Two Demersal Trawl Surveys in

the Gulf of Alaska: Implications of

Survey Design and Methods

P. T. Munro and R. Z Hoff

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

Alaska Fisheries Science Center

March 1995
NOAA Technical Memorandum NMFS

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by the Northwest Fisheries Science Center.

This document should be cited as follows:

Munro, P. T., and R. Z. Hoff. 1995. Two demersal trawl surveys in

the Gulf of Alaska: Implications of survey design and methods. US.

Dep. Commer., NOAA Tech. Memo. NMFS-AFSC-50, 139 p.

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

the National Marine Fisheries Service, NOAA.
NOAA Technical Memorandum NMFS-AFSC-50

Two Demersal Trawl Surveys in

the Gulf of Alaska: Implications of

Survey Design and Methods

P. T. Munro1 and R. Z. Hoff 2

‘Resource Assessment and Conservation Engineering Division

Alaska Fisheries Science Center

National Oceanic and Atmospheric Administration

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

Seattle, WA 98115-0070

2Hazardous Materials Response and Assessment Division

National Ocean Service

National Oceanic and Atmospheric Administration

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

Seattle, WA 98115-0070

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

March 1995
This document Is available to the public through:

National Technical Information Service

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

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ABSTRACT

In 1984 and 1987, demersal trawl surveys of fish stocks in the western and central

Gulf of Alaska were executed by the National Marine Fisheries Service in cooperation with

the Fisheries Agency of Japan. Details of the survey design and sampling methods are

presented. Fishing power differences resulted from differences among vessels, gears, and

trawling procedures. Chartered fishing vessels from the United States and Japan performed

the sampling using modified commercial trawls as sampling gear. Nets used by the U.S.

vessels were standardized to produce data appropriate for estimation of indices of fish

abundance. In both surveys the nets used by the Japanese vessels departed markedly from

that standard. Methods of estimating those fishing power differences, and applying the

estimates to calibrate catches to a standard, have evolved through varied and conflicting

strategies. To decide the most appropriate strategy for estimating and applying Fishing

Power Corrections (FPCs). we defied a notion of negligibility. Based on this notion, final

estimates of FPC were developed.

Introduction ......................................................................................................

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

Methods Common to Both Surveys ................................................................

The Sample Unit............................................. ....................................

Sampling Gear................................................................................ . .

Taking the Sample..................................................................................... . .

Measuring Effort....................................................................................... . .

Survey Design............................................................................................ . .

Allocating Samples in Multispecies Surveys..................................................

Vessels.................................................................................................. . . . . .

Executing the Survey................................................................................ . . .

Processing the Catch..................................................................................................... . . .

Estimation........................................................................................... . . . . .

Methods Peculiar to 1984...................................................................................................... . . . .

Stratification and Survey Design.................................................................. .

Vessels and Gear....................................................................................... . .

Net Mensuration.................................................................................... . . . .

Collecting Data to Estimate Fishing Power Corrections.........................................

Executing the Survey............................................................................... . . .

Methods Peculiar To 1987.................................................................................. . . . . .

Stratification and Survey Design.................................................................. .

Vessels and Gear.................................................................................. . . . . .

Net Mensuration.................................................................................... . . . . .

Collecting Data to Estimate Fishing Power Corrections :

V

CONTENTS

Page

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15

15

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27

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38

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A Comparison Study............................................. . . . .

Executing the survey....................................................... . . . . . .

History of Analyses and Selected Results............................................... . . . . . .

1984 - First Analysis: Adjusting to the Most Efficient Trawl.......................

1984 - Second Analysis: Adjusting to the Standard Trawl.......................

1984 - Third Analysis: the Kappenman Fishing Power Correction Estimator..........

1987 - First Analysis: Adjusting to the Most Efficient Trawl...........................

1987 - Second Analysis: Adjusting to the Standard Trawl............................

1987 - Third Analysis: the Kappenman Fishing Power Correction Estimator............

Second Analysis of the Comparison Study............................................... .

Discussion........................................................................................... . . . . . . . . . .

Data review........................................................................... . . . . . . . . .

Primary Features..................................................... . . . . . .

warnings

Fishing Power Corrections and Comparability of Survey Results........................... .

Considerations for Future Surveys......................................................... . . . . .

Invalid Tows and High Catches................................................ . . . . .

Survey Design and Execution............................................... . . . . .

The Role of Fishing Power Corrections.............................................. .

Critical Ancillary Work................................................................. . . . . . .

Citations........................................................................................................ . . . . . . . . . .

Appendix A: Sampling Gear Specifications......................................................... . . . . .

Appendix B: Estimates for the Scatterplot Comparison of Nylon and Polyethylene

Noreastern Trawls.......................................................... . . . . . . .

Appendix C: Strategies and Formulae for First Round Estimates of Fishing Power

Corrections in 1987......................................................... . . . . . . .

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INTRODUCTION

Bottom trawl surveys of groundfish stocks in the Gulf of Alaska are executed on a

triennial basis as a joint responsibility of the Resource Assessment and Conservation

Engineering (RACE) Division and the Auke Ray Laboratory (ABL). both of the Alaska

Fisheries Science Center (AFSC). A comprehensive survey of such a large area strains fiscal

and logistic resources. The first two such surveys in 1984 and 1987 were cooperative efforts

with the Fisheries Agency of Japan. Japanese participation allowed the entire Gulf of Alaska

to be surveyed in one season.

We shall report design and execution of the 1984 and 1987 surveys of a region of the

Gulf of Alaska from Cape St. Elias to the Islands of Four Mountains. between 144° 30’ and

170° 00’ W long. (Fig. 1). We shall also report the evolution of our treatment of differences in

catch data due to differences in efficiency among the several fishing vessels that served as

sampling systems.

The 1984 and 1987 surveys comprise initial observations in a long-term triennial

series. Bottom trawls were employed to obtain catch-per-unit-effort (CPUE) data as indices of

fish abundance. These indices are used to monitor trends in abundance for a variety of fish

species. Data to estimate a number of biological parameters or changes in those parameters,

including age compositions, were also collected in these surveys.

