NOAA Technical Memorandum NMFS-AFSC-55

Incidental Catches of Salmonids in

the 1991 North Pacific Squid Driftnet

Fisheries

by

J. Pella, R. Rumbaugh, and M. Dahlberg

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Alaska Fisheries Science Center

August 1995
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:

Pella, J., R. Rumbaugh, and M. Dahlberg. 1995. Incidental catches of 

salmonids in the 1991 North Pacific squid driftnet fisheries. U.S. Dep.

Commer., NOAA Tech. Memo. NMFS-AFSC-55, 33 p.

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

the National Marine Fisheries Service, NOAA.
August 1995

NOAA Technical Memorandum NMFS-AFSC-55

by

J. Pella, R. Rumbaugh, and M. Dahlberg

Incidental Catches of Salmonids in

the 1991 North Pacific Squid Driftnet

Fisheries

Auke Bay Laboratory

Alaska Fisheries Science Center

11305 Glacier Highway

Juneau AK 99801-8626

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
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iii

ABSTRACT

In 1991, the National Marine Fisheries Service, as

mandated by the U.S. Driftnet Impact Monitoring,. Assessment,

and Control Act of 1987 and U.S. Public Law 101-627, developed

methods to estimate the incidental catches (bycatch) of

salmonids in the North Pacific squid driftnet fisheries of

Japan, the Republic of Korea (ROK), and Taiwan. Two

analytical approaches were used for the Japanese fishery:

1) simple expansion of average observed vessel-trip bycatch

for total trips by the fleet, and 2) expansion of kernelsmoothed bycatch rates of monitored fishing effort among time

and area strata to account for total fishing effort expended

within strata. Bootstrap resampling was used to determine the

reliability of all estimates. Total salmonid bycatch in the

1991 Japanese squid driftnet fishery was estimated to be

43,700 (vessel-trip method) or 32,100 fish (kernel technique).

With an additional 6,000 (vessel-trip method) or 4,400 (kernel

technique) salmon estimated to have dropped out of the

driftnets during retrieval, total salmonid mortality (bycatch

+ dropouts) was estimated to be 49,700 (vessel-trip method) or

36,500 fish (kernel technique). Total salmonid bycatch in the

squid driftnet fishery of the Republic of Korea was estimated

at 13,500 fish using two-stage expansion. First, observed

bycatch rate of each monitored time and area stratum was

expanded by stratum effort to estimate the stratum bycatch.

Second, the sum of these estimated bycatches for monitored

strata was expanded for effort in unmonitored strata.

Dropouts were estimated at 3,100 fish for a total mortality of

16,600 salmonids. Only 10 salmonids were observed in the

squid driftnet fishery of Taiwan, and the total salmonid

bycatch was evidently small. The salmonid bycatch in these

legal squid driftnet fisheries was minor compared with catches

of Alaskan coastal fisheries in 1991, as it was in 1989 (when

U.S. monitoring began) and in 1990. The 1991 salmonid bycatch

was also small compared with estimates of annual illegal

catches of salmonids from the North Pacific Ocean in recent

years.

V

CONTENTS

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

REGULATED FISHERIES OF JAPAN, REPUBLIC OF KOREA, AND

TAIWAN.......................

Legal Times and Areas of Squid Driftnet Fisheries .

Monitoring Programs . . . . . . . . . . . . . . . .

Bycatch Estimation . . . . . . . . . . . . . . . . .

Japanese Squid Driftnet Fishery . . . . . . . .

Methods . . . . . . . . . . . . . . . . .

Expansion of Vessel-Trip Bycatch . .

Kernel Estimation of Bycatch by Time

and Area Strata . . . . . . .

Bycatch rates by vessel size

class . . . . . . . . . . . .

Bycatch estimates . . . . . .

Species composition of bycatch

Estimation of Salmonid Dropouts . .

Estimation of Salmonid Mortality .

Results . . . . . . . . . . . . . .

Republic of Korea Squid Driftnet Fishery

Methods . . . . . . . . . .

Results . . . . . . . . . .

Taiwanese Squid Driftnet Fishery

CONCLUSION . . . . . . . . . . . . . . . .

CITATIONS ., . . . . . . . . . . . . . . .

.

. .

. .

. .

. .

. .

. .

. .
INTRODUCTION

In 1991, Japan,, the Republic of Korea (ROK), and Taiwan

operated high-seas squid driftnet fisheries in the North

Pacific Ocean. Fishermen from these three countries targeted

various species of squid, especially neon flying squid

(Ommastrephes bartrami). Many species of marine animals,

including fishes, birds, and mammals, were incidentally

caught. Salmonids - chinook (Oncorhynchus tshawytscha), chum

(0. keta), coho (0. kisutch), pink (0. gorbuscha), cherry (0.

masou), and sockeye (0. nerka) salmon and steelhead trout

(0. mykiss) - were among the incidentally caught species.

In this, paper, we estimate the incidental catches

(bycatch) of salmonids by the 1991 high-seas squid driftnet

fisheries of Japan, the ROK, and Taiwan to meet requirements

of the U.S. Driftnet Impact Monitoring, Assessment, and

Control Act of 1987 and of U.S. Public Law 101-627. Estimated

catch (in numbers), by species, taken by the fisheries of

Japan and the ROK are provided. Our analysis used the methods

developed by Pella et al. (1993) to estimate the 1990 salmonid

bycatch of these fisheries. A complete description of these

methods is included along with changes made for the 1991 data

for the convenience of the reader. Only 10 salmonids were

observed in the Taiwanese fishery and this bycatch was

considered negligible for our analysis.

REGULATED FISHERIES OF JAPAN, REPUBLIC OF KOREA, AND TAIWAN

Legal Times and Areas of Squid Driftnet Fisheries

Japan, ROK, and Taiwan all had domestic regulations

establishing boundaries within the North Pacific Ocean where

their licensed squid driftnet vessels could legally operate.

The legal boundaries of the Japanese high-seas squid driftnet

fishery extended north from lat. 20°N, long. 170°E to 145°W,

with the northern boundary changing monthly and longitudinally

(Fig. 1). Monthly movement of the northern boundary was

established to try to minimize the bycatch of salmonids.

Japanese squid driftnet vessels were allowed to operate from

June through December only.

Driftnet vessels from the ROK were allowed to fish

between long. 160°E and 145°W. No southern boundary was

established for the fishery. The northern boundary depended

on longitude and month (Fig. 2). The ROK squid driftnet

fishery was closed between long. 170°E and 145°W from January

through April and in December; however, fishing was allowed

from long. 160°E to 170°E during all months.

The Taiwanese Department of Agriculture also established

legal areas where driftnet fishing was permitted by its

licensed vessels. No southern or western boundaries other

than 200-mile national Exclusive Economic Zones (EEZs) were

Figure 2.--portion of the legal fishing area of the Republic of Korea squid driftnet fishery

northward of lat. 20°N (solid lines).

longitude; western (long.

Northern boundary changed by month and

160°E) and eastern (long. 145°W) boundaries were fixed.

and no boundary to the south was imposed other than national Exclusive Economic

Zones.
4

established; however, an eastern boundary of long. 145°W and

changing monthly northern boundary delimited the fishery (Fig.

3). In contrast to the Japanese fishery, Taiwanese vessels

could fish year-round. Once again, the northern and eastern

boundaries were established to restrict the bycatch of

salmonids.

Monitoring Programs

The U.S. Driftnet Impact Monitoring, Assessment, and

Control Act of 1987 and U.S. Public Law 101-627 requires that

the U.S. Department of State negotiate monitoring and

enforcement agreements with nations that operate high-seas

driftnet fisheries in the North Pacific Ocean. In 1989, the

'United States established pilot observer programs with Japan,

the ROK, and Taiwan to obtain initial driftnet bycatch data on

certain species of anadromous fish, marine mammals, seabirds,

and turtles. In 1990 and 1991, more comprehensive observer

programs were devised to collect data on the bycatch of all

species of fish, marine mammals, seabirds, and turtles.

Data collected by the scientific observers in 1991

consisted of three types taken during each monitored vessel

fishing operation (an operation comprised setting, soaking,

and retrieval of the driftnet) : physical observations,

environmental measurements, and catch. Physical data included

time (month, day, time) and position (latitude and longitude).

Environmental measurements comprised sea-surface temperature,

wind direction, Beaufort state, and ocean swell height.

Physical and environmental, data were recorded for the

beginning and ending of net deployment and retrieval. Catch

information included the number of sections deployed, length

of each section, set direction, catch (in numbers) of the

target species, and bycatch (in numbers of animals) of all

non-target species.

Bycatch Estimation

Japanese Squid Driftnet Fishery

From June through early December 1991, observers were

placed aboard Japanese vessels operating in the North Pacific

squid driftnet fishery as part of a joint monitoring effort by

the United States, Canada, and Japan. The objective of the

observer program was to monitor the fishing operations of

vessels so that the catch of target squid species,

particularly neon flying squid, and the bycatch of non-target

species could be estimated.

Observers were placed on board 59 vessels, or 14% of the

vessels operating in the commercial fishery. Ten observers
Figure 3.--Portion of the legal fishing area of the Taiwanese squid driftnet fishery

northward of lat. 20°N and eastward of long. 140°E (solid lines). Northern

boundary changedby month and longitude, eastern boundary (long. 145°W) was fixed;

and no boundaries to the south or west were imposed other than national Exclusive

Economic Zones.
6

were Canadian, 19 were Japanese, and 30 were from the United

States. The observers were instructed to monitor daily vessel

operations and record the number of individuals of the target

and non-target species entangled and retrieved in the

driftnets. They also recorded the number of animals dropping

out or shaken out of the nets by fishermen during net

retrieval.

Two methods were used to estimate the 1991 bycatch of

salmonids in the Japanese squid driftnet fishery:

1) expansion of average observed vessel-trip bycatch for total

trips by the fleet, and 2) expansion of kernel-smoothed

bycatch rates of monitored effort among time and area strata

to account for total fishing effort expended-within strata.

Stratum bycatches of the second method were also allocated to

species based on kernel-smoothed species composition estimates

from samples of stratum bycatches. A measure of the

uncertainty in the bycatch estimates was obtained in both

cases by bootstrapping (Efron 1982). The bootstrapping

procedures used slightly exaggerate the uncertainty in bycatch

estimates because adjustments for finite populations sampled

were not developed.

Besides the estimate of salmonid bycatch, an estimate of

the number of salmon that drop out of the nets during

retrieval (dropouts), was also desired. Ratio estimation was

used to determine the number of dropouts for the entire fleet,

and the uncertainty of this estimate was evaluated by

bootstrapping.

Methods

Expansion of Vessel-Trip Bycatch.- We estimated total

salmonid bycatch for the 1991 Japanese fishery by classifying

vessel trips into those made by small (25-35 m hull length)

and large (>35 m hull length) vessels. In 1991, a total of

142 small-vessel trips and 286 large-vessel trips occurred

(A. Yatsu, National Research Institute of Far Seas Fisheries,

Fishery Agency of Japan, Shimizu-shi, Shizuoka, Japan, pers.

commun., September 1992). Fifty-nine observers were placed on

15 small vessels (11% of the small-vessel trips) and 44 large

vessels (15% of the large-vessel trips). Observers were on

board the vessels for the entire trip of all 15 small vessels

and 41 of the 44 large vessels (93%). The duration of vessel

trips for which observers arrived or left at sea was unknown,

and possibly some fishing during these trips was not

monitored. The vessel-trip bycatch estimates were not

adjusted for the abbreviated presence of observers for the

three trips. The estimated vessel-trip bycatch averages for

small and large vessels were expanded by corresponding total

numbers of vessel trips to obtain an estimate of total

salmonid bycatch for the fishery by each vessel size class.
7

Estimated total salmonid bycatch for the fishery equaled the

sum of the bycatch estimates for both vessel size classes.

Vessel trips with an observer on board were treated as if

randomly sampled from the "populations" of vessel trips of the

two vessel sizes. Actually, 10 fishing ports were designated

to host specified numbers of observers during the 1991 squid

driftnet fishery, and the vessels that carried observers were

selected through a lottery by the fishermen themselves. The

allocation of observers to each fishing port was roughly

proportional to the size of its fleet. Vessels from different

fishing ports do not fish in distinctive manners, so vessels

were not stratified by fishing port. However, vessels of

different sizes might be expected to have different amounts of

bycatch. Therefore, vessel trips were poststratified by size

class (small and large).

A vessel trip was the primary sampling unit within the

vessel size strata. Operations were the secondary sampling

units within a given vessel trip. Up to 12 sections of net

were fished during an operation, and each net section usually

contained 100-150 net panels (tans) 50 m in length. The net

(all sections) was deployed during late afternoon or evening,

allowed to soak 8-12 hours, and then retrieved the following

morning. With this scenario, one operation was usually

conducted within 24 hours. Occasionally, extremely high

abundances of squid were encountered such that all of the

driftnet that was deployed could not be retrieved in a given

day. Some net sections were then left to soak an additional

night. In one case, several sections of net soaked an

additional 2 days. Hence, two kinds of operations occurred:

daily (all nets deployed were retrieved the following day) and

extended (some nets soaked for up to two additional days).

Extended operations were treated the same as the daily

operations for estimating bycatch.

The daily operations of each month were stratified into

approximate 10-day periods: days 1-10, 11-20, and 21-end of

month. These intervals will be called 10-day periods for

brevity. Fishing operations were monitored in each of 19

10-day periods covering the actual duration of the 1991

fishery, from 1 June to 10 December.

Not all operations of an observed vessel trip were

monitored by the observers. To prevent fatigue, observers

were instructed to skip the sixth day after observing

operations for five consecutive days. Therefore; five of

every six operations should have been observed. However,

daily operations were interrupted by storms of l-9 days, with

frequency depending on the time of year. Daily operations

were also interrupted when a vessel changed locations within

the fishery. Therefore, a variable number of operations were

observed during 10-day periods of a vessel trip even though 5

of every 6 days were to have been observed. The observed
8

daily operations by a vessel during each 10-day period were

viewed as a random sample of all its daily operations for the

period.

planned,

Although a systematic sampling scheme was originally

in reality the daily operations observed within a 10-

day period were closer to a random sample because of the

numerous,, unpredictable disruptions to the schedule.

Finally, the ultimate sampling units for which observers

recorded bycatch were individual net sections. A true random

subset (6 or 7) of the total net sections deployed in an

operation (6-12) was examined for bycatch during an observed

retrieval. The number of sections monitored by an observer

depended on the number of sections deployed. When 6-9

sections of net were deployed, observers were instructed to

randomly monitor 6 sections. If 10-12 sections of net were

deployed, observers monitored 7 sections. Therefore, the

observed net sections of a daily operation were a simple

random sample of the total sections.

To estimate the salmonid bycatch for an observed vessel

trip, the daily operations were stratified into 10-day

periods. The bycatch for a given daily operation was

estimated by multiplying the observed salmonid bycatch rate

 (number of salmon per tan) by the number of tans deployed in

the operation (observed and unobserved sections). Next, the

average daily operation bycatch during each 10-day period of

the given vessel trip was calculated. Then these averages

were multiplied by the corresponding number of daily

operations during the IO-day periods to estimate the bycatch

for each 10-day period during the vessel trip. Finally,

estimates of bycatch for each 10-day period were summed over

10-day periods to obtain the estimated bycatch during the

vessel trip.

Once the estimated bycatch for each observed vessel trip

was available, average bycatch per vessel trip by large and

small vessels was computed. These averages were multiplied by

the corresponding total numbers of vessel trips to estimate

the total bycatch by each vessel size. The estimated bycatch

for vessel size t, Tt, was

(1)

where Nt was the number of vessel trips by vessels of size t

and Xt was the estimated average vessel-trip bycatch of the

observed vessels for size t. The estimates of bycatch for the

two vessel sizes were then combined to obtain the estimate of

bycatch for the entire fleet.

for the entire fleet, T, was

The estimate of total bycatch

(2)
9

Precision of the estimate of total bycatch from the

observed portion of the fleet was evaluated by bootstrap

resampling (Efron 1982). First,, the observed daily operations

within each 10-day period were resampled for each vessel trip.

Then the sections within each daily operation included in the

bootstrap sample were resampled. The resampled observations

were then used to estimate the total bycatch of the observed

vessel trips just as the original observations had been. The

resampling was performed 1,000 times. For each repetition

(b = 1,2,..., 1,000) of the bootstrap, an estimate of salmonid

bycatch (x*bi) for each vessel trip (i = 1,2,...,59) was

calculated.

The sampling distribution of the total bycatch estimate

of the fleet, 9, was estimated by resampling vessel trips and

using the previous bootstrapping results. Consider the bth

resampling. First, the observed trips of the 15 small vessels

were resampled; independently, the observed trips of the 44

large vessels were also resampled. A typical sample omitted

some observed vessel trips and repeated others. Then the

bootstrap average bycatch for small (x*b1) and large (x*b2)

vessels was computed from the previous estimates of vesseltrip bycatch (x*b1) included in the bootstrap sample of vessel

trips. Finally, the bth bootstrap estimate of total bycatch,

T*b, was computed as

(3)

One thousand values of T*b were computed in this fashion. The

standard deviation of the T*b among the 1,000 resamplings is the'

estimate of the standard error of the estimate of the total

bycatch for the fleet, T. The, 5th and 95th

1,000 T*bs provide a 90%

percentiles of the

confidence interval (CI) .

This first approach to estimating total bycatch provides

an estimate and a-measure of precision of this estimate which

are simple to understand, and the basis seems a reasonable

approximation to the sampling process. If the fishing

activities of vessels were not affected by the presence of an

observer, this estimate is unbiased even if imprecise. Some

of the imprecision is due to the differences among vessel

trips in the time of season and areas fished. To account for

the time and area distribution of fishing effort by the fleet

when estimating bycatch, a kernel estimation method was used.

Kernel Estimation of Bycatch by Time and Area Strata.-Bycatch

of the fleet during 1991 was estimated in three stages.

First, bycatch rates of 'small and large vessels were estimated

for every 1o latitude X 1o

longitude area and 10-day period,

hereafter referred to collectively as "strata," in which
10

fishing effort was expended. Second, the bycatch by vessel

size class for each stratum was estimated as the product of

the estimated bycatch rate and the reported fishing effort

expended by vessel size class in the stratum. Third, bycatch

estimates were summed over vessel size class and time and area

strata to obtain the total estimated bycatch for the fishery.

The amount of fishing effort (tans) expended by large and

small vessels in each stratum, hereafter referred to as

"effort," was available. A total of 1,038 strata were fished

by the Japanese driftnet fleet, and of those, some fishing was

monitored by observers in 561 strata (54%). Observer coverage

of effort expended by large and small vessel size classes

ranged from 0% to 100% and 0% to 76%, respectively, among the

strata fished.

The strata in which some effort and associated bycatch

was monitored provide estimates of bycatch rates. The spatial

distributions of bycatch rates of large and small vessels for

each lo-day period from 1 June to 10 December were revealing

(Fig. 4). First, the highest salmonid bycatch rates occurred

between 21 June and 10 July, mainly in the northwest corner of

the fishing area. Second, a marked difference occurred in

bycatch rates between large and small vessels, due in large

part to their distinctive areal distributions. Small vessels

crowded the western portion of the fishery, whereas large

vessels spread out to the east.

Bycatch rates by vessel size class. The amount of

monitored effort varied considerably among the 1o X 1o areas

and 10-day periods, so the resulting variable precision of

observed bycatch rates in the strata had to be considered when

estimating-underlying bycatch rates. Effort in nearly onehalf of the strata in which some fishing occurred was

completely unmonitored, so corresponding bycatch rates were

estimated from information gathered from monitored strata.

The kernel method (Eubank 1988) was used to smooth observed

bycatch rates to estimate the underlying bycatch rates by

vessel size among strata. Kernel estimation was previously

used by Larntz and Garrott (1991) to estimate bycatch rates of

selected bird and marine mammal species in the 1989 Japanese

squid fishery and by Pella et al. (1993) to estimate salmonid

bycatch rates in the 1990 Japanese squid fishery.

The kernel estimate of bycatch rate by a vessel size

class in a particular stratum during any of four monthly

intervals of the fishing season - June, July, August, and

September-December - used observed bycatch rates by the

vessel size class from all strata in which observers monitored

effort and bycatch in that monthly interval. However, the

kernel estimate placed variable emphasis on different strata.'

Observed bycatch rates in nearby strata, including the stratum
Figure 4.--Spatial distribution (1o

 latitude [lat.] X 1o

longitude [long.]) of observed

salmonid bycatch rates (shown as square root of bycatch [numbers of fish] per

1,000 tans) by vessel size class for 10-day periods from 1 June to 10 December.
Figure 4 .--Continued
Figure 4.--Continued
Figure 4.--Continued

16

of concern, were weighted more heavily than those of distant

strata; and observed bycatch rates from strata in which larger

amounts of effort were monitored by observers received greater

weight as well. Specifically, the kernel estimate of the

bycatch rate for vessels of size t in stratum s, I,,, was

computed from the Smax(t) strata in which effort was monitored as

follows:

where ejt is the monitored effort in stratum j of vessel size

t,

rjt is the observed bycatch rate (salmonids per tan) in

stratum j for vessels of size t,

dsj is the distance vector between stratum s and stratum

j, and

k(.) is the kernel function.

The distance vector between stratum s and stratum j

compared their values of three variables - latitude ,

longitude ,

fished stratum.

and time period ( ) - which defined every

at lat.

The variable value for latitude ( ) equaled 1

35°N, and increased by unit steps at each increase of

1

o

 latitude until it equaled 11 at lat. 45°N. The variable

value for longitude ( ) equaled 1 at long. 170°E and increased

in unit steps at each eastward change of 1o until it equaled

46 at long. 145°W. The variable value for time periods 

equaled 1 for 1-10 June and increased by unit steps until it

equaled 19 for 1-10 December. The distance vector between

strata s and j was defined as dsj = 

Finally, the kernel function was defined as

(5)

Values of the three kernel function constants, pl, pZ, and p3,

determine how rapidly the weight given to observed strata

bycatch rates diminishes with distance from the stratum of

concern. The smaller the value for any one of these

constants, the faster the weight decreases with distance along

the corresponding variable - latitude, longitude, or time.

The observed bycatch rates by vessel size varied over

time and area due to underlying variation in bycatch rates, as
17

well as to sampling variation among vessels and operations in

the strata. The possible sampling variation in average

bycatch rate by vessel size for a stratum would have decreased

as the proportion of expended effort in the stratum monitored

by observers increased. For example, if nearly all the effort

expended by the vessel size class in a stratum was observed,

the observed bycatch rate was a very accurate and precise

estimate of the bycatch rate. In the usual, less-favorable

situations, some lesser fraction (possibly zero) of the

expended effort was monitored, so the precision and accuracy

of the observed bycatch rate as an estimate of bycatch rate of

the total expended effort in any stratum would be degraded,

Bycatch rates in stratum s by vessel size class t were

estimated using

(6)

where is the fraction (est/ESt) of the total expended effort

by vessel size class t in stratum s (Est) accounted

for by the monitored effort (est), and

and were defined at Equation (4).

If all the bycatch and reported effort by a vessel size class

were observed, then the estimate of bycatch by the

vessel size class in the stratum was presumably accurate. If

none of the bycatch and effort were observed, then

the estimate of bycatch in the stratum was subject to the

entire sampling error of the kernel estimate. Many strata had

intermediate proportions of effort observed, so the level of

sampling error fell between these extremes.

If some part of the effort was monitored in a stratum,

the observed bycatch rate was included in the kernel estimate

of the bycatch rate for the unobserved effort; that is, the

kernel estimate Equation (4) included all strata in which

effort was monitored. The inclusion of information from the

stratum of interest was in contrast to the method next

described, by which estimates of the kernel parameters were

obtained.

The leave-one-out method of cross-validation was used to

estimate the parameters, pl, p2, and p3, for either vessel size

class. The values of pl, p2, and p3 that minimize the effortweighted sum of squares of vessel size class t,

(7)
18

were the cross-validation estimates. That is, the bycatch

rate by vessel size class for each stratum was estimated by

the kernel estimate (with selected values for pl, p2, and p3)

using only observed bycatch rates of other strata in which

observers monitored bycatch and effort. A search for the

minimizing values of the coefficients was accomplished by the

conjugate gradient method (Press et al. 1986).

Bycatch estimates. Total bycatch in stratum s by vessels

of size class t, Tst, was estimated as

and estimated total bycatch by the fishery was simply the sum

of these estimates over vessel size class and the S*max(t) strata

fished by vessel size class t,

(9)

Sampling variation in estimated total bycatch, T, was

computed by bootstrap resampling as if the observed vessel

trips of each vessel size were a random sample of the

corresponding total vessel trips during a monthly interval.

The trips of large and small vessels were resampled with

replacement. Let the bth bootstrap resampling result in

observed bycatch in stratum s of c*st(b) taken by e*st(b) tans of

effort from vessel size class t. The bootstrap average

observed bycatch rate in stratum s by vessels of size class t

was computed as The bootstrap observed

average bycatch rates for the strata were used to recompute

bycatch rates by vessel size class from Equations (4) - (7).

Finally, the bootstrap estimate of total bycatch by the fleet,

T*b, was computed using the bootstrap values in Equations (8)

and (9). The time-consuming procedure was performed 25 times,

as suggested by Efron and Tibshirani (1986) for obtaining

reliable estimates of standard errors of estimates. The

bootstrap estimates by vessel size class and combined fleet

were plotted in histograms and examined for normality.

Inasmuch as the bootstrap estimates appeared to be

approximately normally distributed, confidence intervals were

then calculated by normal approximation and the bootstrap

standard errors.

Species composition of bycatch. Six- species of salmonids

were present in the bycatch: chinook, chum, coho, pink, and

sockeye salmon and steelhead trout. Scales were obtained from

individual salmonids in the bycatch when possible so that
19

species could be determined (see Koo 1962). When possible,

observers also visually identified the species of salmonids

brought on deck. A total of 1,359 salmonids identified to

species were used to assess the species composition of

bycatches, which was equivalent to 54% of the observed

salmonid bycatch of the monitoring program. The species of

443 of the salmonids were identified by scales, and the

species of the remaining 916 were visually identified.

Reliability of individual observers in species

identification was evaluated by comparing species

determinations from scales with the identification of the same

fish from visual examination by the observers. Generally, if

an observer's correct identification rate for a given species

was high, their identifications of fish without scale samples

for that species were accepted. If, for any given operation,

1) scales were available to identify species in the entire

bycatch, 2) a reliable observer identified species in the

entire bycatch, or 3) a combination of scales and a reliable

observer provided species identifications of the entire

bycatch, then the information was used to estimate the species

composition of the bycatch for that operation. All samples

were aggregated from operations in 1o X 1o areas and 10-day

periods to provide an estimate of the species composition of

the salmonid bycatch in that stratum. If species identified

from scales were always correct, we expect about 14% of the

1,359 individual fish would be misidentified to species, based

on misidentification rates of reliable observers. Probably

many of these misidentification errors within the aggregated

samples were self-compensating and had no effect on our

estimation of species composition of bycatch; that is, a fish

of species x was misidentified as species y but another fish

of species y was misidentified as species x (e.g., see Myers

and Bernard 1993).

Had the species of all the estimated salmonid bycatch in

a stratum been determined from scales or reliable observers,

the species composition of the stratum bycatch would

presumably have been known essentially without error (ignoring

occasional species misidentification by reliable observers).

However, usually only a portion, possibly none, of a stratum

bycatch was so determined. The species composition of the

bycatch of each stratum was estimated by

where was the estimated vector of species

proportions of bycatch from stratum s,

(10)

 was the observed vector of species

proportions of bycatch from stratum s,
20

was the kernel estimate of species

proportions for bycatch of stratum s (described below),

and

 was the proportion of the estimated bycatch for stratum s

for which the species were determined.

If species identification was available for all the estimated

bycatch; then and the estimate was presumably correct

(again ignoring occasional misidentification by reliable

observers). If species identification for none of the bycatch

in the stratum was determined, In this case, the

estimate of species composition includes all the sampling

error inherent in the kernel method of estimation. Most

strata with large estimated salmonid bycatch levels had

species compositions determined for portions of the bycatch,

so the sampling error was intermediate between the two

extremes.

The motivation for the kernel estimate of bycatch rate by

stratum described earlier can also be applied to species

composition. The kernel estimate of species composition in

any stratum used observed estimates of species composition

from all strata for which such information was obtained from

scales and reliable observers.

composition in a stratum,

When estimating the species

variable emphasis is given to the

other strata, depending on their proximity to the target

stratum. Species composition of bycatches from nearby strata

were weighted more heavily than those of distant strata.

Specifically, the kernel estimate of species composition of

bycatch in stratum s, was computed from the Cmax strata in

which species composition was determined for a portion of the

bycatch as follows:

where dsj is the distance between stratum s and stratum j, and

k(.) is the kernel function Equation (5).

The coefficients of the kernel function again were

estimated by cross-validation, this time finding values of the

coefficients which maximized a joint likelihood function for

the aggregated species composition samples of our basic

strata, 1

o

 x 1oareas and 10-day periods. The likelihood

function was maximized with respect to the unknown species

proportions in the Cmax strata of a monthly interval under the

constraint that the proportions were smoothed by Equation

(11) . The joint likelihood function depends on two reasonable

assumptions and one admitted simplification. First, the
21

probability of the recorded frequencies of species in the

aggregate sample of any stratum was assumed to be given by a

multinomial probability function with parameters for aggregate

sample size and unknown species proportions of the stratum

bycatch. Second, aggregated samples among the Cmax strata were

assumed mutually independent. No account was taken of

probable misidentifications of species, which simplified the

analysis; the criteria used for inclusion of speciescomposition samples should have reduced the number of

misclassifications to low levels. If xsh ,was the number of

fish of species h identified in bycatch of stratum s, the

function maximized was

Precision of the estimated species composition of bycatch

from the kernel approach was evaluated by the bootstrap

method. Each of the Cmax species composition samples for each

monthly interval was resampled with replacement to create

bootstrap samples equal in size to those available. Crossvalidation estimates of the kernel coefficients were obtained

for the set of Cmax bootstrap samples. Then the species

proportions were computed for each stratum. The procedure was

performed 25 times to match the earlier bootstrap resamplings

for kernel estimates of bycatch by time and area strata.

Resulting bootstrap estimates of species composition per

stratum were used to allocate the corresponding bootstrap

estimates of bycatch per stratum.

Estimation of Salmonid Dropouts. -During 1991, procedures were

in place to estimate the number of "dropouts" - individual

fish that were entangled when the net was brought out of the

water and that dropped out or were shaken out of the net

before reaching the deck. In each observed operation, two

sections were randomly selected for dropout monitoring. For

these two sections, in addition to recording landed bycatch,
22

observers also recorded the number of dropouts. This

recording procedure does not account for fish that were

entangled in the net during the soaking period but dropped out

unobserved before the net was retrieved.

The total dropout of salmonids for the entire fishery was

estimated as

(15)

where R was the observed ratio of total dropout to total

landed for sections, monitored for dropouts and T was an

estimate of the total landed salmonid bycatch, from the

vessel-trip or the kernel method. Bootstrapping procedures

were once again used to evaluate the precision of the

estimates. The sections that were originally monitored for

dropout were randomly resampled with replacement, and R was

recalculated. This resampling and recalculation was performed

1,000 times (vessel-trip method) and 25 times (kernel method)

to match previous numbers of independent resamplings for total

salmon bycatch estimates. Each bootstrap ratio estimate (R)

was multiplied by a previous bootstrap estimate of total

salmon bycatch (T) to determine a sampling distribution for

D. The standard deviation of the D among bootstrap

repetitions was the estimate of the standard error of the

estimates of dropouts for both the vessel-trip and kernel

methods. A confidence interval for dropouts from the vesseltrip method was obtained using percentiles of the bootstrap

distribution of D. The 25 bootstrap estimates of D from the

kernel method were plotted in a histogram and examined for

normality. Because the distribution appeared normal, normal

approximation methods were used to calculate the confidence

interval for the kernel estimate of dropouts.

Estimation of Salmonid Mortality. -Salmonid mortality' (M) is

the total number of salmonids killed in the Japanese squid

driftnet fishery (dropouts and bycatch combined). Salmonid

mortality was estimated by summing the estimates of bycatch

and dropouts obtained from the vessel-trip and kernel methods:

(16)

Bootstrap estimates of M were obtained by summing bootstrap

estimates of dropouts and bycatch. Again, confidence

intervals were calculated by the percentiles of the bootstrap

distribution of M for the vessel-trip method and the normal

approximation and the bootstrap standard error for the kernel

method. As before, the 25 bootstrap estimates from the kernel

method were examined for normality from a histogram of the data.
23

Results

Estimates of salmonid bycatch using the vessel-trip method

were computed for small and large vessels. The point estimate

of bycatch by the small vessel size class (T1) was 42,600 (90%

CI, 8,400 - 89,000; standard error [SE] = 25,700), and for the

large vessel size class (T2), 1,100 (90% CI, 700 - 1,900; SE =

400), yielding a total bycatch (T) of 43,700 (90% CI, 9,500 -

90,000; SE = 25,700). Stratifying by vessel size was

appropriate, given the difference in bycatch between the two

size classes. Estimated vessel-trip bycatch by observed small

vessels was 0 - 2,236 fish, and by observed large vessels,

0 - 24 fish (Figs. 5 and 6). The 90% confidence intervals of

average bycatch per vessel trip for small (ul) and large (u2)

vessels emphasized the difference: 57 ≤ u1 ≤ 625 for small

vessels and 2 ≤ u2 ≤ 6 for large vessels.

Estimates of bycatch using the kernel approach are given

in Tables 1-3, for small vessels, large vessels, and combined,

respectively. The combined estimates indicate low bycatch

during June (about 2,000), maximum bycatch during July (about

29,000), and little bycatch from August through December

(< 800). An estimated total of 32,100 salmonids (90% CI,

6,100 - 58,100) were taken as bycatch in the Japanese squid

driftnet fishery during the season: small vessels caught

31,100 (90% CI; 5,100 - 57,100), and large vessels 1,000 (90%

CI, 760 - 1,200). Approximate 90% confidence intervals were

calculated from the point estimates and standard errors given

in Tables 1-3. The bycatch was mostly coho (85%), followed by

chum (10%) and pink (3%) salmon.

Most salmonid bycatch occurred in the western half of the

fishery, mainly in the northwestern corner, between long..

170°E and 180° (Fig. 7). During June, most bycatch occurred

between long. 171° and 173°E and between lat. 39° and 40°N.

During July, most of the bycatch occurred between long. 170°E

and 180° and lat. 41° and 42°N. In August-December, bycatch

was much reduced, yet remained concentrated near the

northwestern corner of the fishery.

Total dropouts equaled 13.7% of the total brought on deck,

for net sections monitored for both dropouts and landed

salmonids. The point estimate for total dropouts in the 1991

Japanese squid driftnet fishery was 6,000 fish (90% CI, 1,300

- 12,500; SE = 3,500) (vessel-trip method) and 4,400 fish (90%

CI, 1,100 - 7,700; SE = 1,900) (kernel method). Total

salmonid mortality in the 1991 Japanese squid driftnet fishery

was estimated to be 49,700 fish (90% CI, 10,600 - 102,700) by

the vessel-trip method and 36,500 fish (90% CI, 7,600 -

65,400) by the kernel method.
Figure 5. --Ranked estimated salmonid bycatches (numbers of fish) for observed vessel

trips--small vessels. (Does not include one vessel trip with zero bycatch.)
Figure 6. --Ranked estimated salmonid bycatches (numbers of fish) for observed vessel

trips--large vessels. (Does not include 3 vessel trips with bycatch of one and

14 vessel trips with zero bycatch.)
Table I.-Estimated monthly salmonid bycatch (N) and bootstrap standard errors (SE), by

species, for small vessels of the 1991 Japanese squid driftnet fishery.

Slight inconsistencies in totals are due to rounding.

Table 2.-Estimated monthly salmonid bycatch (N) and bootstrap standard errors (SE), by

species, for large vessels of the 1991 Japanese squid driftnet fishery.

Slight inconsistencies in totals are due to rounding.
Table 3.-Estimated monthly salmonid bycatch (N) and bootstrap standard errors (SE), by

species, for all vessels of the 1991 Japanese squid driftnet fishery. Slight

inconsistencies in totals are due to rounding.
Figure 7.-- Estimated (kernel method) salmonid bycatch (numbers of fish per 1° latitude

[lat.] X 1o longitude [long.] statistical areas) in the 1991 Japanese squid

driftnet fishery.
29

Republic of Korea Squid Driftnet Fishery

As with Japan, the United States and the ROK entered into

an agreement to place scientific observers on board ROK squid

driftnet vessels operating in the North Pacific Ocean in 1991.

From May through December, 23 observers (10 U.S. and 13 ROK)

were placed on board ROK squid driftnet vessels to monitor

fishing operations and record the catch and bycatch of target

and non-target species. Using this information, the bycatch,

dropouts, and total mortality of salmonids in the 1991 ROK

high-seas squid driftnet fishery were estimated.

Methods

The observed salmonid bycatch in the ROK fishery was less

than in the Japanese fishery. In 1991, 258 salmonids were

observed as bycatch in the ROK fishery; 110 were caught within

a single 1o

latitude X 1olongitude statistical area in July.

Interpretation of species identification was much less a

problem in the Korean fishery than it was in the Japanese

fishery. Only 2% of all salmonids observed were not

identified to species. Sixty-two percent of the total

observed (159 of 258 fish) were identified from scales. An

additional 36% (93 of 258) were identified to species by

reliable observers, and 90 of these were pink salmon

identified by one observer who correctly identified pink

salmon in 100% of the cases (24 fish) for which scales

provided a check on observer accuracy. No further account was

taken of possible species misidentification because probable

errors would have had a negligible effect on our analysis.

Due to the rarity of observed bycatch among strata, kernel

smoothing was not practical for estimating salmonid bycatch in

the Korean fishery. Instead, the fishery was divided into

time and area strata (1° latitude X 1o longitude by month),

and bycatch, by species, was estimated for each stratum by

multiplying the observed species bycatch rate by the number of

50-m long net panels (poks) fished in that stratum. The

observed stratum bycatch rate for each species was calculated

by dividing the total number of observed salmon of each

species by the corresponding number of poks deployed for their

capture. Estimated total species bycatch in strata monitored

by observers was obtained by summing over all strata.

Numerous strata within the ROK fishery had no observed

operations during 1991. Although driftnet vessels fished in

501 time and area strata, only 162 strata (32%) had observed

operations. However, the 162 observed strata accounted for

74% of the total poks fished during 1991. Observed effort was

assumed to be a random sample of total effort fished. Total

bycatch for the ROK fishery was estimated by multiplying

estimated total bycatch in observed strata by 1.35 (1 / 0.74).
30

Uncertainty in the species bycatch estimates was evaluated

by bootstrap resampling. Vessels were resampled with

replacement. An estimate of total bycatch of each species for

the entire fishery was calculated from each resampling in the

same manner as the original estimate.

performed 1,000 times.

Resampling was

A 90% confidence interval was obtained

from the 1,000 estimates by the percentile method.

The numbers of salmonid dropouts and salmonid mortality

for the ROK squid driftnet fishery were also estimated.

Dropout procedures followed by the observers of the ROK

fishery were identical to those described earlier for the

Japanese fishery.

Methodology used to estimate dropouts and mortality (and

precision of estimates) for the ROK fishery was the same as

for the Japanese fishery. The ratio estimator, D, Equation

(15) was used to assess the number of salmonid dropouts from

point and bootstrap estimates of total bycatch described

above.. Salmonid mortality (M) was estimated as the sum of the

estimates of bycatch and dropouts.

Results

Point estimates and 90% confidence intervals were

calculated for the bycatch of each salmonid species in the

1991 ROK squid driftnet fishery (Table 4.). The total

estimated bycatch of salmonids in the 1991 ROK squid driftnet

fishery was 13,500 (90% CI, 2,600 - 16,300). Most of the

bycatch consisted of chum and pink salmon.

equaled 23%

Total dropouts

of the total brought on deck, for net sections

monitored for both dropouts and landed salmonids. Estimated

total dropouts in the 1991 ROK squid driftnet fishery was

3,100 fish (90% CI, 590 - 5,800; SE = 1,700). Total salmonid

mortality in the 1991 ROK squid driftnet fishery wasestimated

to be 16,600 fish (90% CI, 3,200 - 21,400).

Table 4.-Point estimates, standard errors, and 90%

confidence intervals of salmonid bycatch, by

species, for the 1991 Republic of Korea squid

driftnet fishery. Slight inconsistency in

total are due to rounding.
31

Taiwanese Squid Driftnet Fishery

Estimation of the bycatch of salmonids in the squid

driftnet fishery of Taiwan was not attempted because only

10 salmonids (8 coho, 1 pink, and 1 unidentified) were

observed caught, thereby indicating negligible bycatch. The

low salmonid bycatch was due to concentration of the Taiwanese

fleet in areas away from high salmonid abundance and use of

gill nets with mesh size too small to be as effective for

salmonids as the larger mesh used by the Japanese and ROK

fisheries. Taiwanese driftnet vessels fished in 858 strata

(1o

 latitude X 1o longitude and 10-day period), and observers

monitored fishing operations in 158 (18%) of these strata.

The 158 monitored strata accounted for 40.5% of the total

piens (50-m net panels) fished by Taiwanese driftnet vessels.

A total of 343 (5.5%) of 6,280 operations were monitored by

observers.

CONCLUSION

Since the beginning of U.S. monitoring in 1989, the

observed salmonid bycatch by the legal squid driftnet

fisheries of Japan, ROK, and Taiwan was small compared with

commercial catches in Alaskan coastal fisheries (Rigby et al.

1991). In 1991, Alaska landings were 188 million salmon as

compared with our estimated total bycatch by legal squid

driftnet fisheries (Japanese bycatch from vessel-trip method +

ROK bycatch) equal to 57,200 fish. In 1990, Alaska landings

were 155 million salmon as compared with 214,000 by the legal

driftnet fisheries (Pella et al. 1993); and in 1989, Alaska

landings were 154 million salmon as compared with the bycatch

of 2,600 salmon by the legal driftnet fisheries (Ignell et al.

1994). However, the salmonid bycatch by illegal driftnet

fishing in the North Pacific Ocean in recent years apparently

has been much greater, although the quantity is more difficult

to ascertain. For example, Pella et al. (1993) used 1988-90

official trade statistics on imports from countries without

legal access to Pacific salmon fishing grounds to infer

substantial illegal catches in these years: the estimated

catch during 1988 was at least 5.5 million salmonids. Pella

et al. (1993) further noted that illegal catches could affect

endangered or depleted Columbia River stocks which occur in

areas of suspected illegal fishing.
THIS PAGE INTENTIONALLY LEFT BLANK
33

CITATIONS

Efron, B. 1982. The jackknife, the bootstrap, and other

resampling plans. Soc. Ind. Appl. Math. CBMS-Natl. Sci.

Found. Monogr. 38, 92 p.

Efron, B., and R. Tibshirani. 1986. Bootstrap methods for

standard errors, confidence intervals, and other measures

of statistical accuracy. Stat. Sci. 1(1):54-75.

Eubank, R. L. 1988. Spline smoothing and nonparametric

regression. Marcel Dekker, New York. 438 p.

Ignell, S. E., L. J.. Simon, and M. L. Dahlberg. 1994.

Estimation of salmonid bycatch in the 1989 Japanese squid

driftnet fishery. U.S. Dep. Commer., NOAA Tech. Memo.

NMFS-AFSC-29, 45 p.

Koo, T. S. Y. 1962. Differential scale characters among

species of Pacific salmon. In T. S. Y. Koo (editor),

Studies of Alaska red salmon, p. 123-136. Univ. of 

Washington Press, Seattle.

Larntz, K., and R. A. Garrott. 1991. Analysis of bycatch of

selected bird and mammal species in the 1989 Japanese neon

squid fishery. (Document submitted to the Scientific

Review of the North Pacific high seas driftnet fisheries,

Sidney, B.C., June 11-14, 1991). 117 p.

Myers, K. W., and R. L. Bernard. 1993. Biological 

information on Pacific salmon and steelhead trout in

observer samples from the Japanese squid driftnet fishery

in 1990. In J. Ito, W. Shaw, and R. L. Burgner (editors),

Symposium on biology, distribution, and stock assessment

of species caught in the high seas driftnet fisheries in

the North Pacific Ocean. Int. N. Pac. Fish. Comm. Bull.

53(111):217-238.

Pella, J., R. Rumbaugh, L. Simon, M. Dahlberg, S. Pennoyer,

and M. Rose. 1993. Incidental and illegal catches of

salmonids in North Pacific driftnet fisheries. In J. Ito,

W. Shaw, and R. L. Burgner (editors), Symposium on

biology, distribution, and stock assessment of species

caught in the high seas driftnet fisheries in the North

Pacific Ocean. Int. N. Pac. Fish. Comm. Bull.

53(111):325-358.

Press, W. H., B. P. Flannery, S. A. Teukolsky, and

W. T. Vetterling. 1986. Numerical recipes. The art of

scientific computing. Cambridge Univ. Press, New York.

818 p.

Rigby, P., J. McConnaughey, and H. Savikko (editors). 1991.

Alaska commercial salmon catches, 1878-1991. Regional

Info. Rep. 51791-16. Alaska Dep. Fish Game, Juneau, 88 p.
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AFSC54 KINOSHITA, R. K., A. GRIEG, and J. M. TERRY. 1995. Economic status of the groundfish fisheries off

Alaska, 1993, 108 p. NTIS No. PB95-252714.

53 WING, B. L., and D. J. KAMIKAWA. 1995. Distribution of neustonic sablefish larvae and associated

ichthyoplankton in the eastern Gulf of Alaska, May 1990, 48 p. NTIS No. PB95-241519. 

52 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, 39 p. NTIS

No. PB95-219424.

51 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 No. PB95-210837.

50 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. 

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

p. NTIS No. PB95-194825.

48 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

47 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.

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

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

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

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

43 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.

42 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.

41 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.

40 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.