertical

migration was observed (Fig. 4, 5). During daytime

hours (0800-1400), juvenile pollock were mainly distributed from 80 to 120 m deep. From 1600 to 2000,

they gradually migrated to 20-60 m. Around midnight

(2200-0200) ,juvenile pollock were concentrated in the

upper 10 m, where they could be caught by surface

plankton nets. At dawn (0300-0400), juveniles descended rapidly to the 60-90-m depth layer and gradually stabilized at about 100 m. Throughout all depths,

age-O pollock exhibited a strong schooling behavior,

remaining in a layer 1O-20-m thick (Fig. 5).

Biological Status

Age-O walleye pollock caught in the northeastern part

of the international water zone had a fork length that

ranged from 29 to 47 mm, with a mean of 40.2 mm, and

a mean body weight of 0.43 g. The dominant length was

in the range of 36-44 mm (Fig. 6A). Slight differences

in individual sizes were observed in fishes collected

from three different locations (Table 2, Fig. 2). Juvenile pollock caught near Bogoslof Island were noticeably smaller; fork lengths ranged from 11 to 17 mm,

with a mean of 13.8 mm, and the mean body weight was

0.03 g (Fig. 6B). The relation between fish length and

age in days was used to back-calculate the age ofjuvenile pollock (Nishimura et al., 1996). Spawning was

estimated to have occurred in late March in the northern part of the basin, and in late May in the southern

part (around BogoslofIsland).
______ Tang et at.: Summer Distribution and Abundance of Age-O Walleye Pollock in the Aleutian Basin 39

r--"'--~-----:""""-----""""I'::'------r------""""-----~r"""161 0

c)

Figure 3

Distribution and relative density (integration value) ofadult walleye pollock in the Aleutian Basin Gune 28-July 24, 1993).

Relations with OtherJuvenile Species

200,-~-----.!._-,:----":-~,-----~~_,-----~~ _

12 14 16 18 20 22 24 02 04 06 08 10

.c

15.. 100

..

Cl

and their mean body weight was 0.017 g. Overall, the

distributional area and density of juveniles of other

species were less than those ofjuvenile pollock.

Figure 4

Day and night variation in distribution ofjuvenile walleye pollock in the northern part of the Aleutian Basin

Guly 7, 1993). Solid lines represent high concentrations; broken lines represent low concentrations.

Midnight

Local time

Noon

Additional juvenile fishes were collected in both the

deepwater and surface samples. These specimens ranged

from 2 to 6 em in length and included Greenland

halibut, Reinhardtius hippoglossoides; ronquil, Bathymaster

spp.; sablefish, Anoplopoma fimbria, and squids (Table

3). Greenland halibut were relatively abundant over a

large area, especially at the first sampling station (Fig.

2), where they contributed 11.3% of totaljuvenile catch

by number. Their body lengths ranged from 26 to 42

mm, with a mean of 33 mm, and their mean body

weight was 0.28 g. In addition, age-O ronquil, possibly

two species, were also abundant there. At the first sampling station, ronquil accounted for 6.1 % of the total

juvenile catch by number. They ranged from 40 to 51

mm FL (x =45 mm), and had a mean body weight of

0.77 g. At the fourth sampling station (near Bogoslof

Island), 72.1 % ofthe juveniles by number were ronquil.

Their body size was smaller, 6-13 mm FL (x =11 mm),
40 NOAA Technical Report NMFS 126

200m

..

I

I

100m

" -I-- ./

... '''' ~.

/

... .. , ...-

"-- B

r '..... 1 .... ""11" ... • .

. .

.' - -:"1"'.

.. ',f

•fo,;......

I 1

"-- A+--+--bo"/'----f---+-----;-

." .

c V'?~ D ,--1---+-----+----+-----'--.

--+--'-+---+-V-,,L- /-+---+---t- 200m

.... • \0

I '

.~

j

.'. ,. ~-t" :.:

• • .' '." 1'<- ,:,~ i .. "+..•(.:~I

.:,: ,.;\.~' .,.'-...~.~~" ::t.)!;';' ....{ ~~'~I:'" 'ti'~1 .,. '.'- '.. .. ,. 10081

i

l/

I

I--F

J_

---+--+----+-- -If----I

20081 _ __I

Figure 5

Echograms of vertical distribution ofjuvenile walleye pollock in the northern part of the Aleutian Basin.

Local time: A, 1510--1535; B, 1650--1720; C, 2105-2125; D, 0140--0205; E, 0615-0640; F, 1050--1115.

Relations with Environmental Factors

At the surface, age-O walleye pollock were generally distributed at temperatures of 7°_9°C, salinities of 32.5%0-

33.0%0, and a DO level of 10 mg/I. At 50 m they were

concentrated at temperatures of3°-6°,salinities of32.6%0-

33.0%0, and DO levels of 8--10 mg/I. Finally, at 100 ill,

highest numbers were seen at a temperature range of 20-

5°, salinities of 32.9%0-33.2%0, and DO levels of 7-10

mg/I. Overall, the highest concentrations of juveniles

were observed at about 100-m depth, at temperatures of

3°-5°, salinities of 32.9%0-33.1 %0, and DO of 8--9 mg/I.

Distributions indicate that juvenile pollock mainly

inhabit the waters between the sharpest thermocline

(25-50 m) and a cold-water pocket at 100-200 m (Fig.

7). However, the vertical migration did not appear to

Table 2

Mean length (FL) and weight (W) ofjuvenile pollock

collected from the survey inJuly 1993. Stations are the

same as in Figure 2.

Date Station FL(mm) W(g)

4 I 36 0.28

7 2a 40 0.38

8 2b 41 0.49

II 3 43 0.54

23 4 13.8 0.03

be affected by the temperature gradient. For example,

dense concentrations could rapidly pass through the

thermocline during the die! migration to the surface
______ Tang et a1.: Summer Distribution and Abundance of Age-O Walleye Pollock in the Aleutian Basin 41

%

N=l64

A

14

12

10

8

6

4

2

o~DfZ~~

~g~~~~~~~~g~~~~:~~~

Forte length (mm)

N=37

B

:J

zsJ

% zoj

15

10

5

o.laz~~

mm~~~~:~~·~~~~~~~~·~~

Forte: length (mm)

Figure 6

Length distribution ofjuvenile walleye pollock in (A) the northeastern part of the Aleutian Basin and (B) the area north

of Bogoslof Island.

layer, where temperatures were 7°_9°C. Water temperatures decreased from east to west in the Aleutian Basin

(Fig. 8). The highestjuvenile distributions were located

in the northeastern basin, and no juveniles were observed to the west along the same 3°C isotherm. When

age-O pollock distribution is compared with the horizontal distribution ofsalinity, the juvenile pollock were

concentrated in areas oflow salinity with relatively dense

isohalines (Fig. 9).

Discussion

The Source of Age-O Pollock

If the most current relationship between target strength

and length of pollock (Foote and Traynor, 1988;

Traynor6) and relevant data (integration values) are·

used to expand our acoustic data to estimate the total

population of age-O pollock in the northeastern part of

the Aleutian Basin, we arrive at a biomass of about

20,000 t, corresponding to 58.2 billion fish. Although

this estimate is crude and possibly low because the

acoustic assessment did not include fish located shallower than the transducer, it implies a large concentration of age-O pollock in the northeastern part of the

Aleutian Basin in an area of 150 x 103 km2. Thus the

northeastern part of the Aleutian Basin appears to be

ail important nursery area for age-O pollock. In light of

6 Traynor, J. J. 1994. Target strength measurements of walleye

pollock and Pacific whiting. Paper for Bering Sea Pollock Cooperative Survey Working Group Meeting.January 1994, Tokyo,Japan.

the present understanding of the population structure

and spawning of Bering Sea pollock (Hinckley, 1987;

Bulatov, 1989; Mulligan et aI., 1989), these juveniles

might have originated from the areas around Bogoslof

Island, the Pribilof Islands, or the Aleutian Basin.

The area near Bogoslof Island is one of the most

important spawning grounds for pollock. But the individual sizes around Bogoslof Island are small, and the

distribution of this population shows a discontinuity at

55°N, 1700 W (Fig. 2, Table 2), which implies that it is

unlikely that they moved directly northwest but instead

migh t have arrived by way of the northeastern shelf.

The main spawning period of pollock on the shelf of

the eastern Bering Sea, around the Pribilof Islands, is

from April to June; only part of the shelf pollock population spawns in early spring (Hinckley, 1987). The

survey data of the Japanese RjV Kaiyo Maru in 1993

showed that the dominant length group was 10 mm in

June, indicating a spawning time of May.3 Thus, on the

basis of the larger individual size of juveniles in the

northeastern part of the basin, it is likely that only a

small number of age-O pollock on the shelf come into

the basin.

Because the major spawning period of pollock in the

southeastern part of the Aleutian Basin is earlier than

that of other spawning stocks (Hinckley, 1987; Sasaki,

1989), and because individual juvenile size decreased

from sampling stations 3 to 1 (Fig. 2, Table 2), we

conclude that most of the age-O pollock collected in

our study came from the southeastern part ofthe Aleutian Basin. If this is true, our data suggest that there is

not only a nursery area but also a spawning ground for

pollock within the Aleutian Basin.
Salinity (%0)

174°£ 178°£ 178°W 174°W

42 NOAA Technical Report NMFS 126

Temperature (Oe)

174°£ 178°E 178°W 174°W

0

100

....-- 3.5, S 200 "-'

.... .... fr '" ,

Q 300 3.5 \

I >3.5 ,

, I

" /

400 3.7, - --'" .... .... ,

< ,

,

, 500 , A

33.4

33.5

33.3

~~'..:

33.0

B

Figure 7

Vertical distributions of (A) temperature and (B) salinity along the 57°N section.

Table 3

Species composition (%) ofjuvenile fish in the Aleutian Basin during the surveyJune 28-July 24,1993. The locations are

the same as in Figure 2. Stations 1-3 were sampled by pelagic trawl and were located in the northeastern part of the

Aleutian Basin; station 4 was sampled by surface plankton net and was located north ofBogoslofIsland. N = number, W = weight.

Sampling station

2a 2b 3 4

Species N(%) W(%) N(%) W(%) N(%) W(%) N(%) W(%) N(%) W(%)

Walleye pollock

Theragra chalcogramma 78.8 69.4 97.6 98.3 98.7 99.0 100 100 27.2 61.6

Greenland halibut

Reinhardtius hippoglossoides 11.3 9.9 2.4 1.7 1.3 1.0

Ronquils

Bathymaster sp. A 6.1 14.8

Bathymaster sp. B 72.1 28.6

Sablefish

Anoplopoma fimlnia 0.7 9.8

Squid

Teuthoidea 3.8 5.9

Survey Methodology

Many surveys on age-O Bering Sea pollock have been

conducted by the United States,Japan, and Russia. The

general conclusion is that age-O pollock are mainly

distributed on the eastern Bering Sea shelf, and that

only a small part of the population is found on the

eastern Bering Sea slope and in the Aleutian Basin

(Haryu, 1980; Lynde, 1984; Haryu et aI., 1985; Traynor,

1986; Bulatov, 1989; Mulligan et aI., 1989; Bakkala et aI.;7
______ Tang et al.: Summer Distribution and Abundance of Age-O Walleye Pollock in the Aleutian Basin 43

.....-r--~,....---.,......,,.------"""l"::--------r-------r------~r---, 61 0

,,- .. .--'.

"";' I

\ . ...../

/

r-'

3 __ 0

00

Figure 8

Relation between the distribution area ofjuvenile walleye pollock and temperature (0C) at 100 m depth. Single

hatching represents the overall juvenile distribution; crosshatching represents the highest-density distribution.

NRIFS;I, 2, 3 Yoshimura4). This may be due to an incomplete understanding of the distribution of age-O pollock and a survey effort largely limited to the shelf. This

conclusion may also be related to the survey methodology. Mulligan et al. (1989) pointed out that valid sampling for any fish species at any stage of development

depends upon the proper use of appropriate gear; abundance ofjuvenile pollock in the Aleutian Basin may be

higher than previously reported because of the use of

inappropriate gear. The sampling gears they used have

a relatively small mouth opening (e.g., Methot trawl

with a mouth of 5 m2) for vertical or oblique sampling,

and net avoidance may have biased the results.

When dense scattering was observed during the acoustic survey in the Aleutian Basin, what was believed to be

dense aggregations of some fish at about lOO-m depth

7 Bakkala, R., K. Wakabayashi,].]. Traynor, H. Yamaguchi, K. Okada,

M. Nelson, T. Sample and M. Alton. 1983. Results of cooperative

U.S.-Japan groundfish investigations in the Bering Sea during

May-August 1979. Northwest and Alaska Fish. Cent., Nat!. Mar.

Fish. Sen'., NOAA, 7600 Sand Point Way N. E., Seattle, WA. Unpub!.

manuscr.

was determined by sampling with the pelagic trawl to be

mainly age-O pollock with a few other species. When the

survey was extended over a large area, a relatively complete picture of the distribution and abundance of ageopollock in the Aleutian Basin was obtained from only

one week of sampling. This indicated that a combination acoustic and trawl survey may be necessary to survey age-O pollock. This method may be especially suitable for surveying juvenile fish over a large area of

relatively deep water. If the target strength ofjuvenile

pollock can be more accurately measured, and the

codend mesh size of our net adjusted for sampling

smaller juveniles, more accurate abundance estimates

of age-O fish may be obtained.

Acknowledgments _

The support of the Bureau of Aquatic Products, Ministry of Agriculture of China, Chinese Academy of Fishery Sciences, China National Fisheries Corp. (NFC) ,

Dalian, Yantai, Qingdao, Shanghai and Zhoushan Deep

Sea Fisheries Corp., as well as the U.S. office ofNFC, is
44 NOAA Technical Report NMFS 126

33.1

"'\

c.·

33.0 1.. 32.9

33'2~~"-~~\ --=: '-"

".

)'

C

.. ) :~.. _ ~.3.3.'1 .' _ .

'-. '... :v/ 33.2.. r" (.0 .r-.. ../ '--... .. " -.. / (' 0 .......... J. ) .._. Gf"', .•,..' ....~A·:1C··-.r -.~_. '-.. - .. _~ ...... '_., t·. VV'[..?

.c:::x~ ,..- .. ..""""" ,~> :". '-

Figure 9

Relation between the distribution area ofjuvenile walleye pollock and salinity (%0) at 100 ill depth. Single

hatching represents the overall juvenile distribution; crosshatching represents the highest-density distribution.

greatly appreciated. We thank the captain, chief engineer, and other crew ofthe R/V BeiDou, and A. Kendall

and A. Matarese of the NMFS Alaska Fisheries Science

Center, who helped us identify some juvenile specimens.

Literature Cited _

Bulatov, O.

1989. Reproduction and abundance of spawning pollock in

the Bering Sea. Proc. Int. Symp. BioI. Manage. Walleye

Pollock, Nov. 1988, Anchorage, Alaska, p. 199-208.

Foote, KG., H. P. Knudsen, G. Vestnes, D. N. MacLennan, and

J. Simmonds.

1987. Calibration of acoustic instruments for fish density estimation: a practical guide. Coop. Res. Rep. Int. Counc.

Explor. Sea 144:1-69.

Foote, KG., andJ..J. Traynor.

1988. Comparison ofwalleye pollock target strength estimate

determined from in situ measurements and calculations based

on swimbladder form. .J. Acoust. Soc. Am. 83(1) :9-17.

Haryu, T.

1980. Larval distribution of walleye pollock in the Bering

Sea, with special reference to morphological changes. Bull.

Fac. Fish. Hokkaido Univ. 31:121-136.

Haryu, T., T. Nishiyama, T. Tsusita, H. Ogi, and S. Mishima.

1985. Kinds and distribution offish larvae in the surface layer

of the Bering Sea in summer. Mem. Kushiro City Museum,

10:7-18.

Hinckley, S.

1987. The reproductive biology of walleye pollock in the

Bering Sea, with reference to spawning stock structure.

Fish. Bull. 85(3):481-498.

Lynde, M.

1984. Juvenile and adult walleye pollock ofthe eastern Bering

Sea: literature review and results of an ecosystem workshop.

In D. H. Ito (ed.), Proceedings of the workshop on walleye

pollock and its ecosystem in the Bering Sea. U.S. Dep.

Commer., NOAA Tech. Memo. NMFS F/NWC-42.

Mulligan, T., K Bailey, and S. Hinckley.

1989. The occurrence of larval and juvenile walleye pollock

in the eastern Bering Sea with implications for stock

structure. Proc. Int. Symp. BioI. Manage. Walleye Pollock.

Nov. 1988, Anchorage, Alaska, p. 471-490.

Nishimura, K, K Mito, and T. Yanagimoto.

1996. Hatch date and growth estimation ofjuvenile walleye

pollock, Theragra chalcogramma, collected in the Bering Sea

in 1989 and 1990. In R. D. Brodeur et al. (eds.), Ecology of

juvenile walleye pollock, Theragra chalcogramma, p. 81-

87. U.S. Dep. Commer., NOAA Tech. Rep. 126.

Sasaki, T.

1989. Synopsis of biological information on pelagic pollock
Tang et al.: Summer Distribution and Abundance of Age-O Walleye Pollock in the Aleutian Basin 45

resources in the Aleutian Basin. U.S. Dep. Commer., NOAA

Tech. Memo. NMFS F/NWC-163:80-182.

Traynor,].].

1986. Midwater abundance of walleye pollock in the eastern

Bering Sea, 1979 and 1982. Int. North Pac. Fish. Comm.

Bull. 45:121-135.

Traynor,].]., and D. Smith.

1996. Summer distribution and abundance of age-zero walleye pollock, Theragra chalcogramma, in the Bering Sea. In R.

D. Brodeur et al. (eds.), Ecology ofjuvenile walleye pollock,

Theragra chalcogramma, p. 57-59. U.S. Dep. Commer.,

NOAA Tech. Rep. 126.

Zhu, D., and S. A. Iversen.

1990. Anchovy and other fish resources in the Yellow Sea and

East China Sea, November 1984-April 1988. Mar. Fish.

Res. 11:1-142.

The Relationship between the Distribution of

One-Year-Old Walleye Pollock, Theragra chalcogramma,

and Sea-Ice Characteristics

TINA WYLLIE-ECHEVERRIA1

Institute ofMarine Science

University ofAlaska

Fairbanks, Alaska 99775

ABSTRACT

Distributions of one-year-old walleye pollock, Theragra chalcogramma, were obtained

from bottom trawl surveys conducted on the southeastern Bering Sea shelf between 1972

and 1992. Coincidentally, seasonal sea-ice characteristics during this period were available

from satellite imagery. These two data sets provided a time series for exploring the relationship between sea ice and fish. Years characterized by heavy ice conditions occurred in 1972-

78, coincident with the highest abundance ofwalleye pollock in the outer domain. Years of

light ice conditions occurred in 1979-87, when walleye pollock were most numerous over

the middle domain. Intermediate ice conditions occurred in 1988-92, when walleye pollock

were most numerous over the middle and inner domains. The link between walleye pollock

distribution and ice cover lies in the relationship of ice and bottom-water temperatures.

One-year-old walleye pollock do not concentrate in waters colder than 2°C. Thus the

condition of seasonal sea ice on the eastern Bering shelf during winter forecasts the

distribution of one-year-old walleye pollock on the shelf the following summer. This shift

can be predicted from the duration of ice cover over the middle domain, "" TM' When ice

remains over the middle domain (south of 59°N) less than 20 weeks, most of the one-yearold walleye pollock will be found in the middle domain. When""TM is longer than 24 weeks,

a higher proportion of fish will be located in the outer domain.

Introduction

One-year-old walleye pollock, Theragra chalcogramma,

occur throughout the eastern Bering Sea shelf, northward to Kotzebue Sound, and into the southeastern

Chukchi Sea (Wolotira et aI.2). Sampling of one-yearold walleye pollock from the northeastern Chukchi Sea

in 1990 (Wyllie-Echeverria, 1995), a year ofreduced ice

cover (ice extended to 75°N, the farthest north in 30

years), suggests that sea ice might influence walleye

pollock distribution. It has been observed that survival

of tanner crab, Chionoecetes opilio, seems related to the

maximum extent of sea ice in April (Somerton, 1982).

In addition, during years of heavy ice cover and more

cold bottom water in the middle domain of the southeastern Bering shelf, distribution of young walleye pollock changed in relation to the 2°C isotherm (Bakkala

and Alton, 1986). Changes in the distribution of walleye pollock are probably a function of systemwide

changes linked to seasonal sea-ice cover.

A dominant characteristic of the Bering Sea shelf in

winter is seasonal ice cover, the extent of which varies

interannually (Overland and Pease, 1982). The amount

of ice cover extending over the shelf affects the biological and physical environment in a number of ways. Ice

affects shelf hydrography through melting and freezing

that locally redistributes salt and heat in the water column, changes vertical stratification, and directly influences shelfwide heat and salt budgets (Schumacher et

1 Present address:jISAO, PMEL, NOAA, 7600 Sand Point Way N.E.,

Seattle, WA 98115-0070.

2 Wolotira, R.j.,jr., T. M. Sample, and M. Morinjr. 1977. Demersal fish and shellfish resources of Norton Sound, the southeastern

Chukchi Sea, and adjacent waters in the baseline year 1976. Northwest Alaska Fish. Cent. Proc. Rep., U.S. Dept. Commer., NOAA,

Natl. Mar. Fish. Serv., Seattle, WA. 292 p.

47
162 159 156

66°N

65

64

63

63

ALASKA

61

60

59

58

57

56

55

168 165

I

I

~I<)0

I %;~

174 171

SOUTHEAS TERN AREA

CENTRAL /

AREA /

50

48 NOAA Technical Report NMFS 126

Figure 1

Study area on the Bering Sea shelf. Station locations from the bottom trawl surveys are indicated by dots. Inner,

middle, and outer domains are bounded by the 50-,100-, and 200-m isobaths. The core area used in the analysis

is the southeastern area, with comparisons to the central area. Seasonal sea-ice characteristics are represented by

ice position along longitude 169°W (dashed line with arrow).

aI., 1979). The extent of cold bottom water in the

middle domain may be directly affected by the amount

ofice cover the previous winter (Takenouti and Ohtani,

1974). Additionally, ice melt generates a relatively strong

current in the marginal ice zone (Muench and

Schumacher, 1985). The timing of the ice-edge phytoplankton bloom, as well as the subsequent spring

bloom, is affected by the ice cover and by the date ofits

retreat (Niebauer et aI., 1995).

The walleye pollock-sea ice interaction is examined

within three hydrographic domains on the southeastern Bering Sea shelf (Kinder and Schumacher, 1982;

Fig. 1). The inner domain, with depths 50 m or less (Z$

50 m), is vertically mixed by tidal action throughout the

year. The middle domain (50 <Z$ 100 m) is mixed by

heat loss in winter, except at the ice edge, where continuous melting produces a surface layer of low-salinity

water. In summer the buoyancy flux from ice melt and

heating stratifies the water column so that a less-saline

surface layer extends down 20-50 meters, depending

on wind conditions. The more saline bottom layer is

colder and results from the previous winter's ice cover

and mixing by tidal currents (Barnes and Thompson,

1938; Takenouti and Ohtani, 1974; Coachman and

Charnell, 1979). The outer domain (>100 m) is a stratified system in summer and winter, influenced by water

intruding onto the shelf from the Bering slope in the

bottom layer, and mixed by winds in the surface layer.

In this paper, I examine interannual variation in the

distribution of one-year-old walleye pollock for the period 1972-92. Discernible distribution patterns emerge

over interannual and longer time periods. The amount
__ Wyllie-Echeverria: The Relationship between the Distribution of One-Year-Old Walleye Pollock and Sea-Ice Characteristics 49

Table I

Distribution of demersal one-year-old pollock on the

central Bering Sea shelf. Percentage of stations in the

inner, middle, and outer domains with pollock in 1972-

1992. Numbers ofstations sampled in parentheses.

(Jle), Suiteland, Maryland. The JIC produces a digitized data set (SIGRID) that presents the weekly concentration (areal coverage) and position ofsea-ice cover.

These data begin 1 January 1972. I extracted data for

the position of ice with concentration of at least 30%

along meridian 169°W from the SIGRID database for

the period 1972-92, and verified them with the weekly

ice charts for the western Arctic (Naval Polar Oceanography Center, 1986). I chose meridian 169°W to represent shelfwide ice conditions because it provides a

mechanism for updating the analysis with a single data

point (latitude of 30% ice cover); it lies over open water

for the entire extent ofthe Bering and Chukchi shelves;

it intersects the shelfbreak at the southernmost reaches

of the shelf; and it borders the area sampled by the

bottom trawl surveys analyzed in this study (Walters and

McPhail, 1982; Walters, 1983; Fig. 1).

During a given year the seasonal pattern ofice growth

and retreat can be characterized by several features.

For the Bering shelf, these include (a) the number of

weeks ice extended over the middle domain (~TM) and

(b) the outer domain (~To); (c) the southern latitude

of ice extent (Ps); (d) how long ice was at this southern

latitude (~Ts); and (e) the date of ice retreat (TR)

(Fig. 3).

Domains of the central Bering Sea shelf

and characteristics of seasonal sea ice also vary over

such time periods and may affect the distribution of

walleye pollock. I discuss the relation between summer

variability in one-year-old walleye pollock distribution

on the shelf and seasonal sea-ice during the preceding

winter, with the intent of developing a method to predict distribution of pollock from sea-ice characteristics.

Methods

One-Year-Old Walleye Pollock

I generated distribution maps for one-year-old walleye

pollock from the data base compiled by the National

Marine Fisheries Service, Resource Assessment and Conservation Engineering Division, Sand Point Way N.E., Seattle, Washington. This data base (RACEBASE) contains

biological data gathered from bottom trawl surveys on the

Bering Sea shelf conducted byJapan, Russia, Canada, and

the United States during summer months between 1972

and 1992 (Mintel and Smith, 1981). Two areas were investigated, the southeastern and the central shelves (Fig. 1). In

order to maximize the time frame of the data set, I chose a

core area ofsoutheastern shelfstations that were sampled

each year, beginning in 1972, for my initial data analysis. I

compared a reduced data set from the central shelf area

with the southeastern area to investigate intra-annual patterns in areal distribution. One-year-olds were considered

to be fish with a fork length ~20 cm.

Three domains, based on the boundaries of the inner, middle, and outer fronts, were analyzed in each

area. The number of stations varied in the early years

and between domains (Tables 1 and 2). Although abundance data exist for this data set, limitations arise because of the variety of ships and gear employed. As a

consequence, a difference in CPUE can be a function

ofgear rather than a real change in abundance (Walters

and McPhail, 1982; Hoff, 1989) and was not used in this

study. In order to standardize the effort between areas

and domains, I divided the number of stations with

one-year-old walleye pollock by the total number of

stations sampled each year. The resulting calculation is

the percentage of stations with one-year-old walleye pollock by area and domain (Table 1, 2). The domain with

the highest percentage ofstations with walleye pollock was

designated the domain of principal distribution for that

year. If the difference between domains was <10%, both

or all were considered the principal domain (Fig. 2).

Sea Ice

Data for the position of the ice edge were extracted

from products of the Navy/NOAA Joint Ice Center

Year

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

Inner % (N)

Not sampled

Not sampled

0(9)

46 (54)

14 (21)

Not sampled

Not sampled

37 (54)

55 (53)

45 (20)

60 (55)

69 (55)

64 (55)

64 (55)

42 (55)

II (55)

67 (55)

35 (55)

85 (54)

82 (55)

36 (39)

Middle % (N)

Not sampled

64 (II)

33 (18)

14 (71)

41 (34)

73 (26)

64 (53)

94 (80)

79 (71)

77 (66)

78 (80)

84 (80)

84 (80)

38 (80)

36 (80)

21 (80)

78 (80)

88 (80)

94 (80)

83 (80)

52 (75)

Outer % (N)

66 (3)

71 (7)

70 (10)

18 (61)

68 (28)

95 (20)

98 (51)

98 (61)

79 (48)

69 (55)

75 (61)

76 (58)

64 (61)

15 (61)

8 (61)

10 (61)

51 (61)

48 (61)

61 (61)

72 (61)

23 (60)
50 NOAA Technical Report NMFS 126

Table 2

Seasonal sea-ice characteristics along longitude 169°W, and the distribution of walleye pollock on the southeastern Bering

Sea shelf: the number ofweeks seasonal ice extended into the middle domain, the date and week when seasonal ice began

to retreat northward, the southernmost extent of the winter ice, and the percentage of stations with one year-old-walleye

pollock in each domain.

Seasonal sea-ice characteristics One-year-old walleye pollock distribution

Middle domain Date of Week of Southern Inner domain Middle domain Outer domain

Year (weeks) retreat retreat latitude % (n) % (n) % (n)

1972 22 May 1 17 56°15' 43 (21) 82 (56) 75 (16)

1973 27 May 8 18 57°15' 4 (24) 14 (49) 70 (23)

1974 28 Apr 16 15 56°00' 21 (28) 31 (70) 41 (27)

1975 28 May 6 18 56°30' 25 (28) 44 (70) 52 (27)

1976 21 May 26 21 56°00' 21 (28) 41 (70) 44 (27)

1977 27 Apr 19 15 57°15' 0(28) 25 (61) 0(27)

1978 19 Mar 28 12 58°00' 11 (28) 41 (70) 19 (27)

1979 3 Mar 20 11 58°45' 82 (28) 73 (70) 85 (27)

1980 21 Mar 18 11 56°45' 61 (28) 49 (70) 13 (23)

1981 17 Mar 17 11 57°45' 29 (28) 63 (70) 15 (27)

1982 23 Mar 23 12 56°15' 43 (28) 66 (70) 56 (27)

1983 20 Mar 15 11 57°30' 36 (28) 69 (70) 22 (27)

1984 21 Apr 3 13 56°30' 46 (28) 61 (70) 19 (27)

1985 19 Apr 30 17 57°15' 75 (28) 59 (70) 11 (27)

1986 20 Apr 8 14 56°45' 43 (28) 46 (70) 19 (27)

1987 20 Apr 21 16 58°30' 29 (28) 30 (70) 0(27)

1988 4 Apr 19 16 57°00' 43 (28) 54 (70) 19 (27)

1989 17 Apr 4 13 58°00 18 (28) 56 (70) 7 (27)

1990 14 Apr 24 16 57°15' 75 (28) 77 (70) 41 (27)

1991 14 Apr 23 16 57°00' 86 (28) 69 (70) 26 (27)

1992 16 May 13 19 56°45' 29 (28) 53 (70) 0(27)

Year

Figure 2

Principal domain (s) of distribution of one-year-old walleye pollock in the southeastern Bering shelf annually,

between 1972 and 1990. When percentages of stations

with pollock were within 10% in two (or all three)

domains, both (or all) domains were considered to be

principal domains.

The relation between ice extent and the cold bottom

water in the middle domain was explored using data

collected with XBT (expendable bathythermograph)

probes during the groundfish surveys (Goddard and

Zimmermann3). The southernmost position of the seasonal ice extent along 169°W was compared to the

average bottom temperature (TB) on the shelf during

summer. The availability of ice data before the summer

sampling season could be used to evaluate future hydrographic conditions.

Results

Sea Ice

The use ofsea-ice characteristics along 169°W as an index

is based on the assumption that they are representative

of ice conditions over the Bering shelf. Comparison

with areal ice coverage between 1600 W and 1800 W

(Chapman and Walsh, 1993) revealed that the southernmost latitude reached by ice along 169°W is significantly correlated with ice-covered shelf area (r = 0.91;

3 Goddard, P., and M. Zimmermann. 1993. Distribution, abundance, and biological characteristics of groundfish in the eastern

Bering Sea based on results of the U.S. bottom trawl survey during

June-September 1991. U.S. Dep. Commer., NOAA, Nat\. Mar.

Fish. Serv., Alaska Fish. Sci. Cent. Proc. Rep. 93-15, 324 p.
__ Wyllie-Echeverria: The Relationship between the Distribution of One-Year-Old Walleye Pollock and Sea-Ice Characteristics 51

75

•

MIDDLE DOMAIN

OUTER DOMAIN

60

55 +4---+-++-+-f-+-+-+-+-l-I--H---+-++-+-H-+-t---l-J-+-+-+---+-+-+-+-f-+-H---+-+-+-+--I---+-++-+-f-+-H-+-H

QJ

-0

;:3

.....

.

.....:l65

"€o

z

70

Week

Figure 3

Weekly ice-edge positions (diamonds) along meridian 169°W in 1976. Characteristics of position and

time used to describe interannual variability in seasonal sea ice in the Bering Sea are noted. D.TM =

duration of ice extent over the middle shelf domain; D.To = duration over the outer domain; Ps = the

position of ice edge at the southern extent; D.Ts= duration at the southern extent; TR = time of ice

retreat. The boundary of the middle domain is latitude 58°50'; the boundary of the outer domain is

57°00'.

p = 0.01). A measurement of the advance and retreat of

the ice edge along one degree of longitude has the

added advantage of permitting the timely updating and

evaluation ofsea-ice conditions from the weekly sea-ice

maps. Areal calculations, in contrast, must await the

release of the SIGRID data base, which is usually delayed by 2-3 years.

Juvenile Walleye Pollock

The distribution ofone-year-old walleye pollock changes

interannually and over longer time periods (Tables 1

and 2; Fig. 2). I investigated intra-annual relationships

in the distribution of pollock to explore the possibility

that the populations may shift between the southeastern and central areas. The percent distributions of oneyear-old pollock, between areas and domains, for the

years 1979-92 were positively correlated. I did not use

the complete data set for this analysis because some

areas were not sampled early in the period. Significant

correlation between the southeastern and central areas

and the inner, middle, and outer domains indicates

that pollock are not switching domains or areas within

years but rather that a shelfwide shift happens between

years.

The distribution of one-year-old walleye pollock did

not exhibit extreme shifts each year, but rather displayed sequences of years in which one domain had

higher values (Fig. 2). Higher percentages of stations

with walleye pollock occurred in the outer domain

between 1972 and 1976 than in the middle or inner

domains. Percentages increased in the middle domain

between 1977 and 1984, and in the inner domain between 1985 and 1991. The middle shelf of the southeastern Bering shelf had the highest concentrations of

one-year-olds in 81% of the years sampled.

Sea Ice and Cold Bottom Water

Sea-ice growth over the shelf is an easily observed, remotely sensed feature that offers insights into some of

the physical conditions in the water column. In each

year, seasonal ice extended beyond 58°30'N and covered the inner domain. In addition, ice occurred over

the middle domain for as long as 28 weeks, and over the

outer domain for up to 13 weeks. The beginning of its
52 NOAA Technical Report NMFS 126

retreat varied from mid-March to the end of May (Table

2). The southern extent of sea ice ranged over two

degrees of latitude, between 58°40', in 1979, to the

shelf break at 56°00', in 1976.

Several trends exist in seasonal sea-ice characteristics. A marked shift in the number of weeks that ice

extended over the middle domain occurred in 1979

and 1988. Ice extended over the middle shelf domain

for an average of 24 weeks between 1972 and 1978, 20

weeks between 1980 and 1987, and 15 weeks between

1989 and 1992 (Table 2). Thus, periods of increasingly

reduced ice cover occurred over this period, punctuated by two years of extremely light ice cover. A late ice

retreat (in May) dominated the early period, followed

by an early retreat (in March), and ending with an

intermediate time of retreat (in April). The transitions

occurred in 1978 and 1984.

The southernmost extent of ice, Ps' was significantly

correlated with average bottom temperatures (r=0.74;

p=0.05). The duration of ice over the middle domain

and the timing of its retreat were also significantly

correlated with bottom temperatures (r=-0.52 and

-0.42, respectively).

The linear model

°C (bottom water) = Ps (0.76) - (40.2),

where °C is the average bottom temperature for the

southeastern Bering shelf in summer, and Ps is the

southernmost latitude reached by seasonal sea ice along

longitude 169°W during the previous winter, can be

used to predict average bottom-water temperatures on

30 '

the southeastern Bering shelf for the following summer. The farther south the seasonal ice advances, the

longer it will remain over the middle domain; thus a

later retreat will result in lower bottom temperatures

on the southeastern Bering shelf.

Sea Ice/ Bottom Temperature/ Pollock

Interactions

The distribution of one-year-old walleye pollock on the

Bering shelf varies in concert with sea-ice conditions

(Fig. 4).

The linear model

% (pollock) = 1.7 (I:-.TM) - 84.2,

where I:-.TM is in weeks, and % represents the percentage of stations in the middle domain where one-yearold walleye pollock were caught by bottom trawl surveys, can be used to predict the percent distribution of

one-year-old walleye pollock (r=-0.65; p =0.01; Table

3). The less time ice remains over the middle domain

(south of 59°) the more one-year-old pollock will be

concentrated in the middle domain (Fig. 5). When

I:-.TM is greater than 24 weeks, pollock are more concentrated in the outer domain (Table 2). The year 1979 is

an exception to the model; fish were distributed

throughout the shelf and reflected the record year class

of 1978 rather than the extremely mild ice conditions.

The average bottom temperature (TB) in the southeastern Bering Sea is significantly correlated with the

90

25';' •\ i ,- II r- n 80

~20111 ' ,j \ :~ i 15;: I~ r.~ rJI"1I' Mnnn-' :~ 1:\ : \ /\ I I ,; : I i 1'\')1 I ;, Q) 10 - I".I' \ /' I I I ' : • I 30

S I:\.i'1 iI! ·1, III! I! ,:, i I:, I I I, I I '" II II!! 20 rf

5 : I., I I I' I",I II " ,I I I 10

, i: :;!ll I ~I' ; I I LJIi I :: i 'I 'I' LJII WI':

A ---L-1·'----J ,!" I l...-- ------!r--------o.i~ I, I I I ,I! ,-1------'.;- I L- I'

'-----~- I I,. ---l..,L-l,'" ............, 0

N ro m 0 N ro m 0 N ro ro ro ro ro ro ro ro ro ro m m m m m m m m m m m m m m m m m m m m m m m m

Figure 4

Number of weeks ice extended over the middle domain (C17~) (bars) and the percentage of

stations with one-year-old walleye pollock in the middle domain (squares). Three periods of

fairly consistent ice cover (1972-78, 1980-87, and 1989-92), were punctuated by years of

extremely limited ice cover.
__ Wyllie-Echeverria: The Relationship between the Distribution of One-Year-Old Walleye Pollock and Sea-Ice Characteristics 53

percentage of stations with pollock in the middle domain of the central shelf area (r=0.43) , but not in the

southeastern area (r=0.14). About 40% of the variability of pollock distributions can be related to the variation in average bottom temperatures on the central

shelf. Warmer overall bottom temperatures result in

increased densities in the middle domain ofthe central

shelf.

Seasonal sea-ice characteristics are useful indicators

of conditions in the water column and of the distribution of walleye pollock the following summer. In particular, the duration of ice cover over the middle domain (between latitudes 56°45' and 59°N) is significantly correlated with bottom temperatures and with

the concentration of one-year-old walleye pollock in

the middle domain of the Bering shelf.

Discussion

Ebbesmeyer et al. (1991), using 40 environmental and

biological variables, detected a general shift in the Pacific climate, beginning in 1977. This shift occurs in ice

cover, air temperature, and sea-surface temperature in

the Bering Sea (Niebauer, 1988). Another significant

climatic feature occurred in 1983-one ofthe strongest

El Nino events of this century (Hollowed and Wooster,

1992; Trenberth and Hurrell, 1994). A large shift in

ground fish recruitment (including walleye pollock)

has been observed in the North Pacific from the mid to

late 1970s (Hollowed and Wooster, 1992), coincident

with the shift in physical features that seems related to

the position and intensity of the Aleutian low-pressure

system (Beamish, 1993; Niebauer and Hollowed, 1993).

Large-scale shifts in the North Pacific apparently extend into the Bering Sea, and the periodic variation in

sea-ice characteristics and one-year-old walleye pollock

distribution operates on the same time scale.

Bakkala and Alton (1986), using the same survey

data as this paper for 1975 and 1979-82, observed a

higher proportion of one-year-old walleye pollock in

the outer shelf area in cold years (1975, 1982) and a

higher proportion in the middle-inner shelf area in

warm years (1979-81), indicating an interannual variation coincident with the bottom-temperature contour

of 2°C. The results from the study reported here were

similar for the same time period. Highest percentages

of stations with walleye pollock occurred in the outer

domain in 1975 (67%) and 1982 (67%), the middle

domain in 1979 (96%) and 1981 (65%), and the inner

domain in 1980 (68%; Table 1). Interannual variation,

however, appears secondary to periods when walleye

pollock are concentrated in the outer (1972-77), middle

(1978-90), or inner (1983-88) domains if a longer

time frame is investigated (Fig. 6).

Comparisons of year-class strength estimated from

annual trawl surveys (1979-91; Wespestad and Dawson,

1993) and the distribution of walleye pollock show a

significant correlation between year-class abundance

and the distributional pattern of the corresponding

year class in the outer (r=0.68) domain, but not the

middle (r=0.16) or inner (r=0.16) domains. The outer

domain is occupied when population levels are high,

but the distributions on the middle and inner shelves

apparently have no relation to density.

The fish examined in this paper are limited to the

catches from the eastern Bering Sea bottom trawl surveys. One-year-old walleye pollock are not limited to

the benthic habitat over the shelf (Bakkala and Alton,

1986), as evidenced by portions of the population 10-

Table 3

Prediction of population distribution based on the number of weeks ice remained over the middle domain,

and the predicted percentage of walleye pollock in the

middle domain ofthe Bering shelf. When TM is greater

than 22 weeks the population increases in the outer

domain.

80

c 70

•

E

8 60

~50 ·

·40

!'!TM

(weeks)

Pollock in middle domain

% 10 15 20 25 30

Weeks loe over Middle Domalll

Figure 5

Linear regression between number ofweeks ice extended

over the middle domain and the percentage of stations

with one-year-old walleye pollock in the middle domain.

Correlation was significant with r= -0.65; p= 0.01.

1

5

10

15

20

25

30

83

76

67

59

50

42

33
54 NOAA Technical Report NMFS 126

JUNE-JULY 1992

CPUE (NO/HAl

NO CATCH

<100

• 100-499

• 500-999

• >1000

+ ••••

ALASKA 62

61

60

59

58

57

56

55

54

MAY-JULY 1981

CPUE (NO/HAl

NO CATCH

• <100

• 100-499

• 500-999

• >1000

v

I

I

JUNE-AUGUST

CPUE (NO/HAl

NO CATCH

<100

• 100-499

• 500-999

• >1000

1973

. +~: . 61

60

59

58

57

56

55

54

61

60

59

58

57

56

55

54

177 OOE 180 OOW 177 OOW 174 OOW 171 OOW 168 OOW 165 OOW 162 OOW 159 OOW 155 OOW

Figure 6

Distribution maps for one-year-old walleye pollock on the Bering shelf. Representative maps from the

cold period (1972-78), the warm period (1979-87), and a warmer period (1988-92) are presented.
__ Wyllie-Echeverria: The Relationship between the Distribution of One·Year·Old Walleye Pollock and Sea-Ice Characteristics 55

cated in midwater by acoustical surveys (Walters et aI.,

1988). The proportion offish in the water column has

been assessed for 1979, 1982, 1985, and 1988, and

shows no consistent portion of the population in the

pelagic versus the demersal habitats (Traynor et aI.

1990).

In conclusion, longer periods of environmental conditions rather than interannual variability appear to

affect the demersal distribution of one-year-old walleye

pollock. Comparative analysis over a twenty-year period

reveals a concurrent shift between areas of distribution

and environmental conditions, related to warm and

cool episodes. The time period examined spanned cool

to warm to warmer conditions, so repeating conditions

did not occur. If cool conditions reappear, as they often

have in the past, one-year-old walleye pollock should

shift their population to the outer domain. This shift

can be predicted from the duration of ice cover over

the middle domain. The outer domain appears to be a

secondary habitat for one-year-olds, occupied when the

middle domain is cold or crowded.

Acknowledgments _

Many people contributed to this work, including those

who collected and processed sea-ice data at the Joint

Ice Center in Suitland, Maryland, and the species data

in the RACE division at National Marine Fisheries Service in Seattle, Washington. People who directly aided

in the analysis, discussion, or review of this work and

whom I would like to thank are Tim Irwin, Gary Walters,

Claire Armistead, Willard E. Barber, Sandy Wyllie

Echeverria, Anne Hollowed, Warren Wooster, andJim

Schumacher. This work was partially funded by Mineral

Management Service, U. S. Department of the Interior,

Anchorage, Alaska, contract No. 14-35-0001-3-559; the

Pacific Marine Environmental Laboratory, Department

of Commerce, Seattle, Washington, of which it is contribution number 1552; and the Joint Institute for the

Study of Atmosphere and Ocean, of which it is contribution number 268.

Literature Cited _

Bakkala, R. G., and M. S. Alton.

1986. Evaluation of demersal trawl survey data for assessing

the condition of eastern Bering Sea walleye pollock. Symposium on biology, stock assessment, and management of

walleye pollock, Pacific cod, and hake in the North Pacific

region. Int. N. Pac. Fish. Comm. Bull. 45:90-120.

Barnes, C. A., and T. G. Thompson.

1938. Physical and chemical investigations in the Bering Sea

and portions of the North Pacific Ocean. Univ. Washington Pub!. Oceanogr. 3(2):35-79.

Beamish, R.].

1993. Climate and exceptional fish production off the west

coast of North America. Can.]. Fish. Aquat. Sci. 50:2,270-

2,291.

Chapman, W. L., and]. E. Walsh.

1993. Recent variations of sea ice and air temperature in

high latitudes. Bull. Am. Meteorol. Soc. 74(1):33-47.

Coachman, L. K., and R. L. Charnell.

1979. On lateral water mass interaction-a case study, Bristol

Bay, Alaska. ]. Phys. Oceanogr. 9:278-297.

Ebbesmeyer, C. c., D. R. Cayan, D. R. McLain, F. H. Nichols,

D. H. Peterson, and K. T. Redmond.

1991. 1976 step in the Pacific climate: forty environmental

changes between 1968-1975 and 1977-1984. In J. L.

Betancourt and V. L. Tharp (eds.), Proceeding of the seventh annual Pacific climate (PACLIM) workshop, April 1990,

p. 115-126. Calif. Dep. Water Resour. Interagency Eco!.

Stud. Program Tech. Rep. 26.

Hoff, R. Z.

1989. Spatial distributions and abundance trends for two species of flatfish in the eastern Bering Sea. M.S. thesis, Univ.

Washington, Seattle. 154 p.

Hollowed, A, and W. Wooster.

1992. Variability of winter ocean conditions and strong year

classes of northeast Pacific groundfish. lCES Mar. Sci.

Symp.195:433-444.

Kinder, T. H., andJ. D. Schumacher.

1982. Circulation over the continental shelf of the southeastern Bering Sea. In D. W. Hood and]. A. Calder (eds.), The

eastern Bering Sea shelf: oceanography and resources, vol.

1, p. 53-93. U. S. Dep. Commer., NOAA.

Mintel, R.]., and G. B. Smith.

1981. A description of the resource survey data-base system

of the Northwest and Alaska Fisheries Center. U.S. Dep.

Commer., NOAA Tech. Memo NMFS F/NWC-18, 111 p.

Muench, R. D., and]. D. Schumacher.

1985. On the Bering Sea ice edge front. ]. Geophys. Res.

90:3185-3197.

Naval Polar Oceanography Center.

1986. Eastern-Western Arctic sea ice analysis 1985. Joint lce

Center, Washington, DC.

Niebauer, H.].

1988. Effects of El Nino-Southern Oscillation and North Pacific weather patterns on interannual variability in the subarctic Bering Sea. ]. Geophys. Res. 93:5051-5068.

Niebauer, H.]., V. Alexander, and S. Henrichs.

1995. A time series study of the spring bloom at the Bering

Sea ice edge. 1. Physical processes, chlorophyll and nutrient

chemistry. Cont. Shelf Res. 15:1859-1877.

Niebauer, H.]., and A. B. Hollowed.

1993. Speculations on the connection of atmospheric and

oceanic variability to recruitment of marine fish stocks in

Alaska waters. In Is it food?: Addressing marine mammal

and seabird declines: workshop summary, p. 45-53. Alaska

Sea Grant Rep. 93-01.

Overland,]. E., and C. H. Pease.

1982. Cyclone climatology of the Bering Sea and its relation

to sea ice extent. Monthly Weather Rev. 110:5-13.

Schumacher,]. D., T. H. Kinder, D.]. Pashinski, and

R. L. Charnell.

1979. A structural front over the continental shelf of the

eastern Bering Sea. ]. Phys. Oceanogr. 9:79-87.

Somerton, D. A

1982. Effects of sea ice on the distribution and population

fluctuations of C. opilio in the eastern Bering Sea. In Proceedings of the international symposium on the genus
56 NOAA Technical Report NMFS 126

Chionoecetes, May 3-6, 1982, p. 157-172. Alaska Sea Grant

Rep. 82-10.

Takenouti, A. Y, and K. Ohtani.

1974. Currents and water masses in the Bering Sea: a review

ofJapanese work. In D. W. Hood and E.]. Kelley (eds.),

Oceanography of the Bering Sea with emphasis on renewable resources, p. 39-57. Occas. Publ. 2, Insl. Mar. Sci.,

Univ. Alaska, Fairbanks.

Traynor,].]., N.]. Williamson, and W. A. Karp.

1990. A consideration of the accuracy and precision of fishabundance estimates derived from echo-integration

surveys. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 189:

101-111.

Trenberth, K. E., and]. W. Hurrell.

1994. Decadal atmosphere-ocean variations in the Pacific.

Clim. Dynamics 9:303-319.

Walters, G. E.

1983. An atlas of demersal fish and invertebrate community

structure in the eastern Bering Sea: part 2, 1971-1977. U.S.

Dep. Commer., NOAA Tech. Memo. NMFS F/NWC40, 152 p.

Walters, G. E., and M.]. McPhail.

1982. An atlas of demersal fish and invertebrate community

structure in the eastern Bering Sea: part 1, 1978-1981. U.S.

Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-35,

122 p.

Walters, G. E., K. Teshima,].]. Traynor, R. G. Bakkala,

]. A. Sassano, D. L. Halliday, W. A. Karp., K. Mito,

.]. Williamson, and D. M. Smith.

1988. Distribution, abundance, and biological characteristics

of groundfish in the eastern Bering Sea; based on results of

the U. S.-Japan triennial bottom trawl and hydroacoustic

surveys during May-September, 1985. U.S. Dep. Commer.,

NOAA Tech. Memo. NMFS F/NWC-154, 400 p.

Wespestad, V. G., and P. Dawson.

1993. Walleye pollock. Condition of groundfish resources in

the Bering Sea-Aleutian Islands region as assessed in

1991. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/

AKC.

Wyllie-Echeverria, T.

1995. Sea-ice conditions and the distribution of walleye pollock (Theragra chalcogramma) on the Bering and Chukchi

Sea shelf. In R. Beamish (ed.), Proceedings on Ocean Climate and Northern Fish Populations. Can. Spec. Publ.

Fish. Aquae Sci. 121:328-342.
Summer Distribution and Abundance of Age-O Walleye Pollock,

Theragra chalcogramma, in the Bering Sea

JIMMIE TRAYNOR and DOUGLAS SMITH

Alaska Fisheries Science Center

National Marine Fisheries Seroice, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

Summer surveys of age-O walleye pollock, Theragra chalcagramma, in the eastern Bering Sea were conducted in

1982, 1984, and 1985. Because of the limited spatial

coverage and the paucity of trawl hauls, the survey in

1982 was viewed as a pilot survey, and few conclusions

resulted. Conversely, the surveys in 1984 and 1985 covered the entire eastern Bering Sea shelf area and included enough hauls to describe the spatial distribution by size of age-O pollock in the shelf region. The two

primary objectives for the surveys were 1) to determine

the feasibility of conducting acoustic abundance estimation surveys for age-O pollock with a low-frequency

echo-sounding system (38 kHz) and 2) to determine

the biological characteristics and distribution of age-O

pollock on the eastern Bering Sea shelf.

In 1982, echo-integration data were collected with a

Biosonics1 Model 101 echo sounder operating at 38

kHz and a Model 120 digital echo integrator. The

system's transducer was mounted in a 2-ft V fin towed

behind the vessel. In 1984 and 1985, no quantitative

echo-sounding system was used, but a Simrad EQ-38

echo sounder, interfaced with a JRC16 color echosounder recorder, was used to obtain relative abundance estimates during the surveys. Age-O walleye pollock were sampled with a Marinovich trawl for all survey

years. The codend was equipped with a 9.6-mm knotless

web liner in 1982 and part of 1984, and with a 3.2-mm

codend liner in 1985 and part of 1984. The vertical

mouth opening of the trawl ranged from 4 to 6 m.

There was not a strong relation between numerical

catch per unit of effort (CPUE) of age-O walleye pollock

and acoustic density determined by echo integration.

Also, there was not a strong relation between the total

weight of the catch and acoustic density. Both results are

probably due to the small size of these organisms relative

to the acoustic wavelength at 38 kHz (about 40 mm).

The CPUE indicated a center of abundance of age-O

pollock in the vicinity of the Pribilof Islands in each of

the three surveys. In 1984 and especially in 1985, a

second peak of abundance was observed in the inner

Bristol Bay area (Fig. 1). Intermediate to high densities

were observed in the Bristol Bay area at approximately

164oW. In general, CPUE's were higher in 1985 than in

1984; the highest was near the Pribilof Islands.

Distribution by size was similar between 1984 and

1985: age-O pollock (>50 mm fork length, adjusted to

size at August 1, assuming a growth of 0.5 mm/d) were

found only in the vicinity of the Pribilof Islands, while

smaller fish were found throughout the survey area. A

closer examination of the size composition in 1985

reveals that the distribution of age-O pollock by size is

fairly complicated (Fig. 2). Up to three size modes were

observed at trawl locations, and two modes were very

common. The inner Bristol Bay area had modal fish

lengths in the 20-30-mm or 30-40-mm size range. On

the outer shelf area southeast of the Pribilof Islands,

size modes were observed in two categories: 30-40 mm

and 50-60 mm. In the vicinity of the Pribilof Islands,

size modes ranged from 20 to 60 mm. In the extreme

northern portion of the area, most fish were in the 20-

40-mm modal size range.

Although it is not possible to determine the spawning date accurately, it is possible to approximate the

spawning time with a number of assumptions. Assuming 1) approximately 20 days from egg release to hatching, 2) that the fish reach 20 mm after about 60 days,

and 3) that the growth rate is approximately 0.5 mm/d

after 60 d, the spawning period associated with each

mode discussed above can be estimated as May 1 for 25-

mm fish, April 10 for 35-mm fish, March 20 for 45-mm

fish, and March 1 for 55-mm fish. The calculations

assume that the growth rate of the age-O pollock is

constant and unaffected by environmental conditions

(temperature) or food availability, which is almost certainly not true. However, the calculations provide an

I Reference to trade names or commercial firms does not imply

endorsement by the National Marine Fisheries Service, NOAA.

57
58 NOAA Technical Report NMFS 126

5600

5800

6000

o

ALASKA

18000 17600 17200 16800 164 00 16000

II

·... ,:: 0 - 999

I}}} 1,000 - 9.999 1::::::::::

,mggm 10,000 - 49,999

1984

Catch rates (number per hour)

17600E

-l!E===¥=:3E:==F=~==I'==~=i==:E==='l===-==F:=E==F""""==F===>=='F'3:=~~~"'3~E,' ~==E::=t==:=E==I'==~=i==:E~~54 00

15600W

1985

Catch rates (number per hour)

I

0-999

mmmm 1,000 - 9,999

;Wkii 10.000 - 49,999

50,000 - 99,999

>100,000

6400N

6200

6000

5800

5600

17600E 18000 17600 17200 16800 16400 16000

1!l===f==E::=t==:=E==I'==~=F':=E==r===""""==F===>=='F'3:===¥=:3===F=3:=$kE::=t==:=E==I'==~=F'='3:=:3r===""""=J 54 00

15600W

Figure I

Catch-rate distributions for age-O walleye pollock in the eastern Bering Sea during 1984 (upper) and 1985 (lower).

Catch rates are in number per hour. The plus signs indicate Marinovich haul locations.
_____ Traynor & Smith: Summer Distribution and Abundance of Age-O Walleye Pollock in the Bering Sea 59

172 168 164 160

•

•

•

• ... ... • • ... •t •• i

• • t • •

• ... -t •

.~

•

176

+

• 2O-30MM

• 3O-40MM

& 4Q-SOMM

• S<>OOMM

58

56 MODAL LENGTHS

54 '-------~----~ _ ____"~~=_.~ _ _____.J

180

60

62 r-----~------,t-----,~----~-~,.--~-----:>i--~--,

•

Figure 2

Modes of length frequency for age-O walleye pollock in 1985. All length

frequencies were adjusted to August 1, assuming a growth rate of

0.55 mm/d. The plus signs indicate Marinovich haul locations.

indication of the range of spawning periods probably

represented by age-O pollock on the eastern Bering Sea

shelf.

The spawning time of pollock is known fairly accurately only for the Bogoslof population, where essentially all spawning occurs in very late February or early

March. There are three areas of spawning in the eastern Bering Sea: near Bogoslof Island, near Unimak

Island, and in the vicinity ofthe Pribilof Islands. Spawning appears to occur in April near Unimak Island and

in April or somewhat later near the Pribilof Islands.

The above scenario indicates that the larger size modes

would come from the Bogoslofspawning group, whereas

the smallersize groups would result from spawning on the

shelf. The presence of the larger size modes near the

shelf and in the vicinity of the Pribilof Islands is consistent with the known current pattern, based on driftbuoy trajectories, which would distribute pollock from

near Bogoslof Island onto the eastern Bering Sea shelf

in the vicinity and to the south of the Pribilof Islands.
Distribution of EarlyJuvenile Walleye Pollock, Theragra chalcogramma,

and Associated Species in the Gulf of Alaska

MORGAN S. BUSBY, RICHARD D. BRODEUR, and MATTHEW T. WILSON

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115

A midwater trawl survey was conducted in July 1991 to

examine the large-scale distribution pattern of late larval and juvenile walleye pollock, Theragra chalcogramma,

and associated fish taxa in the western Gulf of Alaska.

Gear comparisons between the anchovy and Methot

trawls were done to evaluate which sampling trawl was

most efficient at capturing the size range of walleye

pollock available during this time of the year. Both

gears caught similar length distributions in the dominant size class of pollock caught, but the Methot trawl

caught significantly more walleye pollock in the smallest (mostly larval) size classes and had higher standardized catches overall. Based on these results, a grid of 60

stations was sampled with the Methot trawl only.

Although 64 taxa of fishes were collected overall,

most (84%) ofthe larval catch consisted ofjust five taxa

(flathead sole, Hippoglossoides elassodon; walleye pollock;

60

arrowtooth flounder, Atheresthes stomias; Pacific cod,

Gadus macrocephalus; and sculpins, Icelinus spp.). Walleye pollock and Pacific cod were the dominant (>99%)

juveniles collected in the survey. The highest catches of

juvenile walleye pollock (26-52 mm standard length)

were from midshelf waters along the Alaska Peninsula

and near the Shumagin Islands. Recurrent group analysis and two-way indicator species analysis both showed

that juvenile pollock were frequently associated with a

large, heterogeneous grouping of larval and juvenile

fishes, including Pacific cod, flathead sole, arrowtooth

flounder, and other winter-spring spawning species.

The abundance rankings ofthe dominant fishes from the

Methot trawl survey were more similar to the abundance

rankings ofadultsfrom trawl surveys the previous summer

than were those of an ichthyoplankton survey that used

bongo nets several months before the Methot survey.
Size-Related Depth Distribution ofJuvenile Walleye Pollock,

Theragra chalcogramma, in the Western Gulf of Alaska during 1984-1990

MATTHEW T. WILSON

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, WA 98115-0070

MICHIYO SHIMA

School ofFisheries

Box 357980

University of Washington

Seattle, Washington 98195

Analysis of a subset of the juvenile walleye pollock,

Theragra chalcogramma, length and abundance data collected by the Resource Assessment and Conservation

Engineering (RACE) division of the National Marine

Fisheries Service (NMFS) suggests that a size-related

depth distribution exists within age groups. The bottom (n=97) and midwater (n=336) trawl hauls we examined were collected during daylight from August to

early October from 1984 to 1990 in the western Gulf of

Alaska. Our data indicate that hauls where the average

size of age-O pollock was about 60 mm fork length (FL)

were made from 20 to 160 m. However, hauls with an

average age-O pollock size of 100 mm FL were made at

deeper depths (80 to 240 m). High abundances of ageopollock were found at depths of 120 m or less both on

bottom and in midwater, but individuals were found

down to 250 m. No differences in size were found

between age-O pollock collected on bottom and those

collected in midwater. Regardless of whether they are

on bottom or in midwater, age-O pollock tend to undergo an ontogenetic migration into deeper water. The

influences of region and temperature on the size distribution ofjuvenile pollock were also explored.

61
Juvenile Walleye Pollock, Theragra chalcogramma, Distribution and

Association with Other Dominant Species in the Gulf of Alaska

MICHIYO SHIMA

School ofFisheries

Box 357980

University of Washington

Seattle, Washington 98195

ANNE B. HOLLOWED

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.E.

Seattle, Washington 98115

GLENN R. VANBlARICOM

School ofFisheries

Box 357980

University of Washington

Seattle, Washington 98195

The main purpose of our study was to identity areas of

juvenile (ages 0 and 1) walleye pollock, Theragra chalcogramma, habitat and key species associated with juvenile

pollock in the Gulf of Alaska as a preliminary step in

modeling how environmental and fishing variability

affectsjuvenile pollock distribution and, in consequence,

foraging by Steller sea lions, Eumetopias jubatus.

We explored the vertical distribution ofjuvenile pollock collected on select NMFS cruises. Three years (1973,

1982, and 1986) of the NMFS groundfish and shrimp

trawl surveys were selected because they spanned the

late 1970's, when the North Pacific experienced a welldocumented regime shift from cool to warm waters.

Juvenile pollock were divided into two size classes that

roughly represent age groups. Juveniles <150 mm (age

0) were caught at slightly greater bottom depths than

pollock from 150 to 230 mm (age 1) in 1973, but this

trend was reversed in 1982 and 1986. This finding could

have been confounded by the fact that two different

gear types were used between 1973 and the 1980's. In

general, pollock were caught at greater depths in the

Shumagin Islands than around Kodiak Island. When

additional data from only one gear type (high-opening

shrimp trawl) from 1984 to 1990 were examined, both

age groups of pollock were distributed throughout the

water column. Their widespread distribution could be

related to the warm water temperatures measured at

the surface (>9°C) and at bottom (>7°C).

62

Comparison of the NMFS bottom trawl surveys of the

Chirikofregion in 1975, 1984, 1987, and 1990 revealed

that, although the dominant groundfish species remained the same, the relative abundance of each species changed from 1975 to 1990. In 1975, walleye pollock represented 69%, and arrowtooth flounder,

Atheresthes stomias, represented only 10%, ofthe groundfish trawl biomass in the Chirikof region. By 1990,

arrowtooth flounder represented 53% and walleye pollock represen ted only 10% of the groundfish trawl biomass in this region.

Interspecies associations were examined through a

clustering technique (Bray-Curtis method) used on the

shrimp-trawl data. Pollock <150 mm were often found

in association with other nearshore species such as

Pacific herring, Clupea paUasi, and capelin, Mallotus

villosus, un til the early 1980's. This smaller size class of

juveniles was not found together with adult pollock

until the mid-1980's. Pollock between 150 and 230 mm

were found with adults from 1980 and were mostly

associated with more benthic-oriented species. This transition in species associations could result from juvenile

pollock moving to adult pollock habitat or from adults

moving into the nearshore areas commonly inhabited

by juveniles. The latter hypothesis is consistent with

trends in the distribution of age-l fish before and after

the climate shift.
Distribution and Abundance ofJuvenile Walleye Pollock,

Theragra chalcogramma: Summary and Recommendations

for Future Research

DONALD R. GUNDERSON

School ofFisheries

Box 357980

University of Washington

Seattle, Washington 98195

Research to date provides a somewhat fragmented picture of juvenile walleye pollock distribution, largely

because most existing information is based on incidental catches made during bottom trawl or acoustic/

midwater trawl surveys for other species or age groups.

The surveys described in this symposium by Wilson et al.

and Traynor and Smith were the only ones designed with

the principal objective of examining the distribution and

abundance ofjuvenile pollock (age-o in this case).

Nevertheless, the information presented during this

symposium serves to demonstrate thatjuvenile pollock

are found in significant concentrations in both the

midwater pelagic zone (Tang et aI., Wilson et aI.,

McKelvey) and on the seabed (Wyllie-Echeverria, Miyake

et aI., Wilson et aI.). The use of these habitats may vary

within years, because of ontogenetic changes in preferred habitat (Miyake et aI., Wilson and Shima), and

between years, because of environmental variability

(Wilson et aI., Wyllie-Echeverria).

Miyake et aI. reported that walleye pollock larvae and

juveniles (4-38 mm long) were distributed mainly in

the surface layer along the coast of southeastern

Hokkaido.Juveniles migrate offshore as they grow, and

move to the bottom layer. Off southeastern Hokkaido,

juveniles 50-150 mm long are found in significant numbers in waters 40-100 m deep, where they can be captured with a demersal trawl.

Wyllie-Echeverria demonstrated that the spatial distribution of l-year-old pollock in the eastern Bering Sea

shifted significantly during 1972-90, partly because of

interannual changes in ice coverage and its influence

on environmental conditions (surface temperature,

bottom temperature, salinity structure, timing of the

phytoplankton bloom, etc.).

During their survey of the western Gulf of Alaska,

Wilson et aI. found that most age-O fish (averaging 70

mm long) occurred in midwater over the middle and

inner continental shelf, whereas previous investigators

encountered high abundances offshore near the 200-m

isobath, or near the seabed in embayments.

The papers presented at this symposium also indicated that juvenile surveys may eventually provide reliable forecasts of year-class strength. McKelvey found a

significant correlation (r2=O.58) between the abundance

of age-l pollock in Shelikof Strait and Gulf of Alaska

stock assessment estimates from the same year class 2

years later. Miyake et aI. were less successful (r 2=0.21)

in predicting year-class strength from the abundance of

age-O juveniles in trawl surveys off southeastern

Hokkaido, probably because the survey area (designed

for smelt assessment) did not cover the entire range of

this age group in some years.

The vertical and spatial distribution ofjuvenile pollock appears to be quite plastic, resulting from an interplay of many factors (e.g., temperature, light, food, and

predation). Combined midwater trawl!acoustic and

demersal trawl surveys are required to provide a holistic

picture of the distribution ofjuvenile pollock, and-at

least during the initial years of survey developmentthey must extend over a broad geographic area. Improved surveys ofjuvenile pollock are essential ifwe are

to gain a significant understanding of their basic ecology (distribution, feeding, mortality processes, etc.) and

the factors that determine year-class strength.

63

Feeding Habits and Daily Ration ofJuvenile Walleye Pollock,

Theragra chalcogramma, in the Western Gulf of Alaska

NAZlLA MERATII and RICHARD D. BRODEUR

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

ABSTRACT

The stomach contents of age-O walleye pollock, Theragra chalcogramma, collected in the

western Gulf of Alaska during September of 1990 were examined to determine dieI feeding

patterns and variations in feeding habits with respect to time of day, fish size, and geographic area. Most (90%) of the 549 fish examined (length range 49-113 mm standard

length) had food in their stomachs, with an overall median fullness of 1.6% body weight

(BW). Juvenile pollock fed mainly at night, with stomach fullness and prey condition

declining throughout the daylight hours. Daily ration was estimated to range from 7.4% to

8.5% BW/d based on the dieI trajectory ofstomach content weight.

The dominant prey by number were larvaceans and copepods (copepodites and adults),

whereas euphausiids were the dominant prey by weight, constituting >80% of the total

biomass consumed. Despite some apparent trends in the diet with size of predator (i.e.,

copepods gradually decreased and larvaceans and euphausiids increased in importance

with increasing size of pollock), there did not appear to be an abrupt ontogenetic shift in

the diet of these juveniles. However, there were major changes in the diet depending on

geographic area. Stomach samples collected near Kodiak Island contained mainly euphausiids, whereas those collected in the western part of the study area contained large

numbers of larvaceans. There was no discernable pattern in the type of prey consumed at

different times of day.

Introduction

In 1980 a large spawning population ofwalleye pollock,

Theragra chalcogramma, was discovered in ShelikofStrait,

Gulf of Alaska, and soon became the object of an intense multinational fishery (Megrey, 1991). Most Gulf

of Alaska pollock migrate into the strait in late winter

and spawn in deep water (Kendall and Picquelle, 1990;

Kim and Nunnallee, 1990). The eggs hatch in midApril, and larval patches are transported southwest to

nursery grounds along the Alaska Peninsula (Hinckley

et aI., 1991; Wilson et aI., 1996). To better understand

the processes critical to the survival of pollock, investigators continue to study both the biotic and abiotic

factors that may influence year-class success (Schumacher and Kendall, 1991).

If survival during the first summer and fall is crucial

to recruitment of age-2 fish to the fishery, as shown for

some years by Bailey and Spring (1992), then it is important to learn more about their early life history

beyond the larval stage. Fish that survive to the juvenile

stage have passed through critical larval stages where

starvation is thought to be a major cause of mortality.

Thoughjuvenile fish do not appear to be as susceptible

to starvation as larvae are, the amount and quality of

food available may affect their growth rate and distribution, and ultimately their survival (Walline, 1983).

Owing to walleye pollock's considerable commercial

and trophodynamic importance in the North Pacific

Ocean (Springer, 1992), many studies have examined

the predator-prey relationships oftheir early life stages.

Although the feeding habits of young walleye pollock

have been described in detail (Kamba, 1977; Walline,

1983; Clarke, 1984; Krieger, 1985; Lee, 1985; Kendall et

aI., 1987; Nakatani and Maeda, 1987; Grover, 1990,

1991; Takatsu et aI., 1992), most of these studies have

dealt with populations outside the Gulf of Alaska or

I Present address:JISAO, Pacific Marine Environmental Laboratory,

NOAA, 7600 Sand Point Way N.E., Seattle, WA 98115-0070.

65
66 NOAA Technical Report NMFS 126

have examined only early juvenile pollock «30 mm

standard length) collected over a small geographical

area. At present, the only available studies on food

habits of late juvenile age-O pollock in the Gulf of

Alaska ecosystem are those of Walline (1983), Krieger

(1985), and Livingston2 but these studies had a limited

sampling size and in each case, the fish were taken over

a relatively restricted area of the gulf. If feeding in the

juvenile stage is linked to vertical and horizontal distribution patterns and ultimately to survival, then the

knowledge gained from this study may prove useful in

answering questions aboutjuvenile pollock's migratory

behavior and recruitment success.

The objectives of this study are fourfold: 1) to describe the diel feeding chronology of age-O juvenile

walleye pollock in the western Gulf of Alaska; 2) to

estimate daily ration based on diel changes in stomach

2 Livingston, P. A. 1985. Summer food habits of young-of-the-year

walleye pollock, Theragra chalcogramma, in the Kodiak area of the

Gulf of Alaska during 1985. Alaska Fish. Sci. Cent., Nat!. Mar. Fish.

Sen'., 7600 Sand Point Way N.E., Seattle, WA 98115-0070. Unpub!.

manuscript.

fullness; 3) to describe the prey of juvenile walleye

pollock; and 4) to examine changes in prey with respect to time of day, fish size, and geographical area.

These results are discussed in light of what is presently

known about the trophodynamics ofjuvenile pollock in

the Gulf of Alaska and elsewhere in the North Pacific

Ocean.

Materials and Methods

Sample Collection and Processing

Age-O pollock were collected during 6-23 September

1990 aboard the NOAA vessel Miller Freeman in the

western Gulf of Alaska (Table 1; Fig. 1). Several different gear types and fishing strategies were employed to

investigate the distribution and catchability of age-O

pollock (Wilson et aI., 1996). Most hauls were made in

midwater along a grid of stations set up along the

Alaska Peninsula, but opportunistic midwater and nearbottom tows were also made in order to identify acoustic sign and to sample inlets around Kodiak Island and

Table 1

Summary of haul collections used for age-O walleye pollock feeding study. Lengths are means and standard deviations (in

parentheses) offish whose stomachs were examined.

Haul Geographic No. of Standard Time

number region stomachs length (mm) Collection date (ADT)

6 I 26 80.58 (1.19) 8 September 1119

7 I 20 81.21 (1.72) 8 September 2142

8 I 20 90.86 (2.46) 8 September 2340

15 II 20 59.60 (1.28) 9 September 1710

25 II 20 74.90 (1.23) 10 September 1840

26 Il 20 70.70 (1.97) 10 September 2025

31 Il 20 73.81 (2.02) 11 September 0642

34 II 20 72.74 (1.58) 11 September 1143

35 Il 20 70.40 (1.95) 11 September 1307

49 II 20 72.59 (1.12) 13 September 1917

50 II 20 70.24 (1.56) 13 September 2045

71 III 20 73.62 (1.66) 14 September 0959

73 1II 20 72.68 (1.80) 14 September 1124

95 III 21 80.20 (1.74) 16 September 1129

98 III 20 75.24 (2.30) 16 September 1803

100 III 20 69.98 (1.51) 17 September 0152

111 II 20 76.63 (2.38) 18 September 0245

112 II 20 81.02 (2.15) 18 September 0432

113 II 20 79.26 (1.94) 18 September 1031

114 U 20 78.22 (1.79) 18 September 1137

117 Il 21 76.88 (1.65) 18 September 1750

120 Il 21 87.96 (2.96) 19 September 2046

121 II 20 78.93 (1.72) 19 September 2132

122 II 20 81.33 (1.65) 20 September 0013

123 Il 20 94.86 (1.41) 20 September 0200

124 II 20 73.68 (1.44) 20 September 2318

136 III 20 76.21 (1.67) 22 September 0533
__ Merati & Brodeur: Feeding Habits and Daily Ration ofJuvenile Walleye Pollock in the Western Gulf of Alaska 67

o

Area II

other areas (see DeWitt and Clark3 for station locations

and sampIing chronology). The primary midwater gears

used were an 18.6-m high-opening shrimp trawl and a

24.4-m anchovy trawl, which both contained 3-mm mesh

liners (Wilson et aI., 1996). Tows were made throughout the water column in either an oblique or steppedoblique fashion at an average ship speed of 5.9 km/h.

Near-bottom juveniles were sampled with a 37-m

Nor'eastern bottom trawl fitted with a 5-mm mesh liner.

Collections at any particular station were not predetermined as to time of sampling, except for five repeat

3 DeWitt, C., and J. Clark. 1992. Fisheries-Oceanography Coordinated Investigations Shelikof Strait: 1990 field operations. NOAA

Data Rep. ERL PMEL-39, 98 p.

o Stomach Collections

b. Diel Series

o

tows made at the same station over a span of 15 h

(termed the semidiel study).

Mter retrieval of the nets, juvenile pollock were

quickly sorted from the catch, counted, and measured.

Age-O fish were easily differentiated from older age

classes by length distributions; a subsample (a minimum of 20) of these fish was placed in buffered 10%

formalin or 90% ethanol. These fish were approximately 110-140 d old (Brown4).

Because we were interested in characterizing the feeding habits of the main component of the age-O population and wished to maintain a minimum sample size

4 Brown, A. Alaska Fish. Sci. Cent., Nat!. Mar. Fish. Serv., NOAA,

7600 Sand Point Way N.E., Seattle, WA 98115-0070. Unpub!. data.

o

Area I

Gulf of Alaska

Area III

Figure 1

Locations where stomach collections were made in the western Gulf of Alaska during September 1990. The regular

collections are shown as circles and the diel collections are shown as triangles. Also shown are the three areas examined for

geographic variability in food habits.
68 NOAA Technical Report NMFS 126 _

per station (n~20), we chose fish only from stations

where large numbers of juveniles were caught. Age··O

pollock were examined from samples collected over a

broad geographic area to examine regional differences

in diet (Fig. 1). In the laboratory, 20 age-O pollock were

haphazardly chosen from the preserved samples. Fish

were measured to the nearest millimeter (standard

length; SL) and weighed to the nearest 0.001 g, and the

stomach (from the esophagus to pylorus) was excised.

Relative fullness ofthe stomach was visually ranked on a

scale of 0 to 5 as follows: 0 = empty; 1 = trace amount of

food; 2 = 25% full; 3 = 50% full; 4 = 75% full; and 5 =

100% full. The entire stomach contents were then blotted on blotting paper to remove excess moisture and

weighed to the nearest 0.001 g.

The stomach contents were separated under a dissecting scope. The relative condition of the contents

was determined on a scale of 0 to 4, ranging from totally

digested to fresh prey. Gut contents were identified only

to major taxonomic category (order). Each taxon was

enumerated and weighed individually for every stomach.

Data Analysis

To examine variations in feeding intensity, we calculated the percent body weight (%BW) ofthe total stomach content weight (wet weight in grams) for each fish:

%BW = (stomach content weight x 100)

(total body weight - stomach content weight)

For indices to characterize the time of day when ageo pollock feed, we used 1) %BW, 2) fullness code, 3)

digestion code, and 4) percentage ofstomachs containing at least some food. Collection times were grouped

into 4-h intervals to examine diel variation at smaller

increments of time. Because of the highly skewed nature of the %BW values and the number of empty

stomachs, we chose to use the median values and to test

for differences using nonparametric statistics. Diel differences in feeding intensity among time intervals were

tested with a Kruskal-Wallis one-way nonparametric

analysis of variance (ANOVA). When significant differences were observed, we performed an a posterori

pairwise test of adjacent intervals using a Mann-Whitney

Utest to determine the time period in which the changes

occurred (Zar, 1984). To minimize the effects of geographic variability, we used the same analyses to separately examine stomachs collected during the five hauls

(Stations 111-117 in Table 1) at different times but at

the same location (semidiel series).

Daily ration (Rd) was estimated from the diel trajectory ofstomach content weights following the approach

of Sainsbury (1986). We fitted two models to the median values of %BW for each collection period using a

nonlinear least-squares iteration as implemented by the

MAXIMS program (Jarre-Tiechmann, 1992). The first

(Model I) assumes that the ingestion rate is constant,

whereas the second (Model II) assumes that it is linearly related to stomach contents (see Sainsbury, 1986

andJarre et aI., 1991 for derivation of the two models).

Assuming a single feeding period per day, Model I is

defined as

t=F

Rd = {Jdt = J(Fs - Fb ),

t=F.

where Fb is the time at which feeding begins, Fs is the

time feeding stops, andJis the ingestion rate (%BW/h).

Model II contains an additional parameter (S~) to specify

an asymptotic consumption level:

l=Fj

Rd = J[s~ *(E- J2)- J2*St]dt,

t=F.

where E is the instantaneous rate of evacuation (per

hour), J2 is the instantaneous rate of ingestion (per

hour), and St is the stomach contents at time t (Jarre et

aI., 1991). Both models assume an exponential evacuation rate, which has been observed for older juvenile

and adult walleye pollock in laboratory experiments

(Dwyer et aI., 1987; Smith et aI., 1989). These models

have been shown to provide similar estimates to those

from the frequently applied Elliott and Persson (1978)

model, but they do not require independent estimates

of evacuation rate.

The total collection offish, including those from the

diel series, was grouped into several subsets to examine

variability in diet composition. The percent numerical

and percent gravimetric composition were used to characterize the diet composition by three factors: area,

predator size, and time of day. Trends in dietary variability by geographic area were examined by dividing

the collection into three areas (Fig. 1) designated as

Area I (Kodiak Island), Area II (east of Shumagin Islands), and Area III (west of Shumagin Islands). Tc

examine dietary variability by predator size, we divided

the entire range of available age-O pollock lengths into

six size categories «60 mm, 60-69 mm, 70-79 mm, 80-

89 mm, 90-99 mm, and >100 mm). Finally, to examine

diel changes in major prey taxa consumed, we grouped

the collection into three time periods: day (0930-

1930 ADT), twilight (1930-2220 and 0700-0930), and

night (2220-0700). Differences in the presence or absence of major prey taxa among these subsets were

compared individually for each factor (area, size, and
__ Merati & Brodeur: Feeding Habits and Daily Ration ofJuvenile Walleye Pollock in the Western Gulf of Alaska 69

time of day) using the variance test of binomially distributed data. For tests that exceeded the tabulated

0.05 X2 value, we rejected the null hypothesis ofsimilar

diets (Snedecor and Cochran, 1967). Because of the

large number of empty cells, we were not able to test for

interaction effects among these factors. Finally, to quantify absolute changes in the biomass of the major prey

groups among the different times ofday, areas, and size

classes, we also examined the mean weight of prey

consumed at each level within each of the factors.

Results

Because the highest catches of age-O fish occurred in

Area II (Wilson et al., 1996), most of our age-O fish

stomachs were collected in this area (Fig. 1). Age-O

pollock examined for the feeding habits study showed a

similar length-frequency distribution to those from the

overall September 1990 survey (Wilson et al., 1996). No

significant differences in size composition existed between the three areas (Kolmogorov-Smirnov test corrected for multiple tests).

Feeding Intensity

The relation between size of fish and %BW consumed

was examined because size-related variability may be an

important factor in the analyses that follow. We tested

the null hypothesis that %BW was not a function offish

size. We regressed fish size (standard length in millimeters) on %BW, and found that the slope ofthe relationship was not significant (P>0.50); therefore, the meal

size appears to increase isometrically with fish size.

A wide distribution of %BW values was observed (Fig.

2), with a median stomach fullness of 1.64%BW. The

distributions of values for each area were highly

heteroscedastic, including relatively few extreme values, especially in Area III (Fig. 2), but the distributions

were not significantly different among areas (Kolmogorov-Smirnov test).

Box plots of the values of the feeding-intensity parameters for different 4-h periods of the day for the

entire data set revealed that prey condition (digestion

code) was the freshest at night, with a median value of 2.5

at 0200 (Fig. 3a). The overall trend suggested that prey

condition decreased as the day progressed, with the lowest

0.7 ---,-------------------------,

0.6

eu '0 0.5

--o 0.4

c::

o

:e 0.3

o

Q,

~0.2

a.

0.1

o

Percent body weight

LJ Area I (n = 66)

6J Area II (n = 282)

.Area III (n = 131)

Figure 2

Distributions ofrelative stomach content weight (as percent body weight, %BW) ofjuvenile walleye

pollock in each area analyzed. Also given is the number ofstomachs included in each area (N).
70 NOAA Technical Report NMFS 126

0200 06001000140018002200

Time of day (hours)

Overall Diet Composition

median value of 1.5 in the period just before dusk (1600-

2000). Overall differences in prey condition by time were

significant (Kruskal-Wallis test, H=56.46; P<O.OOl). Significant pairwise differences in ranked sums occurred

between time periods 2 and 3 (0400-0800 and 0800-

1200) and 3 and 4 (0800-1200 and 1200-1600; Table 2).

Gut fullness was highest at night, with a median fullness of 5 (Fig. 3b). The gut fullness decreased as the

day progressed, with lowest values at 1000 and after

dusk (2000). We rejected the hypothesis that gut fullness

was equal at different times (H=34.50; P<O.OOl). Significant differences in gut fullness detected by a posteriori

comparisons were found between 0400-0800 and 0800-

1200 and between 0800-1200 and 1200-1600 h (Table 2).

The %BW ofstomach contents demonstrated a trend

similar to that of gut fullness. The highest values were

recorded just before dawn (0400-0800) and decreased

as the day progressed (Fig. 3c). The hypothesis that

%BW was equal at the time periods examined was rejected (H=31.46; P<O.OOl). Subsequent pairwise MannWhitney tests indicated that periods 1 and 2 (0000-

0400 and 0400-0800),2 and 3 (0400-0800 and 0800-

1200), and 3 and 4 (0800-1200 and 1200-1600) had

significantly different %BW values (Table 2).

The semidiel study showed trends similar to those

seen in the overall survey (Fig. 4). Median prey digestion was highest at night (0200) and decreased over

time. Gut fullness and %BW values were highest at

night and decreased steadily until the last sampling at

1745 (Fig. 4). Ranked values for all three feeding-intensity parameters were unequal for all times tested

(Kruskal-Wallis test; P<O.OOl in all cases; Table 3).

Pairwise differences in gut fullness were found between

time periods 1 and 2 (0245 and 0445) and between

periods 4 and 5 (1215 and 1745). Prey-digestion codes

showed significant decreases in every pairwise test except

for time periods 3 and 4 (1000 and 1200). In contrast,

only one significant difference in %BW occurred; it fell

between time periods 4 and 5 (1215 and 1745; Table 4).

Median values of %BW were quite variable, especially

for the times following peak feeding (Fig. 5). Estimates

of daily ration for the two models were 8.45% and

7.37%BW per day for Model I and Model II, respectively (Table 5), at an in situ temperature range of 6-

100e (Wilson et a\., 1996). Although both models gave

virtually identical fits to the data based on lower mean

squared residuals values (Table 5), Model II showed a

better fit at the beginning and end of the feeding

periods, as well as smaller residuals (Fig. 5).

Larvaceans (mostly Oikopleura spp.) were the most important prey numerically, constituting 67.7% ofwalleye

a

a

SS

I

a a a

Tn~

g~tJ

SR

I

a

a

a

c

o

5

B

4

5 A

o

_ 12

J:

.2»

; 9

>-

't:J

,g 6

-

c

Q)

3

Q)

D.

Figure 3

Box plots of (A) digestion code, (B) gut fullness, and (C) percent body weight versus time

of day. Diel sampling was binned into 4-h periods and is represented by the midpoint of the

period. The solid lines indicate 25th, 50th, and

75th percentiles. Error bars mark 10th and 90th

percentiles. Dotted lines indicate mean values.

Circles represent 5th and 95th percentiles. 5unrise (5R) was at 0815 and sunset (55) at 2015.
__ Merati & Brodeur: Feeding Habits and Daily Ration ofJuvenile Walleye Pollock in the Western Gulf of Alaska 71

Table 2

Results of the Mann-Whitney U-test for differences in feeding intensity between adjacent time intervals throughout a 24-

hour period. The data are from the whole survey, including the semidiel study.

Time intervals Gut fullness Digestion code Percent body weight

0000-0400 and U = 2612 U = 2307 U = 1820

0400-0800 P = 0.345 P = 0.685 P = 0.015

0400-0800 and U = 4752 U = 4168.5 U = 5354.5

0800-1200 P <0.001 P = 0.001 P <0.001

0800-1200 and U = 2397.5 U = 3134 U = 2227

1200-1600 P = 0.007 P = 0.873 P = 0.001

1200-1600 and U = 3330.5 U = 3501 U = 3488

1600-2000 P = 0.285 P = 0.090 P = 0.109

1600-2000 and U = 7604.5 U = 6437.5 U = 7680.5

2000-2400 P = 0.415 P = 0.163 P = 0.344

pollock stomach contents (Fig. 6). These were followed

by copepods (19.9% by number) and euphausiids

(10.7%). Other components of age-O pollock diet included fish larvae, decapods (mostly crab megalopae),

chaetognaths, hyperiid amphipods, and euthecosomatous

pteropods, but each contributed less than 2% by number

to the overall diet. Euphausiids contributed 80.8% to the

diet by weight, followed by larvaceans (8.1 %) and copepods (5.4%) (Fig. 6). Fish larvae constituted 0.1 % of the

diet by number and 1.3% by weight. All other taxa made

up less than 2% of the diet by weight.

Prey Composition by Time of Day

Stomach samples collected during the day throughout

the survey area contained mainly larvaceans (84.4% by

Table 3

Results ofthe Kruskal-Wallis test for differences in feeding intensity in adjacent time intervals. The results are

from the whole cruise and the semidiel study.

Gut Digestion Percent

Treatment fullness code body weight

Overall survey H = 34.50 H = 54.75 H = 29.23

P < 0.001 P < 0.001 P < 0.001

Semidiel study H = 54.55 H = 31.46 H = 85.97

P < 0.001 P < 0.001 P < 0.001

number) (Fig. 6). Euphausiids and copepods were the

next important prey items (4.7% and 9.3%, respectively). During twilight hours, the diet composition of

Table 4

Results of the Mann-Whitney U-Test for differences in feeding intensity between adjacent time intervals during the

semidiel study.

Time intervals Gut fullness Digestion code Percent body weight

0245 and 0445 U = 295 U = 296.5 U = 200

P = 0.006 P = 0.005 P = 1.0

0445 and 1030 U = 231 U = 241.5 U = 239

P = 0.365 P = 0.206 P = 0.291

1030 and 1215 U = 241.5 U = 268.5 U = 218

P = 0.236 P = 0.050 P = 0.626

1215 and 1745 U = 298 U = 334 U = 343

P = 0.006 P < 0.001 P < 0.001
72 NOAA Technical Report NMFS 126

10 r---------------,

3r---,---,--,-----.------,,----,

juvenile pollock shifted dramatically to mostly copepods, reflected in the substantial increase of their numerical percentages (82.4%), whereas the number of

larvaceans decreased markedly (3.4%). There were no

pronounced changes in the numerical percentage of

o observed

data --- model I

-- model II

o

Time (h)

A

8

.....

.s:::.

C)

'0:; 6

::

>- "0

0

OJ 4

0

2

SR

oj.

A

4

o~ (I)

"Cl 3

Q~ 0

u

c

0 2

;;

II)

(I)

OJ 1 C

0

B

5 0

Qg ~~ II) 4 ... II)

(I)

c 3

- 2

-

C)

1

0

5 6

o l:b 0

o q,

o o

4

o

o

o

o

80

3

o

o

o

o

o

2

= model 11 = model II

o

o

2

-3 '--_---'--__.L.-_--'-__....l.-__---.JL..-_--.J

o

-2

-1

Predicted value

12 C

.c-.21

(I) 9

==>-

"Cl

0 6

.c

-

c

(I)

3

(I)

D..

0

200 600 1000 1400 1800

400 800 1200 1600

Time of day (hours)

Figure 4

Box plots of (A) digestion code, (B) gut fullness, and (C) percent body weight versus time

of day for the semidiel study (5 collections at

one station). The solid lines indicate the 25th,

50th, and 75th percentiles. Error bars mark

10th and 90th percentiles. The dotted lines

indicate mean values. Circles represen t 5th and

95th percentiles. Sunrise (SR) was at 0815.

Figure 5

(A) Food consumption estimated from the die I trajectory of median percent body weights (circles). Model I

is fitted with the assumption that the ingestion rate is

constant. Model II is fitted with the assumption that

the ingestion rate is linearly related to amount of food

present in the stomach (see text for explanation of

models). (B) The residuals for each of the models.
__ Merati & Brodeur: Feeding Habits and Daily Ration ofJuvenile Walleye Pollock in the Western Gulf of Alaska 73

Prey Composition by Area

Prey Composition by Size

Table 5

Parameter estimates for two models of the diel feeding

cycle ofjuvenile walleye pollock in the Gulf of Alaska

using the median values for stomach fullness (percent

body weight, %BW) during each time period. See text

for explanation of the models.

180

180

Night

Night

102

102

Twilight

TWilight

Day

Day

267

267

B

A

80

til

-;

'ii 60

S

'0

C 40 GI

f

CD

D. 20

Gi 80 .Cl

E

::I

C 60

S

g

'0 40 C

CD

U

..

CD

D. 20

n=

n=

Figure 6

Prey of walleye pollock by time of day in terms of

percentage of (A) total number and (B) total weight of

the major prey categories.

entirelyeuphausiids (98.1% on a numerical basis), although the sample size was limited for this size category. Euphausiids dominated the diet of all size categories by weight, to the almost complete exclusion of

other prey items in the largest size category (Fig. 8).

An examination of the mean weights of stomach

contents by major prey category revealed some interesting trends in consumption (Fig. 9). Larvaceans were

the dominant prey by weight in stomachs collected

during the day, whereas euphausiids were dominant

during the other two periods. Geographic differences

were very pronounced: euphausiids were the dominant

2130

1000

1.10

0.28

5.87

7.37

1.12

Modell!

8.45

1.13

1915

810

1.43

0.25

Parameter Model I

Feeding period begins (h)

Feeding period ends (h)

Ingestion (%BW/h)

Evacuation (%BW/h)

Asymptotic consumption (%BW)

Daily ration (%BW/d)

Mean squared residuals

euphausiids from day to twilight, but numbers increased

at night. At night, prey consumption shifted back to

larvaceans (68.3%). Variations in the gravimetric composition of the diet among time periods were less pronounced, with euphausiids constituting the majority of

the diet by weight for all three time periods.

Diet of age-O pollock in the Kodiak Island region (Area

I) consisted mostly of euphausiids (76.8%), with copepods ranked as the second most important prey item

(10.3%) numerically (Fig. 7). Hyperiid amphipods,

mostly Themista pacifica, made up 2.6% of the diet by

number. In Area II (east of the Shumagin Islands),

pollock diets differed somewhat from those around

Kodiak Island, containing mostly larvaceans (39.9%)

and copepods (39.8%), with a substantial decrease in

the incidence of euphausiids in the diet. Samples collected in Area III (west of the Shumagin Islands) contained mostly larvaceans (92.3%). Copepods numerically contributed 4.8% to the diet in this area. By weight,

euphausiids were the most important prey item in the

diet in all areas, especially Area I.

The smallest fish sampled (49.5-59.9 mm SL) contained high numbers of copepods (41.6%), but

larvaceans and euphausiids were also important (32.8%

and 21.6%, respectively; Fig. 8). Euphausiids contributed the most by weight (72.8%) to the diet of this sizegroup, followed by copepods (20.2%). There was a

gradual numerical shift from copepods to larvaceans

up to the largest size category examined (Fig. 8). Fish

longer than 100 mm SL, however, consumed almost
74 NOAA Technical Report NMFS 126

A A n = 66 322 121 n= 24 114 198 124 54 13

< 60 60-69 70-79 80-89 90-99 > 100

Size category (mm)

114 198 124 54 13

60-69 70-79 80-89 90-99 > 100

Size category (mm)

<60

o

o

B

:c 80 Cl

'ij

iC

S 60

.s

'0

.. 40 c::

Gl

U

Gl

l1. 20

:D 80 .Q

E

:::l

c::

'iii 60 ..

.s

- 40 c::

Gl

U

:. 20

n = 24

100 .--r-""---""~...---- --.:==---r--....--

121

Area III

Area III

322

Area II

Area II

66

Area I

Area I

B

40 Gl

e

Gl

l1. 20

o

:D 80 .Q

E

:::l

c::

'iii 60

S

- 40 c::

Gl

U

Gl

l1. 20

n=

Figure 7

Prey of walleye pollock by geographical area in terms

of percentage of (A) total number and (B) total weight

of the major prey categories. The three areas are shown

in Figure 1.

Figure 8

Prey of walleye pollock by fish size in terms of percentage of (A) total number and (B) total weight of the

major prey categories.

prey in Area I, and euphausiids and lal\laceans combined

were dominant in Areas II and III (Fig. 9). Arnphipods

showed a substantial increase in Area III relative to the

other areas. Euphausiids were the dominant prey for all

size categories, and lal\laceans were important only in the

intermediate categories (70-79 mm and 80-89 mm SL).

Analysis of Dietary Variation

Significant differences in the frequency of the major prey

taxa in the diets of age-O pollock by all three factors were

found (Table 6), Thus we could reject the null hypothesis

that the diets were similar within each factor. Hyperiid

amphipods were the only taxon that did not show significant variation (P=O.05) by time ofday. Comparison by size

of fish demonstrated that copepods, euphausiids, and

decapods showed significant variation, whereas the occurrence oflal\laceans and hyperiid amphipods did not show

significant variation with size of predator (P>O.05) , Areal

analysis showed that the distribution of feeding on copepods, hyperiids, and lal\laceans was highly variable

(P<O,05), whereas the distribution of feeding' on decapods and euphausiids did not show significant variations.
__ Merati & Brodeur: Feeding Habits and Daily Ration ofJuvenile Walleye Pollock in the Western Gulf of Alaska 75

Discussion

<60 60-69 70-7980-8990-99 >100

Fish size (mm)

Figure 9

Prey composition by mean weight for (A) time of

day, (B) sampling area, and (C) fish size category for the dominant prey categories.

Our finding that age-O walleye pollock fed mainly at

night was surprising in light of previous observations

that the larvae feed mainly in daytime or in crepuscular

periods (Kamba, 1977; Kendall et aI., 1987; Canino et

aI., 1991; Canino and Bailey, 1995). Walline (1983),

working with more limited data on stomach fullness of

juvenile pollock from the Bering Sea, concluded that

there was no discernable feeding periodicity, but he

did observe that the percentage of empty stomachs

peaked in the evening hours and then declined at

night, which is consistent with our findings. The increased amount ofwell-digested food and chyme present

in age-O pollock stomachs collected during the day led

Krieger (1985) to suggest that the period of peak feeding switches from day to night during the fall months.

Evidence for diel periodicity in feeding of adult walleye

pollock is equivocal in the results of Dwyer et al. (1987)

for the Bering Sea, but their data suggest that food

items found in the stomachs at night were less digested

than those recovered during the day (Livingston5).

What benefits do juvenile pollock derive from feeding at night rather than during the day, when visual

acuity and contrast are far greater? Bailey (1989) found

that juvenile pollock in the Bering Sea formed dense

aggregations in deep water during the day but migrated

and subsequently dispersed in the surface layer at night.

Similar diel differences in distribution were observed in

acoustic data collected during our sampling in the Gulf

of Alaska (Brodeur and Wilson, 1996). In laboratory

studies, juvenile pollock have been shown to avoid high

light intensities and to seek out relatively low light

levels (Olla and Davis, 1990). Apparently, light levels in

surface waters were sufficient to facilitate feeding even at

night, even though the moon was not full and there was

extensive cloud cover during most of our sampling. A

benefit to foraging in surface waters at night is that many

diel vertically migrating prey, such as copepods and euphausiids, move to the surface at dusk (e.g., Marlowe and

Miller, 1975; Cooney, 1989). Juvenile pollock may also

decrease their risk of predation by spending time at the

surface when major surface-feeding predators such as

seabirds and marine mammals are not foraging (Hatch

and Sanger, 1992; Springer, 1992).

Our estimates of daily ration as a percen tage of body

weight were comparable with the two models, but the

results from Model II, which assumed that ingestion

was proportional to the stomach contents, fit the data

slightly better at the time of initiation and cessation of

feeding. We also found the statistical fit (mean square

errors) of both models to be substantially better when

5 Livingston, P. A. Alaska Fish. Sci. Cent., Nat!. Mar. Fish. Serv., 7600

Sand Point Way N.£., Seattle WA. 98115-0070. PefS. commun.

ill

Night

II

Twilight

Time of day

Sampling area

...j"

//

.. /

)"

.?'-..

/..--- ~-- --~

-- amphipod

- copepod

-- decapod

....... euphausiid

_ ... larvacean

'\

..... \ ...\......... /

__ '. _",L....-

- =~~----

I

............" ......... . ,........,..- -..---~" =-==-=-=-

Day

B

0.06

0.00

0.10 A

0.08

0.05

0.15

0.00

0.10

0.00

0.05

0.04

0.06

0.02

0.03

0.01

c: 0.02

CO

Q)

E

a.

-

o

.....

.c:

.2> 0.04

Q)

.....

.c:

.2>

Q)

c:

CO

Q)

E

.....

.c:

.2>

c:

CO

Q)

E

..-...

S 0.20 C

Q)

a.

-

o

..-...

C)

---

Q)

a.

-

o
76 NOAA Technical Report NMFS 126

Table 6

Results of the chi-square analysis of each factor independently for major prey taxa ofjuvenile pollock. The data are from

the whole cruise, including the diel study.

Prey Time Size Area

Copepods X2 20.8 X2 19.9 X2~18.5

P < 0.001 P 0.001 P < 0.001

Euphausiids X2 107.5 X2 11.3 X2~0.7

P < 0.001 P 0.046 P 0.70

Larvaceans X2~1l.8 X2 7.0 X2~14.8

P 0.003 P 0.22 P< 0.001

Decapods X2~16.0 X2 13.1 X2 3.3

P < 0.001 P 0.02 P 0.19

Hyperiids X2 5.9 X2 3.8 X2 9.1

P 0.05 P 0.57 P 0.01

we used median rather than mean values because ofthe

influence of several very large values, and we recommend using median values when fitting these models.

To be reliable, the models that we fit to the feeding

trajectory require a synchronized switch from feeding

to nonfeeding modes for the entire population

(Sainsbury, 1986). As in many other studies on diel

feeding behavior, our data are most variable during

these transition periods and would benefit from increased sampling intensity during these times.

Previous field studies on the daily ration of age-O

pollock do not exist for comparison with our results.

Springer (1992) summarized a number of laboratory

and field studies of daily consumption rates of walleye

pollock and found them to be quite variable «0.1 % to

8.6%BW) depending on size, temperature, and season.

It is not surprising that our ration estimates are at the

high end of this range, since many of the other field

studies were done on slower-growing adults at the lower

temperatures of the Bering Sea (maximum temperature <4°C) rather than on relatively fast-growing juveniles at 6-lOo

C like those we collected. The laboratory

studies on small juveniles are consonant with our results (Springer, 1992).

All of the taxa that we identified from the stomachs

ofjuvenile pollock were pelagic zooplankton, which is

consistent with previous studies suggesting that age-O

pollock are planktivores feeding mainly on copepods

and euphausiids (Kamba, 1977; Walline, 1983; Grover

1990, 1991; Livingston2). The gradual ontogenetic shift

from copepods to euphausiids documented by Bailey

and Dunn (1979) for pollock from larger size groups

also appears in our data; evidently, the costs offoraging

on copepods exceed the nutritive benefits for larger

walleye pollock. However, we did not note any dramatic

shifts in diet between adjacent size groups that might

indicate an ontogenetic change in habitat (Mahon and

Neilson, 1987). Krieger (1985) found that the consumption of epibenthic prey (e.g., cumaceans, shrimp,

and mysids) increased dramatically during winter, suggesting that age-O fish may be more associated with the

bottom substrate then. Similar foraging on epibenthic

prey was observed among age-l pollock collected in

Shelikof Strait during March and April (Brodeur and

Wilson, 1996). Unlike some other researchers (Lee,

1985; Grover, 1991), we did not find copepod eggs in

the guts of juvenile pollock. This may have been because we examined larger fish and not because there

were no reproductive copepods at this time ofyear. The

low frequency of fish found in the stomachs indicates

that age-O pollock in our study area are neither piscivorous nor cannibalistic on smaller juvenile pollock

(Sogard and Olla, 1994), but the availability of small

fish prey appeared to be limited during our sampling

period (Brodeur, unpub!. data).

The extensive consumption of larvaceans by juvenile

pollock has not been adequately documented, although

larvaceans have been reported as prey of age-O pollock

(Walline, 1983; Krieger, 1985; Lee, 1985; Livingston2).

Larvaceans can be very abundant and aggregated in

coastal waters, particularly in late summer, and thus

may serve as an important link in the food web between

the lower trophic levels and larval and juvenile fishes

(Alldredge and Madin, 1982; Gadomski and Boehlert,

1984; Knoechel and Steel-Flynn, 1989). Larvaceans are

more difficult to recognize and enumerate than crustacean prey because they are digested faster in predator's

stomachs, so our estimates of consumption of these
__ Merati & Brodeur: Feeding Habits and Daily Ration ofJuvenile Walleye Pollock in the Western Gulf of Alaska 77

prey may be conservative. Because oftheir patchy distribution and relatively small size without their large mucous houses, these prey may be inadequately sampled

by plankton-collecting gear. Intact larvaceans may have

a low nutritive value by themselves, but by consuming

and concentrating phytoplankton, they may serve as

concentrated energy sources for both predaceous zooplankton and juvenile walleye pollock (Knoechel and

Steel-Flynn, 1989).

Juveniles collected at twilight contained the most

copepods. This could be attributed to the comigration

of copepods and juvenile pollock to and from the surface during crepuscular periods. Since euphausiids migrate to the surface at night (Alton and Blackburn,

1972; Bailey et al., 1992), pollock are able to prey on

them in the surface layers. Pollock showed significant

differences in consumption of larvaceans by time of

day: the highest percentages of stomachs containing

larvaceans were collected during daylight hours. Though

little is known about their behavior or vertical distribution in the northern Gulf of Alaska, larvaceans were

found in high concentrations outside their gelatinous

houses in windrows at the surface during late afternoon

off California (Alldredge, 1982). If age-O pollock migrate to the surface before dusk, then their chances

of encountering larvaceans increase. Variability was seen

in hyperiid and decapod consumption by time

of day: the highest consumption occurred in twilight

hours. It is unclear how pollock detect these crustaceans, but they may be encountering them during their

crepuscular ascents or descents through the water

column.

Some dietary variability was apparent among differing sizes of pollock. Smaller fish «60 mm) had the

highest percentage of copepods in their stomachs, and

the incidence declined as fish size increased. Larvaceans

were most common in the intermediate size groups but

were not consumed by the larger fishes, possibly because it was not energetically favorable for large fish to

feed on such small prey items. Euphausiids showed

significant variability with fish size, with the highest

mean weight found in larger fish. Though smaller fish

also preyed on euphausiids, the larger juveniles (>80

mm) have larger gape sizes and faster swimming speeds,

which are needed to capture and ingest the larger and

more evasive euphausiids. Decapod consumption increased as fish size increased, which may also be due to

the increased capture efficiency of larger fish.

Though pollock diet varied by area, no differences

were seen in fish condition among the three areas (a

Kolmogorov-Smirnov test showed no significant differences among the three areas for fish we examined; see

also Wilson et al., 1996). If condition index is used as a

long-term indicator of fish growth and survival, then

fish collected in the different areas would be expected

to have similar chances ofsurviving. However, fish caught

in Area II demonstrated a wide range of condition

index values, with several specimens showing very low

values, indicating that some fish may not have received

an adequate daily ration. Although we found that stomach fullness was as high in Area II as in the other areas,

it is difficult to draw clear conclusions without knowing

the migratory patterns ofjuvenile pollock. If these fish

were just passing through or had recently settled in the

area in which they were sampled, they may have been

exposed to different prey fields and to beneficial feeding opportunities which affected their condition more

than their very recent feeding history would indicate.

We need to learn more about the Gulf of Alaska

juvenile pollock population size, distribution, and migration patterns, as well as the patterns of their major

prey, to ascertain whether age-O pollock are food-limited in this environment. With a finer-scale sampling

scheme and data on prey availability and vertical overlap, along with incorporation of environmental data,

we may be able to infer more about juvenile pollock

feeding regimes. Bioenergetic modeling that combines

feeding data with information on pollock juvenile

growth rates may yield some clues about whether juveniles in this population are feeding at or near their

optimal levels.

Acknowledgments _

We wish to thank P. Livingston of the Trophic Interactions Program of the Alaska Fisheries Science Center

for giving us laboratory space during this study. We also

thank M. Canino, A. Kendall, P. Livingston,]. Napp, G.

Stauffer, M. Wilson, and an anonymous reviewer for

useful comments on earlier versions of the manuscript.

This is Fisheries-Oceanography Coordinated Investigations Contribution No. 208.

Literature Cited

Alldredge, A. L.

1982. Aggregation of spawning appendicularians in surface

windrows. Bull. Mar. Sci. 32:250-254.

Alldredge, A. L., and L. P. Madin.

1982. Pelagic tunicates: unique herbivores in the marine

plankton. Bioscience 32:655-663.

Alton, M. S., and C. S. Blackburn.

1972. Diel changes in the vertical distribution of euphausiids, Thysanoessa spinifera Holmes, and Euphausia pacifica

Hansen, in coastal waters of Washington. Calif. Fish Game

58:177-190.

Bailey, K. M.

1989. Interaction between the vertical distribution ofjuvenile

walleye pollock Theragra chalcogramma in the eastern Bering

Sea, and cannibalism. Mar. Ecol. Prog. Ser. 53:205-213.
78 NOAA Technical Report NMFS 126

Bailey, K. M., R. D. Brodeur, N. Merati and M. M. Yoklavich.

1992. Predation on walleye pollock (Theragra chalcogramma)

eggs and yolk-sac larvae by crustacean invertebrates in the

western Gulf of Alaska. Fish. Oceanogr. 2:30-39.

Bailey, K. M. andJ. Dunn.

1979. Spring and summer foods of walleye pollock, Theragra

chalcogramma, in the eastern Bering Sea. Fish. Bull. 77:304-

308.

Bailey, K. M. and S. M. Spring.

1992. Comparison of larval, age-O juvenile and age-2 recruit

abundance indices ofwalleye pollock, Theragra chalcogramma,

in the western Gulf of Alaska. In t. Coune. Explor. Sea J.

Mar. Sci. 49:297-304.

Brodeur, R. D. and M. T. Wilson.

1996. A review of the distribution, ecology, and population

dynamics of age-O walleye pollock in the Gulf ofAlaska. Fish.

Oceanogr. 5 (Suppl. 1):148-166.

Canino, M. F. and K. M. Bailey.

1995. Gastric evacuation of walleye pollock, Theragra

chalcogramma (Pallas), larvae in response to feeding

conditions. J. Fish. BioI. 46:389-403.

Canino, M. F., K. M. Bailey and L. S. Incze.

1991. Temporal and geographic differences in the feeding

and nutritional condition of walleye pollock larvae Theragra

chalcogramma in Shelikof Strait, Gulf of Alaska. Mar. Ecol.

Prog. Ser. 79:27-35.

Clarke, M. E.

1984. Feeding behavior of larval walleye pollock, Theragra

chalcogramma (Pallas) and food availability to larval pollock

in the southeastern Bering Sea. Ph.D. dissert., Univ. California, San Diego, 208 p.

Cooney, R. T.

1989. Acoustic evidence for the vertical partitioning of biomass in the epipelagic zone of the Gulf of Alaska. DeepSea Res. 36:1177-1189.

Dwyer, D. A., K. M. Bailey and P. A. Livingston.

1987. Feeding habits and daily ration of walleye pollock

(Theragra chalcogramma) in the eastern Bering Sea, with

special reference to cannibalism. Can. J. Fish. Aquat. Sci.

44:1972-1984.

Elliott,J. M. and L. Persson.

1978. The estimation of daily rates of food consumption for

fish. J. Anim. Ecol. 47:977-991.

Gadomski, D. M. and G. W. Boehlert.

1984. Feeding ecology of pelagic larvae of English sole

Parophrys vetulus and butter sole Isopsetta isoiepis off the Oregon coast. Mar. Ecol. Prog. Ser. 20:1-12.

Grover,J..J.

1990. Feeding ecology of late-larval and earlyjuvenile walleye

pollock (Theragra chalcogramma) from the Gulf of Alaska in

1987. Fish. Bull. 88:463-470.

1991. Trophic relationship ofage-O and age-I walleye pollock

Theragra chalcogramma collected together in the eastern

Bering Sea. Fish. Bull. 89:719-722.

Hatch, S. A. and G. A. Sanger.

1992. Puffins as samplers ofjuvenile pollock and other forage fish in the Gulf of Alaska. Mar. Ecol. Prog. Ser. 80:

1-14.

Hinckley, S., K. M. Bailey, S.J. Picquelle,J. D. Schumacher and

P.J. Stabeno.

1991. Transport, distribution, and abundance of larval and

juvenile walleye pollock (Theragra chalcogramma) in the western Gulf of Alaska. Can.]. Fish. Aquat. Sci. 48:91-98.

jarre, A., M. L. Palomares, M. L. Soriano, V. C. Sambilay,jr. and

D. Pauly.

1991. Some new analytical and comparative methods for estimating the food consumption offish. Int. Coune. Explor.

Sea Mar. Sci. Symp. 193:99-108.

jarre-Teichmann, A.

1992. MAXIMS-A computer program for estimating the food

consumption offish. Int. Coune. Explor. Sea C.M./G:52, 12 p.

Kamba, M.

1977. Feeding habits and vertical distribution of walleye pollock, Theragra chalcogramma (Pallas), in the early life stage in

Uchira Bay, Hokkaido. Res. Inst. N. Pae. Fish., Hokkaido

Univ., Spee. vol., p. 175-197.

Kendall, A. W.,jr., M. E. Clarke, M. M. Yoklavich and

G. W. Boehlert.

1987. Distribution, feeding and growth of larval walleye pollock, Theragra chalcogramma, from Shelikof Strait, Gulf of

Alaska. Fish. Bull. 85:499-521.

Kendall, A. W.,jr., and S.]. Picquelle.

1990. Egg and larval distributions of walleye pollock Theragra

chalcogramma in Shelikof Strait, Gulf of Alaska. Fish. Bull.

88:133-154.

Kim, S. and E. P. Nunnallee.

1990. Distribution of walleye pollock, Theragra chalcogramma,

spawning in Shelikof Strait, Gulf of Alaska, based on acoustic and ichthyoplankton surveys 1981, 1984 and 1985. Bull.

Korean Fish. Soe. 23:425-438.

Knoechel, R. and D. Steel-Flynn.

1989. Clearance rates of Oikopleura in cold coastal Newfoundland waters: a predictive model and its trophodynamic

implications. Mar. Ecol. Prog. Ser. 53:257-266.

Krieger, K. J.

1985. Food habits and distribution of first-year walleye pollock, Theragra chalcogramma (Pallas), in Auke Bay, southeastern Alaska. M.S. thesis, Univ. Alaska, Juneau, 57 p.

Lee, S. S.

1985. A comparison of the food habits ofjuvenile Pacific cod

and walleye pollock in the southeast Bering Sea. M.S. thesis. Univ. Alaska, Fairbanks, 130 p.

Mahon, R. andJ. D. Neilson.

1987. Diet changes in Scotian Shelf haddock during the pelagic and demersal phases of the first year of life. Mar.

Ecol. Prog. Ser. 37:123-130.

Marlowe, c.]. and C. B. Miller.

1975. Patterns ofvertical distribution and migration ofzooplankton at Ocean Station "P." Limnol. Oceanogr. 20:824-844.

Megrey, B. A.

1991. Population dynamics and management of walleye pollock (Theragra chalcogramma) in the Gulf of Alaska, 1976-

1986. Fish. Res. 1l:321-354.

Nakatani, T. and T. Maeda.

1987. Distribution and movement of larval and juvenile walleye pollock (Theragra chalcogramma) in Funka Bay and adjacent waters. Bull.jpn. Soe. Sci. Fish. 53:1,585-1,591.

Olla, B. L. and M. W. Davis.

1990. Behavioral responses ofjuvenile walleye pollock Theragra

chalcogramma Pallas to light, thermoclines and food: possible role in vertical distribution. J. Exp. Mar. BioI. Ecol.

135:59-68.

Sainsbury, K. J.

1986. Estimation of food consumption from field observations of fish feeding cycles. J. Fish. BioI. 29:23-36.

Schumacher,J. D. and A. W. Kendalljr.

199I. In teractions between young walleye pollock and their

environment in the western Gulf of Alaska. Calif. Coop.

Oceanic Fish. Invest. Rep. 32:22-40.

Smith, R. L.,J. M. Paul, and A.]. Paul.

1989. Gastric evacuation in walleye pollock, Theragra

chalcogramma. Can.]. Fish. Aquat. Sci. 46:489-493.
__ Merati & Brodeur: Feeding Habits and Daily Ration ofJuvenile Walleye Pollock in the Western Gulf of Alaska 79

Snedecor, G. W. and W. G. Cochran.

1967. Statistical Methods. 6th ed. Iowa State Univ. Press,

Ames, 593 p.

Sogard, S. M. and B. L. Olla.

1994. The potential for intracohort cannibalism in age-O walleye pollock, Theragra chalcogramma, as determined under

laboratory conditions. Environ. BioI. Fish. 39:183-190.

Springer, A. M.

1992. A review: walleye pollock in the North Pacific Ocean-how

much difference do they really make? Fish. Oceanogr. 1:80-96.

Takatsu, T, T. Takahashi, T Nakatani, T Maeda and K. Ohkoshi.

1992. Distribution and movement of walleye pollock in early

life stages in Mitsu Bay, Aomori. Nippon Suisan Gakkaishi

58:2235-2243.

Walline, P. D.

1983. Growth oflarval and juvenile walleye pollock related to

year-class strength. Ph.D. dissert., Univ. Washington, Seattle, 144 p.

Wilson, M. T., R. D. Brodeur and S. Hinckley.

1996. Distribution and abundance of age-O walleye pollock,

Theragra chalcogramma, in the western Gulf of Alaska during

September 1990. In R. D. Brodeur et al. (eds.), Ecology of

juvenile walleye pollock, Theragra chalcogramma, p. 11-

24. U.S. Dep. Commer., NOAA Tech. Rep. 126.

Zar,]. H.

1984. Biostatistical analysis, 2nd ed. Prentice-Hall, Inc,

Englewood Cliffs, N.j., 718 p.

Hatch Date and Growth Estimation ofJuvenile Walleye Pollock,

Theragra chalcogramma, Collected in the Bering Sea in 1989 and 1990

AKIRA NISHIMURA, KEI-ICHI MITO, and TAKASHI YANAGIMOTO

National Research Institute ofFar Seas Fisheries

North Pacific Resources Division

5-7-1, Orido, Shimizu 424,Japan

ABSTRACT

Age-O walleye pollock, Theragra chalcogramma, ranging from 6 to 108 mm fork length,

were collected in the Bering Sea in the summers of 1989 and 1990. Trawl surveys showed a

wide distribution ofjuvenile fish in the eastern continental shelf area, and limited distribution in the Aleutian Basin. The ages, in days after hatching, ofjuvenile fish selected from a

wide area were estimated by counting otolith daily growth increments. The mean number of

daily growth increments ofjuvenile pollock collected in 1989 was 98; in 1990 the mean

number was 104. Back-calculated hatch dates of both years ranged between mid-April and

earlyJune, and peaks were observed between late April and mid-May. Allowing for the days

required from spawning to hatching, spawning was estimated to have peaked in middle to

late April. These spawning periods were in agreement with those on the southeastern

Bering Sea continental shelf. Although it is known that pollock spawn from February to

March in the Aleutian Basin area, only a fewjuvenile pollock that had been spawned during

this period were observed. Either the survival rates of basin-spawned fish were very low, or

the fish were transported out of the survey region. Early growth patterns in 1990 were

estimated by the back-calculation method, using daily growth increments.

Introduction

Natural fluctuations in population size are commonly

observed between age classes ofwalleye pollock, Theragra

chalcogramma, and variations of biomass are thought to

largely be dependent upon the strength of these year

classes. However, processes affecting year-class strength

are still largely uncertain. It is known that there are

separate spawning stocks ofwalleye pollock in the Bering

Sea (Hinckley, 1987). One is located in the Aleutian

Basin, and several others are located over the continental shelf. These adult stocks have different biological

characteristics, including spawning period. Pollock

spawn from February to March in the Aleutian Basin

surrounding Bogoslof Island, from April to June in the

southern part of the eastern shelf, and from June to

August in the northern part ofthe eastern shelf (Hinckley,

1987; Mulligan et aI., 1989). The linkages between basin

and shelf components are not well understood.

The otoliths of many fish species contain growth

increments, concentric microstructures generally formed

on a daily basis. Growth increments yield information

about the early life history of the fish, such as hatch

date and growth rates. In previous studies (Nishimura

and Yamada, 1984; Bailey and Stehr, 1988), the increments of larval and juvenile walleye pollock otoliths

were shown to be laid down on a daily basis.

The major objective of this study is to examine the

relationship between surviving juveniles and spawning

stocks by estimating hatch dates in different geographical areas. This paper provides preliminary estimates of

hatch date and growth ofjuvenile pollock collected in

1989 and 1990.

Materials and Methods

Juvenile walleye pollock were sampled at 38 stations in

the Bering Sea between July and October in 1989 and

at 34 stations between July and September in 1990.

Samples were collected with midwater trawl nets deployed from a land-based dragnet trawler (#28 Seiju

Maru in 1989 and #128 Daian Maru in 1990) chartered

as a research vessel for echo-integration midwater trawl

surveys. These surveys were a cooperative effort between the National Research Institute of Far Seas Fish81
82 NOAA Technical Report NMFS 126

eries, the National Research Institute of Fisheries Engineering, and the Alaska Fisheries Science Center. The

forward part of the net body was constructed from

lengthwise ropes, and the mesh size above the codend

was 45 mm in 1989 and 100 mm in 1990. A net liner of

4-mm mesh was attached inside the codend to collect

juvenile fish in both years. Sampling was carried out

within a few hours after sunset by trawling in the subsurface layer at depths of 20-40 m. Towing speeds on

average were 3.4 knots in 1989 and 5.0 knots in 1990.

Net depths and vertical and horizontal openings of the

nets were measured by a SCANMAR net monitoring

system. Average vertical and horizontal net openings

were 28.7 m and 18.9 m in 1989, and 12.0 m and 19.7 m

in 1990. The collected juvenile pollock were frozen and

stored. Fork lengths were measured and otoliths

(sagittae) were dissected out from 40 individuals randomly selected from each station.

The survey area was divided into the Aleutian Basin

area and the southern and northern parts of the eastern continental shelf. The shelf area was divided into

north and south by the 58°N parallel.

Otoliths were measured along the longest axis and

embedded in epoxy resin on a glass slide. Embedded

otoliths were ground with carbon paper (#800-#1200)

perpendicular to the otolith flat plane along the long

axis (frontal section). Polished otolith specimens were

selected if they included the otolith nucleus on the

ground surface and were thin enough to transmit light

under a light microscope. These specimens were polished with lapping film, cleaned, and etched with 0.2 M

EDTA for 4 minutes. The early increments of narrow

width were counted under 600x magnification. As the

increments increased in width, the magnification was

changed to 300x. Hatch date was estimated by subtracting the number of increments from the sampling date.

Otolith specimens from the 1990 samples were used

for early growth estimation. The specimens were photographed, and diameters of 10 increments from the

outer margin were measured on each photograph. Each

diameter was converted to total length by means of an

otolith length-total length relationshop (Nishimura and

Yamada, 1988).

Results

Juvenile Distribution and Size Composition in 1989

Juvenile pollock were distributed extensively on the

eastern continental shelf. In the Aleutian Basin, a small

concentration was observed in the southeastern area

(Fig. 1). Juveniles were collected from early to middle

August on the southern shelf and basin, and from middle

August to late September on the northern shelf. Juveniles from the southern shelf and basin area showed

similar size ranges between 40 and 80 mm, with modes

at about 60 mm (Fig. 2). Juveniles from the northern

shelf were larger, ranging from 40 to 110 mm, with a

mode around 80 mm, because this region was sampled

later.

Juvenile Distribution and Size Composition in 1990

Sampling was conducted between late July and early

August on the eastern continental shelf, and between

late August and late September in the Aleutian Basin.

Juvenile pollock were distributed mainly in the southern part of the eastern continental shelf. A small concentration was observed in the northern shelf and basin

areas (Fig. 1). Although there was not an obvious difference in the modes of fish size between the northern

shelf and the southern shelf (about 50 mm), juveniles

collected from the southern shelf ranged widely in size.

Juveniles collected in the basin area had a larger mode

than those collected in the shelf area, and the size

range extended to 90 mm, again because of later sampling (Fig. 3).

Hatch Date Estimation in 1989 and 1990

Otoliths were taken from 176 fish (size range: 33.2-94.5

mm) from 1989 samples. The fish were selected from 5

stations on the southern shelf, 2 stations on the northern shelf, and 1 station in the southeastern basin. The

otoliths of 156juvenile fish (size range: 32.8-88.2 mm)

from 1990 samples were examined. The fish were selected from 2 stations on the northern shelf, 10 stations

on the southern shelf, and 3 stations in the basin.

A nucleus of about 20 /lm diameter was observed in

the central part of each otolith, and daily growth increments were deposited concentrically around the nucleus

(Fig. 4). The first few increments around the nucleus

were narrow, but increments widened gradually with

increasing age. The increments were clearly observed

by light microscopy. Numbers of daily growth increments ranged from 60 to 139 in 1989 (Table 1). Despite

differences in sampling dates and area, all groups were

estimated to be hatched during almost the same period

from late April to early June in 1989 (Fig. 5). The peak

hatching period was estimated to be in mid-May. The

frequency distribution of hatching dates in 1989 was

unimodal (Fig. 5).

Otoliths from the 1990 samples showed increment

numbers ranging from 60 to 167 (Table 1). As in 1989,

and despite the differences in sampling dates and areas, it was estimated that most of the fish hatched from

mid-April to mid-May in 1990 (Fig. 5). Exceptions in-
______ Nishimura et aI.: Hatch Date and Growth Estimation ofJuvenile Walleye Pollock Collected in the Bering Sea 83

180 160 VI

160 VI

1990

1989

170 VI

170 VI

+ ........

+

+ +

+

180

•

+

+

t

+

\;,

o

+ •

.~ON~ ....~O "---

• =700

• =70

63N

61N

59N

57N

55N

53N D'

51N '

170 E

63N

61N

59N

5IN j

55N

53N

51N

170 E

Figure 1

Catch (no/h) ofjuvenile walleye pollock at sampling stations during 1989 and 1990 summer

surveys. A zero catch is shown by t. The isobaths represent 200 m depth.

eluded the juveniles collected at two stations (24 and

25) on the southern shelf that hatched later (between

late May and early June), and two individuals collected

from the basin that hatched earlier (in late March).

Juvenile fish collected on the southern shelf in 1990

showed a bimodal hatching period. The major mode

was in late April, about 3 weeks earlier: than the major

mode in 1989. The hatching period estimated for the

juvenile fish collected in the basin area was broader

than that for the shelf fish.
84 NOAA Technical Report NMFS 126

10 20

SOUTHERN SHELF

1990

NORTHERN SHELF

1990

30 40 50 60 70 80 90 100 110

o L...-...-J"

20

3

9

6

5

10

15

>- ()

Z

W

::J

(1

W

a::

u.

o L...-........-.....

20 30 40 50 60 70 80 90 100 110

12

SOUTHERN SHELF

1989

NORTHERN SHELF

1989

30 40 50 60 70 80 90 100 110

8

6

4

2

o '-----o._.......--L-AaJ.....

20 30 40 50 60 70 80 90 100 110

3

o L-.......L.l.II...JU

20

9

6

o

- 12

>- ()

Z

W

::J

(1

W

a::

u.

15 20

10

BASIN

1989

15

10

BASIN

1990

5

o '--_.................IAJ

20 30 40 50 60 70 80 90 100 110

5

0"-----

20 30 40 50 60 70 80 90 100 110

FORK LENGTH (mm) FORK LENGTH (mm)

Figure 2

Size composition ofjuvenile walleye pollock collected

from the northern shelf (north of 58°N), southern shelf

(south of 58°N), and basin, in the summer of 1989.

Figure 3

Size composition ofjuvenile walleye pollock collected

from the northern shelf (north of 58°N), southern

shelf (south of 58°N), and basin, in the summer of

1990.

Early Growth Estimation in 1990 BL = 84.0/(1 + e-O.

032 (day-84.7»)

Figure 6 shows back-calculated body length at lO-day

intervals for 39juveniles from the shelf and 22 from the

basin in 1990. Juveniles collected on the shelf show

comparably uniform and favorable growth, whereas

those collected in the basin show substantial variation.

Growth in the basin juveniles began to stagnate between 90 and 120 days after hatching. Growth stagnation was not clear in the shelf samples, because of the

younger age ranges. Assuming a body length at hatching of 4.5 mm, the following logistic growth curves were

calculated for the shelf and basin samples by using

Ricklefs's (1967) method:

BL = 115.5/(1+e-O.

030 (day-lOl.7»)

for shelfsamples and

for basin samples.

Discussion

Nishimura and Yamada (1984) reported that daily

growth increments in the otoliths oflarval andjuvenile

walleye pollock can be seen with scanning electron

microscopy. Bailey and Stehr (1988) incubated eggs

obtained from Puget Sound in the laboratory, and found

that daily growth increments are visible by light microscopy when larvae are fed sufficiently and their growth is

satisfactory. Under conditions of low growth, however,

early growth increments are narrower and impossible

to observe with the resolving power of a light microscope. To get the most accurate counts, it is desirable to
_______ Nishimura et al.: Hatch Date and Growth Estimation ofJuvenile Walleye Pollock Collected in the Bering Sea 85

40 1989

!!!il BASIN

D SOUTH SHELF

30 • NORTH SHELF

20

10

0

Z 0

...J 22 1 11 21 1 11 21 31 10 20

« APR. I MAY I JUN. ::::>

C

> 30 1990

c

z

!!!il BASIN

20 D SOUTH SHELF

• NORTH SHELF

10

Figure 4

Light micrograph of otolith from juvenile walleye pollock (58.5 mm FL), collected in the Bering Sea in the

summer of 1989.

0"""""""'-.......-------------..::::0.............. 22 1 11 21 11 21 31 10 20

APR. I MAY I-J-UN-.-I

ESTIMATED HATCH DATE

use a scanning electron microscope, but preparation

and observation of SEM specimens are extremely time

consuming. On the other hand, counting with the light

microscope is easy and efficient when many specimens

are analyzed. We used 600x magnification to avoid

contamination with immersion oil, which is required

for observation at 1000x, allowing for future SEM observation. Because our otolith specimens were thin and

polished on both sides, the increments were relatively

easily identified. Although a few increments may have

been missed, and counting errors may have occurred,

the degree of error would not greatly affect the results.

Therefore, we consider that gathering substantially more

data by light microscope offered greater value than a

small number of SEM counts.

Hinckley (1987) proposed the existence ofthree separate spawning stocks in the Bering Sea. Spawning occurred between January and March in the basin, between March and June over the southeastern continental shelf and slope, and between June and August over

the northwestern shelf and slope. Our results from

Figure 5

Frequency distribution of estimated hatch date ofjuvenile walleye pollock collected in the Bering Sea in 1989

and 1990.

both 1989 and 1990 showed that juvenile walleye pollock were distributed mainly on the continental shelf

and slope area. On the other hand, only a few juvenile

fish were observed in the Aleutian Basin. This distribution pattern suggests that larvae and juvenile fish

spawned in the basin area may have moved to the shelf

region or that they did not survive in significant numbers in the basin.

The hatching periods estimated by back-calculation

from counts of otolith daily growth increments were

from mid-April to mid-:June in both years, except for

two individuals hatched in March 1990. The time from

spawning to hatching is estimated to be about 25 days at

3° to 4°e (Okada, 1986), and about 14 days at 5° to 6°e

(Haynes and Ignell, 1983; Nakatani and Maeda, 1984;

Nishimura and Yamada, 1984). Water temperature in
86 NOAA Technical Report NMFS 126

Table I

Fork length, otolith daily growth increment number, and estimated hatch date ofjuvenile walleye pollock collected in

1989 and 1990 in the Bering Sea.

Fork length (mm)

Year class umber of

and area specimens Range Average

1989

Northern shelf 44 68.1-94.5 83.7

Southern shelf 96 33.2-77.2 60.9

Basin 36 42.1-78.1 60.4

1989 total 176 33.2-94.5 66.5

1990

Northern shelf 5 40.9-55.3 47.0

Southern shelf 104 32.8-71.7 54.1

Basin 47 49.9-88.2 61.0

1990 total 156 32.8-88.2 55.9

Increment number Estimated hatch date

Range Average Range Average

80-139 118.0 29 Apr-29 Jun 19 May

60-114 89.0 24 Apr-13 Jun 18 May

77-117 99.0 13 Apr-5Jun 7 May

60-139 98.0 13 Apr-26 Jun 16 May

94-112 99.9 11 Apr-29 Apr 22 Apr

60-120 96.2 9 Apr-l0Jun 2 May

93-167 121.9 30 Mar-5Jun 4 May

60-167 104.1 30 Mar-l 0Jun 2 May

OL..---"------"------"------'-----'

o 1~ 1~

AGE IN DAYS AFTER HATCHING

80

SHELF 1990

60

-

E

E 40 -::I:

" 20 0

Z 0

W 0 -O.030(DAY-101.7)

...J BL=115.5/(1 +e

0

Cl 0 30 60 90 120 150

0

aJ

Cl 80

W 0

I- BASIN 1990

«

::!: 60

i=

en

w 40

20 0

0

Figure 6

Back-calculated length at age ofjuvenile walleye pollock collected in the Bering Sea in 1990. Open circles

represent back-calculated body length at 10-day intervals for 39 individuals of the shelf samples and for 22

individuals of the basin samples. The growth equations

were calculated using Ricklefs's (1967) method.

I Wespestad, V. G. 1991. Walleye pollock. In Stock assessment and

fishery evaluation report for the groundfish resources ofthe Bering

SealAleutian Islands region as projected for 1992. North Pacific

Fishery Management Council, P.O. Box 103136, Anchorage, AI<.

99510.

the larval environment is thought to be about 3°C

( ishiyama et al., 1986; Okada, 1986). Allowing 25 days

from spawning to hatching, the spawning period of

these fish was estimated to be from late March to late

May in both 1989 and 1990. This spawning period

coincides with the spawning period on the southeastern continental shelf and slope. These results suggest

the possibility that the juvenile fish collected in this

study originated mainly from the southern shelfspawning stock.

Two fish collected in 1990 were estimated to be

spawned in early March, which matches the spawning

period of the basin stock. From a winter hydroacoustic

survey in 1989, the spawning biomass in the Bogoslof

area was estimated to be about 2 million tons

(Wespestad1). In spite of considerable spawning biomass in the basin, we found only a few (0.7% of the

total) juvenile fish that were estimated to have originated in the basin stock. At several stations in 1989 and

1990, larger fish ranging from 100 mm to 220 mm were

collected by midwater trawling. We also succeeded in

collecting fish larger than 100 mm by bottom trawling

without a net liner at several stations on the shelf.

These larger fish were determined by otolith analysis to

be age-l fish, and we could not find age-O juveniles of

comparable size. These results suggest that the larvae

spawned in the basin area did not survive to the juvenile

stage in 1989 and 1990, and further suggest that some

shelf-spawned juveniles migrated or were transported

off the shelf area into the basin. This concept is sup-
_______ Nishimura et aI.: Hatch Date and Growth Estimation ofJuvenile Walleye Pollock Collected in the Bering Sea 87

ported by satellite drifter trajectories that headed mainly

offshore (Reed and Stabeno, 1990).

The range of the estimated hatching period was almost the same in 1989 and 1990, but the mode in 1990

appeared in late April, about 3 weeks earlier than in

1989. Hatch date distribution in 1990 closely resembles

that observed in 1979 by Walline (1985). Hatch date

frequency distributions showed a unimodal pattern in

1989 and a bimodal pattern in 1990. One explanation

for the bimodal distribution might be a separate spawning period in 1990. At the same time, we cannot rule

out the possibility that selective mortality may influence

hatch date distributions of surviving larvae. Oceanographic information will be necessary to distinguish

between these two possibilities.

In this study, estimated hatch date distribution was

not corrected for mortality. Size-specific mortality rates

of larval and juvenile walleye pollock in the Bering Sea

are unknown. In future analyses, mortality estimates

combined with the effect of advection will be required.

The growth curve estimated for the shelfjuveniles in

this study closely resembles that estimated for the juveniles collected in the Gulf of Alaska (Yoklavich and

Bailey, 1990; Brown and Bailey, 1992). Nishimura and

Yamada (1988) observed geographical differences in

early growth ofjuvenile pollock in the adjacent waters

off Hokkaido. The growth curves obtained in this study

show moderate growth, between the fastest and slowest

growth observed in the waters off Hokkaido.

According to the estimated growth equations, the

asymptotes of growth in the first year are 115.5 mm for

the shelf samples and 84.0 mm for the basin samples.

The first annual rings of one-year-old fish collected in

the Bering Sea were estimated by back-calculation to be

formed between about 120 mm and 170 mm (Nishimura, unpublished data). Considering the growth stagnation before annual ring formation, the estimated

asymptote of 115.5 mm for the shelfsamples appears to

be a reasonable estimation. On the other hand, the

estimated asymptote of 84.0 mm for the basin samples

seems too low. The fate ofjuvenile pollock in the basin will

become the object of additional attention in the future.

Acknowledgments _

We thank Kevin M. Bailey and Richard D. Brodeur of

the Alaska Fisheries Science Center for their comments

on the manuscript. We also thank Jimmie J. Traynor

and the staff of the MACE program of the Alaska Fisheries Science Center, and Masahiko Furusawa and the

staff of the Fishing Boat and Instrument Division ofthe

National Research Institute of Fisheries Engineering,

for their cooperation with the field survey.

Literature Cited

Bailey, K M., and C. L. Stehr.

1988. The effects of feeding periodicity and ration on the

rate of increment formation in otoliths of larval walleye

pollock Theragra chalcogramma (Pallas). ]. Exp. Mar. BioI.

Ecol. 122:147-161.

Brown, A. L., and K M. Bailey.

1992. Otolith analysis of juvenile walleye pollock Theragra

chalcogramma from the western Gulf of Alaska. Mar. BioI.

112:23-30.

Haynes, E. G., and S. E. Ignell.

1983. Effect of temperature on rate of embryonic development of walleye pollock, Theragra chalcogramma. Fish. Bull.

81 :890-894.

Hinckley, S.

1987. The reproductive biology of walleye pollock, Theragra

chalcogramma, in the Bering Sea, with reference to spawning

stock structure. Fish. Bull. 85(3):481-498.

Mulligan, T.]., K M. Bailey, and S. Hinckley.

1989. The occurrence of larval and juvenile walleye pollock,

Theragra chalcogramma, in the eastern Bering Sea with implications for stock structure. In Proceedings of the International Symposium on the Biology and Management of Walleye Pollock, p. 471-490. Lowell Wakefield Fisheries Symposium, Alaska Sea Grant Rep. 89-1.

Nakatani, T., and T. Maeda.

1984. Thermal effect on the development of walleye pollock

eggs and their upward speed to the surface. Bull.Jpn. Soc.

Sci. Fish. 50:937-942.

Nishiyama, T., K Hirano and T. Haryu.

1986. The early life history and feeding habits of larval walleye pollock, Theragra chalcogramma (Pallas), in the southeast

Bering Sea. INPFC Bull. 45:177-227.

Nishimura, A., and]. Yamada.

1984. Age and growth of larval and juvenile walleye pollock,

Theragra chalcogramma (Pallas), as determined by otolith daily

growth increments. ]. Exp. Mar. BioI. Ecol. 82:191-205.

1988. Geographical differences in early growth of walleye

pollock Theragra chalcogramma, estimated by back<alculation

of otolith daily growth increments. Mar. BioI. 97:459-465.

Okada, K

1986. Biological characteristics and abundance of pelagic

pollock in the Aleutian Basin. INPFC Bull. 45:150-177.

Ricklefs, R. E.

1967. A graphical method of fitting equations to growth

curves. Ecology 48:973-983.

Reed, R. K, and P.]. Stabeno.

1990. Flow trajectories in the Bering Sea: Evidence for chaos.

Geophys. Res. Lett. 17:2141-2144.

Walline, P. D.

1985. Growth of larval walleye pollock related to domains

with the SE Bering Sea. Mar. Ecol. Prog. Ser. 21:197-203.

Yoklavich, M. M., and K M. Bailey.

1990. Hatching period, growth and survival of young walleye

pollock Theragra chalcogramma as determined from otolith

analysis. Mar. Ecol. Prog. Ser. 64:13-23.

Diseases and Parasites ofJuvenile Walleye Pollock,

Theragra chalcogramma, from the Gulf of Alaska, 1986-1988

J. FRANK MORADO and DEBRA A. McFEE

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

ABSTRACT

From 1986 to 1988, 1,582 walleye pollock, Theragra chalcogramma, between 40 and 120

mm in length were collected from the Gulf ofAlaska by midwater shrimp trawl for a study on

the diseases ofjuvenile fish. A number of parasites were encountered, varying in prevalence

and distribution in each year of collection. Trematodes were the most often encountered

group of parasites: they were found at approximately all sampled stations and at relatively

high frequencies. The prevalence of most parasites (e.g., digenetic trematodes, nematodes,

myxosporeans, bacterial epibiont) increased with fish size, suggesting that they were acquired through feeding and that they had no deleterious effects on health. The exception

was a microsporidan that infected skeletal muscle. This protistan parasite tended to be more

prevalent in small fish than in larger ones, suggesting that infected fish were removed from

the population either through predation or because the parasitized host died. The geographic distribution patterns of the parasites do not entirely support the existence of a

single population of walleye pollock in the Gulf ofAlaska because neither host nor parasite

distribution can be explained by the region's hydrography.

Introduction

Determination of the disease component of natural

mortality rates for wild fish populations is beset with

many problems. Foremost, the effects of disease vary

considerably with the life-history stage ofthe fish (Munro

et aI., 1983). Second, sample sizes are typically insufficient to determine the incidence of infection; as a

result, prevalence is more often reported. Third, although mortality of wild fishes is the most visible effect,

the relationship of disease to sublethal effects (e.g.,

reduced growth rates and fecundity, diminished avoidance of predators) is unknown and may represent a

significant proportion of natural mortality. Fourth, natural mortality rates are unknown, not constant, and generally vary with age (Munro et aI., 1983). Finally, underlying the previously stated problems is the fact that the

causes of population fluctuations are essentially unknown and unpredictable. This paper reports on the

results of a continuing histopathological baseline study

that attempts to examine the role of disease in the

population dynamics of Gulf of Alaska walleye pollock,

Theragra chalcogramma. Current data on the distribution

patterns and prevalence of infectious and noninfectious diseases ofjuvenile walleye pollock (1986-88) are

reported here.

Methods _

Schools of juvenile walleye pollock, Theragra chalcogramma, were sought using the ship's echo sounder

during 1986, 1987, and 1988 surveys ofjuvenile walleye

pollock in the Gulf of Alaska. Favorable targets were

then sampled with a 24-m high-opening shrimp trawl.

Twenty or more fish were arbitrarily chosen from the

haul at randomly selected stations. Incisions were made

along both sides of the body cavity of all fish, and in

dorsal body musculature offish longer than 70 mm. The

fish were then placed in 10% neutral buffered formalin

and transported to the laboratory. Before processing, the

standard length of each fish was determined and each was

sliced into five cross sections. The five segments were then

dehydrated through a graded ethanol series, embedded

89
90 NOAA Technical Report NMFS 126 . _

in paraffin, sectioned at 3-5 j..lm, and stained with hematoxylin and eosin. All cases were read blind.

Overall parasite prevalence was determined and plotted for each 10-mm increment of fish size. Parasite

distribution and prevalence for each sampled station

were also determined and plotted.

have examined 1,476 of these fish (1986, N=142, or

97%; 1987, N=648, or 98%; 1988, N=686, or 89%). The

size of collected juveniles ranged between 40 and 120

mm, but collections at both ends of the range were few

(Table 1). For this reason, our discussion will focus on

the size groups from 50 through 100 mm.

Results Host Size-Parasite Prevalence Relationships

Samples

During the three-year survey period, 307 hauls (1986,

N=107; 1987, N=1l9; 1988, N=81; Fig. 1) were made in

the attempt to collect sufficient numbers of juvenile

walleye pollock for various studies. In 1986, hauls were

restricted to the central and eastern Alaska Peninsula

regions and around Kodiak Island. In 1987 and 1988,

hauls were made along the entire Alaska Peninsula and

around Kodiak Island. Specimens for disease studies

were collected from 70 hauls (1986, N=6; 1987, N=33;

1988, N=31), resul ting in the collection of 1,582 juvenile walleye pollock during the three-year sampling

period (1986, N=146; 1987, N=661; 1988, N=775). We

Few noninfectious anomalies were encountered in

samples from all the collection years. These anomalies

appeared to be of little significance to this study and

will not be discussed. The most frequent observations

are presented below, along with their prevalence in

each year and in the combined collection.

Bacteria-A filamentous, bacterial epibiont was found

colonizing the rectal epithelium ofjuvenile walleye pollock collected in 1988; it was absent in samples from the

two previous years. The importance of the bacterial

epibiont is unknown because even under the heaviest

infestation, it did not appear to adversely affect the

epithelium of the rectum.

o

153W

Gulf of Alaska

A 1986

o 1987

• 1988

162W 159W 156W

Bering Sea

55N

o

DO

O~.:~ • rfj. 54N -+----r---,--_.__--~-__._-___,---_,_-_.__--~-__._-___,--_,_-_____r--+_

165W

56N

57N

58N

l"igure I

Haul positions for collections ofjuvenile walleye pollock, Theragra chalcogramma, by year.
_______ Morado & McFee: Diseases and Parasites ofJuvenile Walleye Pollock from the Gulf of Alaska 91

Table I

Sample sizes for each lO-mm size category ofjuvenile walleye pollock.

Size (mm)

Year 40 50 60 70 80 90 100 110 120 Total

1986 0 7 28 31 21 29 20 6 0 142

1987 0 98 139 120 III 105 64 5 1 643

1988 2 22 152 198 159 89 22 0 0 644

Total 2 127 319 349 291 223 106 11 1 1429

70,-------------,--------------,

... 1986 <> 1987 • 1988 .. Cumulative

60 .

50 - - - - . - - - -

40

u

0: 30

20 _. - - - •.. - . - . - - . - . - - ... - - - - - - - - .

10 - ..•. - - - - - - - . - - - - . - .

40 50 60 70 80

Size (mm)

90 100 110 120

Figure 2

Relation between fish size and prevalence of a filamentous bacterium in

Gulf of Alaskajuvenile walleye pollock, 1986-88.

The bacterium first became evident in 50-mm fish

(Fig. 2) and remained at the same prevalence for the

subsequen t two size groups. In 1988, the infestation

rate increased markedly in fish larger than 70 mm, to

28%. Overall, prevalence increased slightly in fish between 50 and 70 mm, reached a maximum of 8.5% in

80-mm fish, and then dropped slightly in the 90- and

100-mm groups.

Microsporida-A microsporidan of the genus Pleistophora was routinely found in the skeletal muscle of

juvenile walleye pollock. Infections qualitatively ranged

between light (restricted to isolated muscle fibers) and

severe (including several adjacent muscle fibers). No

host response was directed at the microsporidan until

the surrounding sarcolemma was breached, at which

time a marked granulomatous response appeared.

Microsporidan infections for 1987 and 1988 generally fluctuated between 0 and 11 % for the 50-100-mm

size groups (Fig. 3). In 1986, however, nearly 30% of

the 50-mm group was infected by Pleistophora; microsporidan prevalence dropped to less than 5% in the 60-

mm group, and did not exceed 10% in the subsequent

size groups. Overall prevalence (10%) was highest in

the 50-mm group and gradually declined with increase

in fish size.

Myxosporea-Two myxosporeans were found in the

kidneys ofjuvenile walleye pollock. Neither was identified to species; they were similar in morphology to

myxosporeans of the genera Sphaerospora (Dykova and

Lorn, 1982) and Chloromyxum (Dykova and Lorn, 1988).

Neither was present in sufficiently large numbers to

cause marked changes in parasitized cells of the proxi-
92 NOAA Technical Report NMFS 126

TO ,--------------.----------------,

... 1986 + 1987 • 1988 --- Cumulative

60 - - - -

50 ..•.....•.................•.................•.

40

]

et 30

20

40 50 60 70 80

Size (mm)

90 100 110 120

Figure 3

Relation between fish size and prevalence of a microsporidan,

Pleistophora sp., in Gulf of Alaska juvenile walleye pollock, 1986-88.

mal or distal tubules of the kidney. Sphaerospora sp. was

the most frequently encountered myxosporean, but several mixed infections were observed. Intracellular

Sphaerospora stages were rare, but presumed trophozoites were abundant in the lumena of the proximal and

distal tubules. A large proportion of presumed trophowites adhered to the epithelia of the proximal and

distal tubules. Chloromyxum sp. was typically found attached to the proximal tubule epithelium of the kidney

via a stalk (Morado and Sparks, 1990), but even in the

heaviest of infections, few changes were evident in the

host cells.

Overall, Sphaerospora prevalence increased with fish

size, reaching a maximum (33%) in walleye pollock of

110 mm (Fig. 4); each year's collections generally presented a similar trend. The highest Sphaerospora prevalence (33%) was found in the 1I0-mm group in 1986.

Chloromyxum was much less prevalent than Sphaerospora

(Fig. 5). The cumulative data suggested an infection

pattern of gradual increase with increase of fish size,

but prevalence varied slightly among sizes and years.

The highest Chloromyxum prevalence was observed in

the 100-mm group in 1988.

Digenetic Trematodes-Digenetic trematodes, probably

members of the genera Lepidapedon, Podocotyle, and

Steganoderma (Arthur, 1984), were routinely encountered in fish from all three collection periods (Fig. 6).

All parasites were adults that possessed ova, regardless

of host size. Most were encountered in the intestine,

although they were common in the lumena of the pyloric caeca. Cell and tissue changes were restricted to

the immediate area of parasite attachment; because

infestations were relatively light, no remarkable organ

changes were observed. In 1987 and 1988, trematodes

were significantly prevalent in juvenile walleye pollock

of the 5D-mm group, after which prevalence increased

with fish size in 1987 and decreased with fish size in 1988.

In 1986, trematode infections did not become notable

until the fish reached 80 mm. The highest trematode

prevalence (60%) was observed in lID-mm fish in 1987.

Cestodes-The prevalence of adult cestodes, probably

of the genera A bothrium, Bothriocephalus, and Scolex

(Arthur, 1984), increased with fish size in the 1986 and

1987 samples (Fig. 7); the cumulative data presented a

similar tendency. Cestode prevalences for 1988 remained relatively constant for the 50- to 80-mm groups

and then diminished in larger size groups. In general,

in 1988 cestode prevalence was lower than in 1986 and

1987. Adult cestodes were found attached to the mucosa of the stomach, intestine, and pyloric caeca, but

because infestations were relatively light, no remarkable organ changes were observed.

Larval Nematodes-Larval nematodes, probably members of the genera Anisakis, Contracaecum, and Hyster(}-

thylacium (Margolis and Arthur, 1979), were frequent

in juvenile fish and typically found in the mesenteries

of all organs and tissue, especially the liver. In all in-
_______ Morado & McFee: Diseases and Parasites ofJuvenile Walleye Pollock from the Gulf of Alaska 93

70,----------------,-------------,

... 1986 + 1987 • 1988 ..Cumulative

60 ....

50 .. , - - - ........•......•...........•.. - ..... - . - -

20 - .•..•..•..•..•. - .. - ...

10 . - ... - - -

40 50 60 70 80

Size (mm)

90 100 110 120

Figure 4

Relation between fish size and prevalence ofa myxosporean, Sphaerospora

sp., in Gulf of Alaska juvenile walleye pollock, 1986-88.

70 ,--------------,---------------

... 1986·+ 1987 • 1988 .. Cumulative

60 . - ... - - .....

50 ...

40

ct 30

20 .. -

10 .

40 50 60 70 80

Size (mm)

90 100 110 120

Figure 5

Relation between fish size and prevalence ofa myxosporean, Chloromyxum

sp., in Gulf of Alaskajuvenile walleye pollock, 1986-88.

stances of infection, a host response to the larval nematodes was evident as an encapsulation. Response was

confined to the area immediately around the nematode, but because infections were relatively light, no

remarkable organ changes were observed. In one specimen, the apparent migratory path ofa larval nematode

was observed in the liver. An acute inflammatory reaction directed at host cell damage, the nematode, and

the introduction of bacteria was present at the final

destination of the nematode.
94 NOAA Technical Report NMFS 126 _

70 r-------------,..-----------------,

.. 1986 .. 1987 .1988 .. Cumulative

60 .

50 ... - - ... - .... - ... - - .. - . - .. - - .. - - - - .. - -

40

Oi

>

J: 30

20 ....

10 .

40 50 60 70 80 90 100 110 120

Size (mm)

Figure 6

Relation between fish size and prevalence of digenetic trematodes in

Gulf of Alaskajuvenile walleye pollock, 1986-88.

70 r-------------,..-----------------,

.. 1986 .. 1987 • 1988 .. Cumulative

60 - .

50 - - . - .

20 ...•. - - ...• - . - .. - . - - •.... - ....

10 ..... - - - - - . - . - -

40 50 60 70 80 90 100 110 120

Size (mm)

Figure 7

Relation between fish size and prevalence of unidentified cestodes in

Gulf of Alaska juvenile walleye pollock, 1986-88.

In 1986, nematode prevalence fluctuated considerably, reaching peaks in the 50- and 80-mm size groups

and a low in the 70-mm group (Fig. 8). The pattern of

the cumulative data and that for 1987 indicated that

infection generally increased with increase in fish size.

Nematode prevalence for 1988 presented a curve that

peaked in the 80-mm size group.
_______ Morado & McFee: Diseases and Parasites ofJuvenile Walleye Pollock from the Gulf of Alaska 95

70,------------,-------------------,

... 1986 <Co 1987 • 1988 ... Cumulative

60 .. - - - - - - - - - - - - - - - - - - - - . - . - - - . - - - - - - - - - - - . - - . - -

50 . - - .. - ..... - - - - - - . - . - - . - - - - - - - - - - . - . - - - - - - - - - -

40

:i

0: 30

40 50 60 70 80

Size (mm)

90 100 110 120

Figure 8

Relation between fish size and prevalence of unidentified larval nematodes in Gulf of Alaska juvenile walleye pollock, 1986-88.

Site-Specific Parasite Prevalence and Distribution

Because fewer stations were sampled in 1986 (N=6)

than in 1987 and 1988, the site-specific prevalence and

distribution patterns of the above parasites are presented here only for 1987 and 1988.

Bacteria-Fish colonized by the filamentous bacterium

were collected throughout the survey area. Prevalence

was most remarkable at sites off the central and eastern

Alaska Peninsula, especially around Kodiak Island (Fig.

9). Site-specific prevalence ranged between 3% and 40%.

Microsporida-In 1987, site-specific prevalence ranged

between 3.7% and 50%. Microsporidan infections were

widespread in fish collected along the entire Alaska

Peninsula and at a few sites around the Kodiak Archipelago (Fig. 10). The 1988 microsporidan distribution

pattern was similar to the 1987 pattern, but fewer stations were positive, and site-specific prevalence ranged

from 2.5% to 30% (Fig. 10).

Myxosporea-In 1987, Sphaerospora infections were restricted to stations sampled off the eastern and central

Alaska Peninsula and around Kodiak Island; site-specific prevalence ranged between 5% and 40% (Fig. 11).

In contrast to 1987, in 1988 Sphaerospora infections were

found at stations along the entire length of the Alaska

Peninsula, but overall prevalence was lower (Fig. 11);

site-specific prevalence ranged between 5% and 26.3%.

The distribution patterns of Chloromyxum for 1987

and 1988 were similar to the Sphaerospora patterns of

1987 and 1988 (Fig. 12). Chloromyxumwas less frequently

encountered than Sphaerospora: site-specific prevalence

ranged between 5% and 20% in 1987, and between

4.8% and 20% in 1988.

Trematodes-In 1987, site-specific prevalence ranged

from 5% to 55% (Fig. 13). Trematode-infected fish

were widely distributed throughout the sampling area:

nearly all sampled stations had infected fish. The 1988

distribution pattern is similar to that of 1987, but in

1988, 40% of the fish collected at one station at the far

western end of the survey were infected with digenetic

trematodes. The station was located at Unimak Pass, a

channel between the Bering Sea and the Gulf of Alaska.

Site-specific prevalence in 1988 ranged from 4.8% to 40%.

Cestodes-In 1987, cestode-infected fish were generally found in the central and eastern Alaska Peninsula

regions and around Kodiak Island (Fig. 14); only one

station in the western Alaska Peninsula region had cestode-infected fish. Site-specific prevalence in 1987

ranged between 5.3% and 44.4%. The 1988 distribution was more widespread, but site-specific prevalence

was generally lower (Fig. 14), ranging between 4.5%

and 10%.

Nematodes-In 1987, nematode-infected juvenile walleye pollock were common in the central and eastern
96 NOAA Technical Report NMFS 126

Bering Sea

58N

57N

56N

55N

o 0 , 5

I

Q

GulfofAlaska

162W 159W 156W 153W

54N +---,------r--,------,--,---,----,---.-----,-------r-~--,__-.__-+

165W

Figure 9

Site-specific prevalence of a filamentous bacterium in juvenile walleye pollock,

1988.

Alaska Peninsula regions and around Kodiak Island

(Fig. 15) and less frequent in the western Alaska Peninsula region. In 1988, nematode infection was widely

distributed throughout the survey area, but as before, it

was less common at the western end of the survey.

Prevalence generally seemed lower during 1988: sitespecific prevalence ranged between 5% and 55% in

1987, and between 5% and 23.8% in 1988. Fewer positive stations were found in 1988 than in 1987.

Discussion

Infectious disease affects individuals in different ways.

Death of the host is the most obvious outcome, but a

condition that either directly or indirectly retards growth

or swimming performance is equally important because

it compromises survival (Munro et aI., 1983). Juvenile

fish unable to grow or to school with their cohort are

more susceptible to predation. A loss in swimming efficiency probably affects the host's ability to find and capture prey. Thus an important factor to consider in assessing the impact ofdisease on the condition ofits host is the

organ or tissue affected and the severity of the disease.

Two different myxosporean parasites were found in

juvenile walleye pollock. Both Chloromyxum and

Sphaerospora are obligate parasites that infect kidney

proximal and distal tubule epithelia and are classically

defined as coelozoic even though their initial development occurs within epithelia. Their subsequent developmental stages appear free in the lumena of target

organs (Dykova and Lorn, 1988). Chloromyxum sp. uses a

stalk to attach plasmodia to a parasitized cell (Morado

and Sparks, 1990). In contrast, sporoblasts of Sphaerospora

develop freely in proximal tubule epithelium without

evidence of an attachment structure. Neither parasite

appears lethal, and both generally cause inconspicuous

infections in fish (Lorn et aI., 1985). Little is known

about the pathogenesis of Chloromyxum. In contrast,

Sphaerospora infections tend to be more common in

juveniles than in adults and are reported to be scarce in

feral fish (Lorn et aI., 1985). This study confirms that

Sphaerospora infections are common in juvenile fish,

and also shows that infections are prevalent in wild fish.

The microsporidan parasite Pleistophora sp. develops

within skeletal muscle and causes significant disruption

of myofibers, but death resulting from direct infection

is unlikely. Rather, because of the site of infection,

heavy parasitization can greatly affect muscle function

and thus swimming efficiency. Thus the host's ability to

feed or avoid predation is reduced. This probably explains the general profile of reduced prevalence with

increased fish size.

Most adult digenetic trematodes live in the digestive

tract lumen, attached to intestinal mucosa via muscular

suckers. Large numbers of digenetic trematodes can

reduce the host's fitness to survive, but this is not typical. Even though trematode infections were common,

the number of trematodes per fish was not suffICient to

cause cell or organ changes in infected juvenile walleye
_______ Morado & McFee: Diseases and Parasites ofJuvenile Walleye Pollock from the Gulf of Alaska 97

1987

58N Bering Sea

57N

56N

55N

11.1

23.5 10 Gulf of Alaska

162W 159W 156W 153W

54N +-___r----r-----,--_,_-~-____,--,--,-----~--r---,--,__----r--+

165W

1988

58N

Bering Sea

57N

56N

55N

5

,

4

Gulf of Alaska

162W 159W 156W 153W

54N +-_._-~---=:..,-~-_,_-____,-~~-,--,_-~-~-_,_--.---+

165W

Figure 10

Site-specific prevalence of Pleistophora sp. in juvenile walleye pollock in 1987

(above) and 1988 (below).

pollock. In general, this taxon was the most often encountered parasite; it was found at nearly all sampled

stations and at relatively high frequencies.

In marked con trast, juvenile nematodes must perforate the gut epithelium to find their preferred site of

residence. During penetration and migration to their

preferred site, pathogenic or opportunistic bacteria may

be introduced into the underlying connective tissue

and coelom. When the nematodes reach their preferred site, the host responds by encapsulating them.

Heavily parasitized fish may therefore possess organs or

tissues in which the parenchyma has been significantly
98 NOAA Technical Report NMFS 126 _

1987

58N Bering Sea

57N

56N

55N

0

0 0 Gulf of Alaska

54N

l65W 162W 159W 156W l53W

16

1988

58N

Bering Sea

57N

56N

55N

o

I

Gulf of Alaska

162W l59W 156W 153W

54N +-~------.--,___-_,_-_._-__,_-___y--~-.,__-_,_-~-_____r-____,,___-+_

165W

Figure 11

Site-specific prevalence of Sphaerospora sp. in juvenile walleye pollock in 1987

(above) and 1988 (below).
_______ Morado & McFee: Diseases and Parasites ofJuvenile Walleye Pollock from the Gulf of Alaska 99

1987

58N

57N

56N

55N

o 0 o

Gulfof Alaska

54N +-----,---,--,----,-----r-----,--~-~-___,_--,_-_,___-___,-____,_--t_

165W 162W 159W 156W 153W

58N

57N

56N

55N

1988

Bering Sea

I

4

Gulf of Alaska

162W 159W 156W 153W

54N +----r-____,_----,--,_-~---r--___,_-_r_-___,-~~-,---,___--.---+

165W

Figure 12

Site-specific prevalence of Chloromyxum sp. in juvenile walleye pollock in 1987

(above) and 1988 (below).
100 NOAA Technical Report NMFS 126

1987

58N Bering Sea

57N

56N

55N

16.729.4 20

Gulf of Alaska

54N

165W 162W 159W 156W 153W

1988

58N

Bering Sea

57N

f

56N

(J

7.5

55N

Gulf of Alaska

~O 22.7

15

54N

165W 162W 159W 156W 153W

Figure 13

Site-specific prevalence of unidentified trematodes in juvenile walleye pollock

in 1987 (above) and 1988 (below).
_______ Morado & McFee: Diseases and Parasites ofJuvenile Walleye Pollock from the Gulf of Alaska 101

1987

58N

Bering Sea

57N

56N

55N

o 0 o

Gulf of Alaska

162W 159W 156W 153W

54N +-----,---r----r--r---,---..,----.---~-___,_-----,~-.__-_,_-~-_+_

165W

1988

58N

Bering Sea

57N

56N

55N

4.5

o

o

I

o

Gulfof Alaska

162W 159W 156W 153W

54N +-----,---r----r--r---,---..,----.---~-___,_-____,~-.__-_,_-__.__-_+_

165W

Figure 14

Site-specific prevalence of unidentified cestodes in juvenile walleye pollock in

1987 (above) and 1988 (below).
102 NOAA Technical Report NMFS 126

replaced by connective tissue. Indeed, liver function

and size of Baltic cod, Gadus morhua, parasitized by

juvenile nematodes are inversely proportional to the

level of nematode infection (Margolis, 1970). However,

a correlation between parasitism and tissue function

was not obvious in juvenile walleye pollock.

The major spawning site of Gulf of Alaska walleye

pollock is a trough off Cape Kekurnoi. Eggs and larvae

subsequently drift to the southwest, out of Shelikof

Strait and parallel to the Alaska Peninsula (Kim and

Kendall, 1989; Kendall and Picquelle, 1990; Hinckley et

aI., 1991). But the distribution patterns of parasites in

1987

58N

Bering Sea

57N

56N

55N

o 0 o

Gulf of Alaska

54N -+--~-~-----,--~-~----.------,-----,-------,--~-~-,_-~--+

165W J62W 159W 156W 153W

58N

57N

56N

55N

1988

Bering Sea

,0 10

o

o

o

s

I

a

Gulf of Alaska

162W 159W 156W 153W

54N -+-----,-----,---____,--~-~-,_-~-~-____,--~-~-,_----.----+-

165W

Figure 15

Site-specific prevalence of unidentified nematodes in juvenile walleye pollock

in 1987 (above) and 1988 (below).
Morado & McFee: Diseases and Parasites ofJuvenile Walleye Pollock from the Gulf of Alaska 103

juvenile walleye pollock throughout the survey area

suggest that it is unlikely that recently hatched fish

from the Shelikof Strait spawning population populate

all bays around Kodiak Island, especially Marmot Bay.

It thus appears likely that smaller and possibly distinct

spawning populations exist throughout the GulfofAlaska.

Parasites are often unique to particular ecosystems.

This principle has been frequently used to help identify

various fish stocks (MacKenzie, 1983; Moller and Anders,

1986; Rhode, 1993). Juvenile parasite stages are the

most useful for this purpose because they are acquired

early, remain viable within the host for several years,

and-because they are acquired throughout the host's

life span-may reflect the host's migration through various ecosystems. All the parasites encountered in thisstudy,

but especially the nematodes and digenetic trematodes,

should be identified to species, since this may provide

information on the existence of one or more walleye

pollock stocks in the Gulf of Alaska. Knowledge of the

parasite fauna of exploited fishes may also provide information about stock composition at the time of harvesting.

Literature Cited

Arthur,]. R.

1984. A survey of the parasites of walleye pollock (Theragra

chalcogramma) from the northeastern Pacific Ocean of

Canada and a zoogeographical analysis of the parasite fauna

of this fish throughout its range. Can.]. ZooI. 62:675-684.

Dykova, 1., and]. Lorn.

1982. Sphaerospora renicola n.sp., a myxosporean from carp

kidney, and its pathogenicity. Z. Parasitenkd. 68:259-268.

1988. Chloromyxum reticulatum (Myxozoa, Myxosporea) in the

liver of burbot (Lota Iota L.) and its migration to the final

site of infection. Europ.]. Protistol. 23:258-261.

Hinckley, S., K. M. Bailey, S.]. Piquelle,]. D. Schumacher, and

P.]. Stabeno.

1991. Transport, distribution, and abundance of larval and

juvenile walleye pollock (Theragra chalcogramma) in the western Gulf of Alaska. Can.]. Fish. Aquat. Sci. 48:91-98.

Kendall, A. W.,jr., and S.]. Picquelle.

1990. Egg and larval distributions of walleye pollock Theragra

chalcogramma in Shelikof Strait, Gulf of Alaska. Fish. Bull.

88:133-154.

Kim, S., and A.W. Kendalljr.

1989. Distribution and transport of larval walleye pollock

(Theragra chalcogramma) in Shelikof Strait, Gulf ofAlaska, in

relation to water movement. Rapp. P.-V. Reun. Cons. Int.

Explor. Mer 191:127-136.

Lorn,]., M. Pavlaskova, and 1. Dykova.

1985. Notes on kidney-infecting species of the genus

Sphaerospora Thelohan (Myxosporea), including a new species S. gobionis sp.nov., and on myxosporean life cycle stages

in the blood ofsome freshwater fish. ]. Fish Dis. 8:221-232.

MacKenzie, K.

1983. Parasites as biological tags in fish population

studies. Adv. Appl. BioI. 7:251-331.

Margolis, L.

1970. Nematode diseases of marine fishes. In S. F. Sneiszko,

(ed.l, A symposium on diseases of fishes and shellfishes,

p.190-207. Am. Fish. Soc. Spec. Publ. No.5.

Margolis, L., and]. R. Arthur.

1979. Synopsis of the parasites of fishes of Canada. Bull Fish.

Res. Board Can. 199:1-269.

Moller, H., and K. Anders.

1986. Diseases and parasites ofmarine fishes. Verlag-Moller,

Keil, Germany.

Morado,]. F., and A. K. Sparks.

1990. Preliminary report on the diseases and parasites of

juvenile walleye pollock, Theragra chalcogramma, from the

Gulf of Alaska. In Pathology in marine science, Proc. Third

IntI. Colloq. Pathol. Mar. Aquacult., Virginia Inst. Mar. Sci.,

Gloucester Pt., VA, Oct. 2-6, p. 201-213.

Munro, A. L. S., A. H. McVicar, and R.jones.

1983. The epidemiology of infectious disease in commercially important wild marine fish. Rapp. P.-V. Reun. Cons.

Int. Explor. Mer 182:21-32.

Rhode, K.

1993. Ecology of marine parasites. 2nd ed. CAB International, Oxon, UK.
Growth ofJuvenile Walleye Pollock, Theragra chalcogramma,

in the Gulf ofAlaska

ANNETTE L. BROWN and KEVIN M. BAILEY

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

Recruitment of walleye pollock, Theragra chalcogramma,

in the western Gulf of Alaska can be influenced by

processes affecting survival during the late larval and

earlyjuvenile phases oflife. As part of an ongoing study

offactors affecting recruitment dynamics of pollock, we

monitored length-at-age growth rates by otolith microincrement analysis of age-O pollock caught in annual

surveys from 1985 to 1991. Aggregated seasonal lengthat-age data were used to infer growth rates that could

be compared by geographic region and year. Both geo104

graphic and interannual differences in growth rates

were detected. Mean growth rates can vary by 40%

between years. Intra-annual differences in growth rates

by geographic region can vary by a similar amount.

There were no obvious relationships between growth

rates and gross approximations of age-O juvenile density and sea surface temperature. However, age-O growth

rates in the relatively high recruitment years of 1985

and 1988 were high compared with those in the weak

year classes of 1987 and 1991.
The Growth ofWalleye Pollock, Theragra chalcogramma, and

Structure of Scales during the First Year of Life

OLGA S. TEMNYKH and VLADIMIR 1. RADCHENKO

Pacific Research Institute ofFisheries & Oceanography (TINRO)

Shevchenko Alley, 4

Vladivostok, 690600, Russia

Comparative studies of the juvenile growth of walleye

pollock, Theragra chalcogramma, are of great importance

both in population investigations and for estimating

production. Scientists at TINRO have recently conducted research on walleye pollock growth during the

first year of life which has involved study of the first

annulus on calcified structures and ofjuvenile pollock

size in different regions (Fig. 1).

The great difference in spawning times between areas ofwinter and spring reproduction causes wide variation in the size of juveniles in Far Eastern seas. In

various areas of the Bering Sea, the mean size ofjuveniles in late autumn ranges from 7 to 11 cm. An analogous situation occurs in Pacific waters off Kamchatka

and in the northern Sea ofJapan. In the Sea ofOkhotsk,

juvenile pollock are generally smaller (6.5-10 cm); in

late autumn the mean size ofjuveniles in the south is

12-13 cm. Evidently, differences in mean size (1.5 cm

or more) ofjuveniles in the western Bering Sea and Sea

of Okhotsk can be explained by differences in their

food supply; mesoplankton biomass is over twice as

high in the western Bering Sea as in the Sea ofOkhotsk.

In November-December, and even by the end of the

winter, the sizes ofjuvenile pollock are scarcely different from the sizes of those collected in October. This

indicates a significant slowing of the growth rate of

juveniles in late autumn despite a lack of change in

their feeding. A slight increase in growth is observed

the following spring. The long duration of spawning

and the different growth conditions result in a great

range in juvenile sizes at the end of the first year oflife.

In the western Bering Sea and Sea of Okhotsk during

October-November andJanuary-March, the size distributions of pollock include specimens from 4 to 6 cm as

well as from 12 to 14 cm, which may cause errors in

defining ages by length distribution. Because of the

great temporal extent ofspawning in many areas where

there are differences in transport by currents, body

length increases with distance from spawning grounds.

In the western Bering Sea, the difference in juvenile

Table I

Mean values of parameters measured on the first annulus of walleye pollock scales from different regions of the Sea of

Okhotsk and the Sea ofJapan.

Radius Number of Mean intersclerite

Region (microns) sclerites distance (microns)

Sea of Okhotsk

South Kuril Islands 638.9 13.4 47.7

Okhotksy region 448.4 9.4 47.7

Northeastern Sakhalin 429.6 9.5 45.2

Yamsko-Tauysky region 410.3 8.9 46.1

Southwestern Kamchatka 388.7 8.2 47.6

Northwestern Kamchatka 363.8 8.0 45.7

Iono-Kashevarovsky 363.8 7.4 45.1

Sea ofJapan

Tatarsky Strait 412.5 7.3 56.5

Southwestern Sakhalin 436.0 8.5 50.8

Peter the Great Bay 505.5 10.9 47.5

105
106 NOAA Technical Report NMFS 126 _

-,

.

)

Figure 1

The investigation regions.
____ Temnykh & Radchenko: The Growth of Walleye Pollock and Structure of Scales during the First Year 107

.1976

01977

A 1978

lonoKashevarovsky

o 8 /0

Okhotsky

Y amsko-Tauysky

¥ 6' 8 10 2

N-E Sakhalin

N-W Kamchatka

2

100 S-W Kamchatka

9f}

#50

70

00

50

Q)

u I;{l c

CO

...

(J) 30

1J

Q)

...

.;:

Q)

(3 Southern Kurily 80 (J)

...

Q)

... 10 c

c

CO 00 Q)

50

/f0

JO

2 6 8 /0

Standardized number of sclerites (Ns')

Figure 2

Scleritograms of the first annulus for pollock from different regions of the Sea of Okhotsk, for 1976-78.

mean length is 0.6-1.6 cm in catches from the shelf,

slope, and Aleutian basin waters. Our data indicate that

growth is controlled by factors such as the temperature

of water and food supply interacting with population

density.

The characteristics of the first-year annulus in pollock scales reflect the variability of juvenile growth in

the various areas of the Far Eastern seas (Fig. 2). The

sizes of annuli from the Sea of Okhotsk vary from 364

microns (7.4 sclerites) in the Iono-Kashevarovsky area

to 639 microns (13.4 sclerites) in the south Kuril Islands area (Table 1). The larger walleye pollock juveniles (9.6-13 cm) in the Sea of Japan have a lesser

range of size of the first-year annulus than those from

the Sea ofOkhotsk, mainly because ofgreater intersclerite

distance.The first-year annuli offish from Peter the Great

Bay and the south Kuril area are characterized by an

additional ring in the center. The presence of this ring is

connected with a cessation of growth that occurs as juveniles are vertically displaced to demersal layers.
Distribution, Recruitment, and Survival ofJuvenile Walleye Pollock,

Theragra chalcogramma: What Role Does Behavior Play?

BORI L. OLLA, MICHAEL W. DAVIS, CLIFFORD H. RYER, and SUSAN M. SOGARD

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

Hatfield Marine Science Center

Newport, Oregon 97365

Behavioral responses of age-O juvenile walleye pollock,

Theragra chalcogramma, to both abiotic and biotic factors

may playa major role in controlling distribution, recruitment, and survival. Success in extrapolating results

from laboratory to field depends on how well the laboratory environment permits juveniles to fully express

their behavioral potential. Under laboratory conditions,

the vertical distribution of juveniles was shown to be

influenced by a variety of extrinsic and intrinsic factors.

Distribution above and below a thermocline was influenced by light intensity, food distribution, level of hun108

ger, presence of predators, and ontogenetic differences

in behavioral thermoregulation. Differences in food

availability and distribution were shown to affect the

way in which juveniles either dispersed or aggregated

into distinct schools. Juveniles respond to a hierarchy

of interacting environmental factors, which may be

modified by their internal state and ontogenetic development. Defining the behavioral response ascribed to

anyone factor is insufficient for prediction ofjuveniles'

behavior when faced with multiple factors in the natural environment.
Electron Microprobe Analysis of Otoliths ofJuvenile Walleye Pollock,

Theragra chalcogramma, from Alaska: A Pilot Stock Separation Study

BRENDA L. NORCROSS

School ofFisheries and Ocean Sciences

University ofAlaska, Fairbanks

Fairbanks, Alaska 99775

KENNETH P. SEVERIN

Department of Geology and Geophysics

University ofAlaska, Fairbanks

Fairbanks, Alaska 99775

JOLYNN CARROLL

School ofFisheries and Ocean Sciences

University ofAlaska, Fairbanks

Fairbanks, Alaska 99775

Dissolved oceanic constituents incorporated into the

otoliths of fish may serve as chemical tracers for reconstructing the early life history of marine fish. Wavelength-dispersive spectrometers on an electron microprobe were able to detect Na, Mg, P, S, CI, K, Ca, and Sr

concentrations on the outer margins of otoliths from

57juvenile walleye pollock, Theragra chalcogramma, from

five locations in the Gulf of Alaska and eastern Bering

Sea. Discriminant analyses that used various combinations ofNa, P, K, Sr, and standard length offish showed

that 60%-70% of the samples could be assigned to the

correct capture locality. Although the concentrations

ofsome of the measured elements were correlated with

standard length of the fish, there were measurable differences among localities when concentrations were

length-corrected, mainly because of differences in Na

and Kconcentrations. Elemental composition ofotoliths

potentially could be used to assign fish from a mixedstock fishery to original stocks, information that is greatly

needed for the effective management of fish stocks.

109

Ecology and Growth ofJuvenile Walleye Pollock, Theragra chalcogramma:

Summary and Recommendations for Future Research

R. IAN PERRY

Pacific Biological Station

Department ojFisheries and Oceans

Nanaimo, B.C., V9R 5K6 Canada

There has recently been considerable progress in understanding the ecology and growth of larval walleye

pollock, due to the large programs conducted by the

United States and Japan. Fewer studies have been conducted of the juvenile stage of walleye pollock, however. This stage is usually more dispersed, harder to

sample quantitatively (because of lower abundances

and greater mobility), and is believed to have a lower

instantaneous mortality rate (but a longer duration)

than the larval stage. This lower instantaneous mortality rate suggests that "internal" processes such as feeding and growth (i.e., starvation) may be less important

to population fluctuations than "external" causes of

mortality such as predation. In practice, however, partitioning the major sources of mortality, and therefore

variability in juvenile abundance, is very difficult because of assumed interaction effects such as predator

selection ofstarved or disease-weakened fish. Studies of

the feeding and growth ofjuveniles are therefore crucial to a complete understanding of the population

dynamics of walleye pollock.

This commentary presents a view offuture directions

for research on the ecology, feeding, and growth of

juvenile walleye pollock. It is based on presentations

and discussions that took place during the Juvenile

Pollock Workshop. In the session on ecology and growth,

five papers and three posters were presented, of which

three papers and four abstracts have been included in

this volume. I will first discuss specific points from these

presentations, then discuss broader research issues, and

conclude with suggestions for the types of analyses that

might be considered in future field programs.

Merati and Brodeur (1996) present important information and questions on the diets and feeding activities

ofjuvenile walleye pollock. They conclude that feeding

occurred predominately at night, which is contrary to

the conclusions of previous studies on the larval stages.

How, then, do the juvenile fish detect their prey in

darkness, and how do they maintain contact with their

conspecifics? What might be the reason for the switch

from the larval behavior? Merati and Brodeur's conclusion about the timing of feeding is important for comparisons with prey distributions and for modeling and

behavioral studies. Interestingly, they did not find any

sharp ontogenetic changes in diet, and prey composition changed gradually with time of day and with fish

size. However, they sampled a limited size range of fish

(49-113 mm); broader comparisons should include

sizes of juvenile pollock that inhabit greater depths.

The strongest variations in diet were attributed to location effects, and it would be useful to compare diet

composition with the local concentration of plankton

in each of the areas sampled to determine the degree

of prey selectivity. Also of interest would be an assessment of the nutritive values of the different prey from

the different areas, perhaps coupled with laboratory

studies of prey "desirability."

Nishimura, Mito, and Yanagimoto (1996) present a

tantalizing study of growth estimates and stock origin

based on otoliths. Otoliths provide an excellent means

to develop growth histories of individual fish, to identify the temporal spawning distribution (of survivors),

and thereby to infer possible spawning location and

stock origin. If the spawning areas are sufficiently different in location, as may be the case for the three areas

in the Bering Sea (Hinckley, 1987), then the

microelemental composition ofthe otoliths may also be

used to identify the spawning areas and stock origins of

the fish (e.g., see Norcross et al., 1996). The elemental

composition (e.g., the Sr/Ca ratio) of the otoliths may

also indicate the temperatures experienced by the larvae and juveniles during growth, which-when combined with oceanographic surveys and circulation modeling-could be a powerful tool for reconstructing past

distributions. Development of growth histories would

be particularly useful for comparing the hatch dates of

survivors from different spawning populations that experienced significant oceanographic and biological

events. Such growth histories can be compared with

particle trajectories and feeding dynamics derived from

111
112 NOAA Technical Report NMFS 126

circulation and trophodynamic models. However, the potential for a long spawning period and multiple spawnings

by individual fish (e.g., Sakurai, 1993) might complicate

the process of distinguishing separate spawning populations. Evidence of the variability of growth histories is

provided by Brown and Bailey (1996). Using otoliths of

age-o pollock from the western Gulf of Alaska over the

period 1985-91, they found geographic and interannual

differences in growth rates that range up to 40%.

Shuntov, Temnykh, and Radchenko (Temnykh and

Radchenko, 1996), who also looked at fish ages derived

from scales and estimated from age-length conversions,

caution that growth rates (and therefore size-at-age) may

differ substantially amongjuvenile pollock from different

areas, possibly due to zooplankton densities, temperature,

and population densities ofjuvenile pollock.

Morado and McFee (1996) provide a different view

offeeding and growth in their study of the diseases and

parasites that might inhibit growth ofthe host fish. In a

sense, such parasites can be considered predators, particularly when they increase the mortality of the host.

Parasites may be important to consider if they affect the

growth of fish despite sufficient food, and/or if they

increase the host's susceptibility to predation. They may

also contribute to the identification ofstock origins.

Although not included in its entirety in this volume,

the presentation by alIa et aI. (1996) (also see alIa et

aI., 1995) introduced a number of questions about the

ecology and growth ofjuvenile pollock and the ability

to use this information in management. They showed

that the distribution ofjuvenile pollock above and beIowa thermocline was influenced by a number ofphysical, behavioral, and physiological conditions such as

light, food, level of hunger, presence of predators, and

ontogenetic differences in response to temperature. In

addition, differences in food availability affected the

degree to which juveniles dispersed or aggregated into

distinct schools. The authors concluded that using behavioral responses to single factors was insufficient for

predicting responses to the multiple factors experienced in the real world. Some of these results have

been described in alIa and Davis (1990), Ryer and alla

(1992), and Sogard and alIa (1993).

Recruitment predictions using feeding and growth

characteristics of larval and juvenile fish have been less

common, and perhaps less successful, than predictions

using physical processes alone (e.g., Sinclair, 1988;

Mullin, 1993). Houde (1987) suggested that recruitment variability may be dominated by physical processes (such as transport to or from appropriate areas),

with the remaining variability due to biological processes such as feeding and predation (which themselves

may be physically mediated; e.g., Rothschild and

asborn's [1988] turbulent contact-rate hypothesis).

Since a principal goal of research on walleye pollock

early life history is recruitment prediction, one of the

first tasks should be to estimate which stocks or areas

are likely to have recruitment dominated by physical

processes, and which stocks might be more influenced

by biological processes. Studies on the ecology, feeding, and growth ofjuvenile stages for recruitment predictions should be most profitable in these latter areas.

Circulation models are one tool for assessing the extent

of particle retention and dispersion in an area and

therefore the relative importance of physical and biological processes to recruitment variability.

An important point that can be drawn from all the

presentations in this workshop concerns the long duration of the 'Juvenile" stage in walleye pollock. The

juvenile stage is defined as lasting from about 60 days to

2-3 years of age by the traditional limits, from postmetamorphosis to recruitment to the adult fishery. This

implies a size range from 25 mm to 300 mm, and a

change in capabilities from planktonic to fully nektonic. The juvenile stage therefore includes large

changes in prey and predator fields, and a very large

reduction in numbers. Perhaps this stage should be

viewed as several stages, each with possibly different

ecologies, growth patterns, vertical and horizontal distributions, and relationships with the environment. Consideration should also be given to the concept of a

threshold size or condition factor that is critical for

pollock to survive to the next stage. Habitat (distribution) differences might be used as one criterion for

separating these stages, supplemented with information on diet and behavior, although as alIa et aI. (1996)

pointed out, differences in the latter may be very subtle

but still important. The age ofjuveniles is probably the

best determinant of a particular stage, but age determinations (from otoliths) are time-consuming and expensive for large sample sizes. Length is probably not a

good indicator of stage within the juvenile phase since

it is likely to be influenced by temperature, food, and

other growth conditions that vary among locations and

years. Prey, predator, or competitor fields that remain

with the pollock as they grow through the variousjuvenile stages (such as cannibalism or parasitism) may

have a greater effect on pollock growth and survival

than fields which the juvenile pollock encounter for

only short periods. Their relative effects could be estimated by multiplying some measure of instantaneous

mortality or growth success for a factor by its duration.

None of the papers presented in this session considered animals longer than 130 mm; all examined what

Dwyer et al. (1987) have defined as age-O fish. There is a

need to examine olderjuveniles, possibly those inhabiting deeper depth zones or shallow nearshore areas.

A variety of sampling systems will be necessary to

adequately capture these various juvenile life-history

stages.
__ Perry: Ecology and Growth ofJuvenile Walleye Pollock: Summary and Recommendations for Future Research 113

Gelatinous zooplankton may play several important

roles in the ecology and growth of juvenile walleye

pollock, yet they are often difficult to sample quantitatively. They can be competitors with pollock for zooplankton prey (e.g., copepods) ; they can be prey for

pollock (e.g., Oikopleura); or they may possibly be commensal with pollock by providing a refuge from predators or perhaps a prey-aggregation mechanism (as has

been suggested by Koeller et al. [1986] for haddock in

the northwest Atlantic). Merati and Brodeur (1996)

observe that Oikopleura sp. were a common and abundant prey item in pollock 50-115 mm long (the size

range that they sampled). Large 'Jellyfish" are also very

abundant in summer during acoustic and trawl surveys

of the Bering Sea and Gulf of Alaska, and the associations ofjuvenile pollock with these potential hosts should

be examined. The nutritive value of gelatinous zooplankton should also be examined, since they may serve

as alternate prey when other food is scarce.

The amazing flexibility of behavioral responses by

juvenile pollock to almost all stimuli, as presented by

Olla et al. (1995, 1996) (and by Temnykh and Radchenko, 1996) is potentially a great hurdle in modeling

their ecology and growth. The problem is compounded

by observations that such plasticity seems to develop as

the juveniles become more competent, with fish smaller

than 40 mm being more predictable in their responses.

Determination of the significance of such variability,

and whether it should be considered in populationlevel models, is crucial. Individual-based modeling approaches that incorporate such behavioral plasticity

(e.g., Tyler and Rose, 1994) are one method of determining the importance of this variability to populationlevel characteristics. For example, how critical is the

observation that pollock avoid cold water except when

they are sufficiently hungry? One solution may be to

integrate individual responses to multiple stimuli by

using population-level responses (e.g., population

means, which ignore individual variability). Indices that

represent processes occurring on long time scales may

also integrate overshort-term (behavioral) variability. Such

indices for growth success include otolith daily growth

rings, morphological condition factors, and indices of

nutritional condition such as carbon and nitrogen composition, RNA/DNA ratios, and lipid analyses (e.g.,

Hakanson et al., 1994). Comparison of the variability of

indices (with different time scales) among fish in a school

or sample should also identify how long the school remained together within the same feeding conditions.

Although the ability to detect the condition of individual larvae is reasonably well developed-e.g., by

means of the biochemical indices mentioned abovethe ability to detect food limitation of larval and juvenile fish populations and its effect on survival remains

an active problem in fisheries research (e.g., Mullin,

1993). It is still difficult to determine the effective prey

concentration for larval and juvenile fishes, considering large-scale processes such as vertical migration, and

small-scale processes such as turbulence and predatorprey contact rates. The problem can be expressed in

terms of the carrying capacity of a particular area, such

as the Bering Sea, for juvenile pollock. However, if the

long juvenile period of walleye pollock does represent

several distinct stages, then perhaps there should be

several carrying capacities. The large larval andjuvenile

populations ofwalleye pollock in the GulfofAlaska and

the Bering Sea might make it possible to detect foodlimitation effects and carrying capacity limits in these

areas. Population-level behavioral responses may also

be a valuable and novel method for assessing carrying

capacity and food-limitation concepts. For example,

Olla et al. (1996) noted that school dynamics ofjuvenile walleye pollock will change if they are under stress

from lack of food, so that the interfish distance becomes smaller. Unfortunately, a similar response can

also be elicited in the presence of predators engaged in

active predation, although it was observed that the effect

did not persist long after the predator had departed.

A program to investigate the ecology and growth of

juvenile walleye pollock should consider several approaches. It should first recognise that there may be

several stages within the juvenile phase, possibly each

with different sampling requirements. The distributions

of these stages should be mapped to identify contrasting areas of abundance. It would be particularly useful

for study of carrying capacity problems if these areas

persisted over several years. Condition and growth characteristics offish from each contrasting area could then

be compared with several indices having different time

scales of response to food stress. These include:

• behavioral:

small-scale fish distributions (interfish distances)

• otoliths:

age confirmation

hatch-date analysis (also for confirmation of stock

origin)

elemental analysis

time history of growth, for comparison with oceanographic and meteorological events

• feeding success:

stomach fullness and contents identification (short

timescale)

biochemical: RNA/DNA, proteins, lipids (intermediate timescale)

morphometries, e.g., condition factor (longer

timescale)

stable isotope composition of various body organs,

for possible identification of trophic position and

geographic location offeeding
114 NOAA Technical Report NMFS 126

• parasites:

stock origin

• modeling:

early life-history distributions (circulation modeling)

and growth potential (trophodynamic modeling)

sensitivities to various behavioral strategies

Literature Cited

Brown, A. L., and K. M. Bailey

1996. Growth of juvenile walleye pollock, Theragra chalcogramma, in the Gulf ofAlaska (abstract). In R. D. Brodeur et

al. (eds.), Ecology ofjuvenile walleye pollock, Theragra chalcogramma, p. 104. U.S. Dep. Commer., OAA Tech. Rep. 126.

Dwyer, D. A., K. M. Bailey, and P. A. Livingston.

1987. Feeding habits and daily ration of walleye pollock

(Theragra chalcogramma) in the eastern Bering Sea, with

special reference to cannibalism. Can.]. Fish. Aquat. Sci.

44: 1972-1984.

Hakanson,]. L., S. H. Coombs, and P. Re.

1994. Lipid and elemental composition of sprat (Sprattus

sprattus) larvae at mixed and stratified sites in the German

Bight of the North Sea. ICES]. Mar. Sci. 51:147-154.

Hinckley, S.

1987. The reproductive biology of walleye pollock, Theragra

chalcogramma, in the Bering Sea, with reference to spawning

stock structure. Fish. Bull. 85:481-498.

Houde, E. D.

1987. Fish early life dynamics and recruitment variability.

Am. Fish. Soc. Symp. 2:17-29.

Koeller, P. A., P. C. F. Hurley, P. Perley, and]. D. Neilson.

1986. Juvenile fish surveys on the Scotian Shelf: implications for

year-<:lass assessments.]. Cons. Int. Explor. Mer 43:59-76.

Merati, N., and R. D. Brodeur.

1996. Feeding habits and daily ration ofjuvenile walleye pollock, Theragra chalcogramma, in the western Gulf of Alaska.

In R. D. Brodeur et al. (eds.), Ecology of juvenile walleye

pollock, Theragra chalcogramma, p. 65-79. U.S. Dep.

Commer., NOAA Tech. Rep. 126.

Morado,]. F., and D. A. McFee.

1996. Diseases and parasites of juvenile walleye pollock,

Theragra chalcogramma, from the Gulf of Alaska, 1986-1988.

In R. D. Brodeur et al. (eds.), Ecology of juvenile walleye

pollock, Theragra chalcogramma, p. 89-103. U.S. Dep.

Commer., NOAA Tech. Rep. 126.

Mullin, M. M.

1993. Webs and scales. Physical and ecological processes in

marine fish recruitment. Wash. Sea Grant Program, Univ.

Washington Press, Seattle, 135 p.

Norcross, B. L., K. P. Severin, and]. Carroll.

1996. Electron microprobe analysis of otoliths of juvenile

walleye pollock, Theragra chalcogramma, from Alaska: a pilot

stock separation study (abstract). In R. D. Brodeur et al.

(eds.), Ecology of juvenile walleye pollock, Theragra

chalcogramma, p. 109. U.S. Dep. Commer., NOAA Tech.

Rep. 126.

Nishimura, A., K. Mito, and T. Yanagimoto.

1996. Hatch date and growth estimation ofjuvenile walleye

pollock, Theragra chalcogramma, collected in the Bering Sea

in 1989 and 1990. In R. D. Brodeur et al. (eds.), Ecology of

juvenile walleye pollock, Theragra chalcogramma, p. 81-

87. U.S. Dep. Commer., NOAA Tech. Rep. 126.

Olla, B. L., and M. W. Davis.

1990. Behavioural responses ofjuvenile walleye pollock

Theragra chalcogramma Pallas to ligh t, thermoclines and food:

possible role in vertical distribution. ]. Exp. Mar. BioI.

Ecol. 135:59-68.

Olla, B. L., M. W. Davis, C. H. Ryer, and S. M. Sogard.

1996. Distribution, recruitment, and survival ofjuvenile walleye pollock, Theragra chalcogramma: what role does behavior

play? (abstract). In R. D. Brodeur et al. (eds.), Ecology of

juvenile walleye pollock, Theragra chalcogramma, p. 108. U.S.

Dep. Commer., NOAA Tech. Rep. 126.

Olla, B., M. Davis, and S. M. Sogard.

1995. Behavioral responses of larval and juvenile walleye pollock (Theragra chalcogram1lUl): possible mechanisms controlling

distribution and recruitment. ICES Mar. Sci. Symp. 201:3-15.

Rothschild, B.]., and T. R. Osborn.

1988. Small-scale turbulence and plankton contact rates.

]. Plankton Res. 10:465-474.

Ryer, c., and B. Olla.

1992. Social mechanisms facilitating exploitation of spatially

variable ephemeral food patches in a pelagic marine

fish. Anim. Behav. 44:69-74.

Sakurai, Y.

1993. Reproductive characteristics and spawning strategies

of walleye pollock, Theragra chalcogramma. Sci. Rep.

Hokkaido Fish. Exp. Sta. 42:51-68.

Sinclair, M. M.

1988. Marine populations. Washington Sea Grant Program,

Univ. Washington Press, Seattle, 252 p.

Sogard, S. M., and B. L. Olla.

1993. Effects of light, thermoclines, and predator presence

on vertical distribution and behavioral interactions ofjuvenile walleye pollock, Theragra chalcogramma Pallas. ]. Exp.

Mar. BioI. Ecol. 167:179-195.

Temnykh, O. S., and V. 1. Radchenko.

1996. The growth of walleye pollock, Theragra chalcogramma,

and structure ofscales during the first year of life (abstract).

In R. D. Brodeur et al. (eds.), Ecology of juvenile walleye

pollock, Theragra chalcogramma, p. 105-107. U.S. Dep.

Commer., NOAA Tech. Rep. 126.

Tyler,]. A. and K. A. Rose.

1994. Individual variability and spatial heterogeneity in fish

population models. Rev. Fish BioI. Fish. 4:91-123.
Interdecadal Comparisons ofWalleye Pollock, Theragra chalcogramma,

Cannibalism in the Eastern Bering Sea

PATRICIA A. LIVINGSTON and GEOFFREYM. LANG

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

ABSTRACT

Diet composition and cannibalism of waIleye poIlock, Theragra chalcogramma, in the

eastern Bering Sea were compared over a series of years in the 1970's and the 1980's. No

significant interdecadal differences in diet composition were found. The contribution of

copepods to the diet, as measured by percent weight ofstomach contents, was related more

to predator size than to year. The percent weight of euphausiids in the diet was relatively

constant by year for the 1985-89 period and was similar between the two walleye pollock size

groups «40 cm and ~40 cm) over all the years. Amphipods contributed little to the diet

«4% by weight) for most years regardless of size or year, except for 1974. There was a

definite trend in the percent weight of walleye poIlock in the diet by predator size and by

year. Cannibalism rates were highest in larger walleye poIlock, and rates for both size groups

showed similar declines from 1971 to 1989. In the 1985 to 1989 period, the weight percentage of pollock in the diet was highly correlated (r2=0.99) with an index of age-O and age-1

walleye pollock biomass in each year. This evidence suggests that walleye pollock year-class

strength is currently the main factor that determines the level of cannibalism during

summer. The lack offit ofthe 1970's cannibalism rates to the biomass index ofage-O to agel pollock may be due to several factors, including low sample sizes for some years and

incomplete knowledge about the absolute level of pollock biomass during that period.

Introduction

Walleye pollock, Theragra chalcogramma, is a dominant

component of the groundfish complex in the eastern

Bering Sea (Bakkala, 1993). The large biomass of walleye pollock relative to that of other groundfish species

in the eastern Bering Sea makes pollock important as a

predator on populations such asjuvenile walleye pollock,

and as a competitor with predator populations such as

marine birds and northern fur seals, Callarhinus ursinus.

The food habits, including cannibalism, of walleye

pollock in the eastern Bering Sea have been studied

frequently (Mito, 1974; Livingston et al., 1986, 1993;

Dwyer et al., 1987; Livingston, 1989, 1991, 1993; Mito1).

1 Mito, K-l. 1990. Differences in the weight and composition ofstomach content of pollock between areas in the Bering Sea. Document

submitted to the Annual Meeting of the International North Pacific

Fisheries Commission, Vancouver, Canada, October 1990,

17 p. Fisheries Agency ofJapan, National Research Institute of Far

Seas Fisheries, 7-1 Orido 5 chome, Shimizu, Shizuoka,Japan 424.

Our intent is not to provide a detailed description of

the diet of walleye pollock, but to compare diet composition from two separate time frames. Inspection of

patterns in walleye pollock prey consumption over several years may yield important information on how

pollock use the prey resources available to them. We

examined data obtained during May through September on the eastern Bering Sea continental shelf for two

periods (1970, 1971, and 1974; and 1985 through 1989)

to determine predator size and year-related patterns in

diet composition.

Materials and Methods

A total of 18,422 walleye pollock stomachs were analyzed from samples collected in 1970, 1971, 1974, and

1985-89 (Table 1). These years were analyzed because

they were the only years in which pollock stomach

samples were obtained over a broad area on the eastern

115
116 NOAA Technical Report NMFS 126 . _

Table I

Number ofstomachs collected, number of hauls, and

sampling dates for walleye pollock, Theragra chalcogramma,

collections in the eastern Bering Sea.

Sampling No. No.

Year dates hauls stomachs

1970 7/2-8/2 66 1,678

1971 6/1-9/9 52 1,684

1974 5/5-8/28 116 5,514

1985 6/8-9/30 103 965

1986 5/2-9/30 236 1,927

1987 5/4-9/30 167 2,119

1988 5/2-9/29 218 2,002

1989 6/6-9/29 225 2,533

Total 18,422

Bering Sea shelf during a similar time of year. Samples

from 1970, 1971, and 1974 were collected by Japanese

scientists aboard research vessels; samples from 1985

through 1989 were collected by Alaska Fisheries Science Center (AFSC) scientists. Although data collected

byJapanese and AFSC scientists were available for other

years, only samples from the years presented here were

taken on the shelf during the same time period (May

through September) as our samples from the 1980's.

Sample collection and analysis procedures employed

by the Japanese scientists is described by Mito!; AFSC

scientists used the procedures described by Livingston

(1991).

Diet composition was determined for two size groups

of walleye pollock: <40 cm and ~40 cm. These size

groups were used because previous research on walleye

pollock diet in the eastern Bering Sea showed a sharp

increase in the amount of cannibalism by fish larger

than 40 cm (Dwyer et aI., 1987). We considered the

following five prey categories which together constituted almost 80% ofthe diet by weight over all the years

considered: walleye pollock (10%); copepods (32%);

euphausiids (30%); capelin, Mallotus villosus (1 %); and

amphipods (5%). A sixth category containing miscellaneous and unidentified prey was also used. Diet composition was determined separately for each size group for

six geographic strata. Six strata were defined by depth

zone «50 m, 50-100 m, and 100-200 m) and area

(north of the Pribilof Islands and south of the Pribilof

Islands). In each stratum, percent weight of each prey

species in the diet of each size group of pollock was

calculated as the average of the percentages at all sampling stations. The percent contribution of each species to the diet of each size group was then estimated

for the whole area as the weighted average of the stratum means, with weights being the biomass of the size

group in each stratum. Variation in diet composition in

terms of percent weight was calculated from the between-station variance in diet composition for a particular predator size group in a stratum.

Calculation of prey biomass consumed (see Livingston, 1991 for details) required predator biomass estimates by size and stratum in the eastern Bering Sea,

which were not available for 1970, 1971, and 1974.

Walleye pollock biomass estimates by age from the cohort analysis of Wespestad and Dawson2 were partitioned into strata for 1970, 1971, and 1974, based on

the average proportion of the biomass at age in each

area, determined from bottom trawl surveys during the

1985-89 period.

The index ofage-O and age-1 walleye pollock biomass

available as prey in each year was derived from cohort

analysis estimates of age-2 walleye pollock abundance,

assuming annual instantaneous mortality rates of age-1

fish to be 0.85/yr (Wespestad and Terry, 1984) and of

age-O fish to be 1.2/yr.

Results

Walleye pollock cannibalism occurred primarily in the

middle shelf (50-100 m bottom depth) and outer shelf

(100-200-m) zones in the eastern Bering Sea for all

years of our study (Fig. 1, 2). Geographic extent of

stomach sampling was least in 1974 and 1985 and most

comprehensive in 1989. Percent by weight of pollock in

the diet varied slightly by location among years, although each year showed several stations with high

(>50% by weight) levels of predation on walleye pollock.

Walleye pollock predator biomass was relatively constant throughout all years of the study (Fig. 3). The

largest biomass was seen in 1971 (11 million metric

tons [t]) and the smallest in 1989 (8 million t), wi th an

average of approximately 10 million t. The biomass by

size group was also relatively constant throughout all

years (Fig. 3).

Amphipods represented a relatively small portion of

the diet of both sizes of walleye pollock in all years

except 1974 (Fig. 4). Percent contribution to the diet

by weight ranged from 0% to 23%.

Euphausiid prey were found to be equally important

in the diet of both size groups of walleye pollock in

terms of weight (Fig. 5). However, some interannual

variation in the amount of euphausiids in the diet was

seen, particularly in 1974 relative to the other years.

2 Wespestad, V. G., and P. Dawson. 1992. Walleye pollock. In Stock

assessment and fishery evaluation report for the groundfish resources of the Bering Sea/Aleutian lslands region as projected for

1993, p. 1-1:1-32. North Pacific Fishery Management Council,

P.O. Box 103136, Anchorage, AI<. 99510.
_____ Livingston & Lang: Interdecadal Comparisons of Walleye Pollock Cannibalism in the Eastern Bering Sea 117

....

Alaska

Alaska

1974

PERCENT

(BY WEIGHT)

• < 1 • 1 -25

• 26-50 1 51-75

76-100

1971

PERCENT

(BY WEIGHT)

• < 1 • 1 - 25

• 26-50 1 51-75

76-100

200m

5000

5000

6000N

1970

PERCENT

(BY WEIGHT)

5000 < 1

· 1 -25

• 26-50 1 51 - 75

76-100

\

100m Alaska • 200m

6000N ....

6000N

18000W 17500 17000 16500 16000

Figure I

Geographic location ofwalleye pollock cannibalism in the eastern

Bering Sea by percent weight for the years 1970, 1971, and 1974.
118 NOAA Technical Report NMFS 126

6000N

5500

6000N

5500

1985

PERCENT

(BY WEIGHT)

• < I • , -25

• 26·50

• 5'-75 .76-100

1987

PERCENT

(BYWEIGHT)

• <, • , -25

• 26-50 .51-75

.76-'00

6000N

5500

1989

PERCENT

(BY WEIGHT)

• < 1 • 1-25

• 26- 50

• 51-75 • 76-100

-

1986

PERCENT

(BY WEIGHT) • <, • 1-25

• 26- 50

• 51-75 .76-100

1988

PERCENT

(BY WEIGHT)

• < 1 • , -25

• 26-50 .51-75

.76-100

-

-

180 OOW 17500 17000 165 00 16000

Figure 2

Geographic location of walleye pollock cannibalism in the eastern Bering Sea by percent weight for the years 1985

through 1989.
____ Livingston & Lang: Interdecadal Comparisons of Walleye Pollock Cannibalism in the Eastern Bering Sea 119

Although the average fraction of the diet composed of

euphausiids in the 1985-89 period was more than double

that in the 1970-74 period, there was not a significant

difference between the two means (t test, p>0.05).

The percentage of copepods in the diet of small

pollock appeared greater than in that of large pollock

for every year except 1970 (Fig 6). The average percent

by weight of copepods in the diet of small walleye

pollock over all years (43%) was significantly different

from that oflarge pollock (22%) (t test, p<0.05). Interannual changes in copepod amounts within size groups

were mostly nonsignificant.

Walleye pollock were more important in the diet of

larger pollock than in the diet of smaller ones (Fig. 7)

(t test, p<0.05). Both predator size groups showed a

similar trend in percent by weight of walleye pollock in

12

0::-

c: 1--<40 em

10 ____ >40 em

I --+- Total

III

III

'" 8 E

0

:c

79 84 89

Year

74

O+-~~~~--r-~~~~--,-~~~~~~~~~~--r-~

69

Figure 3

Walleye pollock predator biomass (million metric tons) for the

years 1970, 1971, 1974, and 1985-89. Biomass is shown for predators <40 cm and ~40 cm and for total predators.

0.3

79 84 89

Year

74

0::-

Cl

>-'0; 0.2 ..

a.~ 'C_

o .. .9- :c ~­0.0

Ec:

«.2 0.1 U

~O~~ 69

Figure 4

Fraction by weight of amphipod prey in the diet of walleye pollock by predator size group «40 cm and ~40 cm) for the years

1970, 1971, 1974, and 1985-89. Error bars show the standard

error of the mean. (Data values for large pollock were slightly

offset in the x-direction to prevent overplotting.)
120 NOAA Technical Report NMFS 126

0.6

1:

Cl

~'; 0.4

a.~

"-:= (1)

~:c

.s:.. -0

a.l:

::l 0 w., 0.2 u

74 79

Year

84 89

Figure 5

Fraction by weight of euphausiid prey In the diet of walleye

pollock by predator size group «40 cm and ~40 cm) for the years

1970, 1971, 1974, and 1985-89. Error bars show the standard

error of the mean. (Data values for large pollock were slightly

offset in the x-direction to prevent overplotting.)

0.7

Cl

'iii >.~

~~ ,,-z

0'-

a." ,,-

0. 0

o l: U 0.,

u

0.6

0.5

0.4

0.3

0.2

0.1

1

79 84 89 Year 74

O.j-~~~~.,.-~~.,.-.,.-.,.-.,.-.,.-.,.-.,.-~.,.-.,.-.,.-.,.-~

69

Figure 6

Fraction by weight of copepod prey in the diet of walleye pollock

by predator size group «40 cm and ~40 cm) for the years 1970,

1971, 1974, and 1985-89. Error bars show the standard error of

the mean.

the diet over the study period. Walleye pollock prey

appeared to be less important (<10% by weight) for

both size groups in the 1985-89 period of this study,

but there was not a significant difference between the

average contributions of pollock to the diet in the 1970-·

74 period and the 1985-89 period. The contribution of

walleye pollock in the diet by weight (40% for predator

size groups ;:::40cm, and 12% for groups <40cm) was

highest in 1971. There were, however, large variance

estimates associated with pollock diet fractions, particularly for 1971. The percent by weight of walleye pollock

in the diet for the 1985-89 period was positively correlated (r=0.99) with the biomass index ofage-O and agel walleye pollock present for a given year (Fig. 8). But
____ Livingston & Lang: Interdecadal Comparisons of Walleye Pollock Cannibalism in the Eastern Bering Sea 121

Figure 7

Fraction by weight of walleye pollock prey in the diet of walleye

pollock by predator size group «40 cm and ~40 cm) for the years

1970, 1971, 1974, and 1985-89. Error bars show the standard

error of the mean.

40

35

5,000 6,000

\

74

70

!86

71

1,000 2,000 3,000 4,000

AgeO+1 Biomass Index (1000 ml)

88 !f !87

0+----=-.,-----,..------,-----.,...----,----,

o

;:;- 30

Cl

'0;

'" 25 Gl J:l '"

Q.- g:5 20

~o

C 15 Gl

Gl

E: 10

Figure 8

Percent by weight ofwalleye pollock in the diet ofwalleye pollock

versus the biomass index of age-O+1 walleye pollock for the years

1970, 1971, 1974, and 1985-89. Error bars show the standard

error of the mean.

when all years were considered, the correlation was

much lower (r=0.16).

Walleye pollock biomass consumed by pollock was

highest (3.6 million t) in 1971 and lowest (0.2 million t)

in 1988 (Fig. 9). In general, total juvenile pollock biomass consumed through cannibalism was the lowest

from 1986 through 1989. This trend parallels that of

the percent by weight of walleye pollock in the diet

(Fig. 7).

Copepods and euphausiids together constituted over

50% of the prey biomass consumed by walleye pollock

during all years except 1974, when they made up only

31 % of the biomass consumed (Fig. 9). In 1974, amphipod consumption was highest (23% of prey biomass)
122 NOAA Technical Report NMFS 126

16

z- 14 r::

o

== 12

I

"0 10 Gl

§ 8 I/)

r::

8 6 I/)

I/)

4 o

iii 2

o

1970 1971 1974 1985

Year

1986 1987 1988 1989

IlID Walleye pollock 11II Copepod • Amphipod Euphausiid 0 MiscellaneousI

Figure 9

Walleye pollock, copepod, amphipod, euphausiid, and miscellaneous prey

biomass (million metric tons) consumed by walleye pollock for the years

1970,1971,1974, and 1985-89.

and euphausiid consumption was lowest (6% of prey

biomass) compared with other years. Capelin made up

less than 1% of the prey biomass consumed in all years.

Discussion

Estimates of total walleye pollock predator biomass and

the biomass in each size group «40 cm and ~40 cm)

were relatively constant throughout all years of the

study. Therefore, observed changes in diet composi-·

tion probably reflect the availability or abundance of

prey. Similarly, environmental indices of sea-surface

temperature (Decker et aI., 1995) and February ice

extent (Overland and Pease, 1982; Wilderbuer3) show

that climate during all the years in this study was similar, with below-average sea-surface temperature and average extent ofsea ice. Therefore, observed changes in diet

composition were probably not due to changes in production timing or environmental effects on prey distribution.

There appeared to be no size-related differences in

the consumption of euphausiids. Euphausiids constituted relatively large fractions of the diet for both size

3 Wilderbuer, T. K. 1993. Alaska Fish. Sci. Cent., Nat!. Mar. Fish.

Cent., NOAA, 7600 Sand Point Way N.E., Seattle, WA 98115-0070.

Personal commun.

groups during all the study years except 1974. A large

percentage of amphipods was noted in the diet during

that year; they may actually have been euphausiids,

although that cannot be confirmed.

The greatest differences in diet by predator body size

were seen for copepod and walleye pollock prey. Copepods were a more important item in the diet ofsmaller

walleye pollock, whereas juvenile pollock prey were

more important to larger walleye pollock. Thus small

walleye pollock relied primarily on copepods and euphausiids, whereas larger pollock mainly consumed juvenile pollock and euphausiids. In the years when cannibalism was low (1986-89), euphausiids and copepods

increased in the diet of larger walleye pollock. Examination of the raw data suggests that other prey, such as

shrimp, may also be important during some years.

The most obvious dietary trend implied by these data

is that cannibalism rates were size-dependent, with large

(~40 cm) walleye pollock being more cannibalistic. The

trend toward cannibalism was the same for both size

groups during the two time periods. Although capelin

may have been more abundant in the early 1970's than

in the 1980's (Fritz et aI., 1993), and are consumed by

walleye pollock in the Gulf ofAlaska (Yang, 1993), they

constituted less than 1% ofthe diet by weight during all

the study years. These data contrast with those ofDecker

et ai. (1995), who found a decline in capelin and

hyperiid amphipod consumption by marine birds nest-
____ Livingston & Lang: Interdecadal Comparisons of Walleye Pollock Cannibalism in the Eastern Bering Sea 123

ing on the Pribilof Islands from the pre-1978 period to

the post-1978 period.

Because cannibalism of walleye pollock was highly

correlated with the index ofage-O and age-l abundance

for the 1980's period, it appears that cannibalism rates

during summer were mostly related to the abundance

ofjuvenile walleye pollock in that period. Previous analyses have shown that the main ages cannibalized are ageoand age-l fish, based on prey size frequencies from

the 1980's (Dwyer et al. 1987; Livingston et aI., 1993).

However, no prey size data were available for the 1970's

data considered in this study. It is possible that predation

on older age groups of walleye pollock could have been

the cause of the higher-than-expected percent weight of

walleye pollock in the diet for 1971 relative to age-O and

age-l abundance, but we have no data to support this

contention. The lack of correlation between pollock cannibalism rates and the age-O-l pollock biomass index in

the 1970's could also be due to uncertainty in pollock

biomass estimates from periods prior to the initiation of

shelfwide bottom trawl and hydroacoustic surveys in 1979.

The relation between the index of age-O and age-l

biomass and percent weight in the diet can also be

thought of as a feeding response curve. Within the

range of prey biomass observed while our stomach sampling program has been in effect, the predator response

is in the portion of the curve where prey intake per

predator increases with increasing prey biomass. A nonlinear increase in amount consumed per predator with

increasing prey biomass would indicate that compensatory density-dependent mortality was occurring independent of changes in predator biomass. Unfortunately,

there are not enough data points to sufficiently distinguish whether this is a linear or nonlinear relationship.

Also, we do not have samples from years with very high

prey densities, such as 1978-79 or 1982-83, when large

biomasses of age-O and age-l walleye pollock were

present. Thus we cannot determine at what prey density the predator response curve will level off.

The walleye pollock population as a whole consumes

more euphausiids and copepods than juvenile pollock

prey. Walleye pollock may adjust their diet in the absence of abundant juvenile pollock prey by increasing

the consumption of copepods and euphausiids. This

strategy could make the energy intake of adult pollock

less subject to the interannual variability seen in populations of planktivorous and piscivorous birds in the

eastern Bering Sea that rely heavily on the production

of either copepods or juvenile walleye pollock in a

given year (Springer et aI., 1986). However, the effect

of cannibalism on the overall population dynamics of

walleye pollock in the Bering Sea is a topic of continuing research. A better understanding of adult pollock's

functional feeding response at differing prey densities

is required to make advances in this area.

Acknowledgments _

We are especially grateful to Kei-ichi Mito of the Japanese National Research Institute of Far Seas Fisheries

for so graciously providing us with the groundfish food

habits data collected by their scientists in the eastern

Bering Sea. We also appreciate the helpful comments

provided by the two anonymous reviewers.

Literature Cited

Bakkala, R. G.

1993. Structure and historical changes in the groundfish complex of the eastern Bering Sea. U.S. Dep. Commer., NOAA

Tech. Rep. NMFS 114, 91 p.

Decker, M. 8., G. L. HuntJr., and G. Vernon ByrdJr.

1995. The relationships among sea-surface temperature, the

abundance of juvenile walleye pollock (Theragra chalcogramma) and the reproductive performance and diets of

seabirds at the Pribilof Islands, southeastern Bering Sea. In

R.]. Beamish (ed.), Climate change and northern fish populations, p. 425-437. Can. Spec. Pub. Fish. Aquat. Sci. 121.

Dwyer, D. A., K M. Bailey, and P. A. Livingston.

1987. Feeding habits and daily ration of walleye pollock.

(Theragra chalcogramma) in the eastern Bering Sea, with

special reference to cannibalism. Can.]. Fish. Aquat. Sci.

44:1972-1984.

Fritz, L. W., V. G. Wespestad, and]. S. Collie.

1993. Distribution and abundance trends of forage fishes in

the Bering Sea and Gulf of Alaska. In Is it food?: addressing

marine mammal and seabird declines: workshop summary,

p.30-44. Alaska Sea Grant Report 93-01. Alaska Sea Grant

College Program, Univ. Alaska Fairbanks, 59 p.

Livingston, P. A.

1989. Interannual trends in walleye pollock, Theragra

chalcogramma, cannibalism in the eastern Bering Sea. In

Proc. Int. Symp. BioI. Manage. Walleye Pollock, p. 275-

296. Alaska Sea Grant Rep. 89-1, 789 p.

1991. Groundfish food habits and predation on commercially important prey species in the eastern Bering Sea from

1984 to 1986. U.S. Dep. Commer., NOAA Tech. Memo.

NMFS F/NWC-207, 240 p.

1993. The importance of predation by groundfish, marine

mammals and birds on walleye pollock Theragra chalcogramma

and Pacific herring Clupea paUasi in the eastern Bering

Sea. Mar. Ecol. Prog. Ser. 102:205-215.

Livingston, P. A., D. A. Dwyer, D. L. Wencker, M.-S. Yang, and

G. M. Lang.

1986. Trophic interactions of key fish species in the eastern

Bering Sea. [nt. N. Pac. Fish. Comm. Bull. 47:49-65.

Livingston, P. A., A. Ward, G. M. Lang, and M.-S. Yang.

1993. Groundfish food habits and predation on commercially important prey species in the eastern Bering Sea from

1987 to 1989. U.S. Dep. Commer., NOAA Tech. Memo.

NMF$-AFSC-11, 192 p.

Mito, K-l.

1974. Food relationships among benthic fish populations in

the Bering Sea. M.S. thesis, Hokkaido Univ., Hokkaido,

Japan, 135 p.

Overland,]. E., and C. H. Pease.

1982. Cyclone climatology of the Bering Sea and its relation

to sea ice extent. Monthly Weather Rev. 110:5-13.
124 NOAA Technical Report NMFS 126

Springer, A. M., D. G. Roseneau, D. S. Lloyd, C. P. McRoy, and

E. C. Murphy.

1986. Seabird responses to fluctuating prey availability in the

eastern Bering Sea. Mar. Ecol. Prog. Ser. 32:1-12.

Wespestad, V. G., and]. M. Terry.

1984. Biological and economic yields for eastern Bering Sea

walleye pollock under differing fishing regimes. N. Amer.

]. Fish. Manage. 4:204-215.

Yang, M.-S.

1993. Food habits of the commercially important fishes in

the Gulf of Alaska in 1990. U.S. Dep. Commer., NOAA

Tech. Memo. NMFS-AFSC-22, 150 p.
Changes in the Distribution and Size ofJuvenile Walleye Pollock,

Theragra chalcogramma, as Indicated by Seabird Diets

at the Pribilof Islands and by Bottom Trawl Surveys

in the Eastern Bering Sea, 1975 to 1993

GEORGE L. HUNTJR., ALEXANDER S. KITAYSKY, MARY BETH DECKER

Department ofEcology and Evolutionary Biology

University ofCalifornia

Irvine, Califormia 92697

DONALD E. DRAGOO

Alaska Maritime National Wildlife Refugee

United States Fish and Wildlife Service

Homer, Alaska 99603

ALAN M. SPRINGER

Institute ofMarine Science

University ofAlaska

Fairbanks, Alaska 99775

ABSTRACT

We tested whether the proportion of age-l walleye pollock, Theragra chalcogramma, in

the diets of four species of seabirds-black-legged kittiwake, Rissa tridactyla; red-legged

kittiwake, R brevirostris, common murre, Uria aalge; and thick-billed murre, U. lomviadecreased between the 1970's and the 1980's by examining otoliths present in food samples

obtained from birds breeding on the Pribilof Islands. We examined the distribution of agel walleye pollock on the Bering Sea shelf over the same time period to determine if the

changes in age classes of pollock taken by birds were reflected in data from National Marine

Fisheries Service bottom trawl surveys. We examined the growth rates and sizes of age-O

pollock taken by the birds, and we sought evidence for mechanisms that might have

influenced the distribution and abundance ofjuvenile pollock in the vicinity of the Pribilof

Islands.

We found that the proportion of age-l walleye pollock in seabird diets decreased

significantly from the 1970's to the 1980's. Over the same period, age-l walleye pollock

declined in trawl survey catches in the vicinity of the Pribilof Islands and also in the

southern portion of the shelf edge. Although age-O pollock taken near the end of August

were longer in the 1970's than in the 1980's, growth rates ofage-O pollock in August were

similar in the two periods. We found no significant correlation between the abundance of

. age-l walleye pollock near the PribiiofIslands and in the southern outer domain (strata 32,

42, and 50) and the extent of ice cover along the 1700

W meridian. Likewise, there were no

significant correlations between the number of age-l pollock in strata 32, 42, and 50 and the

number of adult pollock present in these strata in the same or preceding year. We discuss

additional evidence for both interannual and interdecadal changes in the marine environment in the vicinity of the Pribilof Islands. Juvenile walleye pollock were an important

component of the diets of breeding seabirds, but-<:ontrary to our expectations-seabird

reproductive success was not sensitive to the ratio of age-l to age-o pollock in the seabirds' diets.

125
126 NOAA Technical Report NMFS 126

Introduction

The number of young fledged annually by marine birds

breeding on the Pribilof Islands has fluctuated considerably since studies were initiated there in 1975 (Byrd,

1989; Schulmeister, 1991; Springer, 1992; Decker et al.,

1995). Additionally, since the 1970's, breeding populations of some seabird species, particularly black-legged

and red-legged kittiwakes (Rissa tridactyla and R.

brevirostris) , at the Pribilofs have declined (Springer,

1992). The start of the decline in the production of

young coincided with a period ofincreasing sea-surface

temperatures and the maturation ofthe large 1978 year

class ofwalleye pollock, Theragra chalcogramma (Springer,

1992; Decker et al., 1995). Explanations for the decrease in kittiwake reproduction during the 1980's have

focused on two hypotheses (Springer, 1992). The first

emphasized competition between seabirds and adult

pollock for forage fish, in particular age-O and age-l

walleye pollock. The second suggested thatseabird population biology was reflecting changes in prey availability

driven by changes in ocean temperatures. Changes in

the species composition ofseabird prey coincided with

the warming ofsurface waters (Decker et al., 1995), but

the diets ofsome seabirds did not return to their former

species composition when sea-surface temperatures decreased in the mid-1980's. Decker et al. hypothesized that

the birds' diets were indicating a "regime" change in the

southeastern Bering Sea that did not track sea-surface

temperatures. This concept of a regime shift has been

developed independently by others in a number of parallel studies (Beamish, 1995; papers in this symposium).

Throughout the period 1975-90, walleye pollock was

an important prey ofseabirds breeding on the Pribilof

Islands (Decker et al., 1995). Variations in the occurrence and percentage composition of pollock in seabird diets between the 1970's and the 1980's were not

consistent among bird species, or between birds breeding on St. Paul and St. George Islands. Although there

were significant correlations between estimates of abundance of age-l walleye pollock on the eastern Bering

Sea shelf and their presence in seabird diets in two of

six cases examined, Decker et al. (1995) found no

statistically significant positive correlations between seabird production of young and estimated abundance of

age-l pollock either in seabird diets or on the eastern

Bering Sea shelf.

Past examinations of the role of walleye pollock in

the ecologies of Pribilof Islands seabirds relied on aggregated shelfwide estimates of the abundance of age-l

pollock (Springer, 1992; Decker et al., 1995) and did

not differentiate between seabirds' use of age-O and

age-l pollock. We assumed that age-l pollock would be

ofgreater importance to seabirds because ofthe greater

amount of energy contained in each fish, and because

early in the reproductive season, age-O fish might not

be suitable prey because of their small size and their

lack of schooling (Sogard and Olla, 1993). We also

assumed that the abundance of age-l pollock near the

breeding colonies would be of greater importance to

nesting seabirds than would abundances at greater distances from the colonies. Starting with these assumptions, we tested the hypothesis that the production of

young by seabirds on the Pribilof Islands would vary

with the availability of age-l pollock near the islands.

We examined walleye pollock otoliths present in food

samples obtained from birds, and the distribution and

abundance of age-l walleye pollock on the eastern

Bering Sea shelf, using data from National Marine Fisheries Service bottom trawl surveys. We present evidence

that a shift in the distribution and abundance of age-l

pollock was reflected in the diets of four species of

seabirds, and that age-l pollock taken by seabirds in the

1980's differed in size from those taken in the 1970's.

Contrary to our hypothesis, we found no evidence that

variations in the ratio of age-l to age-O pollock in the

diets of seabirds affected seabird reproductive success

on the Pribilof Islands during the study period.

Methods

We investigated the use of walleye pollock by seabirds

nesting on the Pribiloflslands, Bering Sea (Fig. 1) from

1975 to 1979, from 1985 to 1989, and in 1992 and 1993.

During these periods, we and others also gathered information on the production of young by seabirds nesting on the Pribilof Islands (Byrd, 1989; Springer, 1992;

Climo, 1993; Dragoo and Sundseth, 1993; Decker et al.,

1995).

We obtained data on the age classes and sizes of

walleye pollock consumed by seabirds breeding on the

Pribilof Islands by extracting otoliths from samples of

food brought to the colonies on St. Paul and St. George

Islands. Black-legged and red-legged kittiwakes feed

their young by regurgitation. For these species we relied primarily on food regurgitated for chicks to provide samples of what kittiwakes were eating during the

chick-rearing phase of the breeding cycle. In contrast,

thick-billed murres, Uria lomvia, and common murres,

U. aalge, provision their chicks by bringing them single,

large items of prey in their bills. Adult murres themselves eat a variety of prey, including items considerably

smaller than those generally fed to chicks (Hunt et al.,

1981b). For these two species we collected adults near

the colonies to obtain stomach contents, and also examined the prey brought to young. Before chicks

hatched, we collected adults of all four species near the

colonies to obtain food samples. Immediately after collection, we placed food samples in 80% ETOH for
___ Hunt et a1.: Changes in the Distribution and Size ofJuvenile Walleye Pollock in the Eastern Bering Sea 127

64 OON

63 OON

62 OON

ALASKA

61 DON

615

60 OON "="""'"

59 OON 41

58 OON

57 OON

56 OON

55 OON

54 OON - 178 OOW 174 OOW 170 OOW 166 OOW 162 00 158 OOW

Figure 1

The eastern Bering Sea, showing St. Paul and St. George, the largest of the Pribilof Islands, and the strata (numbered

segments) used for assesssing age-l walleye pollock abundance. The outer domain lies between the 200- and lOo-m

isobaths, the middle domain between the 100- and So-m isobaths, and the inner domain shoreward of the So-m isobath.

preservation until they could be examined in the laboratory. By sampling at the islands, we avoided the potential bias that occurs when birds are collected while

they are foraging at aggregations of specific types of

prey. Thus our samples are likely to be representative of

the prey taken by populations ofseabirds nesting at the

Pribilof Islands.

As parts of three separate studies, otoliths were measured to obtain estimates of the sizes of pollock taken

by seabirds. In each study, otoliths were present in food

samples as loose, separate items, and also embedded in

the cranial tissue ofjuvenile pollock, from which they

were extracted. The precision of measurements varied

between studies: from 1975 to 1979, we either measured all otoliths present, or when the numbers of

otoliths of similar size were high, we measured 20 of

representative size, which were chosen haphazardly.

For samples obtained from 1985 to 1989, we measured

all otoliths present. From 1975 to 1979 and from 1987

to 1989, we measured the long axis of otoliths to the

nearest 0.1 mm. Otoliths obtained in 1985 and 1986

were measured to the nearest 0.01 mm. To derive pollock body lengths from otolith lengths, we used the

relationships described by Nishimura and Yamada

(1988): for age-O pollock,

loge otolith length (mm) =

-5.808 + 1.622 loge body length (mm),

and for age-l pollock,
Figure 2

The timing of collections and the number of food

samples available for comparison between the periods

1975-79 and 1985-89.

ence or absence of otoliths of an age class in a stomach

or regurgitated food as our dependent variable, since

individual fish collected by a bird may have come froni

the same size- or age-specific school. Thus we assigned a

degree of freedom to each sample, rather than to each

fish in a sample. We compared the frequency of occurrence of age classes in food samples between the two

decades using a chi-square test for independence.

128 NOAA Technical Report NMFS 126

loge otolith length (mm) =

-1.597 + 0.722 loge body length (mm).

With these equations, an otolith measurement precision of 0.1 mm yields an error in estimating the body

length of age-O pollock of ±4.43 mm.

We estimated the lengths of 5,496 (84% of 6,559)

fish on the basis of measured otolith lengths. The remaining 16% of estimated fish lengths were based on

otoliths whose length was assigned from measurements

of representative otoliths in the sample. We used fish

lengths based on these assigned otolith lengths only to

categorize pollock to age class. In addition to estimating fish lengths from otolith measurements, we measured 73 whole pollock to obtain standard lengths.

Our estimates of fish length based on otoliths are only

approximations. Loose otoliths were exposed to the birds'

stomach acids before collection and preservation in ethanol, and were eroded to varying extents. Otoliths in bird

stomachs that remained imbedded in cranial tissue until

we extracted them were less eroded. We were unable to

estimate the extent to which erosion had taken place. We

therefore assumed that there was no interannual variation

in the extent of erosion before preservation in ethanol for

otoliths of a given size obtained from bird stomachs. All

otoliths were measured within a few months of collection,

except otoliths from 1987, 1988, and 1989, which were

washed in the year of collection and then stored in ethanol until they were measured in 1992.

We determined the age class of a fish based on the

length of its otolith, and on the distribution of otolith

lengths obtained in the same time period. During the

months when we collected otoliths from seabird stomachs, there was a nonoverlapping bimodal distribution

of estimated fish sizes (Hunt et aI., 1981b, Fig. 38), and

we assumed that fish with estimated lengths less than

100 mm in August were age 0 (Nishimura and Yamada,

1988). Since fish were growing throughout the sampling period, there was no single otolith size that we

could use as a criterion for an age class.

To estimate the number of fish represented by our

sample, for all years except 1985 and 1986, we matched

otoliths of similar size, and assumed that each pair of

similar-sized otoliths was contributed by a single fish.

For 1985 and 1986, we halved the total number of

otoliths measured.

To compare the frequency of seabird use of age-O

and age-l pollock between the 1970's and the 1980's,

we restricted our analyses to the period between 20July

and 20 August, a period during which we had a similar

distribution ofsampling effort in the two decades (Fig.

2). For this comparison, we used data on fish lengths

derived from both measured and estimated otolith

lengths, and from measurements ofwhole fish. To avoid

the possibility of pseudoreplication, we used the pres100

80

60

40

20

If)

(1)

a.

E

o

(f) 70

-o

L... 60

(1)

..c

E

50

z

40

30

20

10

1975-1979

1985 -1989

1July 20July 9Aug 29Aug
___ Hunt et at.: Changes in the Distribution and Size ofJuvenile Walleye Pollock in the Eastern Bering Sea 129

We obtained data on the abundance of age-1 pollock

on the eastern Bering Sea shelf from National Marine

Fisheries Service (NMFS) bottom trawl surveys. These

provide a useful index of year-class strength of age-1

pollock, although the strongest year classes may have

been underrepresented because a larger fraction ofthe

age-1 fish were likely to have been too far above the

bottom to be caught (Sample and Bakkala, 1989). In

addition to the traditional strata employed by NMFS,

we requested that a large stratum in the northern outer

domain be divided into five substrata at 1° latitude

intervals (strata 611 to 615, Fig. 1). This division permitted a finer resolution of the distribution of fish within

an area that we expected to be important to foraging

seabirds, as revealed by earlier studies (Hunt et aI.,

1981b) .

We estimated interannual and interdecadal differences in the size of age-O walleye pollock taken by

seabirds by using a Mann-Whitney U-test to compare

the mean size offish, per bird sampled, brought to the

colonies between 10 and 18 August. To minimize the

potential for bias in the data due to interannual differences in the dates on which samples were obtained, we

chose samples obtained between the narrowest range

of dates that would capture a sizable portion of the ageofish taken by the seabirds.

To test whether interannual variations in the sizes of

age-O pollock were caused by differences in growth

rates in August, we determined the arithmetic slope of

the regression ofthe modal size offish on date (Hilborn

and Walters, 1992). To minimize the effects of

interannual variation in sampling dates, we used only

those samples obtained between 1 and 30 August each

year. We excluded data based on estimates of otolith

length derived from representative samples. We tested

for a Pearson correlation between sea-surface temperature and growth rates of age-O pollock by using the

mean values ofAugust surface temperatures for a 5°-by10° grid surrounding the Pribilofs (unpublished NOAA

data from Dan Cayan as described in Decker et aI.,

1995).

We used otoliths collected in different date intervals

for each of our tests to extract maximum information

from our samples without introducing bias due to differences in the distribution ofsampling effort in different years. Thus, for estimates of interannual variation

in size, we selected the shortest possible period within

which we had numerous samples in each year (10-18

August). In contrast, for calculations of growth rates of

age-O fish, we sought the longest period with relatively

similar coverage in each year (1-30 August). We used

the period 20 July to 20 August to calculate the ratios

between age-1 and age-O fish in seabird diets because

there was a seasonal change in the use of the two age

classes, and inclusion of late July samples was less likely

to create bias than the inclusion oflate August samples,

in which age-1 fish were relatively rare.

To correlate interannual changes in the abundance

of age-1 pollock in the southern outer domain and

around the Pribilof Islands (strata 50, 32, and 42) with

variations in winter sea-ice cover, we obtained ice-cover

data from the NOAA/NavyJoint Ice Center. As a proxy

for total ice cover, we used data on the annual maximum latitudinal extent for the "absolute" (near-O%

concentration) ice edge along the 1700W meridian. We

used this meridian because it was close to the Pribilof

Islands, and because it gave an approximation of the

southward extent of ice over the entire eastern Bering

Sea shelf.

We also tested the hypothesis that, because of cannibalism or competition, the number of age-1 pollock

would be inversely related to the number of age-2 and

older pollock present in a stratum. We used data from

the bottom trawl surveys of strata 50, 32, and 42. We

regressed the abundance of age-1 pollock in a stratum

against the abundance of age-2 and older pollock in

that stratum in the same year and in the previous year,

because cannibalism against young pollock is primarily

directed toward age-O fish in the fall and winter (Dwyer

et aI., 1987).

We examined the importance of age-1 pollock to

successful kittiwake and murre reproduction by comparing breeding success (number of chicks fledged per

nest started), as reported in Decker et aI. (1995), with

the proportion of pollock in seabird diets that was agel. For this test, we used the number ofindividual fish of

each age class to calculate proportional use. To minimize the effect of interannual variation in the dates on

which samples were obtained, we restricted this comparison to the period 20 July to 20 August. To test for

differences in breeding success as a function of ratio of

age-1 to age-O pollock, we performed an analysis of

covariance, with breeding success as the dependent

variable and proportion of age-l pollock in the diets of

each of the four bird species on each island as the

continuous independent variable, with bird species as a

factor. We required a minimum of eight prey samples

containing pollock for a given bird species on an island

in a year before those data were included in our analysis.

All statistical analyses were performed with Systat for

Windows version 5.02 (Wilkinson et aI., 1992).

Results

The proportion of age-1 walleye pollock in the diets of

four seabird species (black-legged kittiwake, red-legged

kittiwake, common murre, and thick-billed murre) decreased from 1975-79 to 1985-89 (Fig. 3). For all four

bird species combined, 14% of 1,494 pollock in samples
130 NOAA Technical Report NMFS 126 _

collected in the 1970's were age 1, whereas in the 1980's,

less than 1% of 3,355 pollock were age 1. Furthermore,

fewer age-l pollock were found in food samples of alI

four bird species in 1985-89 than in 1975-79 (Table 1).

Although there was considerable interannual variation

in the number of age-l polIock taken by each seabird

species in the 1970's (Fig. 3), in each case there appeared to be an overalI decline in the use of age-l

polIock between 1975 and 1979. Note that the diets of

surface-foraging kittiwakes and subsurface-foraging murres

showed similar changes. Collections of food samples on

the Pribilof Islands in 1992 and 1993 showed that age-l

Table 1

Change in the occurrence of walleye pollock in seabird food samples from the PribilofIslands between 1975-1979 and

1985-1989. For all tests, df =1.

Proportion Proportion

# of samples ofsamples # of samples of samples

with pollock with age-l with pollock with age-l

Species in 1970's pollock in 1980's pollock X

2 p

Black-legged kittiwake 108 0.61 58 0.09 42.47 <0.001

Red-legged kittiwake 35 0.34 11 0.00 5.10 0.024

Common murre 34 0.65 11 0.09 10.29 0.001

Thick-billed murre 72 0.34 68 0.06 17.71 <0.001

o

1.0

# samples

# pollock

# samples

# pollock

87

11 7 4

276 444 324

t:2j Age 0

,,"~~~2LJ • Age 1

77 78 85

YEAR

76

76 77 78 85 86 87 88 89

TBMU

5 28 39 11 11 8 21 17

6 379 232 959 246 261 459 253

COMU

12 11

14 18

o

o

1.0

1.0

0.5

0.5

87

3

5

4

8

18

240

78 79 86

YEAR

77

RLKI

15 2

31 4

BLKI

27 12 25 35 9 6 22 2 25 3

55 12 91 12436 53 1606 163 9

75 76 77 78 79 85 86 87 88 89

o

0.5

1.0

L)

3 0.5

---I

o

a..

1.1..

o

Z

o

0::

o

a..

o

0::

a..

Figure 3

Proportions of age-O and age-l walleye pollock in the diets of four seabird species breeding on the Pribilof Islands. Data

from birds from both islands are combined. For 1985 and 1986, counts are of otoliths, not fish. BLKI = black-legged

kittiwake; RLKI = red-legged kittiwake; TBMU = thick-billed murre; COMU =common murre.
___ Hunt et al.: Changes in the Distribution and Size ofJuvenile Walleye Pollock in the Eastern Bering Sea 131

Table 2

Use of age-O and age-l walleye pollock by seabirds on the Pribiloflslands in the 1990's. Samples were collected between 24

June and 13 August.

Percent of # of pollock Percent of

# ofsamples samples with age 0 to pollock that

Species Year with pollock age-l pollock 2 years were age 1

Black-legged kittiwake 1992 6 0.50 35 0.11

1993 8 0.37 23 0.15

Red-legged Kittiwake 1992 2 1.00 3 No data

1993 6 0.17 19 0.26

Common murre 1992 14 1.00 103 0.91

Thick-billed murre 1992 42 0.64 404 0.17

walleye pollock had again become an important component of the diets of breeding seabirds (Table 2).

NMFS data on age-1 walleye pollock in the outer

domain indicated dramatically reduced abundance beginning in 1982 in strata 611 and 612, and beginning in

1983 in stratum 50, when compared to earlier years

(Fig. 4). In contrast, age-1 pollock did not decline in

the northern strata of the outer domain (580 to 600 N,

strata 613 to 615). In the vicinity of the Pribiloflslands

(strata 32 and 42), age-1 pollock were relatively scarce

by 1982, but showed no obvious trends in abundance in

either the southern (stratum 31) or northern part (strata

41 and 43) of the middle domain (Fig. 5). Comparison

ofFigures 4 and 5 suggests that the pattern of change in

age-1 pollock near the Pribilof Islands (strata 32, 42,

611, and 612) was more similar to the pattern in the

southern outer domain (stratum 50) than it was to the

patterns found elsewhere in the middle domain (strata

41, 43, and 31). There may have been a northward shift

in the distribution of age-1 pollock in the immediate

vicinity of the Pribilof Islands between the surveys conducted in 1975 and 1979, but the available number of

trawl samples is small (Fig. 6). We have too few samples

from birds in 1979 to determine if there was a parallel

shift in the use of age-1 pollock between birds on St.

George Island and birds on St. Paul Island.

We found that the sizes of age-O walleye pollock taken

by the four species of seabirds decreased between the

1970's (x=69.4 mm) and the 1980's (x=58.4 mm)

(Table 3). Within the 1980's, there was considerable

interannual variation in size of fish taken, with 1986

having the lowest values for any year in which there was

an adequate sample. Growth rates of age-O walleye pollock in August also showed considerable interannual

variation (Table 4). But growth rates were not consistently lower in the 1980's than in the 1970's, as might

have been expected if differences in growth rates were

Table 3

Interannual variation in the size of age-O walleye pollock as estimated from otoliths obtained from kittiwakes and murres at the Pribilof Islands, 10 to 18

August. The difference between the means for the

1970's and 1980's is statistically significant (MannWhitney U=1111, N=72, p<0.001).

# samples

for which mean

fish length Mean length ± standard

Year was estimated (mm) deviation

1976 1 35.9

1977 13 70.2 2.15

1978 12 71.3 2.04

Total 1970's 26 69.4 7.14

1986 9 45.3 2.14

1987 2 59.7

1988 17 66.50 3.31

1989 18 57.3 3.35

Total 1980's 46 58.4 8.26

responsible for the observed interannual variation in

size of age-O pollock taken by seabirds. Ofthe years with

strong year classes for which we have growth data, the

1978 year class was below average (0.70 mm/d vs. the 7-

year average of 0.95 mm/d), whereas the 1988 year

class was the median. We tested the hypothesis that

growth rates of age-O pollock in August were positively

related to August sea-surface temperature, but the relation was not significant (r=0.558, n=7, p=0.193).

We hypothesized that either variations in ice cover

over the eastern Bering Sea shelf or the number of

adult pollock present in a stratum might influence the

number of age-1 pollock present. We regressed the

abundance of age-1 pollock in strata 50, 32, and 611
132 NOAA Technical Report NMFS 126 _

combined against the extent of ice along the 1700W

meridian, but found no significant pattern (r=0.025;

n=12; p=0.939). Likewise, we found no significant relation between the number of age-l pollock in stratum

50, or in strata 32 and 42 combined, and the abundance of age-2 and older pollock in that stratum in the

2000

50

47 Strata 613, 614, and 615

50

43

49 49

51

46

48

51

94

1975 1979 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

o

500

1000

....

1500

.;;-

o

1500

15 Strata 611 and 612

1975 1979 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

1975 1979 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

17 17 15

25 27 26 26 26

Stratum 50

23

53

56

37

o

80

70

60

50

40

30

20

10

o

500

1000

...

Q)

.D

E

::J

C

"C

E

t5

UJ

....

-o

Year

Figure 4

The abundance of age-l walleye pollock in the outer domain of the eastern Bering Sea

shelf, as estimated by the bottom trawl surveys of the National Marine Fisheries Service.

Numbers at the tops of the bars are the numbers of trawls on which the estimates were

based. See Figure 1 for locations of strata.
___ Hunt et aI.: Changes in the Distribution and Size ofJuvenile Walleye Pollock in the Eastern Bering Sea 133

same year (stratum 50: r=-0.019; n=12; p=0.954; strata

32+42: r=0.267; n=12; p=0.401). Likewise, when the

number of age-l pollock was regressed against the number of age-2 and older pollock present in the previous

year, there was no significant relation for either stratum

50 or for strata 32 and 42 combined.

3000

55

Strata 41 and 43

2000

66

63

1000

o

1975 1979 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

o

500

Pribilof Strata 32 and 42

23

127

1975 1979 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

1000

C 1500

oX:

o

.Q

(5

1000

Q)

Jf

15

....

Q)

.0

E

C

"0

Q)

1a

E

w

1975 1979 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

Bristol Bay Stratum 31 800

600

400

200

o

69

68

68

68

68

Year

Figure 5

The abundance of age-l walleye pollock in the middle domain of the eastern Bering

Sea shelf, as estimated by bottom trawl surveys of the National Marine Fisheries Service.

Numbers on the tops of bars are the number of trawls on which the estimate was based.

See Figure 1 for locations ofstrata.
134 NOAA Technical Report NMFS 126

We examined the relation between the ratio ofage-1

to age-O pollock in the diets of the four species of

seabirds in the period 20 July to 20 August and the

breeding success of these species on each island (Fig.7).

A test for homogeneity of slopes showed no significant

interactions between bird species and age-1 pollock in

the diet. An analysis of covariance performed on the

data set as a whole showed no statistically significant

relation between the percentage of age-1 pollock in

bird diets and the number of chicks fledged per nest

(F2,1,17=0.183, p=0,675).

o 0 L.L--,.L-......L---,L----'-----'

o 02 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0

PROPORTION OF AGE-I POLLOCK

Walleye pollock is one of the most important prey species for seabirds breeding in the eastern Bering Sea

(Hunt et al., 1981a; Springer, 1991). At the Pribilof

Islands, age-O and age-1 pollock constituted the single

most important prey taken by surface-foraging blacklegged kittiwakes and subsurface-foraging common

murres during the 1970's (Hunt et al., 1981b).Juvenile

pollock were also important, but not quite so dominant, in the diets of red-legged kittiwakes and thickbilled murres. The proportion of age-I pollock in the

Discussion

\0 \0 BLKl TBMU

O.B 0.8 e

0.6 0.6 eo

1975 1979 00 0

i

e

0.4 , 0 0 0.4 0

8 Vl e o Sf. Poul [so Vl 0.2 0.2 e St. George Is. UJ

u

U

=oJ 0 0 Vl 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0

'"

z 1.0 1.0 0

UJ RLKl COMU UJ

0:: 08 0.8 1Il

0.6 0.6 0 e 0

0.4 0 0.4

0.2 0.2

1975 '97']

15

1000

1500 ,---~--..,---_

500

400

20 200 1975 1979

700 BO

17 70 60G

60 500 Stratum 611 50 1//) _

40

~l 30

200 20

100 10

0 ''175 '979

Year

1000

10

BOO

Stratum 612

1200 ,---~--~-_

'".Q"

o

c.

b 600

Figure 6

Abundance of age-1 walleye pollock in four strata in

the vicinity of the Pribilof Islands in 1975 and 1979.

Estimates of abundance are based on the bottom trawl

surveys by the National Marine Fisheries Service. Numbers at the tops of bars are the number of trawls on

which the estimates were based.

Figure 7

Relation between proportion of age-1 pollock in diets

and breeding success of four species of seabirds at the

PribilofIslands between 1975 and 1989. BLKI = blacklegged kittiwake; RLKI = red-legged kittiwake; TBMU = thick-billed murre; COMU = common murre.

Table 4

Growth rates of age-O walleye pollock during August based on changes in modal size of otoliths obtained from seabird

stomach contents at the Pribilof Islands. p= the significance level of the slope of the regression with respect to 0.0.

Year 1976 1977 1978 1979 1986 1987 1988

# days sampled 5 16 10 5 5 5 14

# fish sampled 8 109 285 101 321 536 186

Growth rate (mm/d) 1.04 0.77 0.70 1.57 0.32 1.33 0.93

P 0.052 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001
___ Hunt et aI.: Changes in the Distribution and Size ofJuvenile Walleye Pollock in the Eastern Bering Sea 135

diets of kittiwakes and murres breeding at the Pribilof

Islands significantly decreased between 1979 and 1985,

but increased again sometime after 1989.

The marked decrease in age-l walleye pollock in the

diets ofseabirds coincided with reduced catches of agel pollock in NMFS bottom trawl surveys near the Pribilof

Islands and along the outer shelf domain to the south

and west of the islands. We assume that the bottom

trawl samples provide valid indices ofinterannual variation in the horizontal distribution and abundance of

age-l pollock, rather than in their vertical distribution.

Because age-l pollock decreased in the diets of both

surface- and subsurface-foraging birds capable of diving to 180 m (Croll et aI., 1992), we interpret the

combined data from seabirds and from bottom trawls

as indicating a change in the horizontal distribution

and abundance of age-l pollock.

Numbers of age-l pollock may have been declining

in the eastern Bering Sea as early as 1973. Springer et

ai. (1986), Springer and Byrd (1989), and Springer

(1991) developed estimates of the abundance of age-l

pollock based on both cohort analysis and bottom trawl

surveys. They showed that, except for the 1978 year

class, the abundance of age-l pollock had a negative

slope from 1973 through 1986. These estimates were

for the eastern Bering Sea as a whole, and did not

address the question of spatial heterogeneity in the

distribution of these fish. Our analyses suggest that this

decline was particularly severe in the southern and

central portion ofthe outer domain (strata 50,611, and

612) and in the vicinity of the Pribiloflslands (strata 32

and 42). A decline in the abundance of age-l pollock in

this region would be expected to adversely affect the

breeding seabirds of the Pribilof Islands because it encompasses major portions of the foraging ranges of

these birds (Hunt et aI., 1981b; Schneider and Hunt,

1984). But despite the prominence of pollock in the

diets of murres and kittiwakes, no species showed a

significant positive correlation between the use of pollock and production of chicks, and the reproductive

success of black-legged kittiwakes on St. George Island

was negatively correlated with their use ofjuvenile pollock (DeckeretaI., 1995).

The decrease in age-l pollock in the diets ofseabirds

is only one of a number ofindications of changes in the

marine environment near the Pribilof Islands. Decker

et. ai. (1995) documented a number of other dietary

changes for Pribilof Island seabirds in the 1980's indicative of a regime change in the southeastern Bering

Sea. This change apparently affected the availability of

prey to seabirds. During the period encompassed by

our study, Coyle and Cooney (1993) found interannual

variation in water mass characteristics near the Pribilof

Islands, as well as changes in chlorophyll levels and

acoustically detected biomass. Evidence from other studies shows that the zooplankton community near the

Pribilof Islands also varied on an interannual scale

(Table 5). In some years Calanus marshallae, a middledomain species (Cooney and Coyle, 1982; Smith and

Vidal, 1984; Vidal and Smith, 1986), was an important

component of the diets of least auklets, Aethia pusilla,

nesting on the Pribilofs (e.g., 1975; Table 5). In other

years, C. marshallae was scarce, and the outer-domain

species Neocalanus cristatus and N. plumchrus (Cooney

and Coyle, 1982; Smith and Vidal, 1984; Vidal and

Smith, 1986) were most common in auklet diets (e.g.,

1978; Table 5; and 1984; Roby and Brink, 1986). Euphausiids, Thysanoessa raschii, apparently increased in

abundance near the Pribilofs between the late 1970's

and the late 1980's, as evidenced by increased observations of feeding on euphausiids by whales (Baretta and

Hunt, 1994) and short-tailed shearwaters, Puffinus

tenuirostris (Hunt et aI., 1996). The data of Coyle and

Cooney (1993) and changes in the species composition

of copepods taken by least auklets near the Pribilof

Islands provide evidence for interannual variability in the

characteristics and possibly the origin (middle vs. outer

domain) of the water surrounding the Pribiloflslands.

It would be useful to know the cause of the decrease

in age-l pollock around the Pribilof Islands and in the

Table 5

Prey of least auklets on St. Paul Island by species. Numbers are percent whole equivalents, i.e., percentages of prey

individuals, whether represented by whole organisms or their parts. Early = 1 May to 10 July; late = 15 July to 30 August.

Data from 1975, 1976, and 1979 from G. Hunt, unpublished; data from 1989 from M. B. Decker, unpublished.