Each survey was executed by two U.S. vessels and one Japanese vessel. The AFSC

chartered the U.S. vessels directly. The Japanese vessels, chartered by the Fishery Agency

of Japan,’ were operated under the direction of AFSC scientists, following standard methods

and taking station assignments from the U.S. principal investigator.
Figure 1 .--Gulf of Alaska.
3

Within each year, variation among participating sampling systems was of sufficient

magnitude to present a strong likelihood that fishing power differed. Differences included

those in vessel size and power, gear, expertise of captains and crews, and survey

responsibilities and assignments. Correction for relative differences in fishing efficiency

became an important consideration.

Inconsistencies in design, implementation, and execution between years also needed to

be evaluated. These included slight changes in strata definitions and sample allocations. a

change in station selection criteria, and changes in how stations were assigned to vessels

after having been selected. However, such inconsistencies were thought to be less influential

on survey results than were differences in vessel fishing power.

 For each survey, we will describe its design, the methods used to execute the survey,

and methods of analysis that were applied to the data. We will provide the information an

analyst needs to render these surveys comparable to each other and to future surveys. Also

of importance is an account of the history of estimation of indices of abundance based on

data from these surveys, particularly the history of the use of fishing power corrections

(FPCs). Data from each survey have advanced through three analyses, each time reflecting a

change in our understanding of the peculiarities of the surveys and changes in estimation

methods, particularly with respect to FPCs. We will state the strategy for estimating and

applying FPCs which has evolved, including a narrow definition of the term “negligible” as it

refers to differences in fishing efficiency. We will outline elements of an ideal multispecies

bottom trawl survey in the Gulf of Alaska. 
4

METHODS

In both the 1984 and 1987 triennial surveys, essentially the Same stratification was

followed. Sample allocation changed with the improved a priori knowledge gained through

the first survey. Nominally, the two surveys were highly consistent in that they both

incorporated the same sample unit, sampling gear, and sampling methods. However,

troubling differences entered through the sample unit itself, namely through differences in

fishing efficiency and the difficulty associated with calibrating such awkward instruments as

commercial vessels and fishing nets.

Methods Common To Both Surveys

The surveys occurred during the summer. They followed a stratified, random design.

Vessels were small U.S commercial fishing boats operating under charter to the AFSC and

larger Japanese boats operating under charter to the Fisheries Agency of Japan. The U.S.

sampling gear was a bottom trawl of commercial design that had been modified to meet

sampling standards. The Japanese vessels used sampling gear that was at variance from

AFSC standards. Catches were made using standard trawling methods, (Wakabyashi et al.

1985, Wilderbuer 1988). Once aboard, data was collected from the catch using standard

methods, (Hughes 1976).

The Sample Unit

The sample unit was the catch made by towing a trawl net over the bottom,

standardized by a measure of the effort expended to make it: that is CPUE. Catch was

measured by mass and counts. Effort was measured as “area swept" by the trawl: the width

of the net opening tunes the length of the tow (Alverson and Pereyra 1969). Dividing catch

by effort produces a measure of fish density which serves as an index of abundance.
5

Sampling Gear

Complete descriptions of the demersal trawls used as sampling gear can be found in

Appendix A. The standard sampling trawl for Gulf of Alaska triennial surveys is a highopening, four-seam Noreastern trawl constructed of polyethylene and rigged with rubber

bobbin roller gear. This particular net was designed to be a rough-bottom, rockfish trawl. It

was chosen because of its capacity to sample species which school slightly above the bottom,

often over rough terrain, such as walleye pollock (Theragra chalcogramma) or rockfish

species (Sebastes spp.) (Feldman and Rose 1981). This trawl became the standard when the

first of the regional triennial surveys was developed for the West Coast of the continental

United States (Gunderson and Sample 1980). At that time this gear design was in common

use among rockfish trawlers in those waters and had proven to be useful in the varied

submarine terrain found there as well as in the Gulf of Alaska. (Trawl design has evolved

and very few vessels still employ this gear.) The Noreastern trawl is also capable of fishing

on flat or soft bottoms but is not considered as effective in sampling species which dwell on

the bottom (Feldman and Rose 1981). The Nor-eastern trawl was originally constructed of

nylon, but during 1984 a switch was made to a polyethylene version with a different

footrope and a lower wing design. These modifications were intended to reduce damage to

the gear.

The Noreastern trawl was towed behind 1.000 kilogram. 1.8 X 2.7 m steel V-doors.

Three 54.9 m (30 fathom) dandylines connected the net to each door. Again, in both

surveys, the Japanese gear configuration deviated from this.

Taking the Sample

Towing methods followed those of AFSC trawl surveys in waters off the West Coast of

the continental United States and the Bering Sea, (Gunderson and Sample 1980,
6

Wakabyashi et al. 1985, Wilderbuer 1988). Before the fishing gear was deployed the station

was surveyed by echo sounder to determine whether the bottom was suitable for trawling.

The tow was attempted whenever there was a chance of successfully completing it, even if

the bottom was considered only marginally trawlable. If the preselected tow site was to be

too rough to trawl, then an additional one-half hour was allowed to search for trawlable

bottom adjacent to the original station. If suitable bottom was not located in the allotted

time the station was abandoned and the vessel moved to the next site.

An ideal tow was defined as having a 30 minute duration, a towing speed of

5.56 km per hour (3 knots), and with a straight course that did not vary in depth. Duration

was measured from the time the net had stabilized on the bottom in its fishing configuration

to the time the winches began retrieval. Proper fishing configuration was determined by

net mensuration gear described below or by the skipper’s judgment. While waiting for the

net to sink to the bottom, the vessel kept no more headway than was necessary to maintain

steerage.

Because most tows deviated to some degree from these standards, valid observations

were defined to be from hauls that lasted a minimum of 10 minutes, varied in depth by 20 m

or less, and were completed without significantly damaging the gear. Damage was

considered significant if it could have caused a major loss of catch, such as a hole in the

codend or a tom belly. There were tows during which the net or the doors hung up but

were considered valid, even though the net may have sustained slight damage. Tows which

 could not be made in a straight line because of rugged bottom topography or because

currents shifted the net or the boat to one side were defined as valid. Under influence of

those same factors, towing depth sometimes varied as much as 20 m. Likewise, some tows

were cut short because of too much change in depth or to avoid damaging the gear on some
7

obstacle revealed by the sonar. Such tows were considered valid if they lasted 10 minutes or

longer. Some tows had the current either directly behind or against and the towing speed

was too much or too little. Such tows were also considered valid.

How hard a trawl tends bottom may be, in part, a function of the scope, the ratio of the

length of the trawl cable let out to the depth of the net. In both of these surveys the amount

of cable let out was left to the best judgment of the skippers.

Measuring Effort

Because the effort expended to make each tow is measured as area swept. the width of

the net opening is necessary information. Data to calculate net width were collected

differently in each year. Those methods will be described in pertinent sections. Distance

fished, the other dimension of the area swept, was inferred from the position of the boat at

the start and end of the tow. We assumed that the distance the vessel travelled during the

tow was also the distance that the net travelled. Start and end positions were measured by

Loran instrumentation.

Survey Design

The geography and, in part, the distribution of demersal fish populations of the central

and western Gulf of Alaska have been previously described (Ronholt et al.. 1978). The highly

varied geography of the Gulf lent itself to a stratified design. Because delineation of large

scale topographic features was often quite sharp, sampling strata could be defined without

the benefit of a comprehensive pilot survey. Some of these features can be discerned even

from a chart with a scale as small as that of Figure 1. Smaller pilot surveys that confirmed

associations of habitat types and species assemblages with particular geographic features

provided adequate background data. (Ronholt et al. 1978, Feldman and Rose 1981).
8

Table 1 and Figures 2 - 5 define and illustrate the sampling strata. There were Eve

basic categories of strata: nearshore. Eats or shelves, gullies or troughs, slope breaks, and the

continental slope. Near-shore strata were those that were bordered by land. Sometimes

nearshore strata were the margins of otherwise broad regions of water less than 100 m in

depth. Flats or shelves were broad areas over which depth changed slowly or not at all.

Gullies or troughs were underwater valleys or canyons, sometimes of quite large dimension.

A slope break is the portion of a shelf or gully that forms the transition zone to the

continental slope. Even though these transition zones may be in the same depth range as

the shelf or gully, they often appeared to have higher fish concentrations, as though there

were some physical or biological attribute that distinguished them. Slope breaks were the

hardest strata to define because there were few natural features to mark the point at which a

shelf or gully changed nature to become more slope-like. Continental slope strata were those

of the survey area that sloped steeply down to the abyss.

These Eve types of strata were further divided by depth contours at 100 m. 200 m,

300 m, 500 m, and 700 m. We did not survey deeper than 1,000 m. Not all classes of strata

occurred in every depth. Waters less than 100 m deep had only nearshore and Eat strata. 

Depths of 100-200 m had Eats, gullies, and occasional slope breaks. Depths of 200-300 m

had flat, gully, slope break, and slope strata. Depths of 300-500 m and depths greater than

500 m had only slope strata. Because we suspected that the east-west fish distribution was

not uniform, we further divided these strata by International North Pacific Fisheries

Commission boundaries (Ronholt et al. 1978). There were a total of 44 sampling strata in

the central and western Gulf of Alaska.
9

Table 1.--Sampling strata in the western and central Gulf of Alaska triennial trawl surveys.

grouped by International North Pacific Fisheries Commission statistical regions.
10

Table 1 .--Continued
Figure 2.--Shumagin INPFC Area with strata boundaries.
12

Figure 3.-Chirikof INPFC Area with strata boundaries.

Figure 5.--Yakutat INPFC Area with strata boundaries.
Allocating Samples in Multispecies Surveys

One critical aspect of a synoptic demersal trawl survey of the Gulf was the need to

assess abundance of a multiplicity of species. Table 2 lists a number of commercially or

ecologically important species that live in the Gulf which are vulnerable to sampling by a

bottom trawl. Several species of commercially important crabs. scallops, and shrimps also

live in the Gulf of Alaska and may be, to some slight degree, vulnerable to a bottom trawl

like the type employed in these surveys;

Optimal allocation for any one species was compromised by conflicts with optimal

allocations for other species. We responded in two ways. First, allocation assessments were

based on a priori data pooled over a number of biologically and economically important

species, Second, we tried to over-sample in as many strata as possible: that is, we tried to

have a higher than optimal concentration of stations. We tried to do this for strata we

believed to be more biologically productive. We hoped that by over-sampling with respect to

most species in a particular stratum, we would avoid undersampling for the one or two

species with high abundance in that stratum. Oversampling proved difficult, however,

simply because of the large size strata in the Gulf.

Vessels

The vessels used in Gulf triennial surveys had to possess the following features:

They must be large enough to provide a stable working platform up to at least moderate gale

conditions. Adequate deck space for handling the fishing gear and for five scientists to

collect data from the catch was also required. Each vessel must possess sufficient power to

pull the net over the bottom at 5.56 km/hr (3 knots). The vessels were required to be able to

stay at sea for as long as 25 days, providing berth space and living quarters for the vessel

crew and a field party of five. The typical vessel operating under charter to the U.S.
16

Table 2.--Some of the commercially important finfish species in the Gulf of Alaska which are

vulnerable to some degree to sampling by a demersal trawl.
17

Government has been between 32 m and 40 m in length and driven by 500 to 850

horsepower engines.

Modem navigation and communications instruments were employed on all boats in

both surveys. Positions were determined by Loran. While this technology is quite accurate,

it is not without error. Errors in position may have been exacerbated in several survey

strata because they were in close proximity to Loran signal stations. Locations of sample

sites and distances fished may be in error for a number of the tows.

Executing :

The surveys were executed during the summer months. This is the feeding season

for most species and we expected to see a wider, more even distribution than the heightened

patchiness associated with spawning aggregations. Summer surveys also allowed the small

charter vessels to avoid as much rough weather as possible. The surveys were collapsed into

as small a time frame as possible so that the resulting data might have as synoptic a

character as could be.

Logistics constrained the Japanese vessel to work from west to east and then back,

east to west. U.S. vessels progressed through the region from west to east, attempting to

synchronize their efforts with that of the Japanese vessel: in each year. one U.S. vessel was

able to stay in the same stratum as the Japanese vessel at the same time during the first

half of the period of Japanese involvement. However, in neither year was there a period

where all three vessels worked in the same stratum. A few strata were represented with

tows made by only one of the boats.
18

Processing the Catch

Methods for collecting data from the catch once it was aboard followed those of other

AFSC trawl surveys (Hughes 1976, Wakabyashi et al. 1985, Wilderbuer 1988). On US.

vessels, subsamples were taken from catches that were greater than 1.000 kg. Catches of

1,000 kg or less on U.S. vessels, and all catches on the Japanese vessels, were processed

completely. Weights and counts were taken for each species once the catch or subsample

had been sorted.

Fork lengths (tip of snout to end of middle rays of the caudal fin) were measured for

each species except squid and grenadiers. Mantle lengths were recorded for squid and the

distance from snout to anus was used as an index of size for grenadiers. Length frequencies

were taken from a randomly selected subsample of a species if it was present in large

numbers. Methods for selecting such subsamples have been described elsewhere (Hughes

1976).

Age structures were collected for a number of species. Individuals were chosen

randomly. After choosing the random subsample, length and weight data were taken as well

as otolith, scale, or fin ray specimens for determining age.

Estimation

Estimating the index of abundance -- Following established methods of estimation

(Gunderson and Sample 1980 Wakabyashi et al. 1985). mean fish density was inferred from

mean CPUE. Mean CPUE was estimated as a stratified, arithmetic mean of observed CPUEs.

weighted by stratum area. This estimator reduces variation due to differences among strata

(Cochran 1977). Though we will not include estimates of abundance indices in this report,

this is the context for the discussions of FPCs and their estimators which follows.
19

Fishing power -- Fishing power is an inherent concept within the notion of CPUE as a

measure of fish abundance. Fishing power is some measure of the effectiveness of a fishing

system in actually catching fish. Following a standard conceptual model for CPUE (Ricker

1975) we let

Here, fishing power is represented by the quantity "q." It is often called the

catchability coefficient. Note that this model of CPUE defines N as ‘encountered N” rather

than the "total N" that one might employ when modelling commercial fishery CPUE. In our

presentation, q simply equates to the probability of capturing a fish that has been

encountered, rather than some ii-action of the total population. Henceforth when we use the

phrase “probability of capture,” we refer to the quantity “q” defined here.

To estimate absolute abundance, we must have a value for q. In past analyses, we

have assumed q = 1. However, data for testing this assumption or estimating q are

currently beyond our technological capability to collect. It seems highly likely that q is quite

different from 1.0, either greater or less than, for most species. Hence it would also seem

likely that estimates of absolute abundance made from demersal trawl data are incorrect.
20

Inference can still be drawn regarding trends in abundance by assuming that q equals

some constant, albeit unknown. With a constant probability of capture, we would be able to

support a hypothesis that change in an observed abundance index reflects either change in

true abundance, change in the distribution of that fish, or both. By assuming constant q.

observed change in abundance indices could be assumed real and not an artifact of change

in relative fishing power.

The probability of capture is a function of many variables. It may be subject to change

due to change in fishing gear. fishing methods, acoustic attributes of a vessel, or some

synchronous combination of these effects. By standardizing sampling gear (the net, doors,

and bridles) and methods of trawling, we tried to hold these effects constant. We then

assumed that the probability of capture was constant with respect to these variables.

Fortunately, though true q is inestimable for these data, relative differences in q are.

Because of this, we can assess the validity of the assumption that q is constant between

vessels and estimate a factor to force this weaker assumption to be true.

It is important to note that the probability of capture may also be subject to change

due to variation in fish behavior with respect to the trawl or change in preferred habitat from

year to year (which may, in turn, be linked to some spurious factor such as shift in prey

distribution or abundance). We were forced to assume that q was constant with respect to

such effects.

Estimating FPCs -- Before a mean CPUE could be calculated, observed catches were adjusted,

for the relative fishing power differences described above. This was done by multiplying the

catches made by one vessel with a correction factor, or FPC. to calibrate them to those of
21

another vessel. The FPC has been estimated in two ways at the AFSC. Early analyses

estimated FPC with FPC’, a ratio of arithmetic

are the mean CPUEs for vessels 1 and 2, respectively. To correct catches for relative fishing

power differences, we multiplied a catch made by vessel 2 with FPC’ which degradesthat

catch from an observation to an estimate of what would have been caught had vessel 1 made

exactly the same tow at exactly 

Final analyses employed a different estimator of a FPC. Concern over the instability of

the ratio of arithmetic means and the disproportionate influence of rare, large catches led to

the development of a new estimator (Kappenman 1992). The Kappenman FPC achieves

robustness by transforming the highly skewed CPUE distributions through a power function
22

into symmetric distributions. The estimator itself is a function of the scale and shape

parameters of the transformed distribution. The critical assumption is that the shapes of the

CPUE distributions of each vessel are identical.

Negligibility -- When there is a true fishing power difference between two vessels, we are

forced to choose between systematic and random error. If we do not correct for the

difference the systematic error in the data produces a bias in the estimated mean CPUE. If

we do correct with an estimate of the difference, we remove the error due to bias, but we

increase the error due to randomness: that is. the variance of the mean CPUE because we

have introduced another random variable. We defined a fishing power difference to be

negligible when the latter error is greater than the former. By this definition. it becomes

possible for a true fishing power difference to be quite large but still be negligible. This

would happen anytime the variance of the difference or the variance of the estimated

difference is high.

We developed this definition because, as analyses of these surveys unfolded, we began

to question the automatic application of an FPC every time we perceived a relative fishing

power difference. Instability of the ratio of means, particularly the sensitivity of this

estimator to infrequent but large CPUE observations, convinced us that our estimates of the

FPC had an extremely high variance (Robson 1960. Robson 1961. Munro 1989). We were

concerned that, by corrupting observed CPUEs into high variance estimates with an

estimated correction factor, we were introducing unacceptably large and unaccounted

uncertainty into estimates of mean CPUE.

Determining negligibility requires computation of the mean square error (MSE) of the

mean CPUE as estimated with and without an FPC. A simple decision rule would then be:
23

‘Use the estimate with the lower MSE.” Unfortunately, we had neither the time nor the

capacity to pursue a study of MSEs at the time of these analyses. Consequently the process

for determining negligibility was entirely ‘subjective; we were without rigorous decision rules.

The magnitude of the FPC estimates themselves did influence our decisions. However,

unless there was some clear factor that could explain a large FPC estimate, such as radical

differences in vessel, gear, or towing characteristics, we deemed even large FPCs negligible.

Generally, given the very unstable nature of a ratio of means, the higher the variance of

mean CPUE for a species the less likely we were to apply an FPC.

When we adopted the Kappenman FPC estimator, we maintained previous

assumptions of negligibility. We did this because at present the variance of the estimate of

the Kappenman FPC is not completely developed and preliminary indications suggest that,

though it is much more stable than a ratio of means, its variance is still high (Kappenman

pers. commun.).

Methods Peculiar to 1984

Stratification and Survey Design

The 1984 survey used a slightly different stratification than that presented above. At

that time stratum 30 included what is now stratum 35. These strata are defined by the

nearshore waters, 0 - 100 m depth. of Kodiak and Afognak Islands. (See Table 1. Fig. 4.)

We considered three potential sampling densities for each stratum. A priori data did

not permit finer division of sampling densities. (These levels are presented below as part of

the station selection methods.) The allocation was based on demersal trawl data from

surveys of the central Gulf of Alaska in 1978 and 1979 (Feldman and Rose 1981). Scarcity

of data forced us to assume that the 1978 and 1979 surveys yielded the mean and the
24

variance of measures of CPUEs that were representative of those in the western Gulf. Table

3 gives station allocation for each stratum. Table 4 shows the assignment of stations to

vessels.

Stations were chosen from a grid of points laid over the survey area. Each point in the

grid was 5 nautical miles from its nearest neighbor along lines of latitude and longitude. For

a stratum expected to have high fish density the sample allocation was one out of every four

points on the grid that fell within that stratum. For a stratum expected to have moderate

fish density the sample allocation was one out of every six points on the grid. For a stratum

expected to have low fish density the allocation was one grid point out of every nine. Once

the number of stations was determined for each stratum their sites were selected

systematically after randomly choosing a starting point,

This selection procedure worked well in the strata of the continental shelf which were

generally broad in shape. However, another selection method was necessary for the

extremely long and slender strata of the continental slope because so few grid points actually

fell in these strata. Slope stations were chosen to correspond to index stations that had

already been established as part of the U.S. Japan cooperative longline survey which had

been taking place annually since 1979 (Sasaki et al. 1982). A total of 30 transect lines

spaced between 35 and 80 km apart were sampled from the Islands of Four Mountains to

Cape St. Elias. Along each transect one or more tows were attempted by each of two vessels

working together. The exact location along the transect, within a depth zone. was randomly

determined, If the longline survey vessel was working a station at the same time as the

trawlers, the two trawlers made sure that their tow sites were at least 3 nautical miles from

the transect. This was done to avoid influencing catch rates observed in the longline survey.

One hundred eighty stations were allocated to the 100-200 m slope strata. Sixty stations
25

Table 3.--Design of the 1984 triennial bottom trawl survey of the Gulf of Alaska:’ Allocated

and successful stations in each sampling stratum.
26

Table 4.-Assignment of stations and numbers of stations successfully sampled by vessel and sampling

stratum in the 1984 triennial bottom trawl survey of the Gulf of Alaska
were planned for each of the three depth intervals between 200 and 700 m. Twenty-nine

stations were planned between 700 and 1,000 m. This resulted in a sampling density in the

slope portion of the survey which exceeded the original allocations. This was accepted

because it would improve the precision of estimates of commercially important rockfish

stocks that dwell on the slope.

Vessels and Gear

The two U.S. vessels, the 37 m F/V Morning Star and the 28 m F/V Ocean Spray, were

both house-forward style crabber/trawlers powered with main engines developing 1,125 and

565 continuous horsepower, respectively. Each U.S. vessel operated with a crew of five,

including the captain and a scientific complement of four or five.

Noreastern trawls were supplied to the U.S. vessels by the AFSC. Two versions were

sent out: one constructed of nylon and another constructed of polyethylene. The nylon net

was an older style that had been used in previous AFSC surveys. Not only did it differ in

material, it also differed slightly in design. The differences between nylon and polyethylene

nets are well documented as part of a gear comparison study (Wilderbuer 1988). Diagrams

of key differences have been reproduced from his thesis in Appendix A. The polyethylene

net, the new standard, was essentially of the same design as the older, nylon net except that

it had a slightly shorter headrope. a slightly longer footrope. and a section of webbing in the

lower wing section had been replaced by a single cable to avoid damage that frequently

occurs to meshes there.

The F/V Daikichi Maru No. 37 was a 50 m, land-based, stem trawler of 349 gross tons

powered by a 2,500 horsepower main engine. The crew numbered 25. including fishing

master and captain. The Daikichi Maru No. 37 had engaged in commercial fishing in these
28

waters for a number of years previous to the survey. Consequently the captain and crew

were very knowledgeable concerning the grounds and the effective use of their gear. The

scientific party consisted of two Japanese scientists and one U.S. scientist.

The Daikichi Maru No. 37 used a considerably different net from the Noreastern. It

was a much larger net with a relatively wider, lower opening. It was a two-seam trawl with

roller gear constructed of automobile tires and bobbins made of steel and rubber. The trawl

was constructed of polyethylene webbing. Mesh sizes were considerably larger than those of

the Noreastern. Rather than a small-mesh codend liner, as used with the standard sampling

gear, the Japanese codend consisted of three layers of webbing, each having 100 mm

meshes. The three layers reduced the effective mesh size. Biologists aboard during

operations felt that this worked fairly well to retain smaller items of the catch, though

perhaps not as well as the liner used on the U.S. nets. This trawl was spread by doors

constructed of wood and steel that weighed 2,931 kg each. The net was joined to the doors

with a combination single and double dandyline arrangement totaling 120 m. As opposed to

the Noreastern. a high-opening rockfish trawl, this net was designed to catch Greenland

turbot Reinhardtius hippoglossoides in a fishery that takes place over steep, hard grounds

and in which there is much less to be gained by maintaining a large vertical opening.

Net Mensuration

Width of the Noreastern trawl mouth was estimated from data collected during other

surveys. Net mensuration studies (Wahtne 1977) indicated, the Noreastem trawl had a

wingtip-to-wingtip spread of 13.4 m. However, this work was done with much smaller

vessels, which had much narrower beams, using notably smaller doors. Based on

unpublished data and qualitative assessments of the vessels, doors, and fishing conditions,

(Wahtne pers. commun.). we concluded that a net width of 18.3 m was more realistic. This
29

path width was used in the first analysis of 1984 data. Later, a more refined estimate of

16.43 m was developed from net mensuration data collected during a gear comparison

experiment (Wilderbuer 1988). In either case, we applied one estimated net width to all U.S.

tows regardless of depth. No variance of that estimate was incorporated in the CPUE

observations .

Height and width of the Japanese trawl was measured by mounting a commercially

available headrope transducer in a sideways configuration on one wingtip and mounting a

sonically reflective plate on the opposite wingtip (Wakabyashi 1984). Area swept was

computed for each tow based on the observed net width. The net width varied between 21

and 29 m and increased as depth increased (Wakabyashi 1984). Tows that were not

measured in this way had their net widths estimated from tows made at similar depths that

did provide mensuration data. No variances of these estimates were incorporated in the

CPUE observations.

Collecting Data To Estimate FPCs

Data to test for fishing power differences which might have resulted between nylon and

polyethylene Noreastern trawls were collected aboard the Morning Star. This vessel

alternated tows between the net types to produce a set of data for each net within each

stratum. The Ocean Spray used only the nylon net.

Differences in relative fishing power were estimated from a subset of the survey data.

The Daikichi Maru No. 37 and the Morning Star worked together along the continental slope

and did some side-by-side tows on Davidson Bank, producing 123 pairs of comparable tows,

from similar places and times. This pairing served only to ensure equal weighting between

U.S. and Japanese data under a variety of fishing conditions. Mean CPUEs for FPC ratios
30

were estimated from pooled observations within each vessel’s subset. Consequently, it was

not true pairing; we had a two-sample situation rather than a single sample of pairs.

Executing the Survey

Eight hundred twenty-three tows were made between the Islands of Four Mountains

(170” W) and Cape St, Elias (143° 30’ W) and between the depths of 16 m and 768 m. Of

these tows, 749 were considered to have produced valid observations. Of these, 334 were

completed by the Daikichi Maru No. 37. 249 by the Morning Star, and 166 by the Ocean

Spray. Table 3 shows the number of tows taken in each stratum. Table 4 shows these tows

broken down by vessel. Table 5 summarizes allocated, attempted, and successful tows over

INPFC areas and depth zones. Table 6 summarizes the biological data taken in the course of

the survey .

The Morning Star began sampling shelf strata in the vicinity of the Semidi Islands

(157° W) on 20 June 1984 and worked westward to the Islands of Four Mountains, arriving

13 July. The Morning Star was joined there by the Daikichi Maru No. 37 on 14 July. The

two vessels worked together in an easterly direction, sampling slope strata and completing

side-by-side tows on Davidson Bank. Both vessels reached the eastern boundary of the

survey area, Cape St. Elias. on 18 August. The Morning Star continued working toward the

east, completing tows in the eastern Gulf as part of the survey run by the ABL. The

Daikichi Maru No. 37 turned back toward the west, completing tows on shelf stations,

ending its work at the Islands of Four Mountains on 2 October. The Ocean Spray began

trawl operations at the eastern boundary of the survey region on 27 June. The vessel

completed tows in shelf strata along the Kenai Peninsula and the east side of Kodiak Island

and on toward the Semidi Islands, arriving there 29 July. The Ocean Spray turned at that
31

Table 5.--Summary of the design of the 1984 triennial bottom trawl survey of the Gulf of

Alaska: Allocated and successful stations in each INPFC Region and depth zone

within each region.
32

Table 6.--Summary of biological data collected during the 1984 Gulf of Alaska triennial

bottom trawl survey. Units are numbers of observations for each species.
33

point and worked northeastwardly through Shelikof Strait and on into lower Cook Inlet,

finishing there on 22 August.

Tables 7 - 10 present data on four measures of vessel and tow performance: towing

speed, distance fished, area swept by the trawl, and scope ratio. Data are pooled across

INPFC areas but segregated by depth. Means in these tables are arithmetic and the standard

error of the mean is the usual sample standard deviation divided by the square root of the

sample size. These tables may be used to assess the degree to which each of the three

vessels adhered to standardized towing procedures. On the Morning Star. a commercially

available headrope transducer was used to determine when the net had reached bottom and

that the tow should commence. No such instrument was available on the Ocean Spray; on

that vessel, we depended on the judgment of the skipper. On the Daikichi Maru No. 37, net

mensuration instruments were used to determine when the net was on bottom and ready to

fish.

Methods Peculiar to 1987

The experience gained during the 1984 triennial survey enabled us to improve the

survey design for 1987. There were slight modifications of the underlying stratification

scheme and sample allocation changed between the two surveys. The sample unit and the

sampling methodology remained unchanged. The sampling gear was also the same, though

only the polyethylene Noreastern trawl was used: no nylon nets were used. None of the

participating vessels had been involved in the 1984 survey. The Japanese vessel, again,

provided its own sampling gear, which was different from the standard and was also quite

different from that used by the Japanese vessel of 1984. Different net mensuration

instruments were used. An important difference was that an independent investigation

provided data for estimating FPCs.
34

Table 7.--Mean trawling speeds and their standard errors for each of the three vessels

participating in the 1984 triennial bottom trawl survey of the Gulf of Alaska. Units

of measure are kilometers per hour. Means and their standard errors were

computed only from tows that were randomly chosen and of acceptable

performance.
3 5

Table 8.--Mean distances fished and their standard errors for each of the three vessels

participating in the 1984 triennial bottom trawl survey of the Gulf of Alaska. Units

of measure are kilometers. Means and their standard errors were computed only

from tows that were randomly chosen and of acceptable performance.
36

Table 9.--Mean efforts, (area swept). and their standard errors for each of the three vessels

participating in the 1964 triennial bottom trawl survey of the Gulf of Alaska. Units

of measure are square kilometers. Means and their standard errors were computed

only from tows that were randomly chosen and of acceptable performance. Net

width was 16.43 m for the U.S. boats.
37

Table l&--Scope ratios and their standard errors for each of the three vessels participating in

the 1984 triennial bottom trawl survey of the Gulf of Alaska. The scope ratio is

the ratio of trawl wire deployed to depth. Means and their standard errors were

computed only from tows that were randomly chosen and of acceptable

performance.
38

Stratifacation and Survey Design 

The stratification scheme used in the 1987 survey is shown in Table 1 and Figures

2 - 5. Stations were reallocated among strata based on the distributions of fish observed in

the 1984 survey. Table 11 gives the allocations by stratum. Table 12 shows how these

allocations were assigned among vessels.

As in 1984 sampling sites were chosen from a grid of points laid over the survey

area. Each point in the grid was 5 nautical miles from its nearest neighbor along lines

running north-south and east-west. Once the number of stations was determined for each

stratum, their sites were selected systematically after randomly choosing a starting point.

This selection procedure worked well in the strata of the continental shelf which were

generally broad in shape. However, another selection method was necessary for the

extremely long and slender strata of the continental slope because so few grid points actually

fell in these strata. We did not use the transect procedure employed in 1984 at the

established longline stations. Instead, the 300-500 m. 500-700 m, and 700-1006 m slope

strata were broken longitudinally into segments, each segment being the equivalent of

85.7 km2 areas, the area represented by each point on the original grid. A set of these areas

was chosen for sampling systematically after randomly choosing a starting area. The tow

was to be taken from the center of each area.

Vessels and Gear

The U.S. vessels were the F/V Lets Go and the F/V Nore-Dick. Both were small,

house-forward stem-trawlers. The Lets Go was 26 m in length and powered by a main

engine that developed 565 continuous horsepower. The Nore-Dick was 24 m in length and
39

Table 11.--Design of the 1987 triennial bottom trawl survey of the Gulf of Alaska: Allocated

and successful stations in each sampling stratum.
40

Table 12.--Assignment of stations and numbers of stations successfully sampled, by vessel

and sampling stratum in the 1987 triennial bottom trawl survey of the Gulf of

Alaska.
41

was powered by a main engine that developed 500 continuous horsepower. Each vessel

carried a crew of four, including captain, and a scientific party of four.

Both U.S. vessels used standard polyethylene Noreastern trawl. Doors and dandyline

characteristics were identical to those used in the 1984 survey. (See Appendix A.)

The Japanese vessel was the F/V Taisei Maru No. 35, a land-based, commercial stem

trawler. This vessel was 50 m in length and displaced 350 gross tons. The vessel was

powered by a 3,400 horsepower main engine. It carried a crew of 24, including the captain

and the fishing master, and a scientific party of two Japanese scientists and one U.S.

scientist.

The Taisei Maru No. 35 used a six-seam trawl of polyethylene webbing with roller

gear constructed of automobile tires and bobbins made out of steel and rubber. See

Appendix A for a more detailed description, It was a low opening net designed to maximize

catches of shortraker rockfish (Sebastes borealis). a species which lives close to the bottom

on grounds that are often both steep and very hard. This net was much larger than the

Noreastern with headrope length of 55.6 m and a footrope length of 65.0 m. Mesh sizes were

greater than those of the Noreastern. though mesh size differences between the Japanese net

and the standard net were not as great as they were in 1984. Rather than a small-mesh

codend liner, as used with the standard sampling gear, the Japanese codend again consisted

of three layers of 100 mm webbing. The trawl doors were curved steel, weighed 3,200 kg

each, and were attached to the net by combination single and double dandylines that each

totaled 156 m in length.
42

Net Mensuration

An acoustic net mensuration system consisting of a headrope unit and two wing

units was used to measure horizontal and vertical dimensions of the Noreastern trawl

following standard methods (Rose and Walters 1990, Wilson and Armistead 1991). Path

widths were either observed or estimated for each U.S. tow., Net mensuration data were

successfully gathered in each of several depths. Table 13 gives mean net widths for each

depth. A number of tows were without net mensuration data because the bottom was too

rough to put the instruments at risk or the instruments were either malfunctioning or not

available. Tows that were not measured in this way had their net widths estimated from

tows made at similar depths that did provide mensuration data. No variance of these

estimates was incorporated in the CPUE observations.

Height and width was measured on the Japanese trawl using the same methods

employed in 1984. Again, path widths of tows with successful observations were used to

estimate mean net width for each depth zone where mensuration information was

unavailable (Table 13). No variance of these estimates was incorporated in the CPUE

observations’,

Collecting Data to Estimate FPCs: A Comparison Study

An independent experiment was done to collect data for estimating FPCs. All three

vessels sampled together in four different depths off the east side of Kodiak Island. Tows in

each depth stratum were taken from areas of very limited size so as not to confound depth

effects with habitat effects. The four areas of the study were felt to be typical of many of the

habitats for which a bottom trawl was an effective sampling gear. The areas were also

chosen in the hope that nonzero catch rates could be observed for as many species as
43

Table 13.--Mean net widths for the vessels and nets involved/in the 1987 Gulf of Alaska

triennial bottom trawl survey.
44

possible. In each depth stratum the three vessels towed side-by-side to reduce the likelihood

that just one vessel would strike a patch of fish and produce a unique and very large CPUE.

thus confounding variation in fish density with relative fishing power differences. A pattern

of repetition was established such that vessels switched position with respect to each other

during every comparative tow.

Tables 14 through 17 show four measures of vessel and tow performance during the

comparison study: towing speed, distance fished. area swept by the trawl, and scope ratio.

Means in these tables are arithmetic and the standard error of the mean is the usual sample

standard deviation divided by the square root of the sample size. These tables may be used

to assess the degree to which each of the three vessels adhered to standardized towing

procedures.

Executing the Survey

A total of 688 randomly selected tows were completed,’ not including those of the

comparison study. Of these 657 met the criteria for a valid tow and were used in estimating

abundance indices. The number of successful hauls for each stratum is shown in Table 11.

Of the successful tows, 399 were made by the Taisei Maru No. 35, 168 by the Lets Go. and

90 by the Nore-Dick. Summaries of this effort are given in Table 18 by depth zone and

INPFC area. Table 19 summarizes the biological data taken in the course of the survey.

The Nore-Dick began sampling shelf strata at 170° W (Islands of Four Mountains) on

31 May and worked eastward, finishing at 143° 30’ W (Cape St. Elias) on 25 July. The NoreDick then participated in the trawl survey of the eastern Gulf, directed by the ABL. The Lets

Go began sampling shelf strata at 170° W (Islands of Four Mountains) on 31 May and worked

eastward to 150° W. arriving there on 8 July. At that point the Lets Go left the survey of the
45

Table 14.--Mean trawling speeds and their standard errors for each of the three vessels

participating in the fishing power experiment in the 1987 triennial bottom trawl

survey of the Gulf of Alaska. Units of measure are kilometers per hour. Means

and their standard errors were computed only from tows that were of acceptable

performance.
46

Table 15.--Mean distances fished and their standard errors for each of the three vessels

participating in the fishing power experiment in the 1987 triennial bottom trawl

survey of the Gulf of Alaska. Units of measure are kilometers. Means and their

standard errors were computed only from tows that were of acceptable

performance.
4 1

Table 16.--Mean efforts, (area swept), and their standard errors for each of the three vessels

participating in the the fishing power experiment in the 1987 triennial bottom

trawl survey of the Gulf of Alaska. Units of measure are square kilometers.

Means and their standard errors were computed only from tows that were of

acceptable performance.
48

Table 17.--Scope ratios and their standard errors for each of the three vessels participating in

the fishing power experiment in the 1987 triennial bottom trawl survey of the Gulf

of Alaska. The scope ratio is the ratio of trawl wire deployed to depth. Means and

their standard errors were computed only from tows that were of acceptable

performance.
49

Table 18.--Summary of the design of the 1987 triennial bottom trawl survey of the Gulf of

Alaska: Allocated and successful stations in each INPFC Region and broken out by

depth zone within each region.
50

Table 19.--Summary of biological data collected during the 1987 Gulf of Alaska triennial

bottom trawl survey. Units are numbers of observations for each species.
51

western and central Gulf to participate in the survey of the eastern Gulf under the direction

of the ABL. The Lets Go returned to the central Gulf on 31 July, working westward from

149° 30’ W to 161° W. arriving there on 16 August. The Lets Go then turned and worked

back eastward to 152° W, finishing there on 6 September. The Taisei Maru No. 35 began

sampling in the Islands of Four Mountains (170° W), on 22 May and worked eastward to 148°

30’ W. arriving there on 8 July. The Taisei Maru No. 35 then worked back westward,

beginning at 147° W on 15 July and ending at 170° W (Islands of Four Mountains) on 29

August. Embedded in these operations was the independent comparison study.

Tables 20 - 23 show four measures of vessel and tow performance: towing speed,

distance fished, area swept by the trawl, and scope ratio. Data are pooled across INPFC

areas but segregated by depth. Means in these tables are arithmetic and the standard error

of the mean is calculated as before.

HISTORY OF ANALYSES AND SELECTED RESULTS

In the years following the 1984 and 1987 surveys the data analyses have undergone

three iterations. The analyses differed in the way FPCs were incorporated into the estimated

mean CPUE, in the FPC estimator itself, or both. Also, with each analysis, corrections in the

data have been incorporated as well as changes in stratum definitions. Results of these

analyses exist in various documents which have been prepared as part of the management

process. We present some of them here, though they are now out of date, to document

estimation methods that produced quantities that were critical to important management

decisions, This will also chart the development of our current strategy for applying FPCs as

well as help frame the next set of questions regarding derivation and use of FPCs.
52

Table 20.--Mean trawling speeds and their standard errors for each of the three vessels

participating in the 1987 triennial bottom trawl survey of the Gulf of Alaska.

Units of measure are kilometers per hour. Means and their standard errors were

computed only from tows that were randomly chosen and of acceptable

performance.
53

Table 21,--Mean distances fished and their standard errors for each of the three vessels

participating in the 1987 triennial bottom trawl survey of the Gulf of Alaska.

Units of measure are kilometers. Means and their standard errors were computed

only from tows that were randomly chosen and of acceptable performance.
54

Table 22.--Mean efforts, (area swept), and their standard errors for each of the three vessels

participating in the 1987 triennial bottom trawl survey of the Gulf of Alaska.

Units of measure are square kilometers. Means and their standard errors were

computed only from tows that were randomly chosen and of acceptable

performance.
55

Table 23.-Scope ratