NOAA Technical Memorandum NMFS-AFSC-52

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Alaska Fisheries Science Center

May 1995

Echo Integration-Trawl Survey of

Walleye Pollock (Theragra chalcogramma)

in the Southeast Aleutian Basin

During February and March, 1994

by

T. Honkalehto and N. Williamson
NOAA Technical Memorandum NMFS

The National Marine Fisheries Service’s Alaska Fisheries Science Center

uses the NOAA Technical Memorandum series to issue informal scientific

and technical publications when complete formal review and editorial

processing are not appropriate or feasible. Documents within this series

reflect sound professional work and may be referenced in the formal

scientific and technical literature.

The NMFS-AFSC Technical Memorandum series of the Alaska Fisheries

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

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

by the Northwest Fisheries Science Center.

This document should be cited as follows:

Honkalehto, T., and N. Williamson. 1995. Echo integration-trawl

survey of walleye pollock (Theragra chalcogramma) in the southeast

Aleutian Basin during February and March, 1994. U.S. Dep.

Commer., NOAA Tech. Memo. NMFS-AFSC-52, 39 p.

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

the National Marine Fisheries Service, NOAA.
May 1995

NOAA Technical Memorandum NMFS-AFSC-52

Echo Integration-Trawl Survey of

Walleye Pollock (Theragra chalcogramma)

in the Southeast Aleutian Basin

During February and March, 1994

U.S. DEPARTMENT OF COMMERCE

Ronald H. Brown, Secretary

National Oceanic and Atmospheric Administration

D. James Baker, Under Secretary and Administrator

National Marine Fisheries Service

Rolland A. Schmitten, Assistant Administrator for Fisheries

by

T. Honkalehto and N. Williamson

Alaska Fisheries Science Center

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

Seattle, WA 98115-0070
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Results from the winter 1994 echo integration-trawl survey of spawning walleye pollock

(Theragra chalcoramma) near Bogoslof Island, covering an area between 166° W. and about

17°W. long., from the Aleutian chain north to 54°30‘N. lat. are presented. The eastern part of

the survey area was inhabited mainly by non-spawning pollock from the 1989 year class that

formed echo sign layers at depths of about 200-500 m. In the middle of the region, over deeper

water, older pollock from a number of year classes and 1989 year class pollock formed dense

spawning aggregations at depths of around 300-600 m. Although some pollock eggs were

collected in one of five bongo net tows, little evidence of pollock spawning was observed before

the end of the survey on 9 March. Despite some recruitment of the 1989 year class and the near

absence of commercial fishing in the central and southeast Aleutian Basin, estimated spawning

biomass declined from its winter 1993 level of about 0.63 million metric tons (t) to about 0.49

million t. The decline was most likely due to the natural mortality of aging year classes, which

had supported the Bogoslof area spawning population at least since 1988.)
INTRODUCTION

Walleye pollock (Theraera chalogramma) are a semidemersal species of fish which

inhabits the North Pacific Ocean from off central California to the Sea of Japan, and ranges

throughout the Bering Sea. They support the largest single-species fishery in the world, with

average annual catches of 5.3 million t since 1978 (FAO 1991, Wespestad 1993a). Until the early

1980s, pollock were harvested in coastal waters of the eastern Bering Sea shelf by countries other

than the United States. Implementation of the Magnuson Fishery Conservation and Management

Act (MFCMA) in 1977 prompted a shift from foreign to domestic fleets inside the 200 mile

fishery conservation zone (FCZ). In 1983, this area was renamed the U.S. exclusive economic

zone (EEZ) and includes most of the Alaska continental shelf and the southeast Aleutian Basin.

Catches on the eastern Bering Sea shelf, which peaked at about 1.9 million t in 1972, have

averaged around 1.3 million t in recent years and are taken exclusively by U.S. vessels.

Subsequent to the MFCMA, new fisheries for pollock arose in the Aleutian Basin. The

displaced foreign fleet initiated a fishery in the central Bering Sea outside both the U.S. and

Russian EEZs (the donut hole (Fig. 1, inset)). Pollock catches in the donut hole rose to 1.45

million t in 1989, exceeding the U.S. EEZ catch but have declined sharply since then (Wespestad

1993a). In 1987, a new domestic fishery began harvesting spawning pollock in the southeast

Aleutian Basin near Bogoslof Island. In 1991, Bogoslof area catches peaked at 264,000 t, while

donut hole catches declined to about the same level.

During the latter half of the 1980s, scientists proposed that there was a connection

between pollock inhabiting the donut hole and those spawning near Bogoslof Island, and were
2

concerned about the effect of donut hole catches on pollock originating from U.S. and Russian

EEZs. Responding to a drop in pollock biomass in the Bogoslof Island region, the North Pacific

Fishery Management Council prohibited U.S. vessels from targeting pollock in the central

Aleutian Basin after 1991, and set pollock bycatch limits near Bogoslof Island to 1,000 t

(Wespestad 1993a).

In February 1991, prompted by a 1990 bilateral meeting between the U.S.S.R. and the

U.S., four high-seas fishing nations (Japan, Korea, Poland, and China) met with the U.S. and

U.S.S.R. to initiate discussions on establishment of a Convention on the Conservation and

Management of the Living Marine Resources of the Central Bering Sea. Through a series of 10

conferences using information from research cruises and the fishery itself, the nations agreed that

pollock populations in the Aleutian Basin were in decline and that unrestricted fishing was harmful

to the entire Bering Sea ecosystem. They proposed a moratorium on fishing in the donut hole

from 1992 to 1995, or until basin stocks recovered to a biomass greater than 1.67 million t. The

Convention agreement, which also established research efforts, monitoring, and enforcement

protocols for the central Bering Sea, was finalized in February 1994 and was signed by each of the

countries that participated in negotiations. It was presented to the U.S. Senate Committee on

Foreign Relations in September 1994, and awaits ratification’.

The relationship between pollock inhabiting the basin and those found associated with the

U.S. and Russian Bering Sea shelves is still unclear. To address this and other management

issues, U.S. scientists from the Alaska Fisheries Science Center (AFSC), in cooperation with

1D. A Colson, Deputy Assistant Secretary of State for Oceans Affairs, U.S. Dept. of State,

Washington D.C., testimony before the Committee on Foreign Relations, United States Senate,

September 28, 1994.
3

scientists from Japan, Russia, Korea, Poland, and China, have conducted echo integration - trawl

(EIT) surveys of spawning pollock in portions of the Aleutian Basin and Bering Sea shelf each

winter since 1988, except 1990. In 1993, as part of the Convention agreement on the donut hole,

participating nations decided that in the absence of other research data the results of the U.S.

winter survey of the Bogoslof area would be used as an index of the total Aleutian Basin biomass.

It was agreed to assume that the biomass of spawning pollock near Bogoslof represented 60% of

the Aleutian Basin pollock biomass. The fishing moratorium will be in effect until the Bogoslof

area spawning biomass exceeded 1 million t.

This report presents results from the 1994 survey of southeast Aleutian Basin waters near

Bogoslof Island conducted between 27 February and 9 March (Fig. 1). The principal objectives

were to collect echo integration data and trawl data necessary to determine the distribution,

abundance, and biological composition of spawning walleye pollock.

METHODS

Sampling Equipment

The survey was conducted on board the NOAA ship Miller Freeman, a 66 m (216 A) stem

trawler equipped for fisheries and oceanographic research. Acoustic data were collected with a

Simrad EK500 quantitative echo sounding system (Bodholt et al. 1989). A Simrad 38 kHz split

beam transducer was mounted on the bottom of the vessel’s centerboard. With the centerboard

fully extended, the transducer was 9 m below the water’s surface. System electronics were
housed in a portable laboratory mounted on the vessel’s weather deck. Data from the Simrad

EK500 echo sounder/receiver were processed using Simrad BI500 echo integration and target

strength data analysis software (Foote et al. 1991) on a SUN workstation.

Midwater echo sign was sampled using a modified Northern Gold 1200 midwater rope

trawl (NET Systems, Inc.). The trawl was constructed with ropes in the forward section and

stretch mesh sizes ranging from 163 cm (64 in) immediately behind the rope section to 8.9 cm

(3.5 in) in the codend. It was fished in a bridleless configuration and was fitted with a 3.2 cm

(1.25 in) mesh codend liner. Headrope and footrope lengths were 94.5 m (310 ft) and 50 m (164

ft), respectively, and the breastlines measured 79.4 m (260.5 ft). Headrope length was measured

between the points of attachment to the breastline; footrope length was measured between the

points where the tom weights are attached, The net was fished with 1.8 m X 2.7 m (6 ft X 9 ft)

steel V-doors (1,000 kg [2,200 lb]), and 227 kg (500 lb) tom weights on each side. Trawl mouth

opening and depth were monitored with a Furuno wireless netsounder system attached to the

headrope of the trawl.

Ichthyoplankton and zooplankton were sampled with a two-tiered bongo net system. A

20 cm bongo frame with 153 pm mesh nets was attached to a wire approximately 0.5 m above a

60 cm (23.6 in) bongo frame with 333 pm mesh nets. A 40 kg lead weight was used as a

depressor. To monitor depth and oceanographic conditions, a Seabird conductivity/temperature/

depth (CTD) profiler was attached to the wire about 0.6 m above the 20 cm bongo frame.
5

Vertical profile measurements of water temperature and salinity were collected at trawl

and calibration sites using a Seabird CTD system. Expendable bathythermographs (XBT)

provided additional temperature profile data.

Sampling Techniques

From 27 February to 9 March, AFSC scientists surveyed westward from 165°51’ W to

169° 53’ W long. along parallel north-south transects (Fig. 1). On the eastern and western ends of

the survey area, transects were spaced 10 nautical miles (nmi) apart. In the central area, pollock

densities were higher and echo sign was distributed such that an adaptive sampling approach was

used. After completing transects 1.0 through 9.0, we doubled back to run transects 6.5 through

10.5 between whole numbered transects, reducing spacing to 5 nmi (Fig. 2). For transects 11 .0

through 15.0, 10 nmi spacing was resumed. Transect 10.0 was cut short by severe weather. The

southern end of each transect extended to the 100 m depth contour on the Aleutian shelf

Northern transect endpoints were similar to those in previous years*, except in the western part of

the survey area, where inclement weather prevented the vessel from conducting transects as far

north. Lack of significant echo sign in the northwestern survey area (north of 54° N lat. and west

of 168° W long.) during winter 1993, and the very low echo sign density along northwestern

transects this year led us to believe that no significant portion of the Bogoslof area spawning

population was missed. However, during the winter 1988-1992 EIT surveys, some pollock sign

was observed in that area.

2

 N. Williamson, Alaska Fisheries Science Center, NMFS, NOAA 7600 Sand Point Way NE,

Seattle WA 98115, unpublished cruise reports MF88-1, MF89-1, MF91-1, MF92-2, MF93-1,

November 1994.
6

Echo integration-trawl operations were conducted both day and night. While the vessel

was transecting, its speed ranged between 4 and 13 knots depending upon weather conditions.

The acoustic system collected echo integration data which, when properly scaled, is used to

estimate pollock density. Midwater trawl hauls were made at selected locations (Fig. 2) to

identify echo sign and to provide biological samples. The vertical net opening for the midwater

rope trawl averaged 20 m at a trawling speed of about 3 knots. Standard catch sorting and

biological sampling procedures, similar to those described in Wakabayashi et al. (1985), were

used to provide weight and number by species for each haul. Pollock were further sampled to

determine sex, fork length (FL), body weight, age, maturity, and gonad weight. Ovary tissue

samples were collected from mature pollock and frozen for stock structure studies. Whole

pollock were frozen for a calorimetric study. Gonadosomatic indices (GSIs) were computed for

all pre-spawning female pollock. The GSI was defined as the ratio of ovaryweight to total body

weight.

When weather permitted, a bongo net tow was conducted each night at approximately

2300 (MT) to collect macrozooplankton samples for the initial conditions of a numerical

simulation model of Bering Sea plankton, The nets were deployed at a constant wire speed of

40 m/min to a maximum depth of 400 m. The winch was stopped and the nets allowed to

stabilize for up to 30 sec. The nets were then retrieved at a wire speed of 20 m/min. The ship’s

speed was adjusted to attempt to maintain a wire angle of 45° during the entire tow. When the

nets reached the surface, they were brought aboard and the samples were washed into the

codends. By random selection, one half of the sample was preserved in formalin. The other half

was sieved into three size classes and frozen for later analysis of nutritional content. Flow meters
7

in the nets recorded the amount of water filtered and the attached CTD recorded the time/depth

history of the tow.

Standard sphere calibrations were conducted in Port Susan, Washington, on 10 February,

in Inanudak Bay, Umnak Island, Alaska, on 25 February, and in Kuliak Bay, Alaska, on 26 March

1994 (Table 1). Acoustic measurements of a copper sphere suspended below the transducer

were made. The standard sphere (60.0 mm diameter) had a known target strength of -33.6 dB.

Split beam target strength and echo integration data were collected with the Simrad EK500

system to describe transducer beam pattern characteristics and other acoustic system parameters.

System parameters were adjusted so that target strength and echo integration measurements

matched the known responses of the standard sphere. No significant changes in the acoustic

system parameters were observed between calibrations.

Data Analysis

Absolute estimates of pollock abundance were derived from acoustic data as follows.

Echo integration data with a horizontal resolution of about 9 m (at 12 knots ship speed) and a

vertical resolution of l-2 m from the transducer to 0.5 m off bottom, or to the lowest extent of

the fish sign, were examined for pollock sign. Estimates of total backscattering strength in the

area represented by each transect were generated. These values were then summedand scaled to

estimate the numbers and weight of pollock for each length (cm) and age (years) category using

pollock size compositions, length-weight relationships derived from trawl catches, a previously

derived relationship between target strength (TS) and fish length (TS = 20 Log FL - 66; Foote

and Traynor 1988), and an age-length key. Although the raw haul size compositions indicated a
8

higher abundance of females in the population, past experience in sampling the Bogoslof area

spawning schools has shown that we cannot easily obtain a representative sex ratio because these

pollock are highly aggregated and stratified by sex. Historical data from Japanese summer

surveys (1977-79) of the entire Aleutian Basin do not support the argument that there are more

females than males in the population. For example, Yoshimura (1992) reports finding a higher

percentage of males in his basin-wide survey conducted during summer 1988. We thus assumed a

50:50 sex ratio for population size compositions and the length-weight relationship. A male

length-weight regression equation obtained by pooling all trawl data was averaged with a pooled

female length-weight regression equation by minimizing the sum of squares between the two

curves using an iterative non-linear approach. Estimates of length-specific numbers and biomass

were summed to produce total estimates for Bogoslof Island pollock aggregations.

RESULTS

Biological and Oceanographic Results

‘Biological data were collected and specimen and tissue samples preserved from 13

midwater rope trawl stations and 5 bongo net tows (Fig. 2, Tables 2 and 3). Pollock was the

dominant fish species captured in midwater trawl hauls (Table 4). Significant numbers of

lanternfish (Myctophidae) and northern smoothtongues (Leuroglossus schmidti) were also

captured. Biological data collected (Table 5) included pollock length and maturity characteristics,

whole pollock, and whole specimens of several other fish species.
9

Pollock caught in trawl hauls ranged from 38 to 67 cm FL. Two size modes were

observed. Pollock catches from bottom depths less than 500 m were dominated by 40-50 cm fish,

whereas those from bottom depths greater than 500 m were composed mostly of 50-60 cm fish.

A single haul (haul 12, 987 m bottom depth, Fig. 2) caught fish with a bimodal length

distribution. Sex composition varied with each haul ranging from 28% to 98% female. In some

areas where two layers of echo sign were observed, each layer was sampled (hauls 3-4 ,7-8).

The proportion of males was higher in the deeper layers, implying that vertical stratification by sex

was occurring. Throughout the survey more females were caught than males, but this was

considered a sampling anomaly (see Methods). For the two nearshore hauls (<500 m bottom

depth), the larger females (>51 cm) were all in a pre-spawning stage whereas 58% of the females

less than or equal to 51 cm were in a developing stage and not expected to spawn soon (Fig. 3a).

Maturity data from the 11 offshore hauls (>500 m bottom depth) indicated that 90% of the female

pollock greater than or equal to 40 cm were in a pre-spawning stage (Fig. 3b). Gonadosomatic

indices from these pre-spawning females exhibited a positive correlation with length (Spearman

r = 0.424, p < 0.00l), with a mean of 0.19 (Fig. 4). Few actively spawning females had been

observed by the time the survey ended on 9 March. We examined all bongo net tows for further

evidence of spawning. Only one, cast 4 (Table 3, Fig. 2) contained a visible quantity of pollock

eggs. We also observed a few female pollock with fairly full ovaries that looked as though they

had partially spawned and were either preparing to spawn again or were resorbing the remainder

of their eggs.

Oceanographic data included 11 CTD casts and 12 XBT casts (Tables 6 and 7, Figs. 5-7).

Most of the temperature data in the area surveyed showed a well-mixed water column with a

slight decline in temperature from 4° C near the surface to about 3.5° C at 300 m and deeper.
10

Salinity increased with depth from about 32 ppt near the surface to 34 ppt at 600 m (Fig. 6). In

the eastern portion of the survey area XBT profiles 6 and 7 (Figs. 5 and 7a,b) indicated a uniform

5° C water column. XBT profiles 20 and 24 showed an influx of cooler water (2° - 3.5° C)

between the surface and 450 m on the western end of the survey area (Fig. 7c).

Pollock Distribution and Abundance

Pollock echo sign was observed throughout the survey area (Fig. 1). In the eastern

portion of the survey area (transects 1 .0-5.0) moderate densities of pollock were distributed

slightly off bottom in water depths averaging around 600 m. They formed a layer from about

200-500 m, deepening to 400-600 m as the water depth increased. Age and maturity data from

haul 1 indicated that these pollock were primarily from the 1989 year class, and were nonspawning. Echo sign on the north end of transect 6.0 resembled echo sign from transects 1.0-5.0.

To the south along transect 6.0, as water depths increased, the sign became denser and remained

well off bottom, which is more typical of southeast Aleutian basin spawning pollock aggregations.

From transect 6.0 westward to transect 10.0, south of 54°N lat., dense concentrations of pollock

echo sign were observed mainly 300-500 m from the surface, but ranging vertically from 250 to

750 m in depth (Fig. 8, “spawners”). Pollock were distributed mostly along the southern third of

each transect within approximately 30 nmi of the Aleutian chain. In shallower water adjacent to

the Aleutian chain, pollock echo sign appeared different than that observed over deeper water,

and consisted of layers at 150-400 m (Fig. 8, “non-spawners”). Information from haul 9 indicated

that these fish were largely from the 1989 year class. In the western portion of the survey area,

echo sign density diminished from transects 10.0 to 12.0, increased significantly along transect

13.0, then decreased again along transects 14.0 and 15.0 (Fig. 1).
11

Beginning at the eastern end of the survey area, pollock sign was usually accompanied by

a thinner scattering layer above at about 200-300 m which was presumed to be a mixture of small

fish and macrozooplankton based on evidence from midwater trawls. We did not specifically

sample this micronekton scattering layer, but we consistently caught lanternfish and northern

smoothtongues in trawls targeting the deeper pollock layer. Along transects 7.0-10.0, pollock

and micronekton layers overlapped. Trawl hauls 4,5, 8 and 13, made in that vicinity (Fig. 2),

caught the largest bycatch of lanternfish and northern smoothtongues. Along the north half of

transect 11.0 and on transects to the west, the micronekton echo sign persisted while pollock sign

dished. The relationship between the two layers is not known. Although evidence exists that

pollock prey on lanternfish and bathylagids during the late summer in nearby slope waters’,

pollock stomach samples during past Bogoslof area surveys have shown little evidence of

predation. Stomachs were usually empty, due either to lack of feeding or to regurgitation after

being trawled from great depths,

Two types of pollock echo sign were identified and quantified from this region. The first

formed the typical spawning aggregations found over deep water in the region surrounding

Bogoslof Island, surveyed since 1988. “These mainly pre-spawning pollock (hauls 2-8 and 10-l 3)

had an estimated biomass of 0.49 million t and numbered around 478 million fish (Table 8). They

ranged in length from 39 to 67 cm with a major mode at 52 cm and a minor mode at 45 cm (Fig.

9a). Their ages ranged from 4 to 24 years with one pronounced mode at 5 years accounting for

19% of the spawning population (Fig. 9b). The second sign type was characterized by pollock

caught in hauls 1 and 9 inhabiting shallower waters along transects 1.0-5.0, the northern portion

3

 G. Lang, Alaska Fisheries Science Center, NMFS, NOAA 7600 Sand Point Way NE, Seattle

WA 98115. pers. commun., November, 1994.
12

of transect 6.0, and near the Aleutian chain They were categorized as a non-spawning portion of .

pollock echo sign found near Bogoslof Island but outside of the main spawning area They had an .

estimated biomass of 0.05 million t and numbered 71 million fish (Table 8). Their predominant

length mode was 44 cm (Fig. 9a), and they were mainly in developing maturity stages in contrast

with fish from the other hauls. Most were age 5 from the 1989 year class, accounting for 50% of

the non-spawning fish (Fig. 9b).

DISCUSSION

In the region surrounding Bogoslof Island, estimated spawning biomass of walleye pollock

has been decreasing since the first survey in 1988 (Fig. 10). The current level is 0.49 million

t--down from 2.4 million t in 1988. Age data from surveys conducted in 1988, 1989, and 1991

indicate that 1978 has been the dominant year class (Fig. 11), while 1992 survey age data showed

the 1978 year class’ dominance less distinctly. Age data collected in 1994 show that the 1989

year class, an above-average year class on the eastern Bering Sea shelf (Wespestad 1993b), is

relatively strong in the Bogoslof area population. No age data were available from the 1993

survey at the time of the writing of this report The average length of fish increased steadily from .

47.2 cm in 1988 to 5 1.4 cm in 1993, underlining the 1978 year class’ dominance of the spawning

stock (Figs. 10 and 12). In 1994, the average length among spawners was 5 1 .0 cm. The decrease

in average length is due to the entry of the 1989 year class in conjunction with the continuing

decline of the 1978+ year class fish.

Age determination for Bogoslof area pollock has become extremely difficult since the first

EIT surveys in 1988 due to the numbers of older fish currently composing the population
13

(Fig. 11). The difficulty in ageing older fish is related to the fact that as fish age, growth slows,

and annual otolith rings crowd closer and closer together. Counting rings becomes more

difficult and therefore is subject to larger errors. While 1988-1991 population-at-age data show a

progression of strong 1978, 1982, and 1984 year classes; the 1992 and 1994 data blurred these

year class relationships for older pollock. For example, in survey year 1994 the pollock

population-at-age curve shows a dip at age 10. We believe the actual numbers representing the

“strong” 1984 year class should be higher and are instead spread over surrounding age classes.

The 1994 Bogoslof area population ages for older fish should thus be used with caution in

modeling and forecasting.

How, when, and from where younger pollock recruit to the Bogoslof area spawning

population is not well understood. The southeast Aleutian Basin is characterized by a near

absence of juvenile pollock compared with surrounding shelf areas, and recruitment to the

Bogoslof spawning stock is not estimated to occur until ages 4-5 (Dawson 1989). By its relative

strength compared with surrounding year classes, the 1989 year class potentially provides the first

chance to track recruitment to the area since the 1984 year class. Cruise results from the winter

of 1991 indicated a few 1989 year class pollock were caught east of the main Bogoslof spawning

area over water greater than 1000 m deep’. During the winter 1992 survey, a few 30-40 cm

pollock were caught in two hauls from greater than 500 m deep waters. In winter 1993,

aggregations of non-spawning pollock that appeared to be mostly the 1989 year class were

observed south and east of the Bogoslof area spawning aggregations. They ranged from 24 to

64 cm with a length mode at 40 cm. This year, non-spawning 1989 year class fish were observed

4

 N. Williamson, Alaska Fisheries Science Center, NMFS, NOAA, 7600 Sand Point Way NE,

Seattle WA 98115, unpublished cruise report MF91-1, November 1994.
14

in the eastern end of the Bogoslof survey area and along the Aleutian shelf over shallower water

than the spawning aggregation. Pollock caught on the western end of the spawning area over

deep water exhibiteda bimodal length distribution, with the smaller mode at 45 cm. These largely

pre-spawning fish were a portion of the 1989 year class that recruited to the spawning population

(Figs. 11 and 12). Why some of the 1989 year class recruited to the Bogoslof area spawning

population while some did not remains unclear, as well as why the two groups were distributed

over different depth habitats. They seemed to inhabit similar temperature and salinity regimes.

Perhaps those that recruited were the first segment of that year class to reach maturity, and others

will follow as they continue to mature. Some of these younger fish may never join the Bogoslof

spawners but remain outside the main aggregations or return to the shelf Others may be part of

an Aleutian Islands “stock’‘-associated with shallow waters near the island chain rather than with

deeper basin waters. Prior to 1993, no large concentrations of non-spawners had been observed

east of the main spawning area in the Aleutian Basin.

Observer catch data from the winter 1994 pollock roe fishery indicated fishing in the

Aleutian Islands region (west of 170° W long.) began in January (Fig. 13). Observer length

frequency data taken on board vessels fishing that location in January matched that from the

Bogoslof area spawning population. Perhaps these vessels were targeting pollock moving into the

Bogoslof area. During the first 2 weeks in February, fishing was concentrated on the Bering Sea

shelf until the offshore roe fishery closed on 18 February, followed by closure of the inshore roe

fishery on 2 March. The Bering Sea offshore closure is reflected by increased fishing in the

Aleutians after 17 February, with peak catches occurring between 17 February and 16 March.

The Aleutian offshore fishery closed on 1 March, and the inshore fishery on 18 March. Pollock

fishing iii the Aleutians after 18 March was attributed to community development quotas (CDQs).
15

Total catch from 20 January through 13 April in the Aleutians between 170° and 177° W long.

was over 57,000 t5

. Of that, about 19,000 t was captured during the survey period between 27

February and 9 March.

Although pollock spawning has been observed in the Aleutian Basin from January through

March (Hinckley 1987), spawning in the Bogoslof Island region usually occurs in late February to

early March (Traynor et al. 1990, Teshima et al. 1991). In 1994, most pollock were in a pre

spawning stage, and pollock eggs were only observed at a single bongo net location, Among

spawners, this year’s average GSI (0.19, Fig. 4) was slightly lower (not significant) than last year’s

(0.22), but so was the average length of fish. The positive correlation between FL and GSI may

account for the difference. Given high average GSI values (compared with GSIs during other

times of the year) and few eggs in the water column in early March, we estimated that most

spawning began shortly after the survey ended on 9 March-similar to spawning timing in 1993,

but later than in previous years. Temperature was probably not a factor in the delay, as water

temperatures were similar to those from past observations for pollock spawning in the southeast

Aleutian Basin (Honkalehto 1990).

In summary, the Bogoslof area spawning walleye pollock population continues to decline

with the biomass now at 0.49 million t. Population decline has been attributed to high fishing

pressure in the Bogoslof region (1987-91), and in the donut hole (1984-91), where fish are

presumed to be part of the same spawning stock (Dawson 1989), as well as to lack of strong

recruitment, and to natural mortality of aging year classes which had supported the population for

5M. Guttormsen, Alaska Fisheries Science Center, NMFS, NOAA, 7600 Sand Point Way NE,

Seattle WA 98115, pers. commun., November 1994.
16

many years. Since 1991, although fishing is no longer a major source of mortality, the population

has declined due to poor recruitment and senescence. This year, we observed some recruitment

of the relatively strong 1989 year class, consistent with projected strength of that year class on the

eastern Bering Sea shelf. Evidence of increased recruitment supports the hypothesis that the

Bogoslof /Aleutian Basin areas are populated by density-dependent pollock population spill-over

from the shelf during years of strong year classes (Wespestad 1993a).

ACKNOWLEDGEMENTS

We would like to thank Denise McKelvey who assisted in drafting the initial cruise report

for this survey. Thanks as well to all other participating scientists on the Bogoslof survey, and to

the captain and crew of the NOAA ship Miller Freeman.
17

CITATIONS

Bodholt, H., H. Nes, and H. Solli. 1989. A new echo sounder system. Proc. IOA. 11(3): 123-130.

Dawson, P. 1989. Stock structure of pollock in the eastern and central Bering Sea, p. 605-642. In

B. R. Melteff (ed.) Proceedings of the International Symposium on the Biology and

Management of Walleye Pollock. Alaska Sea Grant Rept. 89-1.

FAO (Food and Agriculture Organization of the United Nations). 1991. Yearbook of Fishery

Statistics. FAO, Rome, Italy.

Foote, K. G., and J. J. Traynor. 1988. Comparison of walleye pollock target strength estimates

determined from in situ measurements and calculations based on swimbladder form. J.

Acoust. Soc. Am. 83(l): 9-17.

Foote, K.G., H. P. Knudsen, R. J. Komeliussen, P. E. Nordbo, and K. Roang. 1991.

Postprocessing system for echo sounder data. J. Acoust. Soc. Am. 90:37-47.

Hinckley, S. 1987. The reproductive biology of walleye pollock, Theragra chalcogramma in the

Bering Sea with reference to spawning structure. Fish. Bull., U.S. 85(3):481-498.
18

Honkalehto, T. 1990. Results of the echo integration-midwater trawl survey of walleye pollock

Theraga chalcogramma in the eastern Bering Sea in winter 1989. Unpubl. Rep. 32p.

Alaska Fish. Sci. Cent., Natl. Mar. Fish. Serv., NOAA 7600 Sand Point Way NE, Seattle

WA 98115. (submitted to the 37th annual meeting of the INPFC, Vancouver, Canada.)

Teshima, K., T. Honkalehto, and T. Yoshimura. 1991. Maturity condition of pelagic walleye

pollock from the Aleutian Basin of the Bering Sea. Nippon Suisan Gakkaishi 57(8): 1495-

1499.

Traynor, J. J., W. A. Karp, M. Fumsawa, T. Saski, K. Teshima, T. M. Sample, N. J. Williamson,

and Y. Yoshimura. 1990. Methodology and biological results from surveys of walleye

pollock (Theragra chalcogramma) in the eastern Bering Sea and Aleutian Basin in 1988. In

L.L. Low, (ed.), Int. North Pac. Fish. Comm. Proceedings of the symposium on

application of stock assessment techniques to gadids. p. 69-99.

Wakabayashi, K., R. G. Bakkala, and M. S. Alton. 1985. Methods of the U.S.-Japan demersal

trawl surveys. In R. G. Bakkala and K. Wakabayashi (eds.), Results of cooperative U.S.-

Japan groundfish investigations in the Bering Sea during May-August 1979. Int. North

Pac. Fish. Comm Bull. 44:7-26.

Wespestad, V. G. 1993a. The status of Bering Sea pollock and the effect of the “donut hole”

fishery. Fisheries 18(3): 18-24.
Wespestad, V. G. 1993b. Walleye pollock. In Stock Assessment and Fishery Evaluation Report

for the Groundfish Resources of the Bering Sea/Aleutian Islands Regions as projected for

1994. North Pacific Fisheries Management Council, P.0. Box 103136, Anchorage AK,

99510.

Yoshimura, T. 1992. III. Biological sampling. In Sasaki, T., (ed.), Report of cooperative JapanU.S. pelagic pollock investigation in the Aleutian Basin during August-October in 1988.

Special Publication of the National Research Institute of Far Seas Fisheries No. 21: 103 p.
Table 1. Summary of sphere calibrations conducted before, during, and after the 1994 echo Integration-trawl survey

of the southeast Aleutian Basin near Bogoslof Island.

Note: Gain and beam pattern terms are defined in the “Operator Manual for Simrad EK500 Scientific Echo Sounder (1993)’

available from Simrad Subsea A/S, Standpromenaden 50, P.O. Box 111, N-3191 Horten, Norway.

‘The transducer is located approximately 9 m below the water surface.
Table 2. Summary of midwater rope trawl station and catch data from the winter 1994 pollock echo integrationtrawl survey of the southeast Aleutian Basin near Bogoslof Island.

Table 3. Summary of bongo net tow stations from the winter 1994 pollock integration-trawl

survey of the southeast Aleutian Basin near Bogoslof Island.
Table 4. Summary of catch by species in 13 midwater rope trawls from the winter 1994 pollock echo integration-trawl survey of the southeast

Aleutian Basin near Bogoslof Island, Miller Freeman cruise 94-02.
Table 5. Summary of pollock biological samples and measurements from the winter 1994 echo integration-trawl

survey of the southeast Aleutian Basin near Bogoslof Island.
24

Table 6. Conductivity/temperature/depth (CTD) casts conducted during the winter 1994 pollock echo

integration-trawl survey of the southeast Aleutian Basin near Bogoslof Island.

Table 7. Expendable bathythermograph (XBT) casts conducted during the winter 1994 pollock echo

integration-trawl survey of the southeast Aleutian Basin near Bogoslof Island.

Note: XBT drops 1 and 2 occurred prior to the survey.

The maximum depth for probes T-4 , T-6 is 460 m and T-7 is 767 m.
25

Table 8. Population and biomass estimates obtained during echo

integration-trawl survey of the pollock population near Bogoslof Island

in winter 1994.
Figure 1. Relative pollock density along trackline from the winter 1994

pollock echo integration-trawl survey of the southeast Aleutian Basin near

Bogoslof Island. Transect numbers are indicated. Inset: Major features of the

Bering Sea, showing location of the Donut Hole, from Wespestad (1993a).
Figure 2. Rope trawl and bongo tow locations

during the winter 1994 pollock echo integrationtrawl survey of the southeast Aleutian Basin near

Bogoslof Island. Cruise trackline is indicated.
28

Figure 3. Female pollock maturity-length composition from trawl catches

A) in bottom depths less than 500 m, and B) in bottom depths greater than

500 m during the 1994 pollock echo integration-trawl survey of the

southeast Aleutian Basin near Bogoslof Island.


Figure 6. Temperature and salinity profiles of CTD casts made in

the southeastern Aleutian Basin near Bogoslof Island during the

winter 1994 echo integration-trawl survey.
32

Figure 7. Temperature profiles from XBT casts made in the southeastern

Aleutian Basin near Bogoslof Island during the winter 1994 echo

integration-trawl survey.
Figure 8. Echogram of typical pollock scattering layers from the winter 1994 Bogoslof echo

integration-trawl survey. Color gradation from blue to red corresponds to increasing pollock

density, with red indicating the highest density concentrations.

35

Figure 9. Pollock (a) numbers at length and (b) numbers at age from the

1994 winter echo integration-trawl survey of the southeastern Aleutian

Basin near Bogoslof Island.
Figure 10. Biomass estimates and average fork lengths obtained during winter echo integration

-trawl surveys for spawning walleye pollock near Bogoslof Island, 1988-94.

conducted in 1990.

No survey was
37

Millions of fish

Age (years)

Figure 11. Population-at-age estimates obtained during echo integration

trawl surveys of spawning walleye pollock near Bogoslof Island in

winter 1988-89,1991-92 and 94. Major year classes are indicated. No

survey was conducted in 1990, and ages were not available for 1993.

Note y-axis scale differences.
38

Millions of Fish

Figure 12. Population-at-length estimates obtained during echo integration-trawl surveys of

spawning walleye pollock near Bogoslof Island in winter 1988-94. Note y-axis scale differences.

No survey was conducted in 1990.
39

Figure 13. U.S. Domestic Observer Program data showing haul positions where

pollock were caught by the commercial fishing fleet during the Bering Sea /

Aleutian pollock season, 20 January-13 April, 1994. For each 2-week period,

total catch between 170°and 177°W long. is indicated.
RECENT TECHNICAL MEMORANDUMS

Copies of this and other NOAA Technical Memorandums are available from the

National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22167.

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CLARY, J C. (editor). 1995. Poster abstracts and manuscripts from the Third International Conference

on Marine Debris, May 8-13, 1994, Miami, Florida, 108 p. NTIS number pending.

MUNRO, P. T., and R. Z. HOFF. 1995. Two demersal trawl surveys in the Gulf of Alaska: Implications

of survey design and methods, 139 p. NTIS No. PB95-195061.

STARK, J. W., and D. M. CLAUSEN., 1995. Data Report: 1990 Gulf of Alaska bottom trawl survey, 221

p. NTIS No. PB95-194825.

NARITA, R., M. GUTTORMSEN, J. GHARRETT, G. TROMBLE, and J. BERGER. 1994. Summary of

observer sampling of domestic groundfish fisheries in the northeast Pacific Ocean and eastern Bering

Sea, 1991, 540 p. NTIS No. PB95-190963

DORN, M. W., E. P. NUNNALLEE, C. D. WILSON, and M. E. WILKINS. 1994. Status of the coastal

Pacific whiting resource in 1993, 101 p. NTIS No. PB95-176467.

SINCLAIR, E. H. (editor). 1994. Fur seal investigations, 1993, 93 p. NTIS No. PB95-178943.

SINCLAIR, E. H. (editor). 1994. Fur seal investigations, 1992, 190 p. NTIS No. PB95-173472.

KINOSHITA, R. K., and J. M. TERRY. 1994. Oregon, Washington, and Alaska exports of edible

fishery products, 1993, 52 p. NTIS No. PB95-165924.

FERRERO. R. C.. and L. W. FRITZ. 1994. Comparisons of walleye pollock, Theragra chalcogramma,

harvest to Steller sea lion, Eumetopias jubatus, abundance in the Bering Sea and Gulf of Alaska, 25 p.

NTIS No. PB95-155602.

ZIMMERMANN, M. , M. E. WILKINS, R. R. LAUTH, and K. L. WEINBERG. 1994. The 1992 Pacific

west coast bottom trawl survey of groundfish resources: Estimates of distribution, abundance, and length

composition, 110 p. plus Appendices. NTIS No. PB95-154159.

ZIMMERMANN, M., P. GODDARD, T. M. SAMPLE. 1994. Resullts of the 1991 U.S.-U.S.S.R.

cooperative bottom trawl survey of the eastern and western Bering Sea continental shelf, 178 p.

NTIS No. PB95-111589.

SIGLER, M. F., and H. H. ZENGER, Jr. 1994. Relative abundance of Gulf of Alaska sablefish and

other groundfish based on the domestic longline survey, 1989,79 p, NTIS No. PB94-204963.

ZENGER, H. H., Jr., M. F. SIGLER, and E. R. VAROSI. 1994. Assessment of Gulf of Alaska

sablefish and other groundfish species based on the 1988 National Marine Fisheries Service longline

survey, 79 p. NTIS No. PB94-204872.

LOWRY, L. F., K. J. FROST, R. DAVIS, R. S. SUYDAM, and D. P. DEMASTER. Movements and

behavior of satellite-tagged spotted seals (Phoca largha) in the Bering and Chukchi Seas, 71 p. NTIS

No. PB94-180684.

BUCKLEY, T. W., and P. A. LIVINGSTON. 1994. A bioenergetics model of walleye pollock (Theragra

chalcogramma) in the eastern Bering Sea: Structure and documentation, 55 p. NTIS No. PB94-181631.

YANG, M-S., and P. A. LIVINGSTON. 1994. Variations’ in mean stomach content weights of

walleye pollock, Theragra chalcogramma, in the eastern Bering Sea, 32 p. NTIS No. PB94-178084.