Year 1975 early ]975 late 1976 late ]978 early ]978 late 1989 late

Sample size 13 27 51 5 5 27

Prey type

Calanus marshallae 85.3 73.4 21.2 0.0 4.1 0.6

Neocalanus cristatus 0.0 0.0 2.9 32.5 76.3 92.6

Neocalanus plumchrus 0.0 0.0 5.9 59.1 2.8 1.5

Parathemisto libellula 4.5 4.2 2.5 1.5 0.0 0.]
136 NOAA Technical Report NMFS 126 _

southern portion of the outer shelf. There may have

been a change in the availability of their preferred

foods. Euphausiids are an important component of the

diet of age-l walleye pollock in spring, whereas copepods dominate their diets in summer (Dwyer et aI.,

1987). In view of the observations of foraging shearwaters and whales mentioned above, it seems unlikely that

a scarcity of euphausiids was responsible for the decline

in age-l pollock abundance during the 1980's. It is

possible that copepod species composition changed or

biomass declined between the decades. Ifsuch changes

occurred, they could have profoundly influenced the

density of energy available to juvenile walleye pollock in

waters near the Pribilof Islands.

We briefly examined two other classes of non-mutually exclusive hypotheses. One group of hypotheses

focuses on the possible role of cannibalism or competition in determining the distribution and abundance of

juvenile pollock. Another group of hypotheses focuses

on physical factors that might influence the location of

young pollock during their first few mon ths after hatching or during their second year of life.

Adult pollock are a major predator on juvenile pollock, particularly age-O fish in the eastern Bering Sea

(Dwyer et aI., 1987; Bailey, 1989; Livingston, 1989;

Livingston and Lang, 1996). This source of juvenile

mortality is concentrated in the southeastern Bering

Sea shelf and slope area and takes place primarily in

autumn and winter (Dwyer etaI., 1987). Bakkala (1989)

found no correlation between the number of adult

walleye pollock on the eastern Bering Sea shelf and the

subsequent recruitment of age-l pollock. Similarly, we

found no significant relation between the abundance

of age-2 and older pollock and the abundance of age-l

fish in the same or following year for either of the two

strata surrounding the Pribilof Islands, or for stratum

50 along the outer shelf. Because age-l fish move about,

it may be inappropriate to try to explain local variations

in their abundance by means of local estimates of pollock abundance. But cannibalism is not uniformly common throughout the Bering Sea (Dwyer et aI., 1987),

and there is no strong negative relation between the

number of adult pollock and the abundance of age-l

pollock on the entire eastern Bering Sea shelf during

the same period (Springer, 1992). Therefore, it may be

useful to continue studying the possible population

effects of cannibalism at smaller spatial scales.

Physical processes that influence the recruitment and

distribution of juvenile pollock include variations in

water temperature, which may affect spawning distributions, and ice cover and associated low water temperatures, which may affect growth rates and habitat selection by young fish. Addressing the possibility of physical

influences on spawning location, Francis and Bailey

(1983) suggested that during cold-water years spawning

would be limited to the open ocean and outer shelf

waters, from which eggs and larvae would drift to the

northwest, away from the Pribilof Islands. In contrast,

in warm-water years, spawning would occur on the southeastern shelf as far shoreward as the middle domain,

and recruitment would be to midshelfwaters southeast

ofthe Pribiloflslands. Under this scenario, it is possible

that age-O pollock could have drifted toward the Pribilofs

and that in the following year, age-l pollock would be

particularly abundant around the Pribilof Islands. The

mid 1980's was a period of higher water temperatures

(Royer, 1989; Decker et aI., 1995), and one might expect that it would have been a period of high abundance of age-l pollock around the Pribilof Islands.

However, age-l pollock were scarce near the Pribilof

Islands during this period. We need better information

on the source and numbers of age-O pollock arriving at

the Pribiloflslands, their subsequent survival, and their

likelihood of remaining over the first winter. Without

this information, it will be difficult to assess the importance of variation in spawning areas to the recruitment

of age-l pollock to the foraging grounds used by Pribilof

Island seabirds.

Young pollock avoid cold water (Francis and Bailey,

1983), and age-l fish may move to slope waters when

there is extensive ice cover over the eastern Bering Sea

shelf (Ohtani and Azumaya, 1995). We failed to find a

significant correlation between the abundance of age-l

pollock in strata 50, 32, and 42 and the extent of ice

cover along the 1700W meridian. Thus, although there

may be a shift in the relative abundance of young pollock toward the shelf edge when water temperatures on

the shelf are low (see also Wyllie-Echeverria, 1996), the

absolute number of young pollock along the shelf edge

may not vary as a function of ice cover. The abundance

of young pollock there, which is more important to

predators than is their relative abundance with respect

to the middle domain, undoubtedly depends on a variety offactors, ofwhich ice cover on the shelf is but one.

Low growth rates of juvenile fish may reduce their

likelihood of survival because they remain vulnerable

to predation for a longer period, or because, for a given

age, they are in poorer condition (for a review, see

Leggett and DeBlois, 1994). We found no evidence that

age-O fish grew more slowly during the 1980's than the

1970's. Our estimates of growth rates fell within or

above the range ofvalues found by Yoklavich and Bailey

(1990) and Brown and Bailey (1992) for age-O pollock

in the western Gulf of Alaska. The variability in growth

rates found in our study suggests that there was considerable interannual variability in conditions affecting

growth, although we did not find this variability closely

correlated with sea-surface temperature. Perhaps the

roles of interannual variation in copepod species composition (as mentioned above) or abundance, as medi-
___ Hunt et al.: Changes in the Distribution and Size ofJuvenile Walleye Pollock in the Eastern Bering Sea 137

ated by both the source ofthe water around the Pribilof

Islands and water temperature, are worthy of investigation. Size-selective mortality could also influence apparent growth rates, as indicated by comparisons of modal

size groups through time (Baileyl).

We had anticipated that age-l pollock would be important to the reproductive success of seabirds breeding on the Pribilof Islands. Although there were no

breeding failures when age-l pollock constituted 10%

or more of the birds' diets, variation in the ratio of agel to age-O pollock in the diets did not significantly affect

breeding success. This result parallels the finding of

Decker et al. (1995) that from 1975 to 1989 there was

no significant positive correlation between seabird reproductive success and the consumption ofwalleye pollock at the Pribiloflslands. We therefore conclude that

variation in the abundance ofjuvenile walleye pollock

near the Pribilof Islands is unlikely to be the principal

cause of the interannual and interdecadal variations in

seabird reproductive success that have been the focus

of recent studies (e.g., Byrd, 1989; Springer, 1992;

Decker et al., 1995). Rather, we interpret the changes

in pollock use and age-l pollock abundance near the

Pribilof Islands as indicators of more general shifts in

the marine environment which have affected the overall availability of prey to seabirds. The presence of relatively high proportions of age-l pollock in the diets of

Pribilof seabirds in 1992 and 1993, and a return of

chick production to levels comparable to those in the

1970's (Climo, 1993; Dragoo and Sundseth, 1993; Byrd2)

suggest that the marine ecosystem may be changing

again. In the 1970's and the 1990's, elevated levels of

seabird reproduction and use of age-l pollock followed

a period of below-average sea-surface temperatures

(Decker et al., 1995). It is possible that cold surface

temperatures indicate oceanographic conditions that

promote population growth of prey or availability of

prey to seabirds.

Seabirds are constrained to forage in the vicinity of

their colonies when they are raising young. Although

we were able to identify changes in the marine ecosystem near the Pribilof Islands that occurred at time

scales of years to decades, we are unable to explain why

age-l pollock were largely absent from the birds' foraging grounds during the 1980's. We need to understand

the causes of variation in prey availability within these

foraging areas. Research designed to investigate fish

population dynamics for the eastern Bering Sea as a

I Bailey, K. Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, 7600 Sand Point Way N.E., Seattle, WA 98115-

0070. Personal commun.

2 Byrd, G. V. U.S. Fish and Wildlife Serv., Alaska Maritime National

Wildlife Refuge, Homer, AK 99603. Personal commun.

whole may not provide sufficient resolution to answer

these more local questions.

Acknowledgments _

We thank Gary Walters of the National Marine Fisheries Service for the NMFS bottom trawl data, and for

going to considerable effort to customize the analyses

for our needs. Gary Wohl of the NOAA/Navy Joint Ice

Center was most helpful in identifying and providing

appropriate data on sea-ice cover. K. Bailey, G. V. Byrd,

P. Livingston, J. Rice, R. Russell, D. Schneider,

E. Woehler, and two anonymous reviewers offered helpful comments on earlier versions of this manuscript.

Research on the Pribilof Islands in the 1970's was supported by Bureau of Land Management and National

Oceanographic and Atmospheric Administration Outer

Continental Shelf Assessment Program contracts to G.

Hunt. Studies in the 1980's were supported by a National Science Foundation Grant, DPP-8521178, to G.

Hunt. The United States Fish and Wildlife Service supported the work of D. Dragoo. The Alaska Sea Grant

Program supported the collection of food samples in

the 1990's by A. Springer. M. B. Decker was supported

by a National Aeronautics and Space Administration

Global Change Fellowship.

Literature Cited

Bailey, K. M.

1989. Interaction between vertical distribution of juvenile

walleye pollock Theragra chalcogramma in the eastern Bering

Sea, and cannibalism. Mar. EcoI. Prog. Ser. 53:205-213.

Bakkala, R.

1989. Variability in the size and age composition of eastern

Bering Sea walleye pollock. In Proc. Int. Symp. BioI. Manage. Walleye Pollock, Anchorage, 1988, p. 307-322. Alaska

Sea Grant Rep. 89-1.

Baretta, L., and G. L. HuntJr.

1994. Changes in the numbers of cetaceans near the Pribilof

Islands, Bering Sea, between 1975-78 and 1987-89. Arctic

47:321-326.

Beamish, R.J. (ed.)

1995. Climate change and northern fish populations. Can.

Spec. Pub. Fish. Aquat. Sci. 121.

Brown, A. L., and K. M. Bailey.

1992. Otolith analysis of juvenile walleye pollock Theragra

chalcogramma from the western Gulf of Alaska. Mar. BioI.

112:23-30.

Byrd, G. V.

1989. Seabirds in the Pribilof Islands, Alaska: trends and

monitoring methods. M.S. thesis, Univ. Idaho, Moscow,

96 p.

Climo, L.

1993. The status of cliff-nesting seabirds at St. Paul Island,

Alaska, in 1992. U. S. Fish WildI. Servo Rep. AMNWR 93/

15. Homer, AK, 53 p.
138 NOAA Technical Report NMFS 126

Cooney, R. T., and K. O. Coyle.

1982. Trophic implications of cross-shelf copepod distributions in the southeastern Bering Sea. Mar. BioI. 70:187-

196.

Coyle, K. 0., and R. T. Cooney.

1993. Water column sound scattering and hydrography

around the Pribilof Islands, Bering Sea. Cont. Shelf Res.

13:802-827.

Croll, D. A., A.]. Gaston, A. E. Burger, and D. Konnoff.

1992. Foraging behavior and physiological adaptation for diving in thick-billed murres. Ecology 73:344-356.

Decker, M. B., G. L. HuntJr., and G. V. ByrdJr.

1995. The relationship between sea surface temperature, the

abundance of juvenile walleye pollock (Theragra chalcogramma) , and the reproductive performance and diets of

seabirds at the Pribilof Islands, southeastern Bering Sea.

Can. Spec. PubI. Fish. Aquat. Sci. 121 :425-437.

Dragoo, B. K., and K. Sundseth.

1993. The status of northern fulmars, kittiwakes, and murres

at St. George Island, Alaska, in 1992. U.S. Fish Wildl. Servo

Rep., AMNWR 93/10. Homer, AK, 92 p.

Dwyer, D. A., K. M. Bailey, and P. A. Livingston.

1987. Feeding habits and daily ration of walleye pollock

(Theragra chalcogramma) in the eastern Bering Sea, with

special reference to cannibalism. Can]. Fish. Aquat. Sci.

44:1972-1984.

Francis, R. C., and K. M. Bailey.

1983. Factors affecting recruitment of selected Gadoids in

the northeast Pacific and east Bering Sea. In W. S. Wooster

(ed.), From year to year, p. 35-60. Washington Sea Grant,

Univ. Washington, Seattle.

Hilborn, R., and C.]. Walters.

1992. Quantitative fisheries stock assessment, choice, dynamics and uncertainty. Chapman and Hall, New York.

Hunt, G. L.,Jr., B. Burgeson, and G. A. Sanger.

1981a. Feeding ecology ofseabirds of the eastern Bering Sea.

In D. W. Hood and]. A. Calder (eds.), The eastern Bering

Sea shelf: oceanography and resources. Vol. 2, p. 629-

648. Ofc. Mar. Poll. Assess.,Juneau, AK.

Hunt, G. L.,Jr., K. O. Coyle, S. Hoffman, M. B. Decker, and

E. N. Flint.

1996. Foraging ecology of short-tailed shearwaters near the

Pribiloflslands, Bering Sea. Mar. Ecol. Prog. Ser. 141:1-11.

Hunt, G. L.,Jr., Z. Eppley, B. Burgeson, and R. Squibb.

1981b. Reproductive ecology, food and foraging areas ofseabirds nesting on the Pribiloflslands, 1975-1979. U. S. Dep.

Comm., NOAA, OCSEAP, Final Reports of Principallnvestigators 12:1-257.

Leggell, W. G., and E. DeBlois.

1994. Recruitment in marine fishes-is it regulated by starvation and predation in the egg and larval stages? Neth. ].

Sea Res. 32:119-134.

Livingston, P. A.

1989. Interannual trends in walleye pollock, Theragra

chalcogramma, cannibalism in the eastern Bering Sea. In

Proc. Int. Symp. BioI. Manage. Walleye Pollock, Anchorage,

1988, p. 457-469. Alaska Sea Grant Rep. 89-1.

Livingston, P. A., and G. M. Lang.

1996. Interdecadal comparisons of walleye pollock, Theragra

chalcogramma, cannibalism in the eastern Bering Sea. In

R. D. Brodeur et al. (eds.), Ecology of juvenile walleye

pollock, Theragra chalcogramma, p. 115-124. U.S. Dep.

Commer., NOAA Tech. Rep. 126.

Nishimura, A., and]. Yamada.

1988. Geographical differences in early growth of walleye

pollock Theragra chalcogramma, estimated by back-calculation of otolith daily growth increments. Mar. BioI. 97:

459-465.

Ohtani, K., and T. Azumaya.

1995. Influence of interannual changes in ocean conditions

on the abundance ofwalleye pollock (Theragra chalcogramma)

in the eastern Bering Sea. In R.]. Beamish (ed.), Climate

change and northern fish populations. Can. Spec. Publ.

Fish. Aquat. Sci. 121:87-95.

Roby, D. D., and K. L. Brink.

1986. Breeding biology of least auklets on the Pribilof Islands, Alaska. Condor 88:336-346.

Royer, T. C.

1989. Upper ocean temperature variability in the northeast

Pacific Ocean: is it an indicator of global warming? ].

Geophys. Res. 94:18,175-18,183.

Sample, T. M., and R. G. Bakkala.

1989. Assessment ofwalleye pollock of the eastern Bering Sea

based on bottom trawl surveys. In Proc. Int. Symp. BioI.

Manage. Walleye Pollock, Anchorage, 1988, p. 307-

322. Alaska Sea Grant Rep. 89-1.

Schneider, D. C., and G. L. HuntJr.

1984. A comparison of seabird diets and foraging distribution around the Pribilof Islands, Alaska. In D. ettleship,

G. A. Sanger, and A. M. Springer (eds.), Marine birds: their

feeding ecology and commercial fisheries relationships,

p. 86-95. Can. Wildl. Servo Spec. Pub., Ottawa.

Schulmeister, S. D.

1991. Results of productivity monitoring of black-legged kittiwakes, red-legged kittiwakes, and thick-billed murres at St.

George Island, Alaska, in 1990. U.S. Fish Wildl. Servo Rep.,

Homer, AK, 26 p.

Smith, S. L~, and]. Vidal.

1984. Spatial and temporal effects of salinity, temperature

and chlorophyll on the communities of zooplankton in the

southeastern Bering Sea. ]. Mar. Res. 42:221-257.

Sogard, S. M., and B. L. Olla.

1993. Effects of light, thermoclines and predator presence

on vertical distribution and behavioral interactions ofjuvenile walleye pollock, Theragra chalcogramma Pallas. ]. Exper.

Mar. BioI. Ecol. 167:179-195.

Springer, A. M.

1991. Seabird distribution as related to food webs and the

environment: examples from the North Pacific Ocean. In

W. A. Montevecchi and A.]. Gaston (eds.), Studies of high

latitude seabirds 1: Behavioural, energetic and oceanographic aspects ofseabird feeding ecology, p. 39-48. Occas.

Pap. Can. Wildl. Servo 68.

1992. A review: walleye pollock in the North Pacifichow much difference do they really make? Fish.Oceanogr.

1:80-96.

Springer, A. M., and G. V. Byrd.

1989. Seabird dependence on walleye pollock in the southeastern Bering Sea. In Proc. Int. Symp. BioI. Manage. Walleye Pollock, Anchorage, 1988, p. 667-677. Alaska Sea Grant

Rep. 89-1.

Springer, A. M., D. G. Roseneau, D. S. Lloyd, C. P. McRoy, and

E. C. Murphy.

1986. Seabird responses to fluctuating prey availability in the

eastern Bering Sea. Mar. Ecol. Prog. Ser. 32:1-12.

Vidal,]. and S. L. Smith.

1986. Biomass, growth, and development of populations of

herbivorous zooplankton in the southeastern Bering Sea

during spring. Deep-Sea Res. 33:525-556.

Wilkinson, L., M. A. Hill,]. P. Welna, and G. K. Birkenbeuel.

1992. SYSTAT for Windows: statistics, version 5. SYSTAT

Inc., Evanston, IL.
Hunt et al.: Changes in the Distribution and Size of Juvenile Walleye Pollock in the Eastern Bering Sea 139

Wyllie-Echeverria, T.

1996. The relationship between the distribution of one-yearold walleye pollock, Theragra chalcogramma, and sea-ice

characteristics. In R. D. Brodeur et al. (eds.), Ecology of

juvenile walleye pollock, Theragra chalcogramma, p. 47-

56. U.S. Dep. Commer., NOAA Tech. Rep. 126.

Yoklavich, M. M., and K. M. Bailey.

1990. Hatching period, growth and survival of young walleye

pollock Theragra chalco"gramma as determined from otolith

analysis. Mar. Ecol. Prog. Ser. 64:13-23.

Importance of Walleye Pollock, Theragra chalcogramma,

in the Diet of Phocid Seals in the Bering Sea

and Northwestern Pacific Ocean

LLOYD F. LOWRY

Alaska Department ofFish and Game

1300 College Road

Fairbanks, Alaska 99701

VLADIMIR N. BURKANOV

Kamchatka Institute ofEcology and Nature Management

Prospekt Rybakov 19A

Petropavlovsk-Kamchatsky, 683024, Russia

KATHRYN J. FROST

Alaska Department ofFish and Game

1300 College Road

Fairbanks, Alaska 99701

ABSTRACT

In the Bering Sea and northwestern Pacific Ocean all species of phocid seals feed on

walleye pollock. Pollock has been one of the most important foods of harbor seals sampled

at the Pribilof Islands, in Bristol Bay, and in the eastern Aleutian Islands. Ice-associated

seals, particularly spotted and ribbon seals, feed on pollock in the winter and spring when

they inhabit the drifting ice of the Bering Sea. In the Sea of Okhotsk pollock is a significant

food for spotted and ribbon seals. The distribution of ringed seals is more coastal and

northern, but they regularly feed on small gadid fishes and probably eat pollock when their

ranges overlap. Bearded seals feed mostly on invertebrates and bottom fishes, but pollock

have been found in their stomachs. Bearded seals consume more pollock in the southern

part of the Sea of Okhotsk than in the north. In the eastern Bering Sea, harbor seals eat a

wide range ofsizes of pollock, but the other species eat mostly small pollock. Data from the

Sea of Okhotsk suggest that younger seals eat mostly small pollock, whereas older seals also

eat larger fishes. All species of seals also feed on other demersal and schooling fishes, and

invertebrates. Overall, pollock is of most importance in the diets of harbor seals, spotted

seals, and ribbon seals, and oflittle importance to bearded and ringed seals. Although some

estimates have been made of the total amount of pollock consumed by seals, many of those

estimates are based on old and incomplete data and are probably not reliable. Seal predation may affect pollock stocks, and pollock fishing may influence seal populations, but the

possible interactions are complex and poorly understood.

Introduction

Five species of seals of the family Phocidae inhabit

waters ofthe Bering Sea and northwestern Pacific Ocean:

harbor seal, Phoca vitulina; spotted (or larga) seal, P.

largha; ribbon seal, P. fasciata; ringed seal, P. hispida;

and bearded seal, Erignathus barbatus. Harbor seals occur principally in coastal areas south of the normal

extent ofseasonal sea ice, including the Gulf of Alaska,

Bristol Bay, the Aleutian Islands, the Pribilof Islands,

the Commander Islands, the Kuril Islands, and the Sea

ofJapan (Bigg, 1981). Within this broad geographical

141
142 NOAA Technical Report NMFS 126

range there may be two subspecies-Po v. richardsi, occurring along the coast ofwestern North America, and P.

V. stejnegeri, in eastern Asia (Shaughnessy and Fay, 1977).

The closely related spotted seal spendssummer months in

more northern coastal waters, and moves to the drifting

sea ice in winter and spring (Shaughnessy and Fay, 1977;

Lowry et aI., 1994). Ribbon seals also associate with drifting sea ice in spring, and are largely pelagic during other

times of year (Burns, 1981a). Bearded seals and ringed

seals are closely associated with sea ice throughout most of

the year. During winter-spring, bearded seals occur principally in moving pack ice (Burns, 1981b) while ringed

seals prefer stable, shorefast ice (Frost and Lowry, 1981a).

American, Russian, and Japanese investigators have

devoted considerable research effort to the biology of

these seal species in the North Pacific Ocean and the

Bering Sea. Studies of natural history and biology often

produced information on diet composition. The purpose of this paper is to review available information on

the foods eaten by phocid seals in this region, with

emphasis on the dietary importance of walleye pollock,

Theragra chalcogramma.

Methods and Materials

Studies on foods of phocid seals in the region of concern have used a variety of sources for specimen material and various techniques for analyzing samples and

data. Samples collected in Alaska have come principally

from animals taken for subsistence by coastal Eskimos,

and from animals collected for scientific research. Most

of the samples in Russian and Japanese studies have

come from scientific studies conducted in conjunction

with commercial seal harvesting.

Diet has generally been analyzed by examining and

identifying the material taken from stomachs or intestines. In cases where food was well digested, identifications have usually been based on characteristic hard

parts. The sagittal otoliths of walleye pollock are relatively robust and easy to identify (Frost, 1981). Regression

equations have been published and used to estimate the

sizes of pollock consumed from otolith measurements

(Frost and Lowry, 1986). For this paper, pollock lengths

were estimated from otolith lengths by means of the formulas in Frost and Lowry (1981b) as follows:

for otoliths $;10 mm long, fish fork length (cm) =

2.246 (otolith length [mm)) - 0.51

for otoliths >10 mm long, fish fork length (cm) =

3.175 (otolith length [mm)) - 9.77

There are many methods for quantifying diet composition (see review by Pierce and Boyle, 1991). The most

straightforward are the apparent number of prey eaten

(based on counts of whole items or unique parts) and

the percent frequency of occurrence (number of

samples in which a prey occurred, divided by the total

number ofsamples examined, times 100). When stomachs with substantial contents are analyzed, the percent

of total volume made up by a particular prey type can

be calculated. All of these measures are biased because

of differences in detectability and digestibility of prey.

Various techniques have been used to correct for bias

and produce more realistic estimates of the actual

amounts of different prey species that were consumed

(Pierce and Boyle, 1991). Most of the results presented

in this paper will be given as number of prey eaten,

percent frequency of occurrence, or percent volume.

Results and Discussion

Occurrence of Pollock in Seal Diets

Harbor Seals-The most comprehensive study of harbor seal foods was conducted in the Gulf of Alaska by

Pitcher (1980), who examined 269 stomachs containing food. Overall, pollock was the most important prey,

occurring in 21 % of the samples and constituting 21 %

of the total volume. When analyzed by geographical

subarea, pollock ranked number 1 in the northeastern

gulf, Prince William Sound, and along the Kenai coast;

number 2 along the Alaska Peninsula; and number 3

around Kodiak Island.

For other areas in the northwestern Pacific Ocean

and Bering Sea, relatively few harbor seals have been

collected and examined (see review in Lowry et aLl). In

eight seals collected at Otter Island (central Bering

Sea) during April 1979, fish made up 64% of the stomach contents; 44% of the identified fishes were pollock.

Pollock have been recorded as prey in seals sampled at

Port Moller and Port Heiden in Bristol Bay, and at

Akun, Unalaska, and Amchitka Islands in the Aleutians,

but sample sizes were too small to evaluate their relative

importance. Harbor seals in the Commander and Kuril

Islands have been reported to feed on cephalopods and

bottom fishes, but the species of fishes were not identified (Burdin2).

Pollock were found in harbor seal stomach samples

collected in southeast Hokkaido in 1982-83, but were

not one of the top 20 foods in terms of volume or

frequency of occurrence (Wada et aI., 1992).

1 Lowry, L. F., K. J. Frost, D. G. Calkins, G. L. Swartzman, and S.

Hills. 1982. Feeding habits, food requirements, and status of

Bering Sea marine mammals. Council Doc. #19. North Pacific

Fishery Management Council, Anchorage, AK, 292 p.

2 Burdin, A. Kamchatka Institute of Ecology and Nature Management, Petropavlovsk-Kamchatsky, Russia. Unpub!. data.
__ Lowry et al.: Importance of Walleye Pollock in the Diet of Phocid Seals in the Bering Sea and Northwestern Pacific Ocean 143

Spotted Seals-Bukhtiyarov et a1. (1984) presented data

on foods ofspotted seals collected in the Bering Sea in

March-June 1972-78. Pollock were the most important

prey in the central Bering Sea (88% of the identified

fishes in 4 of 5 seals with stomach contents). Pollock

occurred less frequently in seals from the southeastern

Bering Sea (6 of 14) and the northern Bering Sea (1 of

12). Other studies (reviewed in Lowry et a1. l ) also indicate that pollock are not important in the diet of spotted seals-in the northern Bering Sea. Lowry et a1. 3

found pollock in all 12 spotted seals collected in the

central Bering Sea in March 1985, and pollock made

up 58% of all identified fishes. Lowry et a1. also reported pollock in 1 of 3 seals collected in the western

Bering Sea in April 1985.

Most of the information on foods of spotted seals in

the western Bering Sea and Sea of Okhotsk comes from

Russian studies that harvested seals from vessels operating in the pack ice during spring and early summer. In

the western Bering Sea (Gulf ofAnadyr and Karaginskiy

Gulf) during the late 1960's, pollock occurred in 9% of

the spotted seal stomachs examined, and octopus was

the most commonly occurring food (Gol'tsev, 1971).

Bukhtiyarov (1983) reported that pollock made up 5%

of the diet in the Gulf of Anadyr. During the early

1980's, Trukhin et a1. (1991), like Gol'tsev, found that

octopus was the most important food in the Gulf of

Anadyr and that saffron cod (Eleginus gracilis), followed

by pollock, were the most commonly eaten fishes. In

Karaginskiy Gulf, Trukhin et a1. (1991) found that fish

occurred in 75% ofstomachs with food but that pollock

were found in only 7%. Bukhtiyarov (1983) reported

that pollock made up 35% of the overall spotted seal

diet in Karaginskiy Gulf. Bukhtiyarov (1986) estimated

that 24% of the diet of spotted seals in the Bering Sea

consisted of pollock.

In the western Bering Sea, young seals beginning to

feed independently ate mostly small pollock and other

schooling fishes such as sand lance, Ammodytes sp., and

saffron cod (Trukhin, 1986).

In the Sea of Okhotsk, pollock made up 83% of the

total volume ofstomach contents from 19 spotted seals

collected in the pack ice off northern Hokkaido in

April (Wilke, 1954). Pollock also occurred in spotted

seal stomach sampIes collected in northeastern

Hokkaido during the fall of 1992 and 1993, but was not

one ofthe top 20 foods in terms ofvolume or frequency

of occurrence (Wada et a1., 1992).

In the southwestern Sea of Okhotsk, including the

Gulfs of Aniva and Terpeniy near Sakhalin Island,

3 Lowry, 1. F., K. J. Frost, and J. J. Burns. 1986. Assessment of

marine mammal-fishery interactions in the western Gulf ofAlaska

and Bering Sea: consumption of commercially important fishes by

Bering Sea pinnipeds. Final Rep. to NMFS, Contract NA-85-ABH00029,26 p.

Bukhtiyarov (1990) identified 24 species of prey in 222

stomachs with food collected during spring 1974-77.

The most important foods were saffron cod and sand

lance; pollock were found in only 5% of the stomachs.

In the central and northern Sea of Okhotsk the most

important food was pollock (65-91 % frequency of occurrence). The measured amount of pollock in individual stomachs ranged from 100 to 4,000 g (Fedoseev

and Bukhtiyarov, 1972; Popov and Bukhtiyarov, 1975).

In summer, stomachs were examined from spotted

seals taken in scientific collections in coastal areas of

the northern and eastern part of the Sea of Okhotsk,

and the frequency of occurrence of pollock was 6-9%

(Bukhtiyarov, 1984; Burkanov, 1990). Near the

Kamchatka coast in the eastern part of the Sea of

Okhotsk, the most important foods during summer

were salmon, Oncorhynchus spp., and arctic char, Salvelinus alpinus. In the southeastern Sea of Okhotsk near

the Kuril Islands, Panina (1966) found pollock in 10%

of the stomachs she examined.

Information from the Sea of Okhotsk shows clear

seasonal variation in the importance of pollock in the

spotted seal diet. Pollock is most important in spring,

particularly in the northern part. The picture is less

clear for the southern Sea of Okhotsk, where pollock

made up most of the contents in samples from the late

1940's and only a small portion ofsamples from the mid1970's. In summer, pollock is much less important in the

diet in the northern Sea of Okhotsk. In the southeastern

area during summer, salmonids, rather than pollock, make

up most of the spotted seal diet. Overall, Bukhtiyarov

(1986) estimated that pollock made up 30% of the total

mass offood eaten by spotted seals in the Sea of Okhotsk.

In the northern part of the Sea of Japan (Tartar

Strait) the frequency of occurrence of pollock was 85%

(Gol'tsev, 1971; Kosygin and Gol'tsev, 1971). In Peter

the Great Gulf, pollock was not found in the spotted

seal diet (Gol'tsev, 1971).

Ribbon Seals-Frost and Lowry (1980) reported on the

diet of ribbon seals in the Bering Sea from samples

collected during March-June 1976-79. Pollock was the

most important prey item in the south central and

central regions (89% of the total number of fishes and

50% of the total corrected weight, in 9 seal stomachs

with food in the south central Bering Sea; 55% by

number and 28% by weight, in 12 seals with food in the

central Bering Sea). Pollock occurred in only one of

seven seals with food collected in the northern Bering

Sea. In contrast, Shustov (1965) did not find pollock to

be an important food in the ribbon seals he examined

from the central Bering Sea in 1961-63 (32 with food).

Trukhin et a1. (1991) collected samples from ribbon

seals in Karaginskiy Gulf during spring 1982 and found

pollock in 4 of 18 stomachs with food. In the Gulf of
144 NOAA Technical Report NMFS 126

Anadyr, pollock was reported to be the second most

important food based on frequency of occurrence, but

no figures were given. Overall, Bukhtiyarov (1986) estimated that pollock made up 13% of the total mass of

food eaten by ribbon seals in the Bering Sea.

Arsen'ev (1941) examined stomachs with food from

54 ribbon seals collected in the ice of the southwestern

Sea of Okhotsk during April-June and found that pollock was the major prey in all three months. Wilke

(1954) reported that pollock made up 60% of the total

volume of stomach contents in two ribbon seals he

examined from the southern Sea of Okhotsk in April

1949. Bukhtiyarov (1990) examined stomachs of ribbon

seals collected in the southern Sea ofOkhotsk in 1974--77.

Pollock was one of 13 identified species of prey and occurred in 12 of50 stomachs with food. For young seals (1-

2 years old) small pollock were the most important food.

Pollock was also a very important component in the

diet of ribbon seals in the northern Sea of Okhotsk. In

1970-71, 42 of 48 stomachs examined contained pollock (Fedoseev and Bukhtiyarov, 1972) and in 1972-73

pollock occurred in 66 of 94 stomachs (Bukhtiyarov,

1978). Overall, Bukhtiyarov (1986) estimated that pollock made up 69% of the total mass of food eaten by

ribbon seals in the Sea of Okhotsk.

Ringed Seals--Pollock have not been reported from stomachs ofringed seals collected in the Bering Sea (see review

in Lowry et aU). Other small gadids (saffron cod, and

arctic cod, Boreogadus saida) are often very important in

ringed seal diets (Lowry et al., 1980b), so pollock may be

eaten in areas where their ranges overlap.

Pollock is not very important in the diet of ringed

seals in the Sea of Okhotsk (Pikharev, 1946; Fedoseev,

1965; Kosygin et aI., 1984; Trukhin et aI., 1991).

Bukhtiyarov (1984, 1990) examined the stomachs of

more than 125 ringed seals collected in the Sea of

Okhotsk in spring and found that pollock occurred at a

frequency of about 1%. In summer and fall he examined 20 stomachs and found no pollock.

Bearded Seals-In general, bearded seals feed on invertebrates and benthic fishes (Lowry et al., 1980a). A

few pollock were recorded in stomachs of 15 bearded

seals taken north of the Pribilof Islands and near

Nunivak Island in 1975-77.1 Antonelis et ai. (1994)

found pollock in the stomachs of 7 of 74 bearded seals

collected near St. Matthew Island in March-April 1981.

In spring samples taken in the northern and eastern

Bering Sea during 1963-65, Kosygin (1966, 1971) found

pollock in only 1 of 152 stomachs examined. Lowry et

al.I found no pollock in the stomachs of 173 bearded

seals taken in the northern Bering Sea during 1975-79.

Trukhin et al. (1991) found pollock in 3 of 31 bearded

seals collected in April-May 1982 in Karaginskiy and

Anadyr Gulfs. Trukhin et al. stated, however, that pollock play an important role in the diet of bearded seals

in the western Bering Sea.

In the southern Sea of Okhotsk during spring 1974-

77, Bukhtiyarov (1990) found pollock in 5% of the

stomachs examined (n=63). He stated that pollock play

a more important role in the bearded seal diet in the

southern Sea of Okhotsk than they do in other regions.

Pollock were not found in samples from the northern

Sea of Okhotsk in spring (n=72) and summer (n=128)

(Fedoseev and Bukhtiyarov, 1972; Bukhtiyarov, 1984).

Sizes of Pollock Consumed

Pitcher (1981) measured otoliths from stomachs of94

harbor seals collected in the Gulf of Alaska during

1973-78 (Table 1). He reported that the average length

of pollock eaten by harbor seals was 19.2 cm, and noted

that this was significantly smaller than the pollock eaten

by Steller sea lions, Eumetopias jubatus, collected in the

same regions and years (mean 29.8 cm).

Information on sizes of pollock consumed by seals in

other areas is based on relatively few otoliths retrieved

from seal stomachs and intestines (Table 1). Frost and

Table 1

Sizes of pollock eaten by phocid seals in the Gulf of Alaska, Bering Sea, and Sea of Okhotsk.

Fork length (em)

Number of Number of

Species Location seals otoliths Mean Range Source

Harbor seal Gulf of Alaska 94 2,030 19.2 4.2-53.2 Pitcher, 1981

Bering Sea 5 35 24.5 8.2-56.3 Frost and Lowry, 1986

Ribbon seal Bering Sea 19 468 11.2 6.5-34.4 Frost and Lowry, 1986

Spotted seal Bering Sea 2 21 10.9 8.0-15.0 Frost and Lowry, 1986

Sea of Okhotsk 10 26 15.9 2.0-39.1 Burkanov, unpub\. data

Bearded seal Bering Sea 5 56 U.8 6.9-14.3 Antonelis et a\., 1994
__ Lowry et al.: Importance of Walleye Pollock in the Diet of Phocid Seals in the Bering Sea and Northwestern Pacific Ocean 145

Lowry (1986) summarized available data on sizes of

pollock eaten by pinnipeds in the Bering Sea. Their

data showed that ribbon seals and spotted seals ate

almost entirely small pollock, while harbor seals ate all

sizes (Fig. 1-3). Data from spotted seals collected in the

Okhotsk Sea (Fig. 2) showed a wider range in sizes of

pollock eaten (Burkanov, unpubl. data). Antonelis et

al. (1994) reported that bearded seals collected in the

central Bering Sea had eaten only small pollock (mean

length 11.8 cm). Bukhtiyarov (1984, 1990) stated that

all the pollock found in ringed seals collected in the

Sea of Okhotsk in spring were small.

This information suggests that most phocid seals eat

predominantly small pollock. Harbor seals, which eat

all sizes of pollock, are the principal exception. Because

of the exponential relation between pollock length and

weight, however, the few larger fishes that are eaten may

be of considerable nutritional significance. We used the

formula in Frost and Lowry (1981b) to estimate the weights

of pollock eaten by harbor seals in the central Bering Sea

in April 1979. Although only 11 of the 23 pollock eaten

were estimated to be over 30 cm long, those fishes contributed 84% of the estimated biomass consumed.

Quantities of Pollock Consumed

Simple calculations of the amount of food consumed

by pinnipeds are often based on estimates of population sizes and daily food consumption rates. More detailed studies may take into account the size structure

of the pinniped population, metabolic requirements,

and caloric values of various prey (e.g., Perez and

McAlister, 1993). Information on diet composition can

then be used to estimate the amount of a particular

prey that is eaten annually.

The only estimate of the amount of pollock eaten by

harbor seals in the Gulf of Alaska is that by Lowry et al.

(1989), who estimated that 72,800 metric tons (t) were

consumed annually. Their calculations were based on

population size and diet data from the late 1970's provided in Calkins (1986) and Pitcher (1980), and may

not reflect the current consumption level.

Ashwell-Erickson and Elsner (1981) used energy-flow

models to estimate that harbor seals consumed 34,700 t

of pollock in the Bering Sea each year, and that spotted

seals consumed 46,900 t (Table 2). Frost and Lowry

(1980) used diet composition, daily ration, and population size to estimate that ribbon seals could consume

55,000 t of pollock per year in the Bering Sea. Lowry et

al. (1989) produced estimates of pollock consumed by

ribbon seals that were much lower than Frost and

Lowry's (1980) estimates, partly because Lowry et al.

included only the eastern section of the Bering Sea.

Bukhtiyarov (1986) estimated that the average amount

of pollock eaten by a ribbon seal each day in the Bering

Sea was 1.1 kg, and in the Sea of Okhotsk 7.4 kg.

Corresponding estimates for spotted seals were 4.2 and

10,-------------------------------......,

8

f/)

€

'0 6

5

-

0

...

C1)

s:J. 4 E

:::l

Z

2

8 13 18 23 28 33 38 43 48 53

Fork Length of Pollock (em)

Figure 1

Fork lengths of walleye pollock eaten by harbor seals in the Bering Sea (from Frost and

Lowry, 1986).
146 NOAA Technical Report NMFS 126

10 ~--------------------- --,

B A j

, ~:

~t;.

Vl ,. E 6 0 .'

0 /11 0

'1, ..c

E 4 :r :s

z

2

o +---1--+-+--+-+-+-

6 11 16 21 26 31 36

4,----------------------------------,

B

Vl

E

o

o

'0 2

Cll

..c

E

:s

z

6 11 16 21 26 31 36

Fork L.ength of Pollock (em)

Figure 2

Fork lengths of walleye pollock eaten by spotted seals in (A) the Bering Sea and (B) the Sea

of Okhotsk (from Frost and Lowry, 1986, and Burkanov, un pub\. data).

2.2 kg. These figures were used to extrapolate to the

annual amounts eaten by the seal populations in each

area (Table 2). For the Bering Sea his estimate for spotted

seal consumption was nearly twice that derived by AshwellErickson and Elsner (1981), while his ribbon seal estimate

was less than that of Frost and Lowry (1980).

The consumption estimates made by Bukhtiyarov

(1983) indicated that over 900,000 t of pollock were

eaten by seals in the Sea of Okhotsk. Of this, 128,000 t

was attributed to spotted seals and 772,000 t to ribbon

seals. In another set of calculations, Bukhtiyarov (1990)

estimated that during April-June ribbon seals in the
__ Lowry et al.: Importance of Walleye Pollock in the Diet of Phocid Seals in the Bering Sea and Northwestern Pacific Ocean 147

140,.-----------------------------------------,

120

100

III

E

"0 80

(5

...

0

...

Ql

.c 60 E

::l

Z

40

20

o

6 11 16 21 26 31

Fork Length of Pollock (em)

Figure 3

Fork lengths of walleye pollock eaten by ribbon seals in the Bering Sea (from Frost and Lowry, 1986).

Sea of Okhotsk would eat more than 100,000 t of pollock. Sobolevsky (1983) estimated the total amount of

fish eaten each year in that region by spotted seals as

164,000 t and by ribbon seals as 171,000-239,000 t.

These estimates are much less than would be expected

from the pollock consumption calculated by Bukhtiyarov

(1983). For ribbon seals, this may largely be due to the

high value that Bukhtiyarov used for per capita food

consumption. Applying his estimate of 7.4 kg pollock

per day to a diet consisting of 69% pollock results in a

total daily food intake of 10.7 kg. We do not consider

this a realistic average consumption for a seal whose

body weight averages less than 100 kg (Fedoseev, 1973;

Burns, 1981a), because maintenance requirements for

similar phocids are generally 3-6% of body weight per

day (e.g., Markussen et aI., 1990).

The variation in estimates of the amount of pollock

consumed by phocid seals is largely due to differing

assumptions about individual energy requirements, overall diet composition, and population sizes. From the

information presented in previous sections it is evident

that the dietary data available are fragmentary, and

most come from samples collected before the mid1980's. The situation is similar for population sizes.

Most estimates are derived from information collected

Table 2

Estimates of the amount (metric tons x 1000) of pollock consumed by phocid seals in the Gulf of Alaska,

the Sea of Okhotsk, and the Bering Sea.

Culfof Sea of Bering

Species Alaska Okhotsk Sea Source

Harbor seal 34.7 Ashwell-Erickson

and Elsner, 1981

72.8 5.3 Lowry et a!., 1989*

Spotted seal 127.6 85.9 Bukhtiyarov,1983

46.9 Ashwell-Erickson

and Elsner, 1981

19.2 Lowry et a!., 1989*

Ribbon seal 799.2 39.6 Bukhtiyarov, 1983

55.0 Frost and Lowry,

1980

11.4 Lowry et a!., 1989*

'Estimates are for the eastern Bering Sea only.

in the 1970's, and in nearly all cases the surveys they

come from were not adequately designed or conducted

(see reviews in Lowry et aP and Lentfer, 1988). Rea-
148 NOAA Technical Report NMFS 126 _

sonably good information on relative numbers of harbor seals comes from counts of hauled-out animals

(e.g., Pitcher, 1990; Loughlin4), but those figures are

not population estimates because they do not account

for animals not hauled out during surveys. Thus the

data needed to generate current estimates of the quantities of pollock being consumed by phocid seals are

not available, and we consider that previous estimates

are of unknown reliability.

Potential Interactions with Pollock Fisheries

In the late 1970's, pollock was an important food for

harbor seals in the Gulf of Alaska. The commercial

trawl fishery targeting on pollock developed in the late

1970's, peaked in the early 1980's, and has declined

since then. Harbor seal numbers at index areas at

Tugidak Island and Prince William Sound have shown

a major decline in the 1980's (Pitcher, 1990; Frost et aI.,

1994). It is not known whether the pollock fishery has

been a factor in the harbor seal decline.

The 1993 estimate of the exploitable biomass of pollock in the Gulf of Alaska was 1,062,000 t (Hollowed et

aJ.5). The estimate of pollock consumption by harbor

seals in Lowry et aI. (1989) would represent a significant fraction of that pollock stock. However, that consumption estimate was based on diet and population

size data collected during the 1970's, and the amount

currently consumed may be much less.

Estimates of the amount of pollock eaten by seals in

the Bering Sea are also based on relatively old data on

diets and population sizes (Lowry et aI., 1989). Harbor

seal numbers may be stable in that region,4 and the

population status of ribbon and spotted seals is not

known. Nonetheless, if pinnipeds still consume about

150,000 t of pollock in the Bering Sea annually, that

should have little impact on the pollock stock, which

had an estimated 1993 exploitable biomass of about

6,700,000 t (Wespestad6).

4 Loughlin, T. R. 1992. Abundance and distribution of harbor

seals (Phoca vitulina richardsi) in Bristol Bay, Prince William Sound,

and Copper River delta during 1991. Unpub\. Annu. Rep. NMFS

Population Assessment Program, Nat'\. Marine Mammal Lab.,

Alaska Fish. Sci. Cent., NMFS/NOAA, 7600 Sand Point Way N.E.,

Seattle, WA 98115-0070,26 p.

5 Hollowed, A. B., B. A. Megrey, and E. Brown. 1993. Walleye

pollock. In Stock assessment and fishery evaluation report for

groundfish resources ofthe Gulf ofAlaska as projected for 1994, p.

I-54. North Pacific Fishery Management Council, P.O. Box

103136, Anchorage, AK 99510.

6 Wespestad, V. G. 1993. Walleye pollock. In Stock assessment and

fishery evaluation report for groundfish resources of the Bering

SealAleutian Islands region as projected for 1994, p. 1-26. North

Pacific Fishery Management Council, Anchorage, AK.

In recent years, commercial fisheries have harvested

approximately 4 million metric tons of pollock in the

Bering Sea annually, and stock abundance has fluctuated largely as a result of variable year-class strengths

(Springer, 1992). Because the phocid seals discussed in

this paper eat principally small pollock, fisheries should

not have a major effect on seal nutrition unless the

fisheries affect recruitment or survival of young fishes.

Sobolevsky (1983) compared the amount of pollock

eaten annually by all species of pinnipeds (including

Steller sea lions and fur seals, Callorhinus ursinus) in the

Sea of Okhotsk (240,000-280,000 t) to estimates of the

biomass of pollock (4.0-4.5 million t) and concluded

that pinnipeds ate about 6-7% of the total biomass. If

the higher estimate of pollock consumption produced

by Bukhtiyarov (1983) is used, this figure would be 21-

23% for phocid seals alone.

In the Sea ofOkhotsk, pollock are very important in the

ribbon seal diet and are subject to major commercial fisheries. Nonetheless, during the period from 1981 to 1988 the

ribbon seal population in the region is thought to have

doubled from 250,000 to 500,000 (Bukhtiyarov, 1990).

Conclusions

Available data allow a general description of the foods

eaten by phocid seals in most regions of the northwestern Pacific Ocean and Bering Sea (Table 3). Ofthe five

species discussed in this paper, pollock are of greatest

significance in the diets of harbor seals, spotted seals,

and ribbon seals. Bearded seals feed mostly on benthic

invertebrates and fishes, while ringed seals eat mostly

Arctic cod, saffron cod, and crustaceans. For most species, our understanding ofthe diet is based on irregular

and incomplete sampling. We have only limited information to evaluate seasonal, annual, geographical, or

sex/age-related variation in pollock consumption.

Available samples make it evident that prerecruit

pollock are eaten by all species of seals. Harbor seals

and spotted seals also eat larger pollock, while ribbon

seals, ringed seals, and bearded seals may eat only small

pollock. For most species and areas, few otoliths have

been measured, and quantitative data are very limited.

Estimates of the amount of pollock consumed by

seals have been calculated with various techniques that

have produced quite different results. The estimators

mostly used data that are 10-20 years old, and the

estimates therefore cannot be considered reliable measures of current pollock consumption.

Phocid seals (and other marine mammals) consume

large amounts of prerecruit and older pollock in the

northwestern Pacific Ocean and Bering Sea. It is not

clear whether or how this predation may affect pollock

stocks and their ability to withstand fishery harvests.
___ Lowry et al.: Importance of Walleye Pollock in the Diet of Phocid Seals in the Bering Sea and Northwestern Pacific Ocean 149

Table 3

Principal foods ofseals in the Gulf ofAlaska, the Bering Sea, and the Sea of Okhotsk, listed in approximate order of overall

importance. From Pitcher (1980), Lowry et al.,! Bukhtiyarov (1984, 1986, 1990), and Trukhin et al. (1991).

Species

Harbor seal

Spotted seal

Ribbon seal

Ringed seal

Bearded seal

Eastern Western

Gulf of Alaska Bering Sea Bering Sea

Pollock Pollock Octopus

Octopus Octopus Squid

Capelin Pacific cod Rockfishes

Eulachon Sculpins

Herring Sand lance

Pollock Pollock

Capelin Arctic cod

Herring Sand lance

Eelpout Saffron cod

Sculpins Herring

Pollock Pollock

Eelpout Squid

Capelin Saffron cod

Shrimp Shrimp

Flatfish

Saffron cod Capelin

Mysids Sand lance

Sculpins Saffron cod

Shrimp Amphipods

Shrimp

Shrimp Shrimp

Crabs Crabs

Sculpins Octopus

Isopods Clams

Pollock Saffron cod

Northern

Bering Sea

Arctic cod

Capelin

Saffron cod

Sculpins

Herring

Arctic cod

Saffron cod

Sculpins

Pollock

Arctic cod

Saffron cod

Shrimp

Mysids

Amphipods

Shrimp

Clams

Crabs

Sculpins

Arctic cod

Sea of Okhotsk

Pollock

Saffron cod

Sand lance

Shrimps

Octopus

Pollock

Saffron cod

Shrimps

Squid

Euphausiids

Shrimp

Saffron cod

Pollock

Crabs

Shrimp

Clams

Saffron cod

Pollock

Conversely, without a detailed understanding of how

pollock fisheries affect the availability of pollock for

foraging seals, it is not possible to assess whether fishing

has had or may have an impact on seal populations.

Acknowledgments _

We thank the many people who helped with the collection and analysis of seal foods. Many of the samples

collected by U.S. investigators were obtained as part of

the NOAA Outer Continental Shelf Environmental Assessment Program. International collaboration in the

preparation of this paper was aided by the U.S.-Russia

Agreement on Environmental Protection ProjectV.6. and

the U. S. Department ofState. Two anonymous reviewers

made helpful comments on the draft manuscript.

Literature Cited

Antonelis, G. A., S. R. Melin, and Y. A. Bukhtiyarov.

1994. Early spring feeding habits of bearded seals (Erignalhus

barbatus) in the central Bering Sea, 1981. Arctic 47:74-79.

Arsen'ev, V. A.

1941. Feeding of the ribbon seal. Izv. TINRO 20:121-127. [In

Russ.]

Ashwell-Erickson, S., and R. Elsner.

198I. The energy cost of free existence for Bering Sea harbor

and spotted seals. Tn D. W. Hood and J. A. Calder, (eds.),

The eastern Bering Sea shelf: oceanography and resources.

Vol. II, p. 869-899. Univ. Washington Press, Seattle.

Bigg, M. A.

1981. Harbor seal. Tn S. H. Ridgway and R.J. Harrison (eds.),

Handbook of marine mammals, vol. 2. Seals, p. 1-27. Academic Press, Lond.

Bukhtiyarov, Y. A.

1978. Some data about feeding of ribbon and spotted seal in

the seas of the north part of the Pacific Ocean. Tn Tez. dokl.

7-go Vses. SOY. Moskva, p. 52-54. [In Russ.]

1983. Feeding of true seals in the Sea of Okhotsk and Bering

Sea. Otchet NlR "Sostoyanie zapasov, rasprostranenie i

biologiya lastonogikh i kalana" Mo TINRO, Magadan, 113

p. [In Russ.]

1984. Feeding of seals in the northern part of the Sea of

Okhotsk in summer and fall. Tn Morskie mlekopitaushchie

Dalnego Vostoka, p. 23-31. TI RO, Vladivostok. [In Russ.]

1986. Diet oftrue sealsin the Sea ofOkhotsk and Bering Sea. Tn

Tez. dokI. 9-go Vses. sovesch. po izuch., okhrane i raz. ispolz.

mor. mlek., Arkhangelsk, 9-11 Sent. 1986, p. 67-68. [In Russ.]

1990. Feeding of seals in the south part of the Sea of

Okhotsk. Izv. TINRO 112:96-101. [In Russ.]
150 NOAA Technical Report NMFS 126

Bukhtiyarov, Y. A., K]. Frost, and L. F. Lowry.

1984. New information on foods of the spotted seal, Phoca

largha, in the Bering Sea in spring. In F. H. Fay and G. A.

Fedoseev (eds.), Soviet-American cooperative research on

marine mammals, vol. I-Pinnipeds, p. 55-59. U.S. Dep.

Commer., NOAA Tech. Rep. NMFS 12.

Burkanov, V. N.

1990. Data on feeding of spotted seals (Phoca larga PaL) in

summer and fall near the western coast of Kamchatka. In

Morskie mlekopitaushchie, p. 49-56. VNIRO, Moskva. [In

Russ.]

Burns,j.]'

1981a. Ribbon seal. In S. H. Ridgway and R. ]. Harrison

(eds.), Handbook of marine mammals, vol. 2. Seals, p. 89-

109. Academic Press, Lond.

1981b. Bearded seal. In S. H. Ridgway and R. ]. Harrison

(eds.), Handbook of marine mammals, vol. 2. Seals, p. 145-

170. Academic Press, Lond.

Calkins, D. G.

1986. Marine mammals. In D. W. Hood and S. T. Zimmerman

(eds.), The Gulf of Alaska: physical environment and biological resources, p. 527-558. Minerals Manage. Servo Publ.

OCS, MMS 86-0095. (Available from Nat. Tech. Info. Service as PB87-103230, U.S. Dep. Commer., Springfield, VA

22161).

Fedoseev, G. A.

1965. Nutrition of the ringed seal. Izv. TINRO 59:216-223.

[In Russ.]

1973. Morphological-ecological characteristics ofribbon seal

populations and factors affecting the conservation of usable

stocks. Izv. TINRO 86:158-177. [In Russ.]

Fedoseev, G. A., and Y. A. Bukhtiyarov.

1972. Food of seals of the Okhotsk Sea. In Tez. dokl. 5-go

Vses. sovesch. po izuch. mor. mlekop., Chast. 1., Makhachkala, USSR, p. 110-112. [In Russ.]

Frost, K j.

1981. Descriptive key to the otoliths of gadid fishes of the

Bering, Chukchi, and Beaufort Seas. Arctic 34:55-59.

Frost, Kj., and L. F. Lowry.

1980. Feeding of ribbon seals (Phoca fasciata) in the Bering

Sea in spring. Can.j. ZooI. 58:1601-1607.

1981a. Ringed, Caspian, and Baikal seals. In S. H. Ridgway

and R. J. Harrison (eds.), Handbook of marine mammals,

vol. 2. Seals, p. 29-54. Academic Press, Lond.

1981b. Trophic importance ofsome marine gadids in northern Alaska and their body-otolith size relationships. Fish

Bull. 79:187-192.

1986. Sizes of walleye pollock, Theragra chalcogramma, consumed by marine mammals in the Bering Sea. Fish Bull.

84:192-197.

Frost, K j., L. F. Lowry, E. H. Sinclair,]. VerHoef, and

D. C. McAllister.

1994. Impacts on distribution, abundance and productivity

of harbor seals. In T. R. Loughlin (ed.), Marine mammals

and the Exxon Valdez., p. 97-118. Academic Press, N.Y.

Gol'tsev, V. N.

1971. Feeding ofspotted seals. Ecologiya 2:62-70. [In Russ.]

Kosygin, G. M.

1966. Some data on feeding of bearded seals in the Bering

Sea in spring and summer. Izv. TINRO 58:153-158. [In

Russ.]

1971. Feeding of bearded seals, Erignathus barbatus nauticus

(Pallas), in the Bering Sea in spring and summer. Izv.

TINRO 75:144-151. [In Russ.]

Kosygin, G. M., and V. N. Gol'tsev.

1971. Data on the morphology and ecology of spotted seals

in Tatarsky Strait. In Issledovaniya morskikh mlekopitaushchih, p. 238-251. Kaliningrad. [In Russ.]

Kosygin, G. M., S. 1. Lagerev, and A. M. Trukhin.

1984. Data on the biology of pinnipeds in the Bering Sea

(ZRS Zybovo 1982). In NIR po morskim mlekopitaushchim

Severnoi chasti Tikhogo Okeana v 1982/83 g, p.42-

46. VNIRO, Moskva. [In Russ.]

Lentfer,]. W. (ed.).

1988. Selected marine mammals of Alaska. Species accoun ts

with research and management recommendations. U.S.

Marine Mammal Commission, Washington, DC, 275 p.

Lowry, L. F., Kj. Frost, andj.]. Burns.

1980a. Feeding of bearded seals in the Bering and Chukchi

Seas and trophic interaction with Pacific walruses. Arctic

33:330-342.

1980b. Variability in the diet of ringed seals, Phoca hispida, in

Alaska. Can. j. Fish. Aquat. Sci. 37:2254-2261.

Lowry, L. F., K]. Frost, R. Davis, R. S. Suydam, and D. P. DeMaster.

1994. Movements and behavior of satellite-tagged spotted

seals (Phoca largha) in the Bering and Chukchi Seas. U.S.

Dep. Commer., NOAA Tech. Memo. NMFS-AFSC-38, 71 p.

Lowry, L. F., Kj. Frost, and T. R. Loughlin.

1989. Importance of walleye pollock in the diets of marine

mammals in the Gulf ofAlaska and Bering Sea, and implications for fishery management. In Proc. Int. Symp. BioI.

Manage. Walleye Pollock, p. 701-726. Alaska Sea Grant

Rep. 84-1.

Markussen, N. H., M. Ryg, and N. A. Oritsland.

1990. Energy requirements for maintenance and growth of

captive harbor seals, Phoca vitulina. Can. j. Zool. 68:423-

426.

Panina, G. K

1966. Nutrition of sea lion and seals on Kuril Islands. Izv.

TINRO 58:235- 236. [In Russ.]

Perez, M. A., and W. B. McAlister.

1993. Estimates of food consumption by marine mammals in

the eastern Bering Sea. U.S. Dep. Commer., NOAA Tech.

Memo. NMFS--AFSC-14, 36 p.

Pierce, G.j., and P. R. Boyle.

1991. A review of methods for diet analysis in PISClvoroUS

marine mammals. Oceanogr. Mar. BioI. Annu. Rev. 29:409-

486.

Pikharev, G. A.

1946. Nutrition of ringed seal. Izv. T1NRO 22:259-261. [In

Russ.]

Pitcher, K W.

1980. Food of the harbor seal, Phoca vitulina richardsi, in the

Gulf of Alaska. Fish. Bull. 78:544-549.

1981. Prey of the Steller sea lion, Eumetopias jubatus, in the

Gulf of Alaska. Fish. Bull. 79:467-472.

1990. Major decline in number of harbor seals, Phoca vitulina

richardsi, on Tugidak Island, Gulf of Alaska. Mar. Mamm.

Sci. 6:121-134.

Popov, V. N., and Y. A. Bukhtiyarov.

1975. Age related changes in feeding and helminth fauna of

spotted seals in the Sea of Okhotsk. In Morskie mkekopitaushchie: Mat. 6-go Vses. soveschaniya, p. 62-64. Kiev.

[In Russ.]

Shallghnessey, P. D., and F. H. Fay.

1977. A review of the taxonomy and nomenclature of North

Pacific harbour seals. j. ZooI. (Lond.) 182:385-419.

Shustov, A. P.

1965. The food ofribbon seals in the Bering Sea. Izv. TINRO

59:178-183.

Sobolevsky, E. 1.

1983. Marine mammals of the Sea of Okhotsk; distribution,
___ Lowry et al.: Importance of Walleye Pollock in the Diet of Phocid Seals in the Bering Sea and Northwestern Pacific Ocean 151

numbers, and their importance as predators. Biologiya

Morya 5:13-20. [In Russ.]

Springer, A. M.

1992. A review: walleye pollock in the North Pacific-how much

difference do they really make? Fish. Oceanogr. 1:80-96.

Trukhin, A. M.

1986. Information on the feeding ofyoung spotted seals. In Tez.

dokJ. 9-g0 Vses. sovesch. po izuch., okhrane i raz. ispolz. mor.

mlek., Arkhangelsk, 9-11 Sent. 1986, p. 39~399. [In Russ.]

Trukhin, A. M., A. 1. Makhnyr', and V. A. Pavluchkov.

1991. Feeding of true seals in the Bering Sea in spring and

summer. In Biologiya ryb i bespozvonochnykh severnoi chasti

tikhogo okeana, p. 128-142. Vladivostok. [In Russ.]

Wada, K., T. Hamanaka, T. Nakaoka, and K Tanahashi.

1992. Food and feeding of Kuril and Largha seals in southeastern Hokkaido. Mammalia 56:555-566.

Wilke, F.

1954. Seals of northern Hokkaido. J. Mammal. 35:218-224.

Importance ofJuvenile Walleye Pollock, Theragra chalcogramma,

in the Diet of Gulf of Alaska Steller Sea Lions, Eumetopiasjubatus

RICHARD L. MERRICK

National Marine Mammal Laboratory

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.E.

Seattle, Washington 98115-0070

DONALD G. CALKINS

Alaska Department ofFish and Game

333 Raspberry Road

Anchorage, Alaska 99502

ABSTRACT

Walleye pollock, Theragra chalcogramma, were the most common prey ofjuvenile (~4

years old) and adult Steller sea lions, Eumetopias jubatus, in virtually all seasons and areas of

the Gulf ofAlaska sampled during 1975-78 and 1985-86. Other fish prey (e.g., small forage

fish) were consumed only on a seasonal basis. The proportion of sampled animals eating

walleye pollock increased in the Kodiak Island area from the 1970's (49.2%) to the 1980's

(68.7%). This increase resulted from a combination of seasonal increases (particularly for

the May-August time period) and age-specific (particularly forjuvenile sea lions) increases

in the proportion of sea lions eating pollock, coupled with coincident declines in the

consumption ofsmall forage and demersal fish.Juvenile pollock were a major part ofthe sea

lion diet in both the 1975-78 and 1985-86 sea lion collections; juvenile sea lions ate

significantly smaller and relatively more juvenile pollock than adult sea lions. Juvenile

pollock remained a major prey in 1985-86 despite a 48.1 % decrease in abundance of age 2-

3 juvenile pollock between 1975-78 and 1985-86. This paradox could be explained by the

sea lion's preference for small fish, coupled with a disproportionate, simultaneous decrease

in the abundance of alternative small fish prey. Generally low abundance ofjuvenile pollock

and alternative prey throughout most of the 1980's, combined with reduced consumption

of alternative prey, may have contributed to the sea lion population decline in the central

Gulf of Alaska during the 1980's.

Introduction

Steller sea lion, Eumetopiasjubatus, numbers in the central Gulf ofAlaska (GOA) declined 20.5% from 1976 to

1985 (2.5% per year; Merrick et aI., 1987). The cause

for this decline appears related in part to the number

of animals taken incidental to commercial fisheries

(Perez and Loughlin, 1991). The rate of decline accelerated in 1985; the central GOA population declined

by 55% between 1985 and 1989 (18.1 % per year; Merrick

et aI., 1991). Central GOA sea lion declines have continued through 1991-92 at a similar rate (Sease et aI., 1993).

One explanation for the ultimate cause of the recen t

(ca. 1985-93) decline of Steller sea lions is that the

quantity or quality of their prey has declined (Braham

et aI., 1980; Merrick et aI., 1987; Loughlin and Merrick,

1989; Alverson, 1991; Calkins and Goodwin!). Other

proposed causes (e.g., emigration, disease, subsistence

harvest, commercial pup harvests, pollution, predation)

are either not supported by the data or could not exI Calkins, D., and E. Goodwin. 1988. Investigation of the declining

sea lion population in the Gulf ofAlaska. Unpub!. rep. Alaska Dep.

Fish and Game, 333 Raspberry Rd., Anchorage, AI( 99502,76 p.

153
154 NOAA Technical Report NMFS 126 , _

plain declines in areas such as the central and western

Aleutian Islands, where there is little human activity

(e.g., fisheries). Such widespread declines (from Prince

William Sound to the end ofthe Aleutian Islands) point

toward an ecosystem-wide effect.

Circumstantial evidence supports the food-limitation

explanation. Analysis of the proximate cause of the sea

lion decline points toward reduced juvenile survival

(Merrick et aI., 1988; Loughlin and Merrick, 1989; York,

1994) .Juvenile sea lions would be especially susceptible

to increased mortality brought on by reduced food

availability because young animals, especially newly

weaned pups, are inexperienced foragers with limited

range (Merrick and Loughlin2).

Sea lions in the GOA were significantly heavier from

age 1 onward in the 1970's (predecline) than they were

in the 1980's (during the decline).! There is also a

suggestion in the 1980's collection that some animals

were anemic.! Both smaller size and anemia would be

expected in nutritionally stressed animals. A decline in

prey abundance provides the most parsimonious expla·-

nation for GOA population declines of other piscivo--

rous marine predators-harbor seal, Phoca vitulina,

(Pitcher, 1990; Loughlin3) and sea birds (e.g., black-·

legged kittiwakes, Rissa tridactyla; Springer, 1993).

If the cause of the decline from the early 1980's to

the present is food-related, then how has the diet or

prey base changed? Because walleye pollock, Theraf!7a

chalcof!7amma, has been the most common prey ofSteller

sea lions in the GOA since the 1970's (Pitcher, 1981;

Calkins and Goodwin1), an understanding ofthis prey's

role in the sea lion's diet may provide the key to understanding the sea lion's decline in the GOA. But the

importance ofwalleye pollock cannot be understood in

vacuo; the whole diet must be considered. Thus we

discuss the general Steller sea lion diet as determined

from data collected in the GOA in 1975-78 (predecline)

and 1985-86 (during the decline), and compare this

with the limited data available on the abundance of

various sea lion prey.

Methods

The information presented here is a reanalysis of data

obtained from animals collected specifically for studies

2 Merrick, R. L., and T. R. Loughlin. 1993. Foraging behavior of

adult female and young-of-year Steller sea lions in Alaskan waters.

Abs. Tenth Biennial Conf. BioI. Mar. Mammal, November 11-15,

1993, Galveston, TX. (Avail. National Marine Mammal Laboratory

library, 7600 Sand Point Way N.E., Seattle, WA 98ll5-0070).

3 Loughlin, T. R. 1993. Abundance and distribution of harbor

seals (Phoca vitulina richardsi) in the Gulf of Alaska and Prince

William Sound in 1992. Ann. Rep., Mar. Mammal Protection Act

Assess. Prog., Ofc. Protected Res., NMFS, NOAA.

of food habits and related questions. We include only

data from collections made in the GOA in 1975-78 and

1985-86 (Pitcher, 1981; Calkins and Goodwin]). These

data form the only relatively complete data set available

for analysis of temporal or area- specific trends in the

diet of Steller sea lions. Earlier studies in the GOA used

very small sample sizes (Imler and Sarber, 1947; Fiscus

and Baines, 1966), focused on a single site in one

season (Mathisen et aI., 1962), or considered only territorial males (Thorsteinson and Lensink, 1962). The

latter two studies also did not completely identify prey

remains. We have generally not included data for animals incidentally caught in fisheries because the data

may represent a biased sample of food habits. We do,

however, present some of these data (Loughlin and

Nelson, 1986) in our discussion of the importance of

small fish.

Collections in 1975-78 were made in six areas where

sea lion numbers were not declining (Merrick et aI.,

1987)-eastern GOA, Middleton Island, Prince William Sound, Kenai Peninsula, Kodiak Island, and western GOA (Fig. 1). In 1985-86, animals were collected

only in southeast Alaska, Kodiak Island, and western

GOA. At that time, sea lion numbers were declining in

both the Kodiak Island area and the western GOA, but

increasing in southeast Alaska (Merrick et aI., 1987).

Seasonal coverage varied between the two collections

(Table 1). Only in the Kodiak Island area were sea lions

collected throughout the year in both the 1970's and

1980's. Consequently, most of the treatment of temporal

differences in this analysis uses only Kodiak Island data.

A total of 408 animals were collected: 230 in 1975-78,

and 178 in 1985-86. Of these animals, 263 had stomachs with contents: 178 in 1975-78, and 85 in 1985-86

(Table 1). Animals were collected and stomach contents processed as described in Pitcher (1981) and

Calkins and Goodwin.] Prey identification was based on

whole fish, fish otoliths, and cephalopod beaks. Estimates of the fork lengths (FL) and mass of walleye

pollock consumed were calculated from a linear regression of otolith length to FL or mass (Frost and Lowry,

1981) .

Individual species consumed are reported in Pitcher

(1981) and Calkins and Goodwin.! The species identified were

1. Gadids-walleye pollock; Pacific cod, Gadus macrocephalus

2. Pacific salmon-Oncorhynchus spp.

3. Small forage fish-capelin, Mallotus villosus; Pacific

herring, Clupea harengus; Pacific sand lance, Ammodytes

hexapterus.

4. Flatfish-Pacific halibut, Hippoglossus stenolepis;

arrowtooth flounder, Atheresthes stomias; rock sole,

Lepidopsetta bilineata; other Pleuronectidae
__ Merrick & Calkins: Importance ofJuvenile Walleye Pollock in the Diet of Gulf of Alaska Steller Sea Lions 155

r----------------------------~65°N :)...

ALASKA

EASTERN

GOA

PRINCE

WILLIAM

SOUND

WESTERN

GOA

Gulf of Alaska

B R I TIS H 60°

COLUMBIA

I--T"'""-------T"'""------......--------~ 50°

130°

Figure I

Collection areas for study of Steller sea lion diet.

5. Other demersal fish-sculpins, Cottidae; rockfish,

Sebastes spp.; eelpout, Lycodes spp.; Pacific sandfish,

Trichodon trichodon; poacher, Agonidae; skates, Raja

spp.

6. Cephalopods-squid and octopus

In categorizing prey, we recognized that the traditional

presentation of species data adds little to our understanding of how sea lions forage. It is presumed that sea

lions select prey through a combination of their own

foraging behavior and the prey's behavior. Differences

in prey behavior (e.g., schooling, seasonal movements,

diel vertical migration) lead to different daily or seasonal patterns of aggregation. Consequently, prey may

be available only seasonally (e.g., capelin) or to those

sea lions with a foraging ability (e.g., dive depths) that

allows them to encounter the prey. For example, shallow-diving young-of-the-year sea lions may not be able

to dive deep enough to reach most adult pollock. Our

species groupings also account for the different caloric

values of prey (Perez, 1994). For example, the "small

forage fish" group is generally rich in fat, whereas the

gadid group is low in fat but rich in protein. Finally, we

also recognize that these taxa have exhibited similar

trends in abundance since the late 1970's (Low, 1991;

Bakkala, 1993; Fritz et al., 1993; Anderson et al., 1994;

Hollowed et al. 4).

Prey consumption was based on the proportion of

animals found to consume a specific prey (i.e., frequency of occurrence of a prey group in sea lion stomachs). That is, if a prey was found in one-half of all

stomachs with contents that were sampled, its frequency

of occurrence was 50%. We have not presented data on

the numbers of prey found in individual stomachs,

largely because of a lack of data and specimens from

the collections. Without these data, it is difficult to

perform rigorous volumetric comparisons. Fortunately,

most stomachs contained prey from only one group.

The only prey species commonly found mixed with

other taxa was walleye pollock. To clarify the frequency

of consumption of pollock, we present the frequency of

occurrence of pollock both as the number ofstomachs

containing pollock mixed with other prey and the number ofstomachs containing only pollock.

We used Pearson's chi-square statistic for statistical

comparisons offrequencies of occurrence. The level of

significance was set at ex = 0.05.

4 Hollowed, A. B., B. A. Megrey, and E. S. Brown. 1993. Walleye

pollock. In Stock assessmen t and fishery evaluation report for the

1994 Gulf of Alaska groundfish fishery. Gulf of Alaska Groundfish Plan Team, North Pacific Fishery Management Council, P. O.

Box 103136, Anchorage, AK 99510,50 p.
156 NOAA Technical Report NMFS 126

Table 1

Collection of Steller sea lion stomachs with contents by period, area, sea-lion age class, and season.

Season

Period Area Age class Spring Summer Fall Total

1975-78 Southeast Alaska Juvenile

Adult

Sum 0 0 0 0

Eastern Gulf of Alaska Juvenile 1 2

Adult 2 2

Sum 3 0 4

Prince William Sound Juvenile 15 18 33

Adult 17 16 33

Sum 32 0 34 66

Kenai Peninsula Juvenile 25 2 28

Adult 10 10

Sum 35 2 38

Western Gulf of Alaska Juvenile 0

Adult 2 3

Sum 2 0 3

Kodiak Island Juvenile 9 14 10 33

Adult 6 14 10 30

Sum 15 28 20 63

Middleton Island Juvenile 2 2

Adult 2 2

Sum 0 4 0 4

Total 1975-78 Juvenile 50 19 29 98

Adult 35 18 27 80

Sum 85 37 56 178

1985-86 Southeast Alaska Juvenile 1 1

Adult 13 13

Sum 0 14 0 14

Eastern Gulf of Alaska Juvenile

Adult

Sum 0 0 0 0

Prince William Sound Juvenile

Adult

Sum 0 0 0 0

Kenai Peninsula Juvenile

Adult

Sum 0 0 0 0

Western Gulf of Alaska Juvenile 4 4

Adult 0 0

Sum 0 4 0 4

Kodiak Island Juvenile 5 6 5 16

Adult 22 14 15 51

Sum 27 20 20 67

Middleton Island Juvenile

Adult

Sum 0 0 0 0

Total 1985-86 Juvenile 5 11 5 21

Adult 22 27 15 64

Sum 27 38 20 85
__ Merrick & Calkins: Importance ofJuvenile Walleye Pollock in the Diet of Gulf of Alaska Steller Sea Lions 157

Results

Overall Diet

Fish were the primary prey of Steller sea lions in all

areas in both the 1970's and 1980's (94.4% to 87.1 % of

all GOA stomachs with contents; Table 2). Consumption of cephalopods was common (29.2% to 36.5% of

stomachs). However, few sea lions ate just squid and

octopus; most consumers of these taxa also had fish

remains in their stomachs. The cooccurrence of fish

and cephalopod remains may indicate the accumulation of beaks in stomachs through successive feeding

trips, which would lead to underrepresentation ofthose

sea lions feeding only on fish.

Gadids (mostly walleye pollock) were the most common fish taxa consumed (70.8% to 72.9%) in all areas

of the GOA (Table 2, 3). Small collections in the eastern GOA (n=4) and the western GOA (3 in the 1970's

and 4 in the 1980's) also showed walleye pollock to be

the most common prey. Pollock was generally the most

common sea lion prey whether one considers all stomachs containing pollock or those containing only pollock (Table 3). The proportion of animals consuming

pollock was greatest in the 1970's Prince William Sound

and Kenai Peninsula collections (83.3% and 84.2% of

stomachs, respectively). The lowest frequency was in

the Kodiak Island area in the 1970's (49.2%), but the

Kodiak Island area frequency of consumption increased

to 68.7% in the 1980's. This was comparable to southeast Alaska in the 1980's (64.3%).

The only collections where walleye pollock was not

the most common prey were made in the 1970's. In

1977-78, capelin alone was found in 25.0% ofthe Kodiak

Island area stomachs (compared to 23.8% for pollockonly stomachs) and in all four stomachs of animals

collected at Middleton Island in 1976.

Small forage fish were the second most common fish

prey in the 1970's (23.0% ofstomachs; Table 2), whereas

flatfish were the second most common fish prey in the

1980's (15.3%). Pacific salmon was uncommon in all

collections. Cephalopod consumption varied between

areas and years from 15.8% to 42.4% ofstomachs (Table

2). These prey items were generally octopus, except in

Prince William Sound in the 1970's, when squid was

consumed most frequently.

In 1975-78, there were two significant differences

between the Kodiak Island area diet and the remainder

of the GOA collection (Table 2, 3). Pollock was consumed less frequently in the Kodiak Island area (P<O.OI) ,

while salmon were consumed more frequently there

(P=0.01). There were no significant differences in the

frequency ofoccurrence ofthe various prey for the 1980's

Kodiak Island and southeast Alaska collections.

Sizes of Pollock Consumed

Juvenile pollock constituted a major part ofthe pollock

eaten in the Kodiak Island area in both the 1975-78

(n=2,080; mean=29.8 cm FL; sd=11.6; Pitcher, 1981)

and 1985 (n=1,040; mean=25.1 cm FL; sd=1O.3) sea lion

Table 2

Percentages of Steller sea lion stomachs containing various prey taxa from the Gulf of Alaska (GOA) subareas for the

1970's and 1980's. "Gadid" includes walleye pollock and Pacific cod; "Small forage fish" includes Pacific herring, capelin,

and Pacific sand lance; and "Other demersal fish" includes rockfish, sculpins, etc. GulfWide summary includes additional

samples from the eastern GOA, Middleton Island, and the Alaska Peninsula.

Percentage ofstomachs with various prey

Small Other Squid Fish

forage demersal and and

Area Period n Gadid Salmon fish Flatfish fish All fish octopus ceph.

Kodiak Island 1970's 63 52.4 7.9 28.6 7.9 11.1 88.9 22.2 12.7

1980's 67 70.1 2.0 10.4 13.4 1.5 85.1 40.3 25.4

SE Alaska 1970's 0

1980's 14 85.7 7.1 14.3 28.6 0.0 92.9 28.6 21.4

Prince William Sound 1970's 66 83.3 0.0 24.2 0.0 10.6 98.5 42.4 40.9

1980's 0

Kenai Peninsula 1970's 38 86.8 0.0 7.9 5.3 15.8 94.7 15.8 13.2

1980's 0

GulfWide 1970's 178 70.8 3.4 23.0 5.1 12.4 94.4 29.2 24.4

1980's 85 72.9 4.7 11.8 15.3 1.2 87.1 36.5 23.5
158 NOAA Technical Report NMFS 126 , _

Table 3

Percentage of adult andjuvenile Steller sea lion stomachs with contents that included walleye pollock and those containing only

walleye pollock from Gulf of Alaska (GOA) subareas for the 1970's and 1980's. Gulfwide summary includes additional

samples from the eastern GOA, Middleton Island, and the Alaska Peninsula.

1975-78 1985-86

Area Age n With pollock Only pollock n With pollock Only pollock

Kodiak Island Juvenile 33 45.5 27.3 16 75.0 50.0

Adult 30 53.3 20.0 51 66.7 29.4

All 63 49.2 23.8 67 68.7 34.3

SE Alaska Juvenile 0 1 0.0 0.0

Adult 0 13 69.2 23.1

All 0 14 64.3 21.4

Prince William Sound Juvenile 33 84.8 39.4 0

Adult 33 81.8 12.1 0

All 66 83.3 25.8 0

Kenai Peninsula Juvenile 28 82.1 64.3 0

Adult 10 90.0 30.0 0

All 38 84.2 55.3 0

Gulfwide Juvenile 98 68.4 41.8 21 71.4 47.6

Adult 80 70.0 18.8 64 67.2 28.1

All 178 69.1 31.5 85 68.2 32.9

collections. This difference in mean fork· length was

significant (t=13.7, P<O.OI). Over 74% of the fish in the

1985 sample were smaller than 30 cm FL. Mass of the

average-size fish consumed by sea lions fell 37%, from

148 g in 1975-78 to 93 g in 1985.1 The average number

of pollock consumed by sea lions in a feeding trip may

have increased between the two periods. Stomachs

(n=20) examined from the 1975-78 collections contained remains of an average of 49 pollock, whereas 44

stomachs examined in 1985-86 contained an average

of 72 pollocks' remains. Thus the mean mass of pollock

consumed on the most recent feeding trip of these

animals was around 7.3 kg in 1975-78 and 6.7 kg in

1985-86.

The length distribution of fish consumed in 1985

(Fig. 2) indicated a strong mode around 17-19 cm FL;

these would have been l-year-old fish (1984 year class).

A second, weaker mode was apparent around 39-43 cm

FL. This mimicked the GOA size distribution of pollock

in 1985 (Fig. 2; Hollowed et a1. 4). Most (73.1 %) of the

fish consumed were smaller than 30 cm FL, but only

26.8% ofthe mass consumed came from pollock smaller

than 30 cm FL.

Juvenile sea lions in 1985 consumed smaller pollock

(n=344; mean=20.8 cm FL; sd=7.9; Fig. 3) than adult

sea lions (n=720; mean=27.3 cm FL; sd=I1.0; Fig. 3).

The differences were significant (F=92.38; P<O.OI). Juvenile sea lion diets included a significantly greater

proportion of juvenile pollock than did adult diets

(P<O.OI). Of all pollock consumed by 7 juvenile sea

lions, 93.2% were smaller than 30 cm FL, compared

with 65.6% of pollock consumed by 18 adults.

The mean mass of individual pollock consumed in

1985-86 by juvenile sea lions (mean=86.8 g; sd=130.3)

was also smaller than the mean mass consumed by

adults (mean=173.0 g; sd=186.4). Slightly over half of

the walleye pollock mass consumed byjuvenile sea lions

(50.7%) came from fish smaller than 30 cm in length,

while only 21.1 % of the mass consumed by adults was

from juvenile pollock. Stomachs of 11 juveniles examined from 1985-86 contained the remains of an average of 80 pollock (sd=109.8), while 33 adults' stomachs

con tained remains from an average of 70 pollock

(sd=101.8). The mean mass of a meal of pollock consumed by these animals was, consequently, 7.0 kg for

juvenile sea lions and 12.0 kg for adults.

Seasonal and Annual Differences in Prey

Consumption

During the 1970's there were significant (P<0.05) seasonal differences in the consumption of all prey taxa

(Fig. 4). Walleye pollock was consumed most frequently

in spring (January-April) and fall (September-December). Small forage fish and salmon were consumed

almost exclusively in summer, whereas flatfish, other

demersal fish, and cephalopods were consumed most
__ Merrick & Calkins: Importance ofJuvenile Walleye Pollock in the Diet of Gulf of Alaska SteUer Sea Lions 159

50,--------------------,

0.2,------------------, Juvenile

Pollock population

40 --

0.15 . - - - -

c

o

t

&0.1

o

et

30 Q)

.!1

E

::J

Z 20

10

10 15 20 25 30 35 40 45 50 55 60

olu..uJ:uIillL.L.1J..lJ.L.L.1J..lJ.LiliL.L.lJlillL.Llt.ImunnnniJl.wJ.JJIrm.un:dJ.J.J..lnLl..lIII.llllLLLJ..L.L n-rf Jh-, n n en ,~~, ol..J......l--'-L.W..LJ..J.J...L..l..Ll..l.LJ.J..LLUJ...Lilli.J..LLUlll.lllJ..LlJll.llll.1l.Ltl:t±..J.-l..J 5

7 11 15 19 23 27 31 35 39 43 47 51 55 59 63 67

0.05

50,-------------------,

0.2,--------------------,

Adult

Pollock consumed 40

0.15 - - - - - - - - - - .. - - - - - - - .. - - - - - _

c

o

t

&0.1 o

It

30 Q)

.!1

E

::J

Z 20

0.05 10

7 11 15 19 23 2731 3539 43 47 51 55 59 63 67

Fork Length (em)

rllJ o I

5 10 15 20 25 30 35 40 45 50 55 60

Fork Length (em)

Figure 2

Size distributions (em) of the walleye pollock population (upper) and walleye pollock consumed by Steller

sea lions (lower) in the Kodiak Island area of the Gulf

of Alaska in 1985.

Figure 3

Size distributions (em) ofwalleye pollock consumed by

juvenile (upper) and adult (lower) Steller sea lions in

the Kodiak Island area of the Gulf of Alaska in 1985.

frequently in spring or fall. No significant seasonal

differences were found for any of the prey taxa from

the 1985-86 collection.

Between 1975-78 and 1985-86, the proportion of

animals consuming pollock and cephalopods increased

significantly (P<0.05; Table 2, 3). The proportion consuming small forage fish and other demersal fish decreased significantly (P<O.OI).

Walleye pollock was the fish prey most commonly

consumed by sea lions collected in the Kodiak Island

area for all seasons examined in the 1970's and 1980's,

except for summer in the 1970's (Fig. 4). However, the

proportion of animals consuming pollock increased

significantly in summer between 1975-78 and 1985-86

(from 32.1 % to 60.0%; P=0.05). A nonsignificant increase was observed in the spring (from 66.7% to 81.5%

of stomachs; P>0.05), whereas fall consumption remained unchanged at 60%. Between the 1970's and

1980's, summertime consumption of small forage fish

declined significantly (P=0.01) from 60.7% to 20.0%.

Other demersal fish declined in the spring (from 26.7%

to 3.7%; P=0.03) and fall (15.0% to 0.0%; P=0.07), the

only times they were consumed.
160 NOAA Technical Report NMFS 126

100 .--------------------r--------,

__ 17.9 _

55

.Spring (n=15)

GSummer (n=28)

oFall (n=20)

26.7

FI.lfish Other demersal Cephalopod

a

Small forage

a

Salmon

a

60.7

19705

Pollock

66.7

80 Q)

o

c

Q)

L..

::J

o

o

o

-o

>.

o

c

Q)

::J

0-

Q)

L..

-

+-'

c

Q)

o

L..

Q)

a..

100 r---------------,----__

Q) 81.5

0

c 80

Q)

L..

::J

0

0

0

-0

>.

0

c

Q)

::J

0-

Q)

L..

-+-'

c

Q)

0

L..

Q)

a..

19805 • Spring (n =27)

EI Summer (n=20)

pfa,11 !n:=~0l. __

50

G.did Salmon Small forage FI.lfish Other demersal Cephalopod

Prey Item

Figure 4

Frequency of occurrence of Steller sea lion prey items by season for the

Kodiak Island area in 1975-78 and 1985-86. Spring includes January

through April, summer includes May through August, and fall includes

September through December.
__ Merrick & Calkins: Importance ofJuvenile Walleye Pollock in the Diet of Gulf of Alaska Steller Sea Lions 161

Sea Lion Age Differences in Prey Consumption

Pollock was the most common prey for both sampled

juvenile (~4 years old) and adult sea lions. More juveniles than adults had only pollock in their stomachs in

most areas and times sampled (Table 3). Kodiak Island

area juveniles ate small forage fish more frequently

than did adults (P=O.OI; Fig. 5), whereas adults ate

flatfish and cephalopods more frequently than did juveniles (P=O.OI; Fig. 5). Salmon and other demersal

fish were all consumed more frequently by adults, but

the differences were not statistically significant. Similar

dietary differences were observed forjuveniles and adults

in collections outside the Kodiak Island area, except

that there was not a significant age difference in the

consumption ofsmall forage fish.

Between the 1970's and 1980's, the proportion of

juvenile sea lions consuming pollock increased significantly, from 45.5% to 75.0% (P=0.05; Fig. 5). The proportion of adults consuming pollock increased from

53.3% to 66.7%, but the difference was not significant

(P=0.23). The proportion ofjuveniles consuming pollock increased in the spring (from 66.7% in the 1970's

to 80.0% in the 1980's), summer (from 21.4% to 50.0%),

and fall (from 60.0% to 100.0%). Similar increases

were observed for adults in the spring (from 66.7% to

81.8%) and summer (from 42.9% to 64.3%). Adult

consumption in the fall fell from the 1970's to 1980's

(from 60.0% to 46.7%).

In 1985-86, the only fish prey found in 16 Kodiak

Island area juveniles' stomachs were walleye pollock,

Pacific cod, Pacific herring, and Pacific sand lance.

None contained salmon, flatfish, or other demersal

fish.

Discussion

Walleye pollock was the most common prey of Steller

sea lions in virtually all areas, seasons, and years sampled.

This includes areas and times when the Steller sea lion

population was either stable (the 1970's and the 1980's

southeast Alaska collection) or decreasing (the 1980's

Kodiak Island collection). Areas where the biomass of

pollock was not large enough to support large fisheries

for pollock (Prince William Sound and southeastAlaska)

also had high pollock consumption.

The proportion of sampled animals consuming pollock increased in the Kodiak Island area from the 1970's

to the 1980's, particularly during the summer. The

overall proportion of animals consuming pollock was,

however, somewhat affected by the uneven sampling of

sea lions by age and season during the two collections

(Table 1). An alternative approach to estimating the

proportions consuming pollock would be to average

within yearly periods the proportions from each sea

lion age-season combination (e.g.,juveniles in spring).

This yields revised estimates of the proportion ofKodiak

Island animals consuming pollock: 53.0% in the 1970's

and 70.5% in the 1980's. Because these are not substantially different from the original estimates of 49.2% in

the 1970's and 68.7% in the 1980's, it would appear

that the decadal increase in Kodiak area pollock consumption by sampled animals was real.

Another important finding from these analyses is

that small forage fish were consumed only seasonally.

Capelin, sand lance, and herring were most commonly

found in Kodiak Island area sea lion stomachs during

May andJuly-August. In Prince William Sound, herring

were generally consumed only in February (14 of 32

stomachs) ; only 2 of 34 stomachs from October through

November contained forage fish. This seasonal pattern

may be due to increased accessibility of these prey

species at those times of year. These prey represent an

ephemeral resource that is exploited when abundant

(e.g., capelin during the spring spawning run). It remains a possibility, however, that these prey could play

a significant role in the seasonal foraging ecology ofsea

lions (particularly juvenile sea lions). Nonetheless, the

small fish prey most commonly consumed by sampled

GOA sea lions on an annual basis was juvenile walleye

pollock.

Diet Changes and Their Causes

The diet of Kodiak Island area sea lions appears to have

changed between the 1970's and 1980's. The proportion ofsea lions consuming pollock, flatfish, and cephalopod was greater in the 1980's than in the 1970's,

while significant decreases occurred in the proportion

of animals consuming small forage fish and other demersal fish. These changes were most apparent in juveniles. The proportion ofjuveniles consuming walleye

pollock increased by 65%, while the proportion consuming forage fish decreased by 25%. None of the

juveniles sampled in 1985-86 consumed salmon, flatfish, or other demersal fish.

The apparent change in diet of Kodiak Island area

sea lions could represent

1. An artifact of the small sample size or ofsampling

locations

2. A change in prey preferences

3. Increased abundance ofwalleye pollock relative to

the abundance ofsmall forage fish and other demersal fish in the 1980's.

Effects of the sample sizes are difficult to discountsample sizes were small, particularly if only juvenile
162 NOAA Technical Report NMFS 126

100r--------------~------,

0>

()

C

0>

.....

::J

()

()

o

'+-

o

>-

()

C

0>

::J

0-

0>

.....

'+-

+-'

C

0>

()

.....

0>

a..

80

60

40

Juvenile

75

45.5

Pollock Salmon Small forage Flaltish

• 1970's (n =33)

El1980's (n=16)

___ . .16.2 _ l_6.!!

Other demersal Cephalopod

100 ,---------------~----~

80

0>

()

C

0>

.....

::J

()

()

o

'+- 60 o

>-

()

C

0>

::J

0- 40

0>

.....

'+-

+-'

C

0>

()

Q) 20

a..

Adult

66.7

Pollock Salmon Small forage Flaltish

• 1970's (n=30

019805 (n=51)

Other demersal Cephalopod

Prey Item

Figure 5

Frequency of occurrence of prey items in the diets of Kodiak Island area juvenile

and adult Steller sea lions in 1975-78 and 1985-86.
__ Merrick & Calkins: Importance ofJuvenile Walleye Pollock in the Diet of Gulf of Alaska Steller Sea Lions 163

animals are considered. The 1985-86 juvenile sample

(compared to the 1975-78 sample) may have been too

small to include less common prey. However, more

adult sea lions were sampled from the Kodiak Island

area in 1985-86 than in 1975-78, and adults still showed

a reduced consumption ofsalmon, other demersal fish,

and small forage fish.

This analysis was focused on Kodiak Island area collections partly to reduce bias due to sampling area. The

Kodiak Island area samples include animals collected

at the same general times and locations during the

1970's and 1980's. For example, during May 1977 most

of the sea lions collected at the northeast end of the

Kodiak Island Archipelago (e.g., Marmot and Sea Otter

Islands) had consumed capelin. Animals were collected

there in May 1985, and no capelin was found. The

similarity of feeding habits of Kodiak Island area animals to those collected elsewhere in the 1970's and

1980's also suggests that the general dietary patterns

found there are representative of gulfwide consumption patterns.

Increased consumption of pollock in the 1970's could

have been due to an increased preference for this prey.

No data exist to support or refute this hypothesis.

The differences in diet may reflect a change in relative prey abundance. Mean Gulf of Alaska walleye pollock spawner biomass was 0.995 million metric tons (t)

in 1975-78 and increased 46.6% to 1.459 million t in

1985-86. 4 The number of adult fish (~4 years old) was

estimated to be the same in 1975-78 (2,099 billion fish)

and 1985-86 (2,108 billion fish). The biomass increase

was due to the number of large older fish (i.e., 7- and 8-

year-old fish from the 1978 year class).

Total biomass of pollock ~2 years old did, however,

decrease from 1,925 tin 1976-78 to 1,686 t in 1985-86.

This was due to low recruitment. Numbers of 2- and 3-

year-old fish declined by 48.1 % from an average of

2,851 billion fish in 1975-78 to 1,479 billion fish in

1985-86. Although estimates of the biomass of age-l

fish are not available, the numbers of age-2 fish the

following year provide an index. These data suggest

that the abundance of age-l fish declined by at least

29% from 1975-78 to 1985-86.4

These trends in walleye pollock abundance could

best explain increased pollock consumption ifthe mean

size of pollock consumed increased. But the mean size

appears to have remained small, implying continued

reliance on small pollock, even with lower abundance.

One explanation for this paradox could be that sea

lions have a preference for small fish, and alternative

small prey were less abundant (relative to juvenile pollock) in 1985-86 than in 1975-78.

Sea lions do commonly eat small fish. Ten of 13 food

habits studies (Table 4) indicated that Steller sea lions

consumed many small fish (mean fork length <30 cm).

Juveniles ate smaller fish than adults in two of the three

collections where juveniles were distinguished from

adults (Table 4). In the Kodiak Island area, 74% of the

pollock eaten by 25 sea lions sampled were under 30 cm

FL. The data for the Kodiak Island area indicated that

small schooling fish may be more important for younger,

smaller sea lions (e.g., a significantly greater proportion of juvenile sea lions ate small forage fish, and the

mean size of pollock eaten byjuvenile sea lions was significantly smaller than the size consumed by adult sea lions).

Recent changes in the abundance of cephalopods,

noncommercial fish (e.g., capelin), and even some commercial species (e.g., herring) are poorly understood.

Commercial groundfish surveys generally do not include or do not adequately sample many of these potentially important prey species. The available data suggest that, in at least one area ofthe Gulf ofAlaska, some

of these species decreased in abundance after the late

1970's. Anderson et al. 5 found in an analysis of trawl

data collected at Pavlof Bay (Alaska Peninsula) during

1972-92 that the species assemblage there had changed.

Before 1978 the assemblage was dominated by Pandalid

shrimps. During 1978-83 the assemblage shifted

abruptly to one dominated by gadids, and then to flatfish during 1984-92. Large declines were observed in

the abundance of shrimp and several small forage and

demersal fishes (e.g., capelin, Pacific sandfish, and

pricklebacks). The decreased abundance of shrimp,

and the increased abundance of cod and flatfish in

Pavlof Bay during 1978-92 seems to be a common

characteristic of the western and central Gulf of Alaska

(Albers and Anderson, 1985; Hollowed et al. 2; Anderson et al. 5; Wilderbuer and Brown6). Because shrimp,

gadid, and flatfish abundance trends in the Gulf of

Alaska are so similar to those of Pavlof Bay, it may be

that forage fish abundance also decreased during the

1980's in areas outside of Pavlof Bay. This has been

demonstrated from a preliminary analysis of Alaska

Department of Fish and Game shrimp trawl data from

other areas of the GOA (Anderson et al.5).

More data on the abundance of forage and other

demersal fish prey is available from the Bering Sea.

Capelin abundance was high in the 1970's but has since

declined (Wespestad7). Analysis of the pattern of scale

5 Anderson, P.]., S. A. Payne, and B. A.Johnson. 1994. Long-term

demersal community structure changes in Pavlof Bay, Alaska.

Unpub!. manuscr. Kodiak Lab., Alaska Fish. Sci. Cent., P. O. Box

1638, Kodiak, AK 99615,28 p.

6 Wilderbuer, T. K., and E. S. Brown. 1992. Flatfish. In Stock assessment and fishery evaluation report for the 1993 Gulf of Alaska

groundfish fishery. Gulf of Alaska Groundfish Plan Team, North

Pacific Fishery Management Council, P. O. Box 103136, Anchorage, AK 99510,25 p.

7 Wespestad, V. G. 1994. Nat!. Mar. Fish. Serv., NOAA, 7600 Sand

Point Way N.E., Seattle 98115-0070. Personal commun.
164 NOAA Technical Report NMFS 126

Table 4

Fork length (em) of prey from stomachs of Steller sea lions collected in various areas during 1975-86 for all animals, for

juveniles (age ~4 years), and for adults.

Mean FL FL range

Species Area Year n (cm) (cm)

All ages

Walleye pollock Gulf of Alaska 1976-78 102 29.8 5.6-62.9

Shelikof Strait 1983-84 36 40.9 30.0-52.0

Kodiak Island 1985 25 25.1 7.9-54.2

SE Alaska 1985 8 25.5 4.8-55.7

Pribilof Islands 1976-79 2 46.9

Pribilof Islands 1986 2 33.5 20.8-44.5

Central Bering Sea 1981 19 27.4 6.2-60.1

Central Bering Sea 1985 10 21.8 10.5-51.6

Western Bering Sea 1981 31 23.5 8.3-64.2

Eastern Aleutian Islands 1981-82 4 29.9 1.7-42.7

Pacific cod Pri bilof Islands 1986 3 27.5 4.3-59.4

Pacific herring Central Bering Sea 1985 6 27.5 25.0-31.I

Yellowfin sole Prjbilof Islands 1986 5 23.7 9.8-35.7

Juveniles

Walleye pollock Kodiak Island 1985 7 20.8 8.0-54.2

Bering Sea 1981 20 22.4 9.6-56.9

Yellowfin sole Pri bilof Islands 1986 2 25 12.0-32.7

Adults

Walleye pollock Kodiak Island 1985 18 27.9 8.7-53.1

Bering Sea 1981 30 26.9 8.5-63.3

Yellowfin sole Pribilof Islands 1986 3 21.6 9.8-35.7

deposition in sediments at Skan Bay on the north side

of Unalaska Island (eastern Aleutian Islands) indicates

that some small forage fish (probably capelin) disappeared from the area in about 1978 (Francis8). Capelin

also disappeared from seabird diets in the Pribilof Islands beginning around 1978 (Hunt9). Analysis of

bycatch of eulachon, Thaleichthys pacificus, in the eastern Bering Sea indicated that its biomass appeared to

have decreased by an order of magnitude from 1980-

81 to 1985 (Fritz et aI., 1993). Bering Sea biomass of

some other demersal species consumed by sea lions

(sculpins and eelpouts) appeared to decrease from the

late 1970's to the mid-1980's (Bakkala, 1993).

It appears that small forage and other demersal fish

species were decreasing at the time gadid and flatfish

populations were increasing. The declines in the consumption ofsmall forage and demersal fish observed in

the sea lion diet could then have resulted from the

decrease in availability of these fish. Increased con8 Francis, R. 1994. Fisheries Research Insl., Box 357980, Univ. Wa·hington, Seaule WA 98195. Personal commun.

9 Hunt, G. L., Jr. Dep. Ecology and Evolutionary Biology, Univ.

California, Irvine, CA 92717. Personal commun.

sumption of flatfish and gadids (particularly walleye

pollock in the summer) could then have resulted from

their greater availability compared to small forage and

other demersal fish.

Significance of the Change in Diet to the Steller

Sea lion Population Decline After 1985

Studies of other pinniped species have found a relationship between the stock sizes of pinnipeds and their

prey. In Antarctic phocid populations, normal

interannual fluctuations in prey biomass produce relatively small, short-term adjustments in pinniped population sizes due largely to fluctuations in juvenile survival or reproductive rates (Testa et aI., 1991). But the

declines in the Alaskan sea lion population have been

more profound. Between 1985 and 1992, sea lion numbers in the central GOA fell by 70% (Sease et aI., 1993).

Such a change, if driven by a decline in prey abundance, would be similar to that observed only in acute

environmental disturbances like the 1983 EI Nino event.

The effects of that event on prey availability appear to

have resulted in near-total year-class failures for north-
__ Merrick & Calkins: Importance ofJuvenile Walleye Pollock in the Diet of Gulf of Alaska Steller Sea Lions 165

ern fur seals, Callorhinus ursinus, and California sea

lions, Zalophus californianus, in southern California, the

Galapagos Islands, and coastal Peru (Trillmich et aI.,

1991). In western Alaska, however, the effect has been

chronic: central GOA declines that began in the early

1980's have continued at least through 1993.

By 1985, the most common prey ofSteller sea lions in

the Kodiak Island area was still juvenile walleye pollock,

even though juvenile pollock abundance was reduced

in the GOA below 1975-78 levels. Declines in abundance of alternative small prey may have led to increased consumption of pollock. Very low recruitment

of pollock from the 1986-87 and 1989-91 year classes

further reduced available forage. Average recruitment

(as 2-year-old fish) for those five year classes was 291

million fish, 19.7% of that of the 1984-85 year class

(1,479 million fish) and 10.2% of the 1975-78 mean

(2,851 million fish4 ). The smaller mean size of fish

consumed in the 1980's would also mean that sea lions

had to forage longer or catch adult pollock to meet

their energetic needs. l Juvenile sea lions may not, however, have been able to dive deeply enough to catch

adult pollock. Dive studies of young-of-the-year Steller

sea lions have indicated that these young animals rarely

dive deeper than 50 m.2 Whereas juvenile pollock are

commonly found in the upper 50 m, adult pollock are

found far deeper (commonly down to 200 m or more).

Thus, lacking the ability to capture adult pollock, young

sea lions would have been particularly vulnerable to

declines in the biomass ofsmall prey.

In summary, walleye pollock was generally the most

common prey item for the Gulf of Alaska Steller sea

lion population during 1975-78 and 1985-86 in all

areas and seasons sampled. Consumption of walleye

pollock in the Kodiak area increased during this period, particularly for juvenile sea lions. This increase

occurred despite decreases in the abundance of small

pollock. Consumption of alternative prey items (e.g.,

small forage fish) declined in the same period, perhaps

because of a relatively greater decrease in the abundance of these prey. Continued low abundance of both

small pollock and alternative small fish prey after 1986

would have reduced juvenile sea lion foraging success.

This, in turn, would have reduced juvenile survival and

could have produced the sea lion population decline

observed in the Gulf of Alaska during the 1980's and

early 1990's.

Acknowledgments _

We wish to thank H. Braham, A. Hollowed,J. Laake, P.

Livingston, T. Loughlin, M. Perez, E. Sinclair, J.

Thomason, and three anonymous reviewers for their

reviews of this manuscript.

Literature Cited

Albers, W. D., and P.]. Anderson.

1985. Diet of Pacific cod, Gadus macrocephalus, and predation

on the northern pink shrimp, Pandalus borealis, in Pavlof

Bay, Alaska. Fish. Bull. 83:601-610.

Alverson, D. L.

1991. Commercial fisheries and the Stellersea lion (Eumetopias

jubatus): the conflict arena. Univ. Washington Fish. Res.

Inst., Seattle, 90 p.

Bakkala, R.

1993. Structural and historical changes in the groundfish

complex of the eastern Bering Sea. U.S. Dep. Commer.,

NOAA Tech. Rep. NMFS 114,91 p.

Braham, H. W., R. D. Everitt, and D.]' Rugh.

1980. Northern sea lion population decline in the eastern

Aleutian Islands. Fish. Bull. 44:25-33.

Fiscus, C. H., and G. A. Baines.

1966. Food and feeding behavior of Steller and California

sea lions. ]. Mammal. 47:195-200.

Fritz, L. W., V. G. Wespestad, and]. S. Collie.

1993. Distribution and abundance trends of forage fishes in

the Bering Sea and Gulf ofAlaska. In Is it food?: addressing

marine mammal and sea bird declines. Workshop summary,

p. 30-43. Alaska Sea Grant Rep. 93-01. Univ. Alaska,

Fairbanks, AK.

Frost, K.]., and L. F. Lowry.

1981. Trophic importance of some marine gadids and their

body-otolith relationship. Fish. Bull. 79:187-192.

Imler, R. H., and H. R. Sarber.

1947. Harbor seals and sea lions in Alaska. U.S. Fish Wildlife Servo Spec. Sci. Rep. 28, 23 p.

Loughlin, T. R., and R. L. Merrick.

1989. Comparison of commercial harvest of walleye pollock

and northern sea lion abundance in the Bering Sea and

Gulf of Alaska. In Proceedings of the International Symposium on the Biology and Management of Walleye Pollock, p.

679-700. Alaska Sea Grant Rep. 89-1.

Loughlin, T. R., and R. Nelson.

1986. Incidental mortality of northern sea lions in the Shelikof

Strait, Alaska. Mar. Mam. Sci. 2:14-33.

Low, L.-L. (ed.).

1991. Status of living marine resources off Alaska as assessed

in 1991. U.S. Dep. Commer., NOAA Tech. Memo. MFS

F/NWC-211, 95 p.

Mathisen, O. A., R. T. Baade, and R.J. Lopp.

1962. Breeding habits, growth and stomach contents of the

Steller sea lion in Alaska. ]. Mammal. 43:469-477.

Merrick, R. L., R. Brown, D. G. Calkins, and T. R. Loughlin.

1995. A comparison of Steller sea lion, Eumetopias jubatus,

pup masses between rookeries with increasing and decreasing populations. Fish Bull. 93:753-758.

Merrick, R. L., L. M. Ferm, R. D. Everitt, R. R. Ream, and

L. A. Lessard.

1991. Aerial and ship-based surveys of northern sea lions

(Eumetopias jubatus) in the Gulf of Alaska and Aleutian Islands during June and July 1990. U. S. Dep. Commer.,

NOAA Tech. Memo. NMFS F/NWC-196, 34 p.

Merrick, R. L., P. Gearin, S. Osmek, and D. Withrow.

1988. Field studies of northern sea lions at Ugamak Island,

Alaska during the 1985 and 1986 breeding seasons. U.S.

Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-143, 60 p.

Merrick, R. L., T. R. Loughlin, and D. G. Calkins.

1987. Decline in abundance of the northern sea lion,

Eumetopiasjubatus, in Alaska, 1955-86. Fish. Bull. 85:351-365.
166 NOAA Technical Report NMFS 126

Perez, M. A.

1994. Calorimetry measurements of energy value of some

Alaskan fishes and squids. U.S. Dep. Commer., NOAA

Tech. Memo. NMFS-AFSC-32, 32 p.

Perez, M. A., and T. R. Loughlin

1991. Incidental catch of marine mammals by foreign andJV

trawl vessels in the U.S. EEZ of the orth Pacific. U.S.

Dep. Commer., NOAA Tech. Rep. NMFS 104, 57 p.

Pitcher, K. W.

1981. Prey of the Steller sea lion, Eumetopias jubatus, in the

Gulf of Alaska. Fish. Bull. 79:467-472.

1990. Major decline in number of harbor seals, Phoca vitulina

richardsi, on Tugidak Island, Gulf of Alaska. Mar. Mammal. Sci. 6:121-134.

Sease,j. L.,.J. P. Lewis, D. C. McAllister, R. L. Merrick, and

S. M. Mello.

1993. Aerial and ship-based surveys of Steller sea lions

(Eumetopias jubatus) in Southeast Alaska, the Gulf of Alaska,

and Aleutian Islands duringJune andJuly 1992. U.S. Dep.

Commer., NOAA Tech. Memo. NMFS-AFSC-17, 57 p.

Springer, A.

1993. Repon of the seabird working group. In Is it food?:

addressing marine mammal and sea bird declines. Workshop summary, p. 14-29. Alaska Sea Grant Rep. 93-01.

Univ. Alaska, Fairbanks, AK.

Testa,.J. W., G. Oehlert, D. G. Ainley,.J. L. Bengtson, D. B. Siniff,

R. M. Laws, and D. Rounsell.

1991. Temporal variability in Antarctic marine ecosystems:

periodic fluctuations in phocid seals. Can. j. Fish. Aquat.

Sci. 48:631-639.

Thorsteinson, F. V., and C. j. Lensink.

1962. Biological observations of Steller sea lion taken during

an experimental harvest. j. Wildl. Manage. 26:353-359.

Trillmich, F., K. A. Ono, D. P. Costa, R. L. DeLong, S. D. Feldcamp,

j. M. Francis, R. L. Gentry, C. B. Heath, B..J. LeBoeuf, P. Majluf,

and A. E. York.

1991. The effects of El Nino on pinniped populations in

the eastern Pacific. In F. Trillmich and K. A. Ono (eds.),

Pinnipeds and El Nino, p. 247-270. Springer-Verlag,

New York.

York, A.

1994. The population dynamics of northern sea lions, 1976-

85. Mar. Mammal. Sci. 10:38-51.
Northern Fur Seal, Callorhinus ursinus,

Predation onJuvenile Walleye Pollock, Theragra chalcogramma

ELIZABETH H. SINCLAIR, GEORGE A. ANTONELIS,

BRUCE W. ROBSON, ROLF R. REAM, and THOMAS R. LOUGHLIN

National Marine Mammal Laboratory

Alaska Fisheries Science Center

National Marine Fisheries Seroice, NOAA

7600 Sand Point Way N.E., Seattle, WA 98115-0070

ABSTRACT

Juvenile walleye pollock, Theragra chalcogramma, is a primary prey species of adult

female and juvenile northern fur seals, Callorhinus ursinus, during summer in the eastern

Bering Sea. Variability in walleye pollock year-class strength is reflected by the size distribution of walleye pollock consumed by northern fur seals.

Prey were identified from the gastrointestinal contents of 73 northern fur seals collected in the eastern Bering Sea in 1981, 1982, and 1985, and from 847 fecal samples

collected on St. Paul Island (Pribilof Islands, Bering Sea) from 1987 to 1990. Walleye

pollock represented 79% of all prey for all years combined in gastrointestinal tracts, and

78% of the total prey in fecal samples. The frequency of occurrence of pollock in all years

averaged 82% in gastrointestinal tracts and 76% in fecal samples.

Adult female and juvenile fur seals forage primarily on age-O and age-l pollock measuring from 3 em to 20 em, as determined by regression analysis of otolith length versus fish

fork length, accounting for wear on otoliths from digestion. Older age groups of pollock

occurred with greater frequency in gastrointestinal samples when very strong cohorts were

followed by weak year classes, and probably in the absence of other prey resources.

The synchrony of trends in walleye pollock year-class strength with the number and size

distribution of pollock observed in the gastrointestinal tracts of adult female and juvenile

northern fur seals indicates that the variability of juvenile pollock recruitment directly

influences the feeding habi ts of northern fur seals.

Introduction

The Pribilof Island population of northern fur seals,

Callorhinus ursinus, is a significant component of the

Bering Sea ecosystem in summer and fall. The herd

contains approximately 980,000 animals, which is about

72% of the world's breeding population for the species

(Loughlin et a\., 1994). The population is pelagic

throughout the northern Pacific Ocean from November to May. FromJune through October, the seals haul

out on the Pribiloflslands (St. Paul and St. George) in

the Bering Sea to give birth and to breed (Fig. 1). Adult

males fast during the summer breeding season. Most

adult females give birth in June and July, then conduct

cyclic feeding trips throughout the summer; the cycles

average 2.5 days onshore nursing pups and 5.9 days at

sea feeding (Gentry et a\., 1986; Loughlin et a\., 1987).

Juveniles are primarily pelagic for the first few years of

life, returning to the islands intermittently during late

summer or fall.

The decline ofthe Pribiloflsland population of northern fur seals in the late 1970's to early 1980's (York and

Hartley, 1981) and lack of growth through the early

1990's may be related to diet or food availability (Sinclair,

1988; Trites, 1992; Sinclair et a\., 1994). This study

examines patterns in consumption of walleye pollock,

Theragra chalcogramma, by northern fur seals in the

Bering Sea during the 1980's and early 1990's, including the age of walleye pollock eaten relative to pollock

year-class strength.

167
C'> -

OC>

-+ Primary distribution ---

MIGRATORY ROUTES AND BREEDING AREAS

OF NORTHERN .FUR SEALS

Nl

-

C'>

z

a::

;;l

z

..,

III

c:r

;:;' =

"0

o

"'l

...

70 OON

60 00

50 00

-40 00

30 00

20 00

...

June-October

I, •

'"

?

/ ?--' I ~mmander Is

I~) .

J'- __/June-October

•

RUSSIA

.~

Migration route

120 OOE 130 00 1-40 00 150 00 160 00 170 00 180 OOW 170 00 160 00 150 00 1-40 00 130 00 120 00 110 00

Figure 1

Breeding islands and migratory routes of northern fur seals, Callorhinus ursinus (adapted from Loughlin and Miller, 1989).
Sinclair et al.: Northern Fur Seal Predation on Juvenile Walleye Pollock 169

Methods

Information on the diet of northern fur seals from the

eastern Bering Sea was collected from August to October between 1981 and 1990. Fresh (within 2 hours of

death) stomach and intestinal (gastrointestinal, or GI,

tract) contents were collected from 16 juveniles and 57

adult females shot at sea in the vicinity of the Pribilof

Islands in 1981 (nr=7) , 1982 (nr=23) , and 1985 (nr=43;

Fig. 2). Most seals were collected in areas where water

was shallower than 200 m and within the outer domain

ofthe continental shelf (Kinder and Schumacher, 1981).

The GI tract data presented here originally appeared in

Sinclair (1988) and Sinclair et al. (1994).

Feces (scats) were collected on St. Paul Island beaches

where adult female fur seals hauled out in 1987 (nr=228) ,

1988 (nr=85) , 1989 (nr=199) , and 1990 (nr=335). Gastrointestinal tracts and scats were stored frozen prior to

analysis. In 1981 and 1982 prey material was gently

flushed through a series of graded sieves, the smallest

of which was 0.71 mm. Mter 1982, a 0.50-mm sieve was

added to the series to further assure retrieval of all prey

material. Otoliths were stored in 70% ethanol.

Prey were iden tified from remains ofwhole bodies or

hard parts. The importance of a prey species in the

northern fur seal diet was determined by examining

values for percent number and percent occurrence.

Percent number (PN) refers to the number ofindividuals representing a particular prey species as a percentage of the total number of all prey species in all samples.

Percent occurrence (PO) refers to the number ofseals

that consumed a particular prey species, expressed as a

percentage of all samples containing identifiable prey.

Before determining the size of walleye pollock eaten

by seals, we assigned to quality categories otoliths recovered from GI tracts (Sinclair et aI., 1994) to account for

any potential loss of otolith length due to degradation

by stomach acids. Quality categories were further defined for walleye pollock otoliths recovered from scats

by calculation of and correction for the percent loss of

length (Prime, 1979; North et aI., 1983; Dellinger and

Trillmich, 1988; Harvey]). Otoliths in fair or poor condition were counted but not measured. Otoliths in

good or excellent condition were measured to the nearest .05 mm with vernier calipers or an ocular micrometer.

Walleye pollock fork lengths (FL) were estimated based

on Frost and Lowry's (1981) otolith-to-body-length regression formulae for pollock in the Bering Sea:

otoliths >10.0 mm:

Y (FL in cm) = 3.175X - 9.770 (r=0.968)

otoliths $10.0 mm:

Y (FL in cm) = 2.246X - 0.510 (r=0.981)

I James Harvey, Moss Landing Marine Laboratory, P.O. Box 450,

Moss Landing, CA 95039-0450. Unpub!. data.

Ages were then assigned based on length-age relationships described by Smith (1981) and Walline (1983) for

pollock from the Bering Sea. The GI and scat data sets

were kept separate during analysis.

Results

Both PN and PO values indicated that walleye pollock

was the dominant prey item for all years in GI tracts and

scats (Fig. 3, 4). The similarity of walleye pollock PN

and PO values in six of the seven sampling years gives a

strong indication of the importance of walleye pollock

in the diet of northern fur seals, as well as the comparative reliability of both collection methods (Fig. 5).

Juvenile walleye pollock (3-20 cm FL; ages 0 and 1)

were the most numerous and frequently occurring prey

in all years (within sample type) combined (Fig. 6). In

GI tract samples, 72% of prey pollock were from the

age-O group (3-13 cm FL) and 24% were age 1 (13-20

cm FL). Four percent of prey pollock from the GI tracts

were age 2+ (~20 cm FL). In fecal samples, 73% of prey

pollock were from the age-O group, 22% were age 1,

and the remaining 5% were age 2+.

Although age-O pollock were the dominant prey of

seals overall, there were marked differences between

years in the GI tract collections (Fig. 7). In 1981, few

walleye pollock remains were found in GI tracts compared to all other years, and most were from 3- and 4-

year-old fish. In 1982, GI tracts contained primarily ageo pollock; in 1985 they contained primarily age-1 pollock. The age-frequency distribution ofwalleye pollock

in the scats, however, was similar from 1987 to 1990 and

was dominated by age-O juveniles (Fig. 8).

Discussion

The size distribution of walleye pollock in the GI contents of adult female and juvenile northern fur seals

collected in the early 1980's reflected annual year-class

strength estimates for walleye pollock (Fig. 9; Sinclair,

1988; Sinclair et aI., 1994). The 1978 year class (age 3 in

1981) was the strongest ever recorded; the 1980 year

class (age 1 in 1981) was about one-half the estimated

average year-class size; and the 1981 year class (age 0 in

1981) was the weakest observed before 1983. The 1982

and 1984 year classes were strong, but the 1985 year

class was considered below average (Fig. 9a; Wespestad

and Dawson2). Similarly, seals ate primarily 3- and

2 Wespestad, V. G., and P. Dawson. 1992. Walleye pollock. In Stock

assessment and fishery evaluation report for the groundfish resources of the Bering SealAleutian Islands regions as projected for

1993, p.I-2 to 1-32. North Pacific Fishery Management Council,

P.O. Box 103136, Anchorage, AI<. 99510.
170 NOAA Technical Report NMFS 126

r---:----------------------------~59°N

\

@D\,

'\~

\0

D 1981 seal Collections

6. 1982 seal Collections

o 1985 Seal Collections

Middle Shelf Domain

o

o

C}.\

".

".

"6. 6.~

"' .... &.. St. George L----.--.._ ~._-~.._.-// .....

..... - ....

..........., .... - - \

.......: \ ... '" --.

....

....

.........

Oceanic Domain

''''Oa170 " , , _ -' ' - - - - - - .., m....

L- y- ··_····_..._....;......._.t....'...:':....- • ...::t:...·<:,_,tr.:::·.~:...~..:. ... _<m9." ..,,:,-.<J' .J.. 540

165°

Figure 2

The study area showing locations of northern fur seal collections (adapted from Sinclair et a!., 1994).

Oceanic domains are shown as defined by Kinder and Schumacher (1981). Stomachs collected in

1981 off St. Paul Island were empty and not included in the final analysis.

4-year old pollock in 1981 (from the 1977 and 1978 year

classes), age-O pollock in 1982 (from the 1982 year class),

and age-1 pollock in 1985 (from the 1984 year class; Fig.

9b). Lacking discrete annual distribution data on juvenile

walleye pollock, we cannot be certain how much differ··

ences in sampling location may have influenced the relative yearly abundance of walleye pollock and walleye pol··

lock age groups in northern fur seal GI contents. However, the variable recruitment patterns of walleye pollock

were reflected in the GI contents of northern fur seals.

Scat analysis indicated a less direct association (rela·

tive to analysis of GI tract samples) between pollock

year-class strength and seals' consumption of pollock.

Analysis of the 1987-1990 samples indicated that seals

ate primarily age-O walleye pollock despite weak to average pollock year-class strength relative to the early 1980's

(Fig. 9, 10). Fishery surveys and commercial catch data

indicate that pollock year-class size in the late 1980's

may actually have been much larger than the projections presented here2 but revised estimates are not yet

available. If the present estimates prove accurate, we

speculate that seal consumption of age-O walleye pollock as primary prey in the late 1980's may be due to the

absence of alternate prey resources (Sinclair et aI., 1994),

strong localized abundances of age-O fish, or the potential for a skewed concentration of remains of younger

fish in the colons of northern fur seals (Sinclair and

Antonelis, unpub!. data).

The largest series of studies of northern fur seal diet

in the eastern Bering Sea was based on the analysis of
1981 Percent number Percent occurrence

100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 1 1 1 1 1 1

walleye pollock

(Theragra chalcogramma)

Atka mackerel

(Pleurogrammus rnonopterygius)

Salmonidae

:::~t:::::::::::::::::::::::::: :;=;::::::::::::r:;:;:;:;:";:·:>:;:;:·.;·;:;·:;::;

-+-

~H???:}}'??::

I

IT'

walleye pollock

(Theragra chalcogramma)

Atka mackerel

(Pleurogrammus monopterygius)

Salmonidae

100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 1 I 1 1 I I 1 I

100 90 80 70 60 50 40 30 20 10 0

Percent number Percent occurrence

•

Vl S·

I'l

;-

::;.

..

z

o

...

..

::'

...

::s

...

Vl

eo

"tl

...

Q.

..o·

::s

o

::s

'-

e.

;-

;-

'<

"tl

£.

0'

I'l

II';"

gonatid squid

(Gonatus madokai/middendorffi)

gonatid squid

(Gonatopsis borealis!

Berryteuthis magister)

walleye pollock

(Theragra chalcogramma)

walleye pollock

(Theragra chalcogramma)

gonatid squid

(Gonatopsis borealis!

Berryteuthis magister)

gonatid squid

(8erryteuthis magister)

Percent occurrence

10 20 30 40 50 60 70 80 90 100

I -'-

-1-

-1-

-+-

--'-

-1-

i,::;:::

:!~~~~I~~~~~~~~~~I

Percent number

;·:.·:;:~:tr:::?:::::::·:::::::::::::::·:::::::-::·:::::::;:;:;:;::=:::=;:;'::;:::::::;=;:;::" :'::::~:~::::::::::::{

["?H':::"'?::?':':,,H},???:}'},?::?,,?:::}:???:}:?:},????:'?:}::

gonatid squid

(Gonatopsis borealis!

Berryteuthis magister)

gonatid sqUid

(Gonatopsis borealis!

8erryteuthis magister)

gonatid squid

(Berryteuthis magister)

walleye pollock

(Theragra chalcogramma)

Deep-sea smelt

(Leuroglossus schmidti)

walleye pollock

(Theragra chalcogramma)

1985

1982

Figure 3

Percent number (PN) and percent occurrence (PO) values for the three dominant prey species identified in northern fur seal gastrointestinal tracts

collected in 1981, 1982, and 1985 (data originally presented in Sinclair, 1988 and Sinclair et a\., 1994). ....

....
172 NOAA Technical Report NMFS 126

1987 Percent number Percent occurrence

100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100

walleye pollock

(Theragra chalcogramma)

gonatid squid

(Gonatus madokailmiddendortfl)

gonatid squid

(Gonatapsis borealisl

Be"yteuthis magister)

lwalleye pollock

(Theragra chalcogramma)

gonatid squid

(Gonatus madokailmiddei<dorlli)

gonatid squid

(Gonatapsis borealis!

Berryteuthis magister)

1988 Percent number Percent occurrence

walleye pollock

(Theragra chalcogramma)

gonalid squid

(Gonatus rnadokailmiddendorlli)

Pacific sandlance

(Ammodyt86 hexapterus)

:;:;.:.;.;.:.:.:.: : : :.., .,'.

100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 L-J_-'--"-_l--'---'-_-'--"-_L-+-'-_-'--L---l'--'----'-_l-...L---l_

walleye pollock

(Theragra chalcogramma)

Pacific sandlance

(Ammodytes hexapterus)

gonalid squid

(Gonatus rnadokailmiddendorll,)

1989 Percent number Percent occurrence

100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100

walleye pollock

(Theragra chalcogramma)

gonatid squid

(Gonatus rnadokailmiddendortfl)

deep-sea smell

(Leuroglossus schmidti)

I::;:::::::; ..•:.:.:.;:;.:.:•..: :::::::::;::::::..

E_

-

walleye pollock

(Theragra chalcogramma)

gonatid squid

(Gonatus rnadokailmiddendortfl)

gonatid squid

(Gonatopsis borealisl

Berryteuthis magister)

1990 Percent number Percent occurrence

100 gO 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100

walleye pollock

(Theragra chalcogramma)

gonatid squid

(Gonatapsis borealisl

Be"yteuthis magister)

gonatid squid

(Gonatus rnadokailmiddendortfl)

::::;:;:::: ; ,..: :::., ::::::::;::':.:.;.;.:.:.;:::::::::::: """:':'::::::;:;:;:;:::: walleye pollock

(Theragra chalcogramma)

gonatid squid

(Gonatus madokailmiddendortfi)

Pacific sandlance

(Ammodyt86 hexapterus)

Figure 4

Percent number (PN) and percent occurrence (PO) values for the three dominant prey species identified in northern fur

seal scats collected in 1987-90.

stomach contents of 1,800 animals collected pelagically

from June to October in 1960, 1962-64, 1968, 1973,

and 1974 (Kajimura, 1985; Perez and Bigg, 1986). Walleye pollock was the principal prey species for all years

combined in the pelagic studies, yet it ranked less than

25% PO, compared with PO values of 79% for all years

combined in the 1980's studies. Capelin, Mallotus villosus,

was absent both in this study and in Sinclair et al.

(1994), yet it ranked from first to third most important

prey identified in northern fur seal stomachs from the

earlier pelagic collections (Kajimura, 1985). The absence of capelin in all 7 years of this study is noteworthy, since samples were collected in the same region

and season as in the pelagic studies. Herring, Clupea

pallasi, was also a principal prey species of northern fur

seals in the pelagic studies (Kajimura, 1985). Yet her-
Sinclair et al.: Northern Fur Seal Predation on Juvenile Walleye Pollock 173

Figure 6

Percent number by age group of prey pollock from the

gastrointestinal tracts (1981, 1982, and 1985) and scats

(1987-90) of northern fur seals. The abbreviation OL

represents larval-size walleye pollock (less than 5 em in

body length). The OL group is considered part of the

age-O group in PO and PN calculations.

5+

5+

4

4

GI Tracts

All years

Pollock n = 2659

2 3

123

Pollock age group

OL o

0+,,-..,

100

Scats

75 All years

Q; Pollock n = 7365 .a

E

:J

c::: 50 E(1)

(1)

a.. 25

1985

100

o PO Q; 75

.a

E

:J

c::: 50 E

'"

a..

'" 25

0

OL 0

1982

Pollock in GI tracts

1981

o

o

100

100

Q; 75 .0

E

:J

c:::

E

Ql

50 Ql

a..

25

75 ...

Ql

.a

E

:J

c:::

E

Ql 50

Ql

a..

25

1987 1988 1989

Pollock in Scats

1990

Figure 5

Percent number (PN) and percent occurrence (PO)

values for walleye pollock, Theragra chalcogramma, in

gastrointestinal tracts and scats of northern fur seals,

1981-90.

ring was present in only one GI tract and 3% ofthe scats

in this study. Such changes in the consumption of fish

probably reflect the dynamics of the Bering Sea ecosystem over the last 20 years. The composition of potential

fur seal prey changed from being dominated by herring and capelin through the early 1970's, to a pollockdominated system more recently (NMFS3).

The size of Bering Sea walleye pollock (Smith, 1981)

and the size of pollock consumed by northern fur seals

(Swartzman and Haar, 1983) may have decreased during the 1970's. Swartzman and Haar further suggest

that more small pollock became available in the Bering

Sea ecosystem during the 1970's. A 'Juvenation"

(Swartzman and Haar, 1983) of walleye pollock in the

Bering Sea cannot be confirmed by northern fur seal

diet studies because prey size was not consistently recorded before the 1980's. If such a size reduction has

occurred, it may explain the high occurrence ofjuvenile pollock in fur seal diets in the 1980's and 1990's

and the marked reduction of other small schooling

3 NMFS. National Marine Fisheries Service. 1993. Fina! conservation plan for the northern fur seal (Callorhinus ursinus) . Prepared

by the National Marine Mammal Laboratory. Alaska Fisheries Science Center, Seattle, and the Office of Protected Resources, ationa! Marine Fisheries Service, Silver Spring, MD. 80 p.
174 NOAA Technical Report NMFS 126

100

1981

Q; 75

.0

E n = 39 :>

c 50 C

Q)

Q)

a. 25

0

OL 0 2 3 4 5+

100

1982

Q; 75

.0

E n =1191 :>

c 50 -

c

Q)

0

Q;

a. 25

0

OL 0 2 3 4 5+

100

1985

Q; 75

.0

E n = 1429 :>

c 50 C

Q)

0

Q;

a. 25

0

OL 0 1 2 3 4 5+

Pollock age group

Figure 7

Interannual variation in the percent number by age

group of prey pollock from the gastrointestinal tracts

of northern fur seals (data originally presented in

Sinclair, 1988 and Sinclair et aI., 1994).

prey. Seals may have found a newly abundant but unreliable resource in walleye pollock because ofits variable

recruitment rates (Smith, 1981) and because it may

have expanded at the expense of other prey species,

such as Pacific herring (Wespestad and Barton, 1981;

Bakkala et aI., 1987). During years oflow abundance or

availability of juvenile pollock, fur seals can switch to

other prey (capelin and herring). In spite of this capability, their food resources may be limited if these alternate prey species are displaced or depleted (Sinclair,

1988; Sinclair et al., 1994).

Summary _

Our analysis indicates a marked shift in northern fur

seal diet in the eastern Bering Sea since the 1970's; this

shift includes increased consumption of pollock, de-
100 -, 1987 100 1988

J ....

Q)

.0 75"

- n =1968 75-1 - n =240 E

:::J

C

....

c 50., - Q) 50

Q)

Q..

25-1

- 25

j

- I

rJJ

S'

o I 01 I'l

I I I I I I I I I I I ;;

::j' OL 0 1 2 3 4 5+ OL 0 1 2 3 4 5+ I'

...

Z

0

...

... 100 ::r 100-, I'

1989 1990 ...

="'l

::: .... ... Q)

75 n =3992 rJJ

.0 75 n =1165 I'

E e:.

:::J ."

... C I'

.... Q.,

III C 50 50 ... Q) 0'

Q) 0

= Q.. c...., = 25 :::

25

- < -- I'e,

;'

01 01 e:.

I I I i I I I I ;'

OL 0 1 2 3 4 5+ OL 0 1 2 3 4 5+ '<I'

Pollock age group

."

Pollock age group 0'

I'l

:0:-

Figure 8

Interannual patterns in the percent number by age group of prey pollock from the scats of northern fur seals.

-\11
176 NOAA Technical Report NMFS 126

100

a

90

80

.r::. _o..... g>x 70 Q)1Jl

... c: iii .Q 60 00:=

00.0

~- u(j; 50 .!..o

mE

>-:::J 40 ~c:

Uc:

.20

"O.~ 30

0..-:::J

a. 0 20 a.

10

0

1978 79 80 81 82 83 84 85

100 b

90 • 1981 GI Tracts

0 1982 GI Tracts 80

U [ill 1985 GI Tracts .2

"0 70 a.

>-

Q)

... 60 a.

'0

...

Q) 50

.0

E

:::J 40 c:

"E

Q) 30 Q)

a.. 20

10

0

1978 79 80 81 82 83 84 85

Pollock year class

Figure 9

(a) Estimates of walleye pollock year-class strength, 1978-85, based on

midwater acoustic and bottom trawl surveys of walleye pollock at age 1

(Wespestad and Dawson2); compared to (b) the estimated abundance of

specific pollock year classes in northern fur seal gastroin testinal tracts collected in 1981, 1982, and 1985. Adapted from Sinclair et al. (1994).
Sinclair et aI.: Northern Fur Seal Predation on Juvenile Walleye Pollock 177

creased consumption of other small schooling prey,

and perhaps a decrease in the size of prey pollock. This

study demonstrates the dominance of age-O and age-l

pollock in adult female andjuvenile fur seal diets over a

7-year period, and the strong influence of pollock yearclass strength on the age frequency of pollock in GI

tract samples collected in the early 1980's. For undetermined reasons, pollock year-class strength played a less

important role in the age frequency of pollock in seal

scat samples collected in the late 1980's relative to all

100 a

90 0

.c x 80 ... I/)

Ole:

e: 0

...

Q) .-= 70

(;):0

1/)::-

I/) Q) 60 .!l!.o

E

Q)

Cll e::l 50 >-e:0 o -+= 40 o Cll =:5 00.

a.. 0 30 a.

20

10

0

1984 1985 1986 1987 1988 1989 1990

100 b

90 • 1987 Scats 1989 Scats

80 u

g

0 70 a.

>-

a. 60

'0

Ci> 50 .0

E

:l

e: 40

'E

Q) 30

Q)

a.. 20

10

0

1984 1985 1986 1987 1988 1989 1990

Pollock year class

Figure 10

(a) Estimates of walleye pollock year-class strength, 1984-90, based on

midwater acoustic and bottom trawl surveys of walleye pollock at age 1

(Wespestad and Dawson2); compared to (b) the estimated abundance of

specific pollock year classes in northern fur seal scats collected in 1987-90.
178 NOAA Technical Report NMFS 126

other sample years. In order to fully evaluate patterns

in consumption ofwalleye pollock by northern fur seals,

we need more refined estimates of pollock year-class

strength and movements ofjuvenile and adult walleye

pollock, as well as estimates of the abundance and

availability of noncommercial fish and squid.

Acknowledgments _

We extend our thanks to T. Antonelis, D. Benoit, M.

Byrd, A. Hanson,]. Harvey, H. Huber, A. Mallek, and E.

Paez, who helped collect scats; to A. York, who conducted the 1990 field census and scat collections; to R.

Johnson, who helped count and measure walleye pollock otoliths in the laboratory; and to K. Bailey, E.

Brown, G. Duker, L. Fritz,]. Lee, L.-L. Low, and two

anonymous reviewers, who improved the quality of this

manuscript.

Literature Cited

Bakkala, R. G., V. G. Wespestad, and L.-L. Low.

1987. Historical trends in abundance and current condition

of walleye pollock in the eastern Bering sea. Fish. Res.

5:199-215.

Dellinger, T., and F. Trillmich.

1988. Estimating diet composition from scat analysis in otariid

seals (Otariidae): Is it reliable? Can.]. ZooI. 66:1865-

1870.

Frost, K.]., and L. F. Lowry.

1981. Trophic importance of some marine gadids in northern Alaska and their body-otolith size relationships. Fish.

Bull. 79(1):187-192.

Gentry, R. L., G. L. Kooyman, and M. E. Goebel .

1986. Feeding and diving behavior of northern fur seals. In

R. L. Gentry and G. L. Kooyman (eds.), Fur seals: maternal

strategies on land and at sea, p. 61-78. Princeton Univ.

Press, Princeton, NJ.

Kajimura, H.

1985. Opportunistic feeding by the northern fur seal

(Callorhinus ursinus). InJ. R. Beddington, R.]. H. Beverton,

and D. M. Lavigne (eds.), Marine mammals and fisheries, p.

300-318. George Allen and Unwin Ltd., Lond.

Kinder, T. H., and.J. D. Schumacher.

1981. Hydrographic structure over the continental shelf of

the southeastern Bering Sea. In D. W. Hood and]. A. Calder

(eds.), The eastern Bering Sea shelf: oceanography and

resources, Vol. 1, p. 31-52. U.S. Dep. Commer., NOAA,

Office of Mar. Poll. Assess.

Loughlin, T. R, G. A. Antonelis,]. D. Baker, A. E. York,

C. W. Fowler, R. L. DeLong, and H. W. Braham.

1994. The status of the northern fur seal population on the

U.S. west coast in 1992. In E. H. Sinclair (ed.), Fur seal

investigations, 1992. U.S. Dep. Commer., NOAA Tech.

Memo. NMF$-AFSC-45.

Loughlin, T. R.,.J. L. Bengtson, and R. L. Merrick.

1987. Characteristics of feeding trips of female northern fur

seals. Can.]. ZooI. 65 (8) :2079-2084.

Loughlin, T. R., and R V. Miller

1989. Marine mammals. In F. G. Johnson and R. R Stickney

(eds.), Fisheries, p. 296-333. Kendall/Hunt Publishing Co.,

Dubuque, Iowa.

North, A. W.,]. P. Croxall, and D. W. Doidge.

1983. Fish prey of the Antarctic fur seal Arctocephalus gazella at

South Georgia. Br. Antarct. Surv. Bull. 61:27-37.

Perez, M. A., and M. A. Bigg.

1986. Diet of northern fur seals, Callorhinus ursinus, off western North America. Fish. Bull. 84(4):957-971.

Prime,]. H.

1979. Observations on the digestion of some gadoid fish

otoliths by a young common seal. Int. Coune. Explor. Sea.

CM 1979, N:14.

Sinclair, E. H.

1988. Feeding habits of northern fur seals in the eastern

Bering Sea. M.S. thesis, College of Oceanography, Oregon

State Univ., Corvallis, 94 p.

Sinclair, E. H., T. R. Loughlin, and W. G. Pearcy.

1994. Prey selection by northern fur seals (Callorhinus ursinus)

in the eastern Bering Sea. Fish. Bull. 92(1):144-156.

Smith, G. B.

1981. The biology of walleye pollock. In D. W. Hood and]. A.

Calder (eds.), The eastern Bering Sea shelf: oceanography

and resources, Vol. I, p. 527-552. U.S. Dep. Commer.,

NOAA, Office of Mar. Poll. Assess.

Swartzman, G. L., and R. T. Haar.

1983. Interactions between fur seal populations and fisheries

in the Bering Sea. Fish. Bull. 81 (1):121-132.

Trites, A. W.

1992. Northern fur seals: Why have they declined? Aquat.

Mamm. 18:3-18.

Walline, P. D.

1983. Growth of larval and juvenile walleye pollock related to

year-class strength. Ph.D. dissert., Univ. Washington, Seattle, 144 p.

Wespestad, V. G., and L. H. Barton.

1981. Distribution, migration, and status of Pacific herring.

In D. W. Hood and]. A. Calder (eds.), The eastern Bering

Sea shelf: oceanography and resources, Vol. I, p. 509-

525. U.S. Dep. Commer., NOAA, Office of Mar. Poll.

Assess.

York, A. E., and.J. R. Hartley.

1981. Pup production following harvest of female northern

fur seals. Can..J. Fish. Aquat. Sci. 38:84-90.
Juvenile Walleye Pollock, Theragra chalcogramma,

Bycatch in Commercial Groundfish Fisheries in Alaskan Waters

LOWELL W. FRITZ

Resource Ecology and Fisheries Management Division

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

ABSTRACT

Juvenile walleye pollock, Theragra chalcogramma, «30 cm total length) caught incidentally in trawl fisheries for groundfish off Alaska are generally discarded because they are too

small to be processed into surimi or fillets. Catch rates of20-29-cm pollock (ages 2-3) have

varied greatly over time and area, but these fish account only for 1.8% of the Gulf ofAlaska

(GOA) pollock and 4.5% of the eastern Bering Sea (EBS) pollock measured by observers

during 1977-92. Few pollock smaller than 20 cm are caught by groundfish fisheries. Most

juvenile pollock are caught by pollock fisheries in both the GOA (98%) and EBS (96%).

Bycatch rates of pollock <30 cm by pollock fisheries are similar for midwater and bottom

pollock fisheries. However, annual average rates (1977-92) in the EBS (4.8% for both trawl

types) have been double those in the GOA (2.0% for midwater and 1.8% for bottom trawls).

Bycatches ofjuvenile pollock varied depending on the age structure of the pollock

population, season, locations fished, and, to some extent, the behavior of the fishery.

Comparison of catch-at-age and population estimates between 1964 and 1991 suggest that

catches of young pollock generally varied directly with their abundance. In the GOA,

exploitation rates of 2-3-year-old pollock averaged about 1%, with levels as high as 3% from

1982 to 1984. In the EBS, exploitation rates on 2-3-year-olds averaged about 2% from 1964

to 1972 and 1980 to 1990 (maximum of 4%), but ranged between 11 % and 21 % from 1973

to 1979, apparen t1y because the foreign fishery was targeting on small fish at the time.

Juvenile pollock in the GOA were generally found in embayments on the Kodiak

Archipelago and along the Alaska Peninsula. Pollock trawl fisheries along the edge of the

GOA continental shelf, especially in the western GOA, took fewer juvenile pollock than

those located closer to shore. In the EBS, juvenile pollock were more commonly found

along the mid to outer continental shelf northwest of the Pribilof Islands than southeast of

the islands.

Introduction

Juvenile walleye pollock, Theragra chalcogramma, is an

important component of the diets of many marine

mammals, birds, and fish in the North Pacific Ocean

and Bering Sea (e.g., Hunt et aI., 1981; Lowry et aI.,

1988; Livingston, 1993), and adult pollock support one

ofthe largest commercial fisheries in the world (Megrey

and Wespestad, 1990). It has been speculated that commercial harvests of pollock, including bycatches ofjuveniles, have contributed to recent declines in many marine mammal and bird populations in the North Pacific

and Bering Sea. In this paper, I describe the bycatch of

juvenile pollock by federally managed groundfish fisheries in the eastern Bering Sea (EBS) and GulfofAlaska

(GOA). Conclusions about whether the described levels ofjuvenile pollock bycatch contribute to the marine

mammal and bird population declines, however, are

left to the reader.

Juvenile pollock are defined as fish <30 em in total

length (TL), which corresponds to 0-2-year-old and

some 3-year-old fish (Hollowed et al. l ; Wespestad and

I Hollowed, A. B., B. A. Megrey, and W. &Irp. 1992. Walleye pollock. In Stock assessment and fishery evaluation report for the

groundfish resources of the Gulf ofAlaska as projected for 1993, p.

1-1-1-69. orth Pacific Fishery Management Council, P.O. Box

103136, Anchorage, AK 99510.

179
180 NOAA Technical Report NMFS 126

Dawson2). Wespestad and Dawson2 reported that 3-

year-old pollock in the EBS averaged 34.5 cm in length,

and that 29% were mature. In the GOA, 3-year-old

pollock averaged between 29 and 38 cm in length (Hollowed3) and approximately 20% were mature (Hollowed and Megrey4). In this paper, I address the following questions. 1) Which groundfish fisheries catch juvenile pollock, when, and where? 2) Has the juvenile

pollock bycatch varied over time, and how does this

relate to the age structure of the population? 3) Does

the bycatch ofjuvenile pollock differ between the Gulf

of Alaska and the eastern Bering Sea?

Methods

Observers aboard commercial fishing vessels in the

North Pacific have collected data on species and size

composition of catches since the mid-1970's (see Nelson

et al., 1981 for a discussion of sampling methodology).

For this study, pollock length-frequency data collected

between 1977 and 1992 in the EBS (International North

Pacific Fisheries Commission [INPFC] statistical areas

51 and 52) and the GOA (INPFC statistical areas 61-

64) were tabulated by three seasons and various target

fisheries. Seasons were defined as January-April (approximately the current Bering SealAleutian Islands

"A," or pollock-roe season); May-July (early "B," or

non-roe season); and August-December (late "B" season). Only data collected aboard trawl vessels were

used. Longlines and pots are not used in the directed

pollock fisheries, and in other fisheries these gear generally account for less than 0.2% of the pollock catch in

the Gulf of Alaska and Bering Sea each year. Observers

collect and report length-frequency data from a sample

of the entire catch, which consists of both retained and

discarded species and sizes. However, no data were

available that describe the length-frequency ofjust the

discarded portion of the catch.

Pollock length frequencies were assigned to target

fisheries based on a series of hierarchical catch-composition criteria of individual hauls (Berger5; Table 1).

Under the criteria in Table 1, a single haul may satisfy

2 Wespestad, V., and P. Dawson. 1992. Walleye pollock. In Stock

assessment and fishery evaluation report for the groundfish resources of the Bering SealAleutian Islands regions as projected for

1993, p. 1-1-1-32. North Pacific Fishery Management Council,

P.O. Box 103136, Anchorage, AK 99510.

3 Hollowed, A. B., 1994. Alaska Fisheries Science Center, MFS,

NOAA. 7600 Sand Point Way N.E., Seattle 98115-0070. Personal

commun.

4 Hollowed, A. B., and B. A. Megrey. 1990. Walleye pollock. In

Stock assessment and fishery evaluation report for the groundfish

resources of the Gulf of Alaska as projected for 1991, p. 22-

89. North Pacific Fishery Management Council, P.O. Box 103136,

Anchorage, AK 99510.

the definition of more than one target species, but is

assigned to the last fishery in the list for which it meets

the criteria. For example, if a haul from the GOA comprised 21 % deepwater flatfish, Pleuronectidae; 40%

sablefish, Anoplopomafimbria; and 37% rockfish, Sebastes

spp., it would be defined as a rockfish haul despite

meeting the criteria for both deepwater flatfish and

sablefish fisheries. These hierarchical definitions and

criteria were developed with considerable attention to

what data (hauls) were included and excluded, in order to best capture the distribution and catch of fisheries as a whole.

Catch-at-age and numbers-at-age data for both the

EBS and GOA pollock populations were obtained from

recent assessments ofthe two stocks. Assessments ofthe

EBS pollock population2 and the GOA populationsl

were sources of data on catches and numbers of pollock

at age.

Data on numbers of pollock caught and discarded by

EBS fisheries from 1990 to 1992 were obtained from

Michael Guttormsen, NMFSjAFSC. He combined

weekly processor reports (retained and discarded

weights of each species supplied by fish processors) and

observers' estimates of catches to yield retained and

discarded catches of all species, by time and fishery. Estimates of discarded and total pollock catch for each GOA

fishery in 1990-92 were obtained from Hollowed et al.6

Results and Discussion

Catch of Pollock by Fishery

Target pollock fisheries in the GOA accounted for more

than 85% of the pollock caught by weight in the GOA

(with the Pacific cod, Gadus macrocephalus, trawl fishery

contributing 10%), while pollock fisheries in the EBS

accounted for more than 95% ofthe total pollock catch

there in 1990-92 (Table 2). All fisheries generally discard pollock <30 cm long (Wespestad and Dawson,

1992). Fisheries other than the directed pollock fishery

may keep pollock if the pollock fishery is open, but

otherwise must discard all pollock bycatch. Consequently, size distributions of discarded pollock may

vary considerably among fisheries and at various times.

Discard data have been collected for Alaskan groundfish fisheries since 1990 (Table 2). Estimates of pollock

5 Berger, J. D. 1992. NMFS Observer Program, Alaska Fish. Sci.

Cent., Nat!. Mar. Fish. Servo OAA, 7600 Sand Point Way N.E.,

Seattle WA 98115-0070. Personal commun.

6 Hollowed, A. B., B. A. Megrey, and E. Brown. 1993. Walleye

pollock. In Stock assessment and fishery evaluation report for the

groundfish resources of the Gulf of Alaska as projected for 1994, p.

1-1-1-54. North Pacific Fishery Management Council, P.O. Box

103136, Anchorage, AK 99510.
_______ Fritz: Juvenile Walleye Pollock Bycatch in Commercial Groundfish Fisheries in Alaskan Waters 181

Table 1

Hierarchical catch-composition criteria for individual hauls used to assign pollock length-frequency data to target

fisheries. Fisheries are listed from least to most restrictive; each haul was assigned to the most restrictive fishery possible.

Trawl fishery

Eastern Bering Sea

1. Other

2. Atka mackerel

3. Sablefish

4. Rockfish

5. Pollock (bottom2)

6. Flatfish

7. Pacific cod

8. Greenland turbot

9. Pollock (midwater2)

Gulf of Alaska

1. Other

2. Shallow flatfish 3

3. Deep flatfish4

4. Sablefish

5. Pollock (bottom2)

6. Rockfish

7. Pacific cod

8. Pollock (midwater2)

Criteria

Does not meet any of the following criteria

Atka mackerel 20% of retained groundfish l

Sablefish 20% of retained groundfish

Rockfish 35% of retained groundfish

Pollock 20% of retained groundfish

All flatfish except Greenland turbot and Pacific halibut 40% of retained groundfish

Pacific cod 40% of retained groundfish

Greenland turbot 35% of retained groundfish

Pollock 95% of total catch

Does not meet any of the following criteria

Shallow flatfish 20% of retained groundfish

Deep flatfish 20% of retained groundfish

Sablefish 20% of retained groundfish

Pollock 20% of retained groundfish

Rockfish 35% of retained groundfish

Pacific cod 40% of retained groundfish

Pollock 95% of total catch

I Retained groundfish defmed as catches of allocated groundfish species. Includes pollock, Pacific cod, Atka mackerel, sablefish, flatfish

(except Pacific halibut), and rockfish in the Bering Sea and Gulf of Alaska.

2 Data from both groups in each area were pooled and then separated on the basis of trawl type (bottom or midwater/pelagic trawl).

3 Shallow flatfish = rock sole, Alaska plaice, English sole, butter sole, slender sole, sand sole, yellowfin sole, and starry flounder.

4 Deep flatfish = Greenland turbot, flathead sole, rex sole, Dover sole, deepsea sole, and arrowtooth flounder.

Table 2

Total and discarded catch (metric tons) of pollock by trawl fisheries in the Gulf ofAlaska and eastern Bering Sea, 1990-92.

Annual fishery percentages by region are listed in parentheses.

Total catch Discards

Fishery 1990 1991 1992 1990 1991 1992

Gulf of Alaska

Midwater pollock 53,505 (63) 80,497 (75) 62,116 (66) 4,621 (27) 2,468 (15) 4,998 (32)

Bottom pollock 20,038 (24) 13,965 (13) 20,846 (22) 3,184 (19) 2,062 (13) 1,628 (10)

Pacific cod 8,482 (10) 9,330 (9) 7,985 (8) 6,548 (38) 8,676 (54) 6,886 (44)

Rockfish 1,950 (2) 1,390 (1) 545 (<1) 1,666 (10) 1,237 (8) 406 (2)

Deepwater flatfish 492 (<1) 1,666 (2) 1,404 (1) 299 (2) 1,208 (8) 1,086 (7)

Others 119 «1) 657 (<1) 1,008 (1) 666 (4) 382 (2) 665 (4)

All 84,586 107,505 93,904 16,984 16,033 15,669

Eastern Bering Sea

Midwater pollock 1,238,470 (94) 1,128,431 (74) 1,243,584 (90) 71,690 (62) 72,371 (45) 94,297 (69)

Bottom pollock 40,910 (3) 325,800 (21) 96,330 (7) 6,787 (6) 24,181 (15) 5,835 (4)

Pacific cod 25,730 (2) 43,610 (3) 19,590 (1) 25,649 (22) 34,914 (22) 14,131 (10)

Rock sole 6,300 (<1) 20,040 (1) 10,301 (1) 6,325 (6) 18,066 (11) 9,018 (7)

Yellowfin sole 1,870 (<1) 8,060(<1) 12,865 (1) 1,871 (2) 7,154 (4) 12,014 (9)

Others 2,090 (<1) 4,113 (<1) 1,690 (<1) 2,653 (2) 3,574 (2) 1,407 (1)

All 1,315,370 1,530,054 1,384,360 114,975 160,260 136,702
182 NOAA Technical Report NMFS 126 _

discards by pollock fisheries ranged between 4,500 and

7,800 metric tons (t) in the GOA in 1990-92 (or between 5% and 10% of all pollock caught by weight),

and between 78,000 and 100,000 t in the EBS (between

5% and 8% of all pollock caught by weight). The difference in magnitude of pollock discards between the two

regions reflects the fact that pollock catches were 12-15

times greater in the EBS than in the GOA between 1990

and 1992. In the GOA, Pacific cod trawl fisheries discarded the most pollock from 1990 to 1992 (ranging

between 6,500 and 8,600 t per year), followed by

midwater (2,500-5,000 t per year) and bottom pollock

(1,600-3,200 t per year) fisheries. In the EBS, midwater

pollock fisheries discarded the most pollock (between

72,000 and 94,000 t per year, or 45%-69% of the total

EBS pollock discarded annually between 1990 and

1992), followed by bottom trawl fisheries for Pacific cod

(14,000-35,000 t per year) and pollock (6,000-24,000 t

per year).

Fisheries observers measured more than 11 million

pollock from the GOA and EBS between 1977 and 1992

(Table 3). These data represent the length-frequency

distribution of the total catch. From 1977 to 1992,

average catch rates ofjuvenile pollock «30 cm TL) by

trawl fisheries were more than twice as high in the EBS

(slightly less than 5% of all pollock measured) than in

the GOA (approximately 2% of all pollock measured).

In the GOA, pollock <20 cm were rarely caught by any

trawl fishery «0.08% of all pollock measured) and 99%

of the pollock <30 cm were caught by either midwater

or bottom trawl pollock vessels; juvenile pollock made

up only 1% or less of the catches of all other trawl

fisheries. In the EBS, pollock trawl fisheries accoun ted

for 96% of the juvenile pollock measured by observers.

Only about 2% ofthe pollock caught by the EBS Pacific

cod trawl fishery between 1977 and 1992 were <30 cm,

while juvenile pollock made up over 13% ofthe pollock

caught by EBS flatfish fisheries.

Annual Variability

Comparison of annual (Fig. 1, 2, 3) and aggregate

(Table 3) length-frequency distributions of pollock

caught by midwater and bottom trawl pollock fisheries

(Fig. 1,2) and Pacific cod trawl fisheries (Fig. 3) in the

GOA and EBS between 1977 and 1992 suggest that 1)

catch rates of pollock <30 cm TL have been similar for

Table 3

Numbers (and percentages) of walleye pollock, by length, measured by observers aboard foreign, joint venture, and

domestic trawl vessels participating in various fisheries .in the Gulf of Alaska and eastern Bering Sea from 1977 to 1992.

Gulf of Alaska fisheries

Midwater Bottom All trawl

pollock pollock Rockfish Pacific cod fisheries

Pollock

length Number Percent Number Percent Number Percent Number Percent Number Percent

1-9 cm 7 <0.01 1 <0.01 0 0.00 0 0.00 9 <0.01

10-19 741 0.06 295 0.07 16 0.05 6 0.02 1,089 0.07

20-29 21,596 1.97 7,552 1.73 115 0.35 251 0.98 29,630 1.86

30-39 167,879 15.33 57,293 13.12 5,791 17.42 3,993 15.62 237,043 14.86

40+ 904,993 82.63 371,432 85.08 27,316 82.18 21,319 83.38 1,327,325 83.21

All 1,095,216 436,573 33,238 25,569 1,595,096

Eastern Bering Sea fisheries

Midwater Bottom All trawl

pollock pollock Pacific cod Flatfish fisheries

Pollock

length umber Percent Number Percent Number Percent umber Percent Number Percent

1-9 cm 67 <0.01 207 <0.01 0 0.00 124 0.12 398 <0.01

10-19 17,059 0.25 11,094 0.43 268 0.20 3,524 3.36 31,951 0.33

20-29 306,467 4.55 109,365 4.32 2,821 2.07 10,228 9.76 429,290 4.49

30-39 1,841,797 27.33 462,831 18.27 20,907 15.31 7,904 7.55 2,342,754 24.49

40+ 4,572,722 67.86 1,949,396 76.96 112,547 82.43 82,971 79.21 6,760,703 70.68

All 6,738,112 2,532,893 136,543 104,751 9,565,096
_______ Fritz: Juvenile Walleye Pollock Bycatch in Commercial Groundfish Fisheries in Alaskan Waters 183

100

90

80

70

60 c:

.,

u so- Q;

0..

40

30

20

10

0

100 --- -------.~

90 __

80

70

60

50

40

30

20

10

0-

Year

B. Eastern Bering Sea

Figure I

Percent length frequency of walleye pollock caught by midwater pollock fisheries

in (A) the Gulf of Alaska and (B) the eastern Bering Sea by 10-cm length ranges.

Numbers of pollock measured each year ranged from 5,591 to 265,678 (median =

38,408) in the Gulf of Alaska, and from 66,655 to 757,861 (median = 408,498) in

the eastern Bering Sea. Data from the Gulf of Alaska in 1988 were not used

because of small sample size.
184 NOAA Technical Report NMFS 126 _

A. Gulf of Alaska

100

90

80

70

.... 60 r:::

Q)

C,) so- a;

0.. 40

30

20-

10

0

100

90

80

70

.... 60 r:::

Q)

C,) 50

a;

0.. 40

30

20-

10

0

77 78 79 80 81 82 83 84 85 86 Year 87 88

89 90

91 92

~ernBering Sea

-------------------

77 78 79 80 81 82

83 84 85 86

Year 87 88

92

I 0 10-19 em • 20-29 em 30··39 em .40+ em .1

Figure 2

Percent length frequency of walleye pollock caught by bottom pollock fisheries in (A) the Gulf of Alaska and (B) the eastern Bering Sea by 10-cm length

ranges. Numbers of pollock measured each year ranged from 916 to 76,418

(median = 20,712) in the Gulf ofAlaska, and from 42,660 to 272,505 (median

= 100,542) in the eastern Bering Sea. Data from the Gulf of Alaska in 1988

were not used because of small sample size.
______ Fritz: Juvenile Walleye Pollock Bycatch in Commercial Groundfish Fisheries in Alaskan Waters 185

A. Gulf of Alaska

100

90

80

70

... 60 c:

III

u 50

Cii

c. 40

30

20

10-

0

77 78 79 80

81 82

83 84 85 86 Year 87 88

89 90

91 92

B. Eastern Bering Sea

I0 10-19 em • 20-29 em 30-39 em .40+ em I

91 92

77 78 79 80 81 82 83 84 85 86 Year 87 88

89 90

100

90-

80

70

60

50

40

30

20

10

o

...

c:

III

U

Cii

c.

Figure 3

Percent length frequency of walleye pollock caught by Pacific cod trawl

fisheries in (A) the Gulf of Alaska and (B) the eastern Bering Sea by 10-cm

length ranges. Numbers of pollock measured each year ranged from 151 to

11,040 (median = 626) in the Gulf ofAlaska, and from 468 to 24,760 (median

= 7,714) in the eastern Bering Sea. Data from the Gulf of Alaska in 1988 were

not used because ofsmall sample size.
186 NOAA Technical Report NMFS 126

midwater and bottom trawl pollock fisheries in each

region; 2) midwater pollock fisheries have had higher

catch rates of pollock 30-39 cm TL than bottom trawl

fisheries; and 3) there has been considerable annual

variability in the catch rates ofjuvenile pollock. In the

GOA, juvenile pollock were most commonly caught in

1977-78, 1980, 1985-86, and 1990-91 by the midwater

pollock fishery (Fig. lA); in 1978 and 1985-86 by the

bottom pollock fishery (Fig. 2A); and in 1977-78, 1983,

1986, and 1991 by the Pacific cod trawl fishery (Fig.

A. Gulf of Alaska

.<:

III

ii:

-o 4 III

c:

iii 3

3A). In the EBS, juvenile pollock were most commonly

caught in 1977-80, 1985-86, and 1990-92 by the

midwater (Fig. IB) and bottom (Fig. 2B) pollock fisheries and the Pacific cod trawl fishery (Fig. 3B).

One possible explanation for this pattern of bycatch

rates is variability in the size of pollock year classes in

each region (Fig. 4). Since 1975, strong year classes

were spawned in the GOA in 1975-79, 1984-85, and

1988; in the EBS, 1978, 1982, 1984-85, and 1989-90

produced strong year classes. Years with high juvenile

62 64 66 68 70 72 74 76 78 M 82 M 86 88 90

35

B. Eastern Bering Sea

30

25

.<:

III ii: 20

-o

III

c: 15

a;

10

II II 11.1 62 64 66 68 70 72 74 76 78 80 82 86 88 90

Year class

Figure 4

Year-class strength of the walleye pollock populations in (A) the Gulf of

Alaska and (B) the eastern Bering Sea, assessed as 2-year-olds. Data from

age-structured population assessments conducted by Hollowed et al. l and

Wespestad and Dawson. 2
______ Fritz: Juvenile Walleye Pollock Bycatch in Commercial Groundfish Fisheries in Alaskan Waters 187

pollock bycatch in each region follow (allowing for

growth) years when strong year classes were spawned,

suggesting that catch rates ofjuveniles by trawl fisheries

vary directly with their abundance.

The relation between catches and numbers of 2-3-

year-old pollock in the GOA and EBS from 1964 to

1990 are shown in Figures 5 and 6. In both regions,

catches ofjuveniles increased directly with their abundance. The slope of the regression line in Figure 5A was

0.009 (F=22.00, P<O.OOI), suggesting that on the average, fisheries in the GOA took about 1% of the 2-3-

year-old pollock each year from 1964 to 1991. The scarcity of pollock <20 cm TL «0.08% ofall pollock measured

in observer samples; Table 3) suggests that catch rates of

O-I-year-old pollock in the GOA are very low.

From 1973 to 1979 in the EBS, catch rates of 2-3-

year-old pollock were higher than during the combined 1964-72 and 1980-90 periods (Fig. 5B, 6B). During the "low" catch period, an average of 2.18% of the

2-3 year-old pollock were caught each year, and catches

increased directly with abundance (lower regression

line in Figure 5B: F=17.93, P<O.OOI). During the "high"

catch period from 1973 to 1979, trawl fisheries caught

between 11 % and 21 % of the 2-3-year-old pollock

(F=43.82, 1'=0.001). Exploitation rates of pollock 4 years

and older in the EBS were also higher from 1973 to

1979 (4 of 7 years had rates >20%) than from 1964 to

1972 or from 1980 to 1990 (all years but 1990 <15%).

The length-frequency distributions, however, indicate

that the foreign fishery active from 1973 to 1979 either

A. Gulf of Alaska (1964-911 0.16

0.14

:2 0.12

-

0

'" 0.1 c

;§. 0.08

.<:

;l 0.06 U'"

0.04

0.02

0

0 2 4 6 8 10 12

5 B. Eastern Bering Sea

30 35 40

1964-72

1980-90

5 10 15 20 25

o +---........,..:........................;:::;;,-...-..:,.......-r...-............-r;;,..;T...;-,................ ................~~........~"""'

o

4

:2

;;:'"

'0 3

c'"

;§. 2 .<:

;l

u'"

Population (billions of fish)

Figure 5

Fishery catches and populations of 2- and 3-year-old walleye pollock in

(A) the Gulf of Alaska and the (B) eastern Bering Sea assessed as 2-yearolds. Data from age-structured population assessments conducted by

Hollowed et al. l and Wespestad and Dawson. 2
188 NOAA Technical Report NMFS 126

targeted small fish, possibly of the 1972 year class, or

could not separate them from the mix of older year

classes spawned from 1965 to 1969. The data in Figure

6B suggest that the fishery since 1980 has been more

successful at avoiding small fish, because catch rates of

2-3-year-olds have been less than 5% (mostly less than

3%). Furthermore, catch rates 2-3 years after the large

year classes spawned in 1978, 1982, and 1984-85 did

not approach the "high" rates observed in 1973-79.

Even though pollock <20 cm were about four times

8

7

6

5

-

c:

"

CJ 4 t

l1.

3

2

0

30

25

20

E

CJ

" 15

t

l1.

10

5

0

A. Gulf of Alaska

B. Eastern Bering Sea

Figure 6

Percentage of the populations of 2-3-year-olds and 4+ year-old walleye pollock

caught by fisheries in (A) the Gulf of Alaska and (B) the eastern Bering Sea.

Data from age-structured population assessments conducted by Hollowed et

al.I and Wespestad and Dawson. 2
______ Fritz: Juvenile Walleye Pollock Bycatch in Commercial Groundfish Fisheries in Alaskan Waters 189

more common in the EBS length-frequency samples

«0.34% of all pollock measured; Table 3) than in the

GOA, their scarcity suggests that catch rates ofO-l-yearolds in the EBS are also low.

Average catch rates of 2-3-year-old pollock in the

GOA from 1964 to 1991 (0.94%) were less than onehalf of those during the EBS "low" periods (1964-1972

and 1980-90; 2.18%) and only 5%-10% of those during the EBS "high" period (1973-79; II %-21 %). Two

factors contribute to these regional differences injuvenile pollock bycatch rates. First, pollock year-class

strength has varied less in the EBS than in the GOA

(Fig. 4). In the EBS, about one-half (14 of 29, or 48%)

of the year classes spawned during 1962-90 were larger

than the long-term mean of 10.194 billion fish (at age

2), whereas only 5 classes (17%) were smaller than half

the mean. In contrast, only about one-fourth ofthe year

classes (8 of 28, or 28%) spawned in the GOA from

1962 to 1989 were greater than the long-term mean of

2.205 billion fish (at age 2), whereas more than onehalf (17 of 28, or 61 %) were less than half this size.

Thus, 2-3-year-old pollock were more commonly encountered and caught in the EBS than in the GOA,

reflecting the relation between abundance and catch

shown in Figure 5. Second, the average (1964-90) exploitation rate for all pollock 2 years of age and older

was five times higher in the EBS (9.20%) than in the

GOA (1.83%).

Livingston (1993) compared estimates of pollock removals (by size in 1985) by groundfish, marine mammals, marine birds, and the commercial fishery in the

EBS. She reported that the 1985 commercial fishery

accounted for almost all removals (by weight) of pollock >40 cm, and most removals offish >25 cm. Pollock

cannibalism far exceeded all other removals of age-O

(<10 cm) pollock both in terms of numbers and biomass. Biomass removals of age-l (10-19 cm) pollock

were evenly distributed between groundfish (principally arrowtooth flounder, Atheresthes stomias; walleye

pollock; and Pacific cod), marine mammals (predominately northern fur seals, Callorhinus ursinus) , and marine birds, and far exceeded removals by the fishery.

This pattern ofremovals by the fishery and other predators of pollock <20 cm has probably been the same for

the entire 1977-92 period (Fig. 1-3). When the fishery

has caught pollock <30 cm in great numbers (1977-80,

1991), the fish have primarily ranged from 20 to 29 cm

(2-year-olds) .

Seasonal Variability

Bycatch rates ofjuvenile pollock vary not only annually

because of changes in pollock age structure and in the

fishery, but also seasonally as the fish grow and recruit

to the gear. Pollock are spawning during most of the

January-April period in the Bering Sea, and they aggregate along the shelf edge or over deep basin waters

where they tend to segregate by size, allowing the fishery to target large, mature, roe-bearing females. 2 Small

fish between 25 and 45 cm dominated the catches during this period in the late 1970's (Fig. 7A). Since 1984,

however, few pollock smaller than 30 cm have been

caught in January-April (except for 1986, because of

the large 1984 year class, and 1992, because of the 1989

and 1990 year classes), and most pollock caught measured 45-55 cm (see also Fig. IB). Large, recently recruited (3-year-old) year classes are evidenced by modes

in the mid-30-cm range, such as the 1984, 1985, and

1989 year classes in the respective 1987, 1988, and 1992

January-April length-frequency data.

By early summer, spawning aggregations have dispersed and pollock tend to be less segregated by size.

This results in a greater bycatch of small fish in MayJuly compared to January-April (Fig. 7B). Large year

classes of 2-year-old pollock are often unavoidable by

the fishery, as evidenced by large catches of the 1978

year class in 1980 and the 1989 year class in 1991 (modes

in the mid-20-cm range). In 1992, the 1989 year class

appeared again as the large mode in the mid-30-cm

range.

During August-December (Fig. 7C), modal lengths

were greater, and fewer years had significant numbers

of pollock <30 cm than in May-July, as the 2-3-year-old

recent recruits grew during the year. TheJanuary-April

pattern ofmodal lengths increasing from the late 1970's

through the mid-1980's and stabilizing through 1991

was also evident in the August-December data. Between 1991 and 1992, however, the modal length declined 10 cm because of the large number of 1989 yearclass fish and the relative scarcity of older year c1assesparticularly those spawned in 1978, 1982, and 1984-

that sustained the fishery during the 1980's.

Spatial Variability

In the EBS, there are spatial as well as temporal components to the patterns ofjuvenile pollock bycatch. NMFS

surveys conducted during the summer in the EBS have

found that most juvenile pollock are north and west of

the PribilofIslands (57°N, 1700W). Four combined bottom and midwater trawl surveys conducted in the past

12 years found that 60%-90% of the 1- and 2-year-old

pollock were northwest ofthe Pribilof Islands. Fisheries

have also caught more juvenile fish there than in the

southeastern portion of the EBS shelf between Unimak

Pass (54°30'N, 165°W) and the PribilofIslands.

Locations fished seasonally by the pollock fishery

have changed considerably between 1977 and 1992
190 NOAA Technical Report NMFS 126

A. January-April B. May-July C. August-December

92 92

91 91

90 90

89 89

88 88

87 87

86 86

85 r:- 85 r:-

84 e: 84 e:

83 83

82 82

81 81

80 80 15 15 79

- 79 c 10 78 .. - 10 78 c l::! .. .. 78 l::! 77 ... 77 ..

... 77 0 60 70 10 20 30 40 50 60 70 0

10 20 30 40 50 60 70

length (eml length (em)

30 40 50

length (eml

15

1: 10 ..

l::!

..... 0

10 20

Figure 7

Percent length frequency of walleye pollock caught by midwater pollock fisheries in the eastern Bering Sea in (A)

January-April, (B) May-July, and (C) August-December.

(Fritz7). In the late 1970's, pollock fisheries inJanuaryApril were located predominately on the edge of the

continental shelf northwest of the Pribilof Islands (Fig.

8A), and pollock <20 em were present throughout this

area (Fig. 8B). Since the discovery of the spawning

concentration of pollock near Bogoslof Island (54°N,

I68°W) in 1986, the fishery has increasingly moved to

locations in the eastern Aleutian Basin and on the

southeastern Bering Sea shelf (Fig. 9A). The Bogoslof

Island pollock roe fishery is represented by the concentrated fishing grounds north of Umnak Island

(53°15'N,I68°30'W), southwest of the trawl locations

on the shelf. No fish smaller than 20 em were caught in

the deep waters of the eastern Aleutian Basin, and only

small numbers were found near the shelf break to the

northeast in 1991 (Fig. 9B). Therefore, the shift in

fishing locations in the late I980's,7 changes in pollock

age structure (Fig. 4), and changes in the fishery (targeting on spawning concentrations of large fish) all

contributed to the January-April decline in juvenile

7 Fritz, L. W. 1993. Trawl locations of walleye pollock and Atka

mackerel fisheries in the Bering Sea, Aleutian Islands, and Gulf of

Alaska from 1977-92. AFSC Proc. Rep. 93-08, Alaska Fish. Sci.

Cent., Nat!. Mar. Fish. Serv., NOAA, 7600 Sand Point Way N.E.,

Seattle, WA 98115-0070.

bycatch observed since 1977. In 1992, the Bogoslof

Island district (NMFS statistical area 518) was closed to

directed pollock fishing. As a result, the fleet redirected

its "A" season effort onto the EBS shelf, where juvenile

pollock bycatch can be higher than over deep basin

waters (as evidenced by catches of the 1989 year class,

or the mode in the 30-cm range; Fig. 7A).

Locations fished have also contributed to the higher

bycatch of smaller pollock in summer, compared with

January-April, since 1977. In summer, the pollock fishery typically works the entire outer shelf area from

Unimak Pass to Zhenchug Canyon (58°N,175°W;

Fig.lOA). However, small fish were caught more frequen tly northwest of the Pribilof Islands than on the

southeastern shelf (Fig. lOB).

In the GOA, the pollock fishery since 1990 has fished

predominantly in the gullies on the south and east sides

of Kodiak Island, along the southern side of Shelikof

Strait, in Shelikof Gully extending southwest of Kodiak

Island, and-to a lesser extent-in the western GOA

along the shelf break (Fig. I IA). The few pollock <20

em caught by the fishery from 1990 to 1992, however,

were caught around Kodiak Island, and none were

caught in the western GOA on the shelf break (Fig.

lIB). These results agree with surveys that found small

pollock primarily in embayments along the Alaska Pen-
_______ Fritz: Juvenile Walleye Pollock Bycatch in Commercial Groundfish Fisheries in Alaskan Waters 191

A

f

i'-

60N

I

I

I

i

I

/

!

I

180W

o

o

l70W 160W

B :.

,e ,. • I , C

60N r--7:~--------------~ / \~ -

f

i

f

I

!

I

i

I

t---------__ 55N

! ---------

iI'

n

I{

Figure 8

Trawl locations of the walleye pollock fishery in the eastern Bering Sea in January-April

1979 where (A) length-frequency data were collected (circles) and (B) pollock <20 cm

were caught (dots). The 1,000-m isobath is shown.
192 NOAA Technical Report NMFS 126

i

0/

o i""_'Qq;;,~O '-.~--.."

'c;"r~\" ..:

\0

,"0

'(

'-

;

180W 170W 160W

;

r,,\

60N ;----7--------------------

f

i

0/'.I

1"0'

._....__..~_..... .. ..

\

rJ

?

f

f··

B

--._~-_.

Figure 9

Trawl locations of the midwater walleye pollock fishery In the eastern Bering Sea in

January-April 1991 where (A) length-frequency data were collected (circles), and (B)

pollock <20 em were caught (dots). The 1,000-m isobath is shown,
_______ Fritz: Juvenile Walleye Pollock Bycatch in Commercial Groundfish Fisheries in Alaskan Waters 193

A

!

1

I

.I

; o

........_- ......-..-............. __...._._...s.~~_.

o

0: o

180W 170W 160W

,..

.,-';--',..'

• •

•

•• • • •• ?

..-. .~'. --+-..., - --~ .. ...

j

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

!

..- !-_ ..

•••

•

B

I

Figure 10

Trawl locations of the midwater walleye pollock fishery in the eastern Bering Sea in JuneSeptember 1991 where (A) length-frequency data were collected (circles) and (B) pollock

<20 em were caught (dots). The 1,000-m isobath is shown.
194 NOAA Technical Report NMFS 126

!

!

f

i

i

....;

, •./l.•.-

.-~.. /

SSN ........ ~.~ ~., ., .

o 0

......"·.',,0·: , ,

......" .. ... ..

" ,,~

,.

-' .. f

.'

\.. i

60N '~.).

......., ..;.

!

-.

h~:·~o~,.!,if~··- !

A

170W l60W lSOW

SSN

/

.-......

:,.'/

•

-

..., .

,

...._.. \~

!

!

i

.' j '- ";' ..... " ..~>

i ........ ., :L

60N

i\~~:-=·r ":, 'f-'-'~ . !

B

Figure 11

Trawl locations of the midwater walleye pollock fishery in the Gulf of Alaska in 1990-92

where (A) length-frequency data were collected (circles), and (B) pollock <20 cm were

caught (dots). The 1,000-m isobath is shown.
_______ Fritz: Juvenile Walleye Pollock Bycatch in Commercial Groundfish Fisheries in Alaskan Waters 195

insula and on Kodiak Island, not along the shelf break

(Smith et aI., 1984; Walters et aI., 1985; Shippen8; Spring

and Bailey9).

For two reasons, the rates discussed in this paper

could be considered minimal estimates of the catch of

juvenile pollock by trawl groundfish fisheries. First, only

30% of the trips made by vessels shorter than 125 ft

carry observers. While this could lead to underestimates of the true rates in both the EBS and GOA, GOA

rates may be affected more since small vessels catch a

larger proportion ofthe pollock in the GOA than in the

EBS.! Second, observers do not sample every haul, nor

can they sample hauls that are not brought on board. It

has been reported that codends full of fish have been

dumped rather than brought on board. The number of

times this has occurred and the size composition of the

catches are unknown, but the underreporting problem

this would create would exist even if 100% of vessels

carried observers. The magnitude of underreporting of

juvenile pollock catches probably increases when the

population is dominated by young fish.

Acknowledgments _

I thank the organizers of the juvenile pollock symposium for asking me to prepare this paper for presentation, as well as J. Berger, G. Duker, R. Marasco, R.

8 Shippen, H. H. 1988. Progress report on the young-of-the-year

project, 1984-87. Alaska Fish. Sci. Cent., Nat!. Mar. Fish. SeIV., 7600

Sand Point Way .E, Seattle, WA 98115-0070. Unpub!. rep., 65 p.

9 Spring, S., and K. Bailey. 1991. Distribution and abundance of

juvenile pollock from historical shrimp trawl sUIVeys in the western

Gulf of Alaska. AFSC Proc. Rep. 91-18, Alaska Fish. Sci. Cent.,

Nat!. Mar. Fish. SeIV., 7600 Sand Point Way N.E., Seattle, WA

98115-0070.

Merrick, R. Methot, and V. Wespestad, whose comments improved the manuscript.

Literature Cited

Hunt, G. L.,Jr., B. Burgeson, and G. A. Sangar.

1981. Feeding ecology and seabirds of the Bering Sea. In D.

W. Hood and]. A. Calder (eds.), The Eastern Bering Sea

shelf: oceanography and resources, Vol. II, p. 629-

647. USDOC, NOAA, OCSEAP, Office of Mar. Poll. Assess., Seattle. Univ. Washington Press.

Livingston, P. A.

1993. Importance of predation by groundfish, marine mammals and birds on walleye pollock, Theragra chalcogramma,

and Pacific herring, Clupea pallasi, in the eastern Bering

Sea. Mar. Eco!. Prog. Ser. 102: 202-215.

Lowry, L. F., K.]. Frost, and T. R. Loughlin.

1988. Importance of walleye pollock in the diets of marine

mammals in the Gulf ofAlaska and Bering Sea, and implications for fishery management. In Proceedings of the International Symposium on the Biology and Management of

Walleye Pollock, November 14-16, 1988, Anchorage AK, p.

701-728. Alaska Sea Grant Rep. 89-1.

Megrey, B. A., and V. G. Wespestad.

1990. Alaskan groundfish resources: 10 years of management

under the Magnuson Fishery ConseIVation and Management Act. N. Am.]. Fish. Manage. 10(2):125-143.

Nelson, R.,Jr., R. French, and]. Wal!.

1981. Sampling by U.S. obseIVers on foreign fishing vessels in

the eastern Bering Sea and Aleutian Islands region, 1977-

78. Mar. Fish. Rev. 43(5):1-19.

Smith, G. B., G. E. Walters, P. A. Raymore Jr., and

W. A. Hirschberger.

1984. Studies of the distribution and abundance ofjuvenile

groundfish in the northwestern Gulf of Alaska, 1980-82:

Part I. Three-year comparisons. U.S. Dep. Commer., NOAA

Tech. Memo. NMFS F/NWC-59, 100 p.

Walters, G. E., G. B. Smith, P. A. RaymoreJr., and

W. A. Hirschberger.

1985. Studies of the distribution and abundance ofjuvenile

groundfish in the northwestern Gulf of Alaska, 1980-82:

Part II. Biological characteristics in the extended

regions. U.S. Dep. Commer., NOAA Tech. Memo. NMFS

F/NWC-77, 95 p.
Walleye Pollock, Theragra chalcogramma,

in the Western Pacific Ocean and Bering Sea Ecosystems

V. P. SHUNTOV and E. P. DULEPOVA

Laboratory ofApplied Bioceanology

Pacific Research institute ofFisheries and Oceanography (TINRO)

Schevchenko Alley, 4

Vladivostok, 690600, Russia

1 Mean value excluding a maximum catch of 3.8 x 106 t in one

year.

Table I

Annual mean consumption and production ofjuvenile

walleye pollock (in million metric tons) in the Sea of

Okhotsk and the western Bering Sea during the 1980's.

corresponding consumption of squid and fish is 5.6

and 17.6 x 106 t. We estimate that less than 5% of the

total fish consumed on an annual basis were juvenile

pollock. The biomass ofjuvenile pollock lost to cannibalism was estimated to be approximately 0.6 x 106 t in

the Sea of Okhotsk and 0.9 x 106 in the western Bering

Sea. Thus, although cannibalism may be an important

source of mortality for juvenile pollock, the adult pollock stocks probably inflict a greater proportional mortality on other forage fishes and crustaceans.

Table 1 shows the importance of juvenile walleye

pollock as prey for other fishes in the Sea of Okhotsk

and the western Bering Sea. These data show that the

consumption by predators in both ecosystems exceeded

the amount taken by intensive commercial fisheries.

The complex interactions among the trophic levels in

these regions, including self-regulation of abundance

by adult pollock, have important implications for the

rational management of these ecosystems.

Because ofits high abundance, walleye pollock, Theraf!7a

chalcogramma, is one of the key nekton species in both

the pelagic and demersal communities of many far

eastern Russian ecosystems. It is a main competitor of

other plankton-eating fishes and invertebrates and a

predator on small nekton. At the same time, it is an

important food source for many higher-level predators.

In the 1980's, the proportion oftotal fish biomass made

up by walleye pollock was 35%-40% in the western

Bering Sea (80%-90% of the epipelagic biomass), 40%

in the Sea of Okhotsk (75%-85% of epipelagic biomass), 50% in the Pacific waters off Kamchatka (90% of

epipelagic biomass), 20% (summer) to 55% (winter) near

the Kuril Islands, and 35%-40% in the Sea ofJapan.

During the 1980's, many thousands of pollock stomachs were examined by TINRO personnel. The annual

ration of walleye pollock was determined to be 12.6

times its body weight. The average annual food consumed by walleye pollock in the western Bering Sea in

the 1980's was estimated to be 189-252 x 106 t in the

following proportions: copepods (45%), euphausiids

(30%), amphipods (5%), chaetognaths (2%), fishes

(8%), squid (3%), and other prey (7%). In the Sea of

Okhotsk, an estimated 126-189 x 106 t were consumed:

euphausiids (45%), copepods (15%), amphipods

(10%), chaetognaths (5%), squids (3%), fishes (8%),

and other taxa (14%). The total annual consumption

for the Sea ofJapan was estimated to be 50-88 x 106 t.

For the epipelagic communities in which walleye pollock occur, the adult pollock share was 80%-90% ofthe

total annual fish consumption. Consequently, changes

in the abundance of walleye pollock affect the food

supply of other species in the pelagic nekton community. During the 1980's, walleye pollock annually consumed 4.5 x 106 t ofsquid and 12 x 106 t ofsmall fishes

in the Sea of Okhotsk. In the western Bering Sea, the

196

Source

Adult pollock (cannibalism)

Other predatory fishes

Marine mammals, birds

Total predation

Fisheries catch

Walleye pollock production

Sea of Okhotsk

0.6

2.2

0.4

3.2

1.8

5.0-7.5

Bering Sea

0.9

4.5

1.5

6.0

2.01

10.0-12.5
Juvenile Walleye Pollock, Theragra chalcogramma,

as Food for Teleosts in the Gulf of Alaska Ecosystem

MEl-SUN YANG, PATRICIA A. LIVINGSTON, and RICHARD D. BRODEUR

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.E.

Seaille, Washington 98115-0070

Analysis of stomach contents of groundfish collected

during the summer of 1990 in the Gulf ofAlaska showed

that juvenile walleye pollock, Theragra chalcogramma,

was eaten by arrowtooth flounder, Atheresthes stomias;

Pacific halibut, Hippoglossus stenolepis; sablefish,

Anoplopoma fimbria; Pacific cod, Gadus macrocephalus;

and adult walleye pollock. Arrowtooth flounder and

Pacific halibut ate the most walleye pollock by weight

(66% and 57% of their diet, respectively). Juvenile

pollock was a large component in the diet of arrowtooth

flounder and Pacific halibut at most of the stations

sampled. Consumption of age-O, age-I, and age-2 pollock by these two groundfish was also spread throughout the area, indicating a broad distribution and availability of all age groups ofjuvenile pollock to predators.

Additional stomach samples of potential predators

were collected from March through May and in September to examine predation on juvenile pollock during other times of the year. Adult pollock collected

during the spring were mostly in spawning condition,

and predation on juveniles was not observed. In September, juvenile pollock were eaten by eight different

taxa; adult pollock, arrowtooth flounder, and Pacific

sandfish, Trichodon trichodon, had the highest incidences

of age-O juvenile pollock in their diets. Pacific cod and

Pacific halibut consumed older age groups of pollock.

The size distribution of age-0 pollock in predator guts was

similar to that from a concurrentjuvenile trawl survey.

Estimates of the total amount of walleye pollock consumed by groundfish predators in the Gulf of Alaska

were produced using diet composition and predator

biomass estimates from the 1990 Gulf ofAlaska bottom

trawl survey. A 9-month feeding season was assumed in

these estimates, and daily rations were calculated from

estimates of annual growth increments and assumptions about gross conversion efficiency. Although Pacific halibut consumed the most pollock in terms of

biomass, most pollock they ate were adults. Arrowtooth

flounder was the most important groundfish predator

on juvenile pollock in terms of biomass and number

consumed. Most of the pollock biomass consumed by

arrowtooth flounder consisted of age-l and age-2 fish.

Age-O pollock were eaten by arrowtooth flounder and

walleye pollock.

197
Prerecruit Walleye Pollock, Theragra chalcogramma,

in Seabird Food Webs of the Bering Sea

ALAN M. SPRINGER

Institute ofMarine Science

University ofAlaska

Fairbanks, Alaska 99775

Prerecruit walleye pollock, Theragra chalcogramma, are

important prey ofseabirds during the nesting season at

St. Matthew Island, the Pribilof Islands, and Bogoslof

Island in the eastern Aleutian Islands (Fig. I). Limited

data suggest that they might also be important to thickbilled murres, Uria lomvia, on the Near Islands in the

western Aleutian Islands. Elsewhere, juvenile pollock

are common, if not important, in seabird diets at Cape

Peirce and Cape Newenham in Bristol Bay. They are

probably common prey at Karaginskii Island and

Verkhoturov Island in the western Bering Sea and at

other colonies in the Aleutian Islands, but no data are

available on diets at those locations.

Juvenile pollock were common prey of murres caught

during summer in commercial gill nets offshore in the

southeastern and northwestern Bering Sea (Fig. 2).

Ages 1+ and 2+ pollock are apparently important prey

ofseabirds at the ice edge in winter, having been found

in 60%-100% of five species of seabirds sampled in

March-April 1976. As indicated by otolith lengths, the

fish from the ice edge ranged in size from about 125-

250 mm (about 15-130 g).

The occurrence of pollock in the diets of black-legged

kittiwakes, Rissa tridactyla, on the Pribilof Islands tends

to be inversely proportional to the occurrence of Pacific

sand lance, Ammodytes hexapterus (Fig. 3). Years when sand

lance were common in diets were probably years when

sand lance were highly abundant, and the birds probably

shifted from pollock. Pollock may also have been less

available at the surface where kittiwakes fed in those years.

198

The proportion ofjuvenile pollock in diets of blacklegged kittiwakes on St. Paul Island has been generally

proportional to modeled estimates of pollock abundance based on survey information (r2=0.38 for age 0+,

and 0.47 for age 1+). There has been no such relation

on St. George Island.

An inverse relation between productivity of piscivorous kittiwakes and planktivorous least auklets, Aethia

pusilla, was seen in 3 years of overlapping measurements on St. Paul Island. This, and a similar inverse

relation between the abundance of age-l+ pollock and

the abundance of copepods around St. Matthew Island,

support the hypothesis that pollock is a strongly interacting member of the pelagic food web of the Bering Sea.

Acknowledgments _

Support for the author on the Pribilof Islands in 1992

and for the preparation of this abstract was provided by

Alaska Sea Grant with funds from NOAA, Office of Sea

Grant, Grant No. NA90AA-D-SG066, Project M/81-01,

and from the University of Alaska with funds appropriated by the state. Additional support in the field has

been provided by the North Pacific Universities Marine

Mammal Research Consortium. A travel grant to attend

the symposium was provided by the School of Fisheries

and Ocean Sciences, University of Alaska, Fairbanks.

Contribution No. 1474, Institute of Marine Science,

University of Alaska Fairbanks.
Springer: Prerecruit Walleye Pollock in Seabird Food Webs of the Bering Sea 199

Bering Sea

.

..

Chukchi Sea

:~~80

o

80 " 60

,..~.~ .~~o:-li-W

North Pacific Ocean

Gulf ofAlaska

Figure 1

Importance of prerecruit pollock to nesting seabirds in the Bering Sea. Values are percentages of estimated total prey

biomass (BUG = black-legged kittiwakes; RLKI = red-legged kittiwakes; COMU = common murres; TBMU = thick-billed

murres) .
200 NOAA Technical Report NMFS 126

Chukchi Sea

North Pacific Ocean

Figure 2

Locations where pollock were recovered from seabirds caught in fishing nets during summer.

The pollock were 90-240 mm long. Isobaths in m.
Springer: Prerecruit Walleye Pollock in Seabird Food Webs of the Bering Sea 201

80

(i)

,-... 40

'-'

Q)

()

I:: 20

::s

()

() 0 0

....

0 40 ;>,

()

I::

Q)

::s 30 0-

Q)

....

20

10

0

Pollock

0 St George

• StPaul

Ir rI I J

Sand Lance

o St.George

• St.Pau!

I I • r n r

40....---------------~

•

•

20 40 60 80

Frequency of Pollock

o+--~-.__~-.:...___r-~-__....._-..._~

o

Figure 3

Relation between prerecruit pollock and sand lance in

the diets of black-legged kittiwakes on the PribiiofIslands.
Relationship Between Sea-Surface Temperature,

Abundance ofJuvenile Walleye Pollock, Theragra chalcogramma,

and the Reproductive Success and Diets of Seabirds

at the Pribilof Islands, Southeastern Bering Sea

MARY BETH DECKER, GEORGE L. HUNTJR.

Department ofEcology and Evolutionary Biology

University ofCalifornia

Irvine, California 92697

G. VERNON BYRD JR.

Alaska Maritime National Wildlife Refuge

2355 Kachemak Bay Drive

Homer, Alaska 99603

Between 1975 and 1990, the reproductive success of

seabirds breeding at the Pribilof Islands fluctuated

widely, as did sea-surface temperature and juvenile walleye pollock, Theragra chalcogramma, populations. We

explored how variations in seabird reproductive success and diets may have been influenced by fluctuations

in sea-surface temperature and juvenile walleye pollock

abundance. We hypothesized that changes in the temperature regime of the southeastern Bering Sea affected the availability of key prey required for successful seabird reproduction. By inspecting annual variations in seabird diets and reproductive success, we sought

evidence for interdecadal changes in the marine environment near the Pribilof Islands. Low-frequency fluctuations in ocean temperatures have been found in the

northeastern Pacific Ocean and Bering Sea. Sea-surface

temperatures were relatively cool in the 1970's, with

1978 being the last cool year in that decade. Sea-surface

temperatures were warmer than average from 1979 to

1984, and another cool period began in 1985.

The reproductive success of both surface-foraging

black-legged kittiwakes, Rissa tridactyla, and red-legged

kittiwakes, R. brevirostris, showed no significant correlation with sea-surface temperatures. However, both species of kittiwakes had poorer reproductive success after

1978. Kittiwake reproductive success was generally below average during the warm period from 1979 to 1984,

and remained below average during the cool period

beginning in 1985, except for 1988. Reproductive success of subsurface-foraging thick-billed murres, Uria

lomvia, was negatively correlated with annual sea-sur202

face temperature when data from populations on both

islands were combined. The reproductive success of

thick-billed murres breeding on St. Paul Island appeared

to decline after 1978, whereas reproductive success on

St. George Island was greatest in the cool period starting in 1985.

We examined an alternative hypothesis: that seabird

reproductive success was correlated with the abundance

of age-l pollock in the Bering Sea. Correlations of

annual reproductive success and the number of age-l

pollock were not statistically significant for any of the

three seabirds over the study period.

Each species of seabird showed similar changes in

diet over the period of our study. Capelin, Mallotus

villosus, was eaten during the cool period from 1975 to

1978, but after 1978, capelin was no longer taken by

kittiwakes or murres. After 1978, hyperiid amphipods

were less important in the diets of black-legged kittiwakes and thick-billed murres. Pacific sand lance,

Ammodytes hexapterus, increased in the diets of blacklegged kittiwakes and thick-billed murres after 1978.

Pollock consumption, although important for seabirds,

showed no consistent or significant change through the

study period. Similarly, the percentage of pollock in

seabird diets was not correlated with reproductive success, except for a significant negative correlation for

black-legged kittiwakes on St. George Island.

In summary, seabirds nesting on the Pribilof Islands

showed decreased reproductive success (black-legged

kittiwakes and red-legged kittiwakes) and changes in

food habits (black-legged kittiwakes and thick-billed
____________ Decker et al.: Reproductive Success and Diets of Seabirds at the Pribilof Islands 203

murres) beginning in about 1978. These changes coincided with the appearance of the unusually large 1978

year class of walleye pollock and the beginning of a

period of above-average sea-surface temperatures that

lasted until 1984. Seabird reproductive success and diets did not return to pre-1979 values after 1984, which

suggests that the changes in the marine ecosystem to

which the birds responded lasted longer than the periods of warm and cool surface temperatures observed

between 1975 and 1990.
Seabird Predation onJuvenile Walleye Pollock, Theragra chalcogramma,

and Other Forage Fishes in the Gulf of Alaska

JOHN F. PIATT and SCOTT A. HATCH

National Biological Survey

Alaska Fish and Wildlife Research Center

U.S. Fish and Wildlife Service

1011 E. Tudor Road

Anchorage, Alaska 99503

Juvenile walleye pollock, Theragra chalcogramma, is an

important component of summer diet for a variety of

common seabirds in the northwestern Gulf of Alaska.

As indicated by 18,163 prey items collected mostly in

the 1980's from tufted (Fratercula cirrhata) and horned

(F. corniculata) puffin chicks at 20 different colonies,

the importance of juvenile pollock varies geographically, from little or no use in the north central Gulf of

Alaska and Kodiak Island areas, to about 5%-25% of

diets in the Semidi and Shumagin Islands, to about

25%-60% of diets at colonies farther west on the continental shelf and in the eastern Aleutian Islands. The

proportion of pollock in puffin chick diets varies annually, and is correlated with independent estimates of

204

juvenile pollock cohort strength. Juvenile pollock also

constituted about 5%-75% of diets of adult tufted and

horned puffins; common murres, Uria aalge; marbled

murrelets, Brachyramphus marmoratus; and black-legged

kittiwakes, Rissa tridactyla, during the 1980's. This contrasts with diets of these five species during the 1970's

when capelin, Mallotus villosus, made up 30%-90% of

diets, and pollock was relatively unimportant (0%-15%).

Concurrent with these changes in diet, breeding success and populations of some seabird species declined

through the 1980's. Varying diets and population parameters ofseabirds appear to reflect long-term changes

in oceanography and fish populations in the Gulf of

Alaska.
Predator-Prey Relationships ofJuvenile Walleye Pollock,

Theragra chalcogramma: Summary and Recommendations

for Future Research

GEORGE R. LILLY

Department ofFisheries and Oceans

Post Office Box 5667

St.John's, Newfoundland, AlC 5Xl Canada

The presentations in this session provided reviews and

new information about the sizes and quantities ofjuvenile walleye pollock eaten by various predators, and

analyses of the importance of walleye pollock to the

well-being ofsome of those predators. In addition, the

predators were viewed as sampling tools which could

provide information on the distribution, size, and relative abundance ofjuvenile walleye pollock.

Summary of Presentations

Because of its high abundance and large biomass, walleye pollock plays an important role in the trophodynamics of the North Pacific (Shuntov and Dulepova).

Pollock consume large quantities of zooplankton (especially copepods and euphausiids) and lesser quantities of fish and squid, and are themselves preyed upon

by a variety of fish, seabirds, and marine mammals.

Walleye pollock has long been recognized as highly

cannibalistic in some geographic areas. Studies in the

eastern Bering Sea during May-September of several

years in the early 1970's and late 1980's have shown that

the level of cannibalism, as measured by percent weight

of stomach contents, was higher in the 1970's than in

the 1980's (Livingston and Lang). The change over

time was found to be related to the abundance of age-O

and age-1 walleye pollock. The shape of the feeding

response curve remains uncertain; data are required from

additional years, especially years of very high juvenile

abundance. Other important prey included copepods (especially for fish <40 cm) and euphausiids. During years

when cannibalism was low, the percent contribution of

other prey increased. This may be interpreted as evidence that larger walleye pollock can compensate during times when smaller conspecifics are scarce, but such

an interpretation requires additional information. One

would have to assume as well that the level of consumption of all prey combined did not change. In response to

a question, it was noted that there are no independent

data on euphausiid abundance. It is therefore not possible to determine if the increased level of predation on

euphausiids in the 1980's reflected an increase in euphausiid abundance. Predation bywalleye pollock on capelin was low in both the 1970's and the 1980's.

Walleye pollock are preyed upon by many fish in

addition to larger conspecifics. A study in the Gulf of

Alaska in summer 1990 showed that walleye pollock was

very important in the diets of arrowtooth flounder and

Pacific halibut, less important for sablefish, and of relatively low importance for Pacific cod and walleye pollock (Yang et al.). Arrowtooth flounder were the major

consumers ofjuvenile walleye pollock, but larger walleye pollock were also important consumers because of

their high numbers and their tendency to feed on

small, age-O individuals. Most of the juvenile walleye

pollock eaten were age O.

Juvenile walleye pollock, primarily of ages 0 and 1,

are eaten by murres, puffins, kittiwakes, and other piscivorous marine birds. In the Gulf ofAlaska, the importance ofjuvenile walleye pollock in the diet of puffins

varies geographically and annually (Piatt and Hatch).

There is also longer-term variability. Juvenile walleye

pollock was an important prey offive species of birds in

the 1980's but was relatively unimportant for those

species in the 1970's, when capelin was an important

prey. This change in diet was concurrent with changes

in breeding success.

In the Bering Sea,juvenile walleye pollock are known

to be important prey of seabirds during the nesting

season at certain well-studied sites such as St. Matthew

Island and the PribilofIslands (Springer). Their importance to birds at many colonies is not well known because of a low level of study. Walleye pollock may also

be important prey for birds away from the colonies, in

both summer and winter, but feeding data are sparse.

There is evidence that juvenile walleye pollock may

compete for prey with planktivorous birds.

205
206 NOAA Technical Report NMFS 126

Studies of seabirds on the Pribilof Islands revealed

that, beginning in about 1978, some species showed

both a reduction in the quantity of capelin and hyperiid

amphipods in the diet and a decline in reproductive

success (Decker et al.). Two seabird species also ate

fewerjuvenile walleye pollock, but there was no positive

correlation between the consumption of pollock and

seabird reproductive success. These changes coincided

with the appearance of the unusually large 1978 year

class of walleye pollock, prompting the hypothesis that

cannibalism and other predation by adult walleye pollock might reduce prey availability to birds. The decline

in reproductive success also coincided with the beginning of a period of above-average sea-surface temperatures, prompting the hypothesis that temperature-mediated changes in distribution had reduced prey availability. Reproductive success and feeding on walleye

pollock did not increase when sea temperature cooled,

prompting the suggestion of a "regime" change.

Analyses ofjuvenile walleye pollock's importance to

predators such as seabirds is complicated by variability

in reproductive success among species and among colonies within a species. Additional insight might be gained

by narrowing the focus ofthe studies. For example, one

can determine the importance ofspecific age classes of

juvenile walleye pollock in the diets of the seabirds, and

examine seasonal and annual variation in the geographic

distributions of specific age classes ofjuvenile walleye

pollock. The proportion of age-l walleye pollock in the

diet of seabirds on the Pribilof Islands declined between the 1970's and 1980's (Hunt et aI.). During the

same period the catches of age-l walleye pollock declined in bottom-trawl surveys near the islands. These

changes in age-l walleye pollock were not correlated

with the extent ofsea ice or with potential cannibalism,

as measured by the abundance of adult walleye pollock

near the Pribilof Islands. In addition, changes in the

ratio of age-l to age-O walleye pollock in the seabird

diets did not explain a significant portion of the variability in seabird reproductive success.

Walleye pollock is also important prey for certain

marine mammals. Age-O and age-l juveniles dominated

the prey of northern fur seals in the vicinity of the

PribilofIslands in the 1980's (Sinclair et aI.). The age

distributions reconstructed from otoliths found in gastrointestinal tracts of northern fur seals caught at sea in

1981-85 reflected the relative strength of the 1978-85

year classes ofwalleye pollock. In contrast, otoliths found

in scats in 1987-90 revealed a dominance of age-O juveniles in all years. Northern fur seals collected in the

1960's and early 1970's had been feeding to a great

extent on capelin and herring, both of which were

uncommon in collections in the 1980's. This change in

diet from the 1960's to the 1980's reflects ecosystem

changes recognized in many other studies.

Long-term (decadal) changes in diet have also been

documented for Steller sea lions in the Gulf of Alaska

(Merrick and Calkins). In the Kodiak Island area, the

proportion of animals that consumed walleye pollock

increased from the 1970's to the 1980's. The proportion that consumed alternate small fish prey declined

dramatically. It is hypothesized that a decline in abundance ofsmall schooling fish, coupled with a reduction

in year-class strength ofwalleye pollock, may have been

responsible for the observed reduction in juvenile sea

lion survival and decrease in individual weight, and may

have contributed to the population decline of Steller

sea lions in the Kodiak Island area.

A review of the literature on diets of phocid seals

throughout the northwestern Pacific Ocean and Bering

Sea revealed that walleye pollock is an important prey

of harbor, spotted, and ribbon seals, but a minor prey

of bearded and ringed seals (Lowry et aI.). Most of the

walleye pollock eaten are small «20 cm), but larger

individuals may be consumed, especially by harbor seals.

Additional information on the distribution and relative abundance ofjuvenile walleye pollock comes from

bycatches in various commercial fisheries, especially

the walleye pollock fishery (Fritz). Few walleye pollock

<20 cm are caught in these fisheries, so the information

provided is for fish that are larger and older than most

of those taken by natural predators. Catch rates of

juvenile fish vary seasonally and with geographic area.

Annual changes in catch rates generally reflected

changes in the abundance of young fish, but were also

influenced by changes in fleet behavior.

Recommendations _

Recommendations were compiled from the formal presentations and general discussions. Many of these recommendations are of a general nature and similar to

those put forward following other symposia or workshops on multispecies interactions (e.g., Mercer, 1982;

Sissenwine and Daan, 1991) and the biology ofjuvenile

fishes (e.g., de Lafontaine et aI., 1992).

Measuring the Intensity of Predation on Walleye

Pollock

Who Are the Predators?-The workshop included studies of predation on walleye pollock by several species of

fish, seabirds, and marine mammals. There was no attempt to review the feeding habits of all abundant or

large predators in the North Pacific, so it is not clear if

any important predators may have been neglected. If

such a review has not been conducted elsewhere, it may

be informative to summarize the feeding habits ofgroups
__ Lilly: Predator-Prey Relationships ofJuvenile Walleye Pollock: Summary and Recommendations for Future Research 207

such as cetaceans, squids, and seabirds away from colonies, including those seabirds that breed in the Southern Hemisphere but feed in the North Pacific during

the summer.

Temporal and Spatial Heterogeneity-It is clear that

there can be considerable spatial and temporal heterogeneity in the distribution of predators and prey, and

in the quantity and type of prey consumed by predators. Data should be collected on time and space scales

that are as fine as practical. Data can be aggregated

during analysis ifsuch aggregation is more appropriate

to the specific study. Distribution of walleye pollock

and the occurrence ofwalleye pollock in predator stomachs should be reported by age (or size group) .

Importance of Predator Size and Sex-Numerous studies have emphasized that foraging strategy and diet

composition may vary with predator size (or age) and

sex, partly because animals of different sizes or sexes

may have different capabilities, and partly because they

may have different geographic distributions. Data collection and analysis should include consideration of

predator size and sex.

Functional Feeding Response-In large natural ecosystems, the relation between predators' feeding rates and

the abundance of prey can be determined only by monitoring these variables over time. A time series may have

to continue for many years to capture information during periods of low and high abundance of both predator and prey. Existing time series of predator feeding,

such as the study of cannibalism in walleye pollock,

should be continued. There may be opportunities for

creating new time series.

Many studies of functional feeding response use a

proxy for feeding rate. In some studies, feeding is expressed as percentage contribution (by weight) of the

specific prey to the total stomach contents ofthe predator. This percentage depends not only on the quantity

of the prey of interest but also on the quantity of all

other prey. A more appropriate measure would be independent of the level of predation on other prey. For

predatory fish, one could use, for example, the average

weight of the prey of interest in stomachs of the predator, taking into account variability associated with predator size.

Importance of Walleye Pollock to Predators

Compensation in Diet-Several populations of predators have been found to be in decline. In some instances these declines can be traced to low performance at specific points in the life cycle (e.g., reduced

juvenile survival in Steller sea lions, and low fledging

success in some birds), and in some instances the declines are associated with reductions in individual fitness (e.g., weight at age in Steller sea lions). Tests of

hypotheses regarding the influence of changes in abundance or availability ofjuvenile walleye pollock as prey

must take into account the availability and suitability of

other prey. Can the predator compensate for low availability ofjuvenile walleye pollock by preying more intensively on alternate species? When examining such

hypotheses, one perhaps should not express diet as

percentage of stomach content weight, because this

measure reveals changes in diet composition, not

changes in consumption ofspecific prey.

Compensation in diet may occur because the predator can increase its rate of feeding on a prey that has

not changed in abundance or availability. There is also

the possibility that some prey or competitor of walleye

pollock has become more abundant because of the

decline in walleye pollock. To understand the biological mechanisms behind compensation, more independent information is required on the abundance of other

prey such as macrozooplankton (particularlyeuphausiids); squid; and small schooling fishes such as capelin

and Pacific sand lance.

An associated question is whether changes that have

been observed in the diet ofsome predator populations

in the past two decades result from broad-scale ecosystem changes or regime shifts. If the large-scale phenomena reported elsewhere have strong local effects, it

will be important to understand the causes of these

phenomena and the geographic area over which they

operate. Since much ofthe evidence in support oflongterm ecosystem changes comes from analyses of predator diets, it may be helpful to thoroughly document the

spatial and temporal variability in the diet sampling

that has been conducted for each predator. This may

help resolve apparent inconsistencies, such as the contrast between a decline in the level of feeding on capelin by seabirds and northern fur seals near the Pribilof

Islands, and the consistent low level offeeding on capelin by walleye pollock in the eastern Bering Sea.

Importance of Spatial Scale-To expand upon an earlier point, some predators, particularly seabirds foraging around colonies, have limited ranges. In order to

assess how prey availability influences predator wellbeing (e.g., breeding success), it may be insufficient to

determine prey abundance on large spatial scales, such

as the eastern Bering Sea. It may be necesssary to determine the abundance of specific age classes of walleye

pollock and other prey on local scales (such as the

foraging areas around seabird breeding islands), and

to ascertain the factors that influence prey abundance

in these specific geographic areas. Factors that may be
208 NOAA Technical Report NMFS 126

influential include 1) changes in other stages of the

walleye pollock life cycle (e.g., spawning location, abundance oflarger conspecifics); 2) changes in abundance

or distribution of other organisms (e.g., prey such as

copepods and euphausiids); and 3) changes in the

physical environment (e.g., ice cover, temperature).

The area around the Pribilof Islands may be a good

choice for additional, well-integrated, multidisciplinary

studies.

Effect of Predators on Dynamics of Walleye

Pollock

The quantities of walleye pollock consumed by various

species of fish, seabirds, and marine mammals have

been estimated (Springer, 1992; Livingston, 1993; and

abstract by Shuntov and Dulepova), but for various

reasons the reliability of these estimates is unknown.

The number or biomass ofwalleye pollock estimated to

be removed by specific predators is often large, but how

this removal contributes to total mortality of walleye

pollock is difficult to determine, partly because of uncertainty about the magnitude of the removals and

partly because of uncertainty about the abundance of

walleye pollock at ages 0-2.

The information on consumption of walleye pollock

comes from a variety of predators sampled at different

times and places (see, for example, Livingston [1993]

and various papers in these proceedings). If practical, it

may be very useful to select a single year for extensive

and intensive study of feeding by all (major) predators

on walleye pollock, and to accompany these studies

with broad-scale trawl and acoustic surveys to determine the distribution and abundance of walleye pollock of ages 0-3. Such a study would make it easier to

quantify the removal of walleye pollock by age, and to

reconstruct cohorts to the beginning of the year by

combining the information on abundance (from surveys) , removals by predators, and discards from commercial fisheries. This exercise would help reveal the

most important contributors to mortality of juvenile

walleye pollock.

Predators as Sampling Tools

There is much to be learned about the distribution and

relative abundance ofjuvenile walleye pollock of specific ages or sizes by examining the diets of predators

(see, for example, Hatch and Sanger [1992] and the

paper by Hunt et al.). It may be useful to create maps

illustrating the locations where predators have found

walleye pollock. There could be a map for each 3- or 4-

month period ofthe walleye pollock's life up to age 3 or

4. Such maps could be compared with the information

coming from surveys, and may prove useful in modeling exercises and the planning of further studies.

Of course, one would like to know the extent to

which information on prey abundance and distribution

derived from predator feeding studies may be biased by

the feeding behavior ofthe predator. Such information

could come from studies comparing the prey taken by

the predator with the prey available in the environment

(see, for example, Sinclair et al. [1994]). Studies of

prey selectivity are not common for predators feeding

in the open ocean because of difficulties in simultaneously measuring the distribution and abundance of a

variety of potential prey, particularly when such prey

might include plankton, nekton, and various types of

benthos.

Literature Cited

de Lafontaine, Y, T. Lambert, G. R. Lilly, W. D. McKone, and

R.J. Miller (eds.).

1992. Juvenile stages: the missing link in fisheries research.

Report of a workshop. Can. Tech. Rep. Fish. Aquat. Sci.

1890, 139 p.

Hatch, S. A., and G. A. Sanger.

1992. Puffins as samplers ofjuvenile pollock and other forage fish in the Gulf of Alaska. Mar. Ecol. Prog. Ser. 80:

1-14.

Livingston, P. A.

1993. Importance of predation by groundfish, marine mammals and birds on walleye pollock Theregra chalcogramma and

Pacific herring Clupea pallasi in the eastern Bering Sea. Mar.

Ecol. Prog. Ser. 102:205-215.

Mercer, M. C. (ed.).

1982. Multispecies approaches to fisheries management

advice. Can. Spec. Publ. Fish. Aquat. Sci. 59, 169 p.

Sinclair, E., T. Loughlin, and W. Pearcy.

1994. Prey selection by northern fur seals (Callorhinus uTsinus)

in the eastern Bering Sea. Fish. Bull. 92:144-156.

Sissenwine, M. P., and N. Daan.

1991. An overview of multispecies models relevant to management of living resources. ICES Mar. Sci. Symp. 193:

6-11.

Springer, A. M.

1992. A review: walleye pollock in the North Pacific-How

much difference do they really make? Fish. Oceanogr.

1:80-96.
An Individual-based Model of the Juvenile Stage ofWalleye Pollock,

Theragra chalcogramma, in the Western Gulf of Alaska

SARAH HINCKLEY, BERNARD MEGREY

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.E.

Seattle, Washington 98115-0070

ALHERMANN

Pacific Marine Environmental Laboratory

7600 Sand Point Way, N.E.

Seattle, Washington 98115-0070

As part of an effort within NOAA's Fisheries-Oceanography Coordinated Investigations Program, we have

been developing a biophysical model of the early life

history ofwalleye pollock, Theragra chalcogramma, in the

western Gulf of Alaska. Combining an individual-based

biological life-stage model with a three-dimensional hydrodynamic model, the model is process-oriented,

mechanistic, and spatially explicit. Its purpose is to

investigate how recruitment variability relates to the

interaction of mechanisms affecting growth and survival of the different life stages of pollock.

The individual-based model tracks fish separately

throughout the study period and the region. It follows

each fish from spawning through the egg, yolk-sac larval, feeding larval, and O-age juvenile life stages, and

through the processes (such as mortality, feeding, and

growth) of each stage. An individual-based approach

was chosen for several reasons. First, it was thought that

modeling the "average" fish (as do most traditional

deterministic models) may be misleading; that it was

important, instead, to understand the history of unique

individuals, particularly those that survive to recruitment. Second, the environment of these fish was not

homogeneous (an important assumption of aggregate

models). Each fish traces a unique trajectory through

space, which is strongly affected by mesoscale circulation features and results in differing exposure to environmental variables (i.e., temperature, salinity, predators, and prey). These different trajectories result in

unique growth histories and probabilities of survival

among individuals. Third, mechanisms and behaviors

thought to be important to recruitment variability can

be included with much more detail in individual-based

models than is possible in aggregated models.

The initial effort to model the juvenile stage had

several objectives. First was to use the model to validate

the horizontal distribution ofjuveniles seen in the field

data. In the model we treat juveniles as passively advected particles. In examining the divergence between

modeled and real horizontal distributions, we hope to

examine the question of how much of the horizontal

distribution that we need to explain by directed horizontal swimming behavior. The second major objective

in the preliminary version of the model was to identify

gaps in our knowledge about the juvenile life stage.

The juvenile-stage model has several components. It

predicts daily consumption as a probabilistic function

of body size-i.e., a developmental response. No functional response (consumption as a function of prey

density) information was available for this life stage of

pollock. Metabolic rates ofjuveniles had to be obtained

by interpolation between the larval stage and older

juvenile stages. Daily growth is based on the difference

between daily consumption and daily metabolic costs.

Mortality ofjuveniles is neglected in this preliminary

version ofthe model to reduce the number ofindividuals that need to be followed. Mortality is not critical in

the process that we are examining here (advection).

Depth of individuals in the water column affects the

direction of horizontal advection due to vertical shear

and the effects of bottom topography. It is therefore

important to include a prediction of depth of each

individual at each time step for correct advection. Determination of the depth of individuals varies by life

209
210 NOAA Technical Report NMFS 126

stage in the model. Eggs are spawned and hatch. deep

in the water column, and yolk-sac larvae rise quickly to

the upper 60 meters. When feeding larvae reach 6 mm,

they begin dieI migration, which increases in amplitude

as they increase in length. The depth distribution and

vertical migration ofjuvenile pollock are not well understood. We therefore used two simple schemes to

predict individual juvenile depths. We then used each

scheme to examine the resulting horizontal distribution of the juvenile stage. In the first scheme, average

depth increased with increasing juvenile size; in the

second scheme, the average depth ofjuveniles was constant. In both schemes, normally distributed random

noise was added to the depth of an individual at each

time step.

Modeled length-at-age curves agree well with empirical length-at-age curves derived from otolith data. An

increased variability in length with increasing age is

seen in the model output, which agrees with what is

observed in the data. Length frequencies from field

data from September 1990 are normally distributed

with a mean of 71 mm (range 55-110 mm). Modeled

length frequencies are also normally distributed with a

mean of76 mm (range 45-100 mm).

The actual distribution of juvenile pollock in September 1990 was concentratedjust east ofthe Shumagin

Islands. The distribution was patchy, and the highest

concentrations were inshore. The modeled distribution ofjuveniles using the first scheme of depth determination showed good agreement in degree of patchiness and the location ofthe highest concentration (east

of the Shumagins), but the center of the distribution

was somewhat farther offshore. Advection along bottom contours significantly affected the distribution, because near-bottom oceanic flows tend to follow bathymetric contours. When the second scheme was used,

the distribution ofjuveniles was farther inshore. In this

scheme,juveniles were generally shallower in the water

column, and the flow and thus their advection were less

influenced by bottom topography.

In conclusion, many of the features of actual juvenile

pollock distributions (in September) were reproduced

by the model. It is interesting that treating juveniles as

passively advected particles did not result in large discrepancies between modeled and real distributions, at

least for September 1990. This suggests that active,

directed horizontal swimming behaviors during the juvenile (or earlier) stages may be less important than

advective processes over all of the early life stages in

determining September distributions. The directed

swimming behaviors that do exist appear to transport

juveniles toward inshore areas.
On the Relationship ofjuvenile Walleye Pollock, Theragra chalcogramma,

Abundance to Adult Recruitment and Linkages with the Environment

BERNARD A. MEGREY

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

I examined the relationship of trends in juvenile pollock abundance to recruitment time series of walleye

pollock, Theragra chalcogramma, in ShelikofStrait, Alaska.

I also explored the relative importance of various biological and physical factors to abundance ofjuvenile

pollock. Two recent findings are that there is a relation

between survival offirst-feeding larval pollock and local

wind mixing, and that year-class strength is usually set

by September following the spring spawning. Based on

these findings, the analysis focused on the period from

January through August. The primary biological data

for the analysis consisted of an age-2 recruitment time

series covering the period 1962-89, and egg, larval, and

juvenile abundance time series dating from 1981 to

1989. A suite of physical data, which overlapped the

biological data, consisted of various time series describing atmospheric and oceanic features ofthe region. These

included estimates of turbulent wind mixing of the oceanic mixed layer, volume transport, simulated subsurface

drifter trajectories, and sea-surface temperature. Univariate

and multivariate statistical techniques were used to qualitatively and quantitatively determine how the biological

and physical variables relate to juvenile abundance.

Results suggested that age-O and age-l abundance

estimates are related to freshwater input and an index

of storminess; linear regression models accounted for

95% and 96% of the variation in those dependent

variables, respectively. The influence of these factors

was consistently demonstrated over the larval,juvenile,

and adult life stages. From these results several hypotheses were proposed: spring wind mixing in Shelikof

Strait affects larval survival, large-scale atmospheric circulation directly affects the Shelikofregion; baroclinicity

may generate features conducive to larval survival; and

effects of the physical environment on predation and

behavior may affect juvenile survival. Another important finding is that significant processes (yet unknown)

may be occurring during the summer months.

211
Importance of Cannibalism in the Population Dynamics

of Walleye Pollock, Theragra chalcogramma

VIDAR G. WESPESTAD

Alaska Fisheries Science Center

National Marine Fisheries Seroice, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

TERRANCE]. QUINN II

Juneau Center, School ofFisheries and Ocean Sciences

University ofAlaska, Fairbanks

11120 Glacier Highway

Juneau, Alaska 99801

A spawner-recruit time series of nearly 30 years suggests

a strong density dependence in eastern Bering Sea

pollock recruitment (Fig. 1). The underlying cause of

this density dependence is believed to be cannibalism

ofjuvenile pollock by adults. To test the effect of cannibalism on population size and yield we used the historic

age-structured population data to simulate population

trends with varying fishery removals of adult pollock.

We relate cannibalism to Ricker parameter ~. Abundance at time t during the early life history is denoted

N(t). The dynamics of the population during the early

life stage are related to spawning and mortality. It is

assumed that

1) T, the age of recruitment, is constant;

2) the number of eggs, No' is proportional to spawning

stock S, and No = fS, where fis average net fecundity;

3) the total mortality, Zt' at life stage t between times 0

and T contains both density-independent (q) and

density-dependent effects, represented by Zt = a + bS.

Assumption 3 implies that the spawning stock inhibits

the population of young fish in some way before recruitment. One mechanism is cannibalism, which is

known to occur in eastern Bering Sea pollock. The

Ricker spawner-recruit relationship,

was used to model pollock, where recruits at age 2 (R)

are expressed in terms of egg production (No) instead

of spawning stock (S). Productivity parameter a is re212

lated only to density-independent mortality parameter

a. Parameter is proportional to density-dependent

parameter b and inversely proportional to fecundity

parameter! For our purposes, we call the "cannibalism" parameter since we have no evidence to suggest

major changes in average net fecundity.

Population parameters for the simulations include

estimates of abundance and fishing mortality from cohort analysis for 1964-92 for ages 2 to 12+, where the

last age is a plus group. We set T=2 for the starting age

of recruitment since this was the starting age in the

data. Full-recruitment fishing mortality, Fhist' was estimated for each year t as the average fishing mortality

for ages 4 and 5. Selectivity was estimated by age and

year. Natural mortality, M a, was set to 0.45 at age 2 and

0.30 for ages 3 to 12+. Annual estimates of egg production, No' were calculated with maturity rna and fecundity

fa values at age. The Ricker spawner-recruit relationship was fitted to the egg production and recruitment

estimates from the cohort analysis. We used egg production rather than spawning stock in developing the

spawner-recruit relationship because the ratio of recruitment to egg production is an estimate of early life

survival. Estimates of parameters are

Ina = 9.84 ± 0.25(±1 S.E.),

= 1.53 X 10-15 ± 0.18 x 10-15 , and

&=0.55.

The latter is an estimate ofthe standard deviation about

the spawner-recruit line on a log scale. Thus, on the

original scale, a coefficient of variation of about 0.55
______Wespestad & Quinn: Importance of Cannibalism in the Population Dynamics of Walleye Pollock 213

.78 R=4.15'S'EXP(-0.173'S)

.65 .. 84 & 82

.72

66 .67

.85

.74

.64

• 7j 75

.83

.86

.88

.87

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Spawners (billions)

20

18

16

14 'iii

c:

12

M

CI)

10 '"

..

.. 8 ':;

t;

CI)

a: 6

4

2

0

0

Figure 1

Eastern Bering Sea pollock spawner-recruit relationship, 1964-88. Numbers are years.

about the spawner-recruit line is estimated. In the regression, the value of r 2 was 73%, the amount of variability in the data explained by the spawner-recruit

relationship. The cannibalism coefficient was significantly different from zero (P<O.OOI), suggesting empirically that cannibalism is an important component

of pollock dynamics.

We constructed a simulation to explore the interaction

of cannibalism and fishing mortality on the population

from 1979 to 1992, the most recent years of management under the Magnuson Act and the years that are

supported by trawl-survey estimates of abundance. We

used starting estimates of abundance for ages 2 to 12+

in 1979 from cohort analysis, as well as egg production

in 1977 and 1978. We calculated recruitment at age 2 in

year t+2 from egg production in year t as

mortalities Fhist for Ft , the cannibalism coefficient from

the spawner-recruit analysis, age- and year-specific selectivities, and the starting abundance values described

above. We analyzed twelve scenarios, including the base

case: three sets of cannibalism coefficients (p's) crossed

with four fishing mortality scenarios. The three sets of

p's represented weak, medium, and strong levels of

cannibalism (density dependence) aI}d corresponded

to 0.5, 1.0, and 1.5 times the historic ~. As we were not

interested in the effect of density-independent mortality on the population, we estimated a value of In a that

gave the best fit to the spawner-recruit model by using

the recruitment-egg production estimates. For a given

~, this estimate is

for the years 1979 to 199? If a and are replaced with

their estimates &. and ~, then the recruitments obtained are identical to those obtained from cohort analysis. Mortality and abundance at later ages and years

were projected from the typical age-structured model

equations.

We constructed a base simulation to reproduce the

cohort analysis results. We used the historical fishing

where the overline denotes the average of the variable

beneath it, over the range of years used in the spawnerrecruit analysis. The four fishing mortality scenarios

were no fishing (Ft=O) , fishing at half the historical rate

(Ft=O.5 Fhist)' fishing at the historical rate (Ft=Fhis) ' and

fishing at 1.5 times the historical rate (Ft=1.5 Fhi). For

each scenario, we summarized the results in terms of

total biomass across age, total yield across age, egg

production, and recruitment trends over time.
214 NOAA Technical Report NMFS 126 . _

Results suggest that fishing has little effect on pol1ock

recruitment if density dependence is either strong or

weak, but can exert some influence when density dependence is moderate (Fig. 2). Variation in recruitment pattern through density dependence affects biomass trends (Fig. 3). At observed, moderate levels of

density dependence, increasing or decreasing fishing

from observed levels would not have altered the population trend, and the level of fishing would have no

effect in the most recent years. Ifstrong density dependence operated in the eastern Bering Sea pollock population, the rate of removals would have less effect, and

the population would rise to a higher level and fal1

more rapidly and to a lower level than under moderate

density dependence. With weak density dependence, recruitment is greater in the midyears of the period examined than that observed, and the resulting biomass trend

indicates greater stability (Fig. 2, 3). In general, recruitment is increased by fishing under the observed moderate

density dependence, but it is only slightly increased under

strong and weak density dependence. Yield is enhanced at

intermediate population levels. It appears from this analysis that maintaining high population levels is detrimental

and causes population oscillations, especially if density

dependence is strong.
_____Wespestad & Quinn: Importance of Cannibalism in the Population Dynamics of Walleye Pollock 215

Moderate Density Dependence Fishing Level

___ 0

~0.5

-.-1.5

-+- Observed

82 83 84 85 86 898887 90 91

u; 30 c: 25

:c 20 .:.:

u

0 15

"0

Co 10

('oj

CIl 5 Cl

0

79 80 81

Strong Density Dependence

III

c:

o

:c

.:.:

u

.2

"0

Co

('oj

CIl

Cl

50

40

30

10

o ~--~-_.

o

CX) CX)

N

CX)

('I)

CX)

LO

CX) coCX)

CX)

CX) enCX) o

en en

Weak Density Dependence

III

c:

:c

.:.:

u

o

"0

Co

('oj

CIl

Cl

30

25

20

15

10

5

0

79 80 81 82 83 84 85 86 87 88 89 90 91

Year

Figure 2

Estimated recruitment trend of eastern Bering Sea pollock, 1979-91, under three levels of density dependence and four levels of fishing: 0, 0.5 observed, observed (1.0), and 1.5 observed.
216 NOAA Technical Report NMFS 126

Moderate Density Dependence Fishing Level ___ 0

-x-0.5

20

--.-1.5

;;- ~Observed

c: 15 0

'E 10 VI

VI

co

E

0 5

i:a I

Ot79 80 81 82 83 84 85 86 87 88 89 90 91

Strong Density Dependence

25

;;- 20 c:

15 :g

VI

VI 10 co

E

0

: -I

i:a

79 80 81 82 83 84 85 86 87 88 89 90 91

Weak Density Dependence

25

;;- 20

c:

.2

'E 15

VI

VI 10 co

E

0

i:a 5 -

o 1--+-------1

79 80 81 82 83 84 85 86 87 88 89 90 91

Figure 3

Estimated biomass trend of eastern Bering Sea pollock, 1979-91, under three levels of density dependence and four levels

of fishing: 0, 0.5 observed, observed (1.0), and 1.5 observed.
Stock Synthesis Model ofWalleye Pollock,

Theragra chalcogramma, Including Predation

PATRICIA A. LIVINGSTON and RICHARD D. METHOT}

Alaska Fisheries Science Center

National Marine Fisheries Seroice, NOAA

7600 Sand Point Way N.£.

Seattle, Washington 98115-0070

Predation is often thought to be an important influence on the population dynamics of some groundfish

populations. Because most traditional stock assessment

models do not have a way to incorporate predation into

the assessment process, predation is often entered as a

constant rate across time. The stock synthesis model

offers a framework for incorporating predators as "fisheries" in a single-species catch-at-age analysis. We investigated the effect of including predation by adult walleye pollock, Theragra chalcogramma; Pacific cod, Gadus

macrocephalus; and northern fur seals, Callorhinus ursinus,

into a stock assessment ofwal1eye pol1ock in the eastern

Bering Sea. These predators were thought to be the

main sources of predation mortality for age-1+ walleye

pol1ock. Predators were entered into the model by defining 1) a time series of predator abundance (Le.,

effort) over the whole modeled period, 2) a series of

consumption data (i.e., catch per unit of effort) for

each predator for each year where data on food habits

were available, and 3) the age composition of pollock

consumed for each year where data on food habits were

available. Instantaneous annual predation mortality rates

estimated from cod and fur seal were small, ranging

from about 0.02 to 0.04. However, mortality rates of

age-1 pollock due to cannibalism were high. Adding

predation and particularly cannibalism to the model

improved the fit of the stock-recruitment relation at

age 1 by increasing the amount of variation accounted

for by the relation by a factor of 2. The largest outliers

from the relation were years when the physical environment favored larval survival. Environment and predation both appear to play an important role in pollock

year-class recruitment.

J Present address: Northwest Fisheries Science Center, NMFS, NOAA,

2725 Montlake Blvd. E., Seattle WA 98112-2097.

217
Natural Mortality Estimates ofJuvenile Walleye Pollock,

Theragra chalcogramma, in the Gulf of Alaska

ANNE B. HOLLOWED, KEVIN M. BAILEY, and RICHARD D. BRODEUR

Alaska Fisheries Science Center

National Marine Fisheries Service, NOAA

7600 Sand Point Way N.£.

Seattle, WA 98115-0070

interval in days. Natural mortality for 15-60-mm pollock was set equal to 0.02. Age in days for a given size

range was determined from a length-age relationship

derived from otolith microstructure analysis:

where SL is the standard length, Yl = 4.52, Y2 = 90.2,

a =-0.81, = 0.026, and age is in days. Model estimates

of total abundance were adjusted to fit the survey indices of abundance by estimating a "catchability" coefficient (Q) for each survey with the following simple

form:

where Nis the estimated number at age t (days) and dis

the duration of the life stage. The central assumption

here is that research survey estimates of population

abundance are directly proportional to the true abundance. Estimates oftotal pollock abundance were made

for three age groups: 53 d (late larvae, 15 mm), 167 d

(age 0), and 639 d (age 2). Embryonic stage durations

were taken from previous studies, and larval stage durations were estimated from Equation 2.

Model parameters in Equation 3 were adjusted to

minimize the mean square error between observed and

estimated pollock abundance at the three life stages.

Three parameters were adjusted to tune the expected

survey indices to the observed survey indices: Q for the

larval and age-O surveys and M for juvenile pollock. All

other model parameters were fixed at the values shown

in Table 1.

Estimates of Q were 1.90 for larval surveys and 0.24

for age-O surveys. The value of 1.90 would indicate that

the initial abundance of eggs used to initiate the model

was underestimated, the natural mortality schedule was

Juvenile walleye pollock, Theragra chalcogramma, is an

important component of the marine ecosystem in the

Gulf ofAlaska, serving as primary prey for many marine

fishes, mammals, and birds. Although the amount of

juvenile pollock consumed by various predators has

been estimated, and survey indices ofjuvenile pollock

abundance are available, estimates of the total abundance of age-O walleye pollock have not been advanced.

This paper presents an exploratory method for converting indices of abundance to total abundance, provides

a life-history table, and gives a brief analysis of ontoge··

netic change in abundance.

Indices of abundance can be converted to total abun··

dance estimates if natural mortality, availability, and

net selectivity are known. The Fisheries Oceanography

Coordinated Investigations (FOCI) program has en·

deavored to produce estimates of natural mortality for

larval and early juvenile pollock, and net selectivity of

plankton gear for these early stages. We used the FOCI

estimates of natural mortality, in conjunction with larval and juvenile abundance indices, to estimate a life

table for Gulf ofAlaska pollock during their first 3 years

of life. We assumed that natural mortality was known

for egg and larval stages and that survey selectivity and

availability were unknown. Egg mortality was set equal

to the mean egg mortality (0.208) measured for 1981

and 1985-91. Mortality during the early larval period

was estimated from mortality estimates for three size

categories (yolk-sac-6 mm, 6 mm-7 mm, 7 mm-15

mm) estimated from a logistic fit to observed mortality

rates provided by K. Bailey (unpubl. data). The y intercept of the logistic equation was set at the maximum

daily instantaneous mortality (0.38) for the youngest

age group observed, yielding the equation

M = 0.38 - Y/[1 + EXP(-A *(~- x»)], (1)

where M = mortality, y = 0.362, = -7.172, 1.=-0.208,

and x was the midpoint of the age range for the size

218

SL = [Yl~ + (Y2~ - Yl~) *(1- EXP(-a *age»)/

I

1- EXP(-a*l50)]il,

Nt =Q *[Ntod *EXP(-M*d)],

(2)

(3)
_______ Hollowed et aI.: Natural Mortality Estimates ofJuvenile Walleye Pollock in the Gulf of Alaska 219

Table 1

Life table for Gulf of Alaska walleye pollock for ten life stages. Initial abundance, estimates of natural mortality, and stage

duration based on FOCI research. Italicized mortality value indicates a free parameter in the modeling exercise.

Stage M daily Duration (d) Initial abundance d since hatch

Egg 0.208 12.5 132,536,728,571,429 0.001

Yolk-sac 0.239 8.8 9,843,977,075,822 12.5

6mm 0.119 5.9 1,201,606,971,406 21.3

7mm 0.057 5.2 595,448,606,975 27.2

7-15 mm 0.020 25.5 442,710,068,451 32.4

15-60 mm 0.020 110 265,845,438,80 I 57.9

Age 0 0.014 207 29,456,514,255 167.9

Age 1 0.005 265 1,624,037,094 374.9

Age 2 0.00082 365 431,673,865 639.9

Age 3 320,015,797 1004.9

Tons consumed

13,820

254,977

253,276

83,346

too high, or spawning occurred outside of Shelikof

Strait. The value of 0.24 for the age-O survey suggests

that the natural mortality schedule used was too low,

that the age-O surveys missed a large fraction ofjuveniles, or both. Juvenile mortality was estimated to be

0.014, a value higher than previously reported.

The life table for juvenile pollock provided a reasonable life-history trajectory (Table 1) in which the expected number of age-2 pollock at the beginning of the

year corresponded to the average observed year classes

of pollock. The proposed mortality schedule indicated

that the biomass of pollock at age 0 would exceed the

biomass at age 1 or 2. If one assumed that the mortality of

juvenile pollock >15 mm was all due to predation, the

biomass ofjuvenile pollock consumed would be 592,000 t.

Time series of observed pollock recruitment show

that the 1985 and 1988 year classes were average to

above average, and that the 1981, 1986, 1987, and 1989-

91 year classes were all below average. The observed

and predicted abundance of late larvae showed a good

fit to the 1981, 1985, and 1988 year classes, whereas

predictions of the 1986, 1987, and 1992 year classes

were higher than the observed survey index (Fig. 1).

This suggests that natural mortality was higher than

expected in 1986, 1987, and 1991.

The model overestimated the survey index of age-O

and age-2 pollock in 1981, and underestimated the

survey index of age-O and age-2 pollock in 1985 and

1988 (Fig. 1). This pattern suggests that in 1981 natural

mortality was higher than expected during the first

summer. Likewise, the results suggest that in 1985 and

1988, natural mortality was lower than expected during

the first summer. These results indicate that significant

adjustment (positive or negative) to year-class strength

can occur during the summer of the first year of life,

well beyond the larval stage.
220 NOAA Technical Report NMFS 126

';n' 2500·

= II Prediction :5 2000

e II Survey Index

e

e

'"....

... 1000 eo

.... 500 e

=z 0

81 82 83 84 85 86 87 88 89 90 91

25 '"= 20 e

= 15 ..

OIl

-<... 10 eo

..

.. 5

e

z=

81 82 83 84 85 86 87 88 89 90 91

Il:IPredicted

IIIStock Assessment

';n' 3 r----;=========;---------------~

=

:5 2

e

2

.§

'3

..

'"

..

OIl

-< 0

81 82 83 84 85 86 87 88 89 90 91

Year Class

Figure I

Comparison of observed and predicted indices of juvenile pollock abundance in the

Gulf of Alaska for IS-mm fish (upper), age-O fish (middle), and age-2 fish (lower).
Using Mechanistic Models to Test Hypothesized Causes

for Steller Sea Lion, Eumetopiasjubatus, Declines

MILO ADKISONI and MIGUEL PASCUAL2

Fisheries Research Institute

Box 357980

University of Washington

Seattle, Washington 98195

The most contentious issue in the decline ofthe Steller

sea lion, Eumetopias jubatus, in the Bering Sea and the

Gulf ofAlaska is the possibility that the walleye pollock,

Themgra chalcogramma, fishery contributes to the decline. Some have suggested that the commercial fleet is

reducing the forage available to the sea lions, and that

the resultant energetic stress has reduced survival and

reproductive success. Others have argued that pollock

abundance was greater during the period of the decline than previously. We initially attempted to link the

historical data on the food available to Steller sea lions

to their trends in abundance using a mechanistic model

with three components: 1) a prey-capture model linking the abundance and spatial distribution offood species to the amount of fish consumed by individuals in

specific sea lion colonies, 2) a bioenergetics model

relating the amount of fish consumed to the survival

and reproductive success of an individual sea lion, and

3) a population dynamics model relating individual

survival and reproductive rates to changes in sea lion

abundance.

We abandoned this approach because abundance

data for many of the fish species and species size classes

eaten by sea lions were sparse. Instead, we restricted

ourselves to the population dynamics component of

the model and compared how the various hypotheses

about the causes of Steller sea lion declines explain the

observed declines. Using this methodology, we were

able to eliminate a delayed effect of 1970's sea lion

harvests, short-term environmental fluctuations, and

transient effects of perturbations in population age

structure as potential causes. The only remaining hypothesis capable of explaining sea lion declines was a

significant long-term degradation in environmental conditions. Unfortunately, this hypothesis is so broad as to

include many divergent scenarios, only one of which is

a reduction in food intake.

We attempt to narrow this ambiguity by using the

bioenergetics component of our original model. We

determine how large a reduction in caloric intake would

be necessary to reduce survival and reproduction to the

levels necessary to produce the observed population

declines.

1 Present address: College of Fisheries and Ocean Sciences, University of Alaska, Fairbanks, l1l20 Glacier Hwy.,juneau, AK 99801.

2 Present address: Dept. of Zoology, Box 351800, University ofWashington, Seattle, WA 98195-1800.

221

Population Dynamics and Modeling ofJuvenile Walleye Pollock,

Theragra chalcogramma: Summary and Recommendations

for Future Research

~LL~J.OVERHOLTZ

Northeast Fisheries Science Center

National Marine Fisheries Service, NOAA

Woods Hole, Massachusetts 02543

The session on population dynamics and modeling reviewed a variety of research perspectives taken by modelers working on the dynamics of juvenile pollock,

Theragra chalcogramma, in the North Pacific and Bering

Sea ecosystems. The general theme of the session focused on biological and environmental mechanisms

that are important in the overall scheme of population

change in these marine systems. Topics included environmental effects on pollock recruitment, the magnitude of natural mortality, and compensation and its

relation to the natural mortality rates ofjuvenile pollock. Other analyses focused on the survival and growth

of juvenile pollock from the perspective of the individual organism, and on how additional sources on

predation mortality might change our perspective on

overall pollock abundance. Several researchers also concentrated on cannibalism as an important mechanism

in juvenile pollock population dynamics. Another study

attempted to relate declines in Steller sea lions, Eumetopiasjubatus, to changes in juvenile pollock abundance

by studying alternative hypotheses with simulation.

Hinckley et al. gave an overview of their bottom-up

modeling approach to juvenile pollock dynamics. The

model was individual-based, concentrating on the effects of predation, prey availability, fish growth, physical factors, and other important mechanisms controllingjuvenile abundance. Participants felt the approach

was useful because details and important mechanisms

could be easily incorporated into this model structure.

The senior author discussed ways of incorporating additional complexity into the method. Some discussion

centered on whether accumulation of individual responses is the best method for obtaining aggregate

population results.

Megrey's presentation focused on the importance of

various biological and environmental factors as regulators of juvenile pollock abundance in Shelikof Strait.

Results suggested that spring weather conditions may

affect larval survival, predation rates, and behavioral

traits. The models accounted for a large percentage of

the variation in larval and juvenile abundance and may

lead to a better understanding of the mechanisms controlling recruitment in this species. Discussion centered

on future areas of investigation, methods, and the next

generation of models that will be developed.

Wespestad and Quinn reviewed a study on the underlying causes of density dependence in Bering Sea walleye pollock. One current hypothesis is that cannibalism is

a strong regulatory mechanism influencing this stock.

Simulations under varying fishing mortality regimes were

used to ascertain the level of population response. Results

suggested that yields are highest at middle levels ofstock

size and would be much more variable at high pollock

abundances, presumably because of heavy cannibalism.

For their study of the sources of natural mortality of

juvenile pollock in the Gulf of Alaska, Hollowed et al.

chose a life-stage approach addressing the period from

prerecruit surveys to the time of recruitment. By estimating an average catchability factor for a set ofsurvey

years (1981 and 1985-92), the researchers calculated

annual fluctuations from this constant Qand used them

to examine different recruitment and predation hypotheses. This approach appeared promising, although

perhaps data hungry, to participants because it allowed

for a direct focus on causes of variation in year-class

strength of walleye pollock.

A study by Livingston and Methot incorporated predation rates into a stock synthesis model of Bering Sea

pollock. The model included the impact of predation

by adult pollock, Pacific cod, and northern fur seals.

Age-specific estimates of predation mortality were produced, and model results suggested that predation rates

by northern fur seals and Pacific cod were low, whereas

cannibalism may produce a fairly high mortality rate in

juvenile pollock.

A study of the mechanisms controlling abundance of

juvenile pollock offJapan was reported by Wada and

Yabuki (not published here). They analyzed the mor223
224 NOAA Technical Report NMFS 126

tality rate ofjuveniles relative to food type and availability, and whether adult pollock abundance might significantly influencejuvenile survival. Results suggested that

when food was in high abundance, especially euphausiids, mortality rates were low. Correlation analyses indicated that cannibalism was not an important regulatory

mechanism of year-class strength.

The presentation by Adkison and Pascual centered

on the causes of decline in the Steller sea lion population ofthe Bering Sea. They used a simulation model to

study various hypotheses. They initiated various model

scenarios and examined results for causal links. Results

suggested that general demographic responses, previous harvests, and short-term environmental processes

were not important. Longer-term environmental conditions did appear to be a possible cause of the decline,

but mechanisms were not readily apparent. Future work

will center on the energetics of the species and how

survival and reproduction might be affected.

The session successfully integrated important information and studies of the pivotal species in these two

large North Pacific ecosystems. The breadth and quality of modeling approaches to study how walleye pollock interact with important biological mechanisms and

environmental forcing was impressive. Results should

lead to a better understanding of the relevant networks

and linkages in these systems. The directions of future

research on this timely subject appear to be centered

on the right set of questions and hypotheses, and a

better level of understanding seems assured.
Milo Adkison

College of Fisheries and

Ocean Sciences

University of Alaska, Fairbanks

11120 Glacier Highway

Juneau, AK 99801

William Aron

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Kevin Bailey

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Christine Blackburn

Director, Alaska Groundfish Data

Bank

P.O. Box 2298

Kodiak, AK 99615

Richard Brodeur

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Annette L. Brown

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Troy Buckley

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Vladimir N. Burkanov

Kamchatka Institute of Ecology and

Nature Management

Russian Academy of Sciences

Prospekt Rybakov 19A

Petropavlovsk-Kamchatskiy,

683204 Russia

1 Current addresses.

Workshop Participants1

Morgan S. Busby

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Mary Beth Decker

Department of Ecology and

Evolutionary Biology

University ofCalifornia

Irvine, CA 92697

Elena P. Dulepova

Pacific Research Institute of

Fisheries and Oceanography

(TINRO)

Shevchenko Alley, 4

Vladivostok, 690600 Russia

Lowell Fritz

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Thomas]. Gross

University of Alaska

P.O. Box 55846

Fairbanks, AK 99705

Donald Gunderson

School of Fisheries

Box 357980

University of Washington

Seattle, WA 98195

Albert]. Hermann

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Sarah Hinckley

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Kenneth Holland

Minerals Management Service

949 E. 36th Ave.

Anchorage, AK 99508

Anne Hollowed

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

George L. HuntJr.

Department of Ecology and

Evolutionary Biology

University of California

Irvine, CA 92697

Yasuhiro Kawasaki

Hokkaido National Fisheries

Research Institute

Katsurakoi 116

Kushiro, Hokkaido 085,Japan

Alexander S. Kitaysky

Department of Ecology and

Evolutionary Biology

University of California

Irvine, CA 92697

Masat Kobayashi

Center for Coastal Physical

Oceanography

Old Dominion University

768 52nd Street

Norfolk, VA 23529

Han-Lin Lai

Northeast Fisheries Science Center

NMFS, NOAA

Woods Hole, MA 02543

Geoffrey M. Lang

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

George R. Lilly

Department of Fisheries and

Oceans

P.O. Box 5667

St.John's, Newfoundland

AIC 5Xl Canada

225
226 NOAA Technical Report NMFS 126

Patricia Livingston

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Elizabeth Logerwell

Department of Ecology and

Evolutionary Biology

University of California

Irvine, CA 92697

Thomas Loughlin

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Debra A. McFee

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Denise R. McKelvey

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Bernard A. Megrey

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Nazila Merati

jISAO, PMEL, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Richard Methot

NWFSC, NMFS, NOAA

2725 Montlake Blvd. E.

Seattle, WA 98112-2097

Hiroya Miyake

Hokkaido Wakkanai Fisheries

Experimental Station

Wakkanai, Hokkaido 097,japan

Frank Morado

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Kazuya Nagasawa

Salmon Ecology Section

National Research Institute of Far

Seas Fisheries

Orido, Shimizu, Shizuoka 424,

japan

jeffrey Napp

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Akira Nishimura

National Research Institute of Far

Seas Fisheries

North Pacific Resources Division

5-7-1, Orido, Shimizu 424,japan

Brenda Norcross

Institute of Marine Science

University of Alaska

Fairbanks, AK 99775-1080

Bori Olla

AFSC, NMFS, NOAA

Cooperative Institute for Marine

Resource Studies

Hatfield Marine Science Center

Newport, OR 97365

William J. Overholtz

Northeast Fisheries Science Center

NMFS, NOAA

Woods Hole, MA 02543

james Overland

PMEL, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Yoshioki Ozeki

National Research Institute of Far

Seas Fisheries

2-12-4 Fukuura

Yokohama 236,japan

Brent C. Paine

North Pacific Fishery Management

Council

605 W. 4th Avenue

Anchorage, AK 99510

Miguel Pascual

Department of Zoology

Box 351800

University of Washington

Seattle, WA 98195

Ian Perry

Pacific Biological Station

Department of Fisheries and

Oceans

Nanaimo, B.C., V9R 5K6 Canada

john F. Piatt

Alaska Fish and Wildlife Research

Center

U.S. Fish and Wildlife Service

1011 East Tudor Road

Anchorage, AK 99503

Vladimir I. Radchenko

TINRO, Vladivostok

Shevchenko Alley, 4

Vladivostok 690600, Russia

RolfR. Ream

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Bruce W. Robson

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Yasumori Sakurai

Research Institute of North Pacific

Fisheries

Hokkaido University

Minato-cho 5-1-1

Hakodate, Hokkaido 044,japan

Michiyo Shima

School of Fisheries

Box 357980

University ofWashington

Seattle, WA 98195

Elizabeth Sinclair

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070
Michael P. Sissenwine

NMFS,NOAA

1335 East-West Highway

Silver Spring, MD 20910

Alan Springer

Institute of Marine Science

University of Alaska

Fairbanks, AK 99775

Gary Stauffer

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Olga S. Temnykh

Pacific Research Institute of

Fisheries and Oceanography

(TINRO)

Shevchenko Alley, 4

Vladivostok, 690600 Russia

jimmie Traynor

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Tokio Wada

NRIFS

2-12-4 Fukuura

Yokohama 236, japan

Kasutoshi Watanabe

Hokkaido National Fisheries

Research Institute

Katsurakoi 116

Kushiro, Hokkaido 085,japan

Matthew Wilson

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Tina Wyllie-Echeverria

jISAO, PMEL, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Mei-Sun Yang

AFSC, NMFS, NOAA

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

Hideo Yoshida

Hokkaido Central Fisheries

Experimental Station

Hokkaido Prefectural Government

Hamanaka 238, Yoichi-cho

Hokkaido 046, japan

Workshop Participants 227
NOAA TECHNICAL REPORTS NMFS

The major responsibilities of the National Marine Fisheries Service (NMFS) are to monitor and assess the abundance

and geographic distribution of fishery resources, to understand and predict fluctuations in the quantity and distribution of

these resources, and to establish levels for their optimum use. NMFS is also charged with the development and implementation of policies for managing national fishing grounds, with the development and enforcement of domestic fisheries regulations, with the surveillance of foreign fishing off U.S. coastal waters, and with the development and enforcement

of international fishery agreements and policies. NMFS also assists the fishing industry through marketing services and economic analysis programs and through mortgage insurance and vessel construction subsidies. It collects, analyzes, and publishes statistics on various phases of the industry.

Recently Published NOAA Technical Reports NMFS

117. Distribution, abundance, and biological characteristics of groundfish off the coast of Washington,

Oregon, and California, 1977-1986, by Thomas A. Dark

and Mark E. Wilkins. May 1994,73 p.

118. Pictorial guide to the groupers (Teleostei:

Serranidae) of the western North Atlantic, by Mark

Grace, Kevin R. Rademacher, and Mike Russell. May 1994,

46 p.

119. Stocks of dolphins (Stenella spp. and

Delphinus delphis) in the eastern tropical Pacific: a

phylogeographic classification, by Andrew E. Dizon, William F. Perrin, and Priscilla A. Akin. June 1994, 20 p.

120. Abundance and distribution of ichthyoplankton

along an inshore-offshore transect in Onslow Bay,

North Carolina, by Allyn B. Powell and Roger E. Robbins.

June 1994, 28 p.

121. Marine flora and fauna of the eastern United

States, Platyhehninthes: Monogenea, by Sherman S.

Hendrix. August 1994, 107 p.

122. The sununer flounder, scup, and black sea

bass fishery ofthe Middle Atlantic Bight and southern

New England waters, by Gary R. Shepherd and Mark Terceiro. August 1994, 13 p.

123. NOAA Coastal Change Analysis Progra.rn (CCAP): Guidance for regional hnplenlentation, by]. E.

Dobson, E. A. Bright, R. L. Ferguson, D. W. Field, L. L.

Wood, K. D. Haddad, H. Iredale III, ]. R. Jensen, V. V.

Klemas, R.]. Orth, and]. P. Thomas. April 1995,92 p.

124. Effects of the Cessation of Sewage Sludge

DUnlping at the 12-Mile Site: 12-Mile DUnlpsite SYnlposiUnl, Long Branch, New Jersey, June 1991, by Anne

L. Studholme, John E. O'Reilly, and Merton C. Ingham (editors). October 1995, 257 p.

125. Population Characteristics of Gulf Menhaden,

Brevoortia patronus, by Douglas S. Vaughan, Eldon ].

Levi, and joseph W. Smith. February 1996, 18 p.

Copyright Law

Although the contents if the-,e reports hwe not been copyrighted

and may be reprinted erztire!y, riference to -,ource i-, appreciated.

The National Marine Fisheries Service (NMFS) does

not approve, recommend, or endorse any proprietary

product or proprietary material mentioned in this

publication. No reference shall be made to NMFS,

or to this publication furnished by NMFS, in any advertising or sales promotion which would indicate or

imply that NMFS approves, recommends, or endorses any proprietary product or proprietary material mentioned herein, or which has as its purpose an

intent to cause directly or indirectly the advertised

product to be used or purchased because of this

NMFS publication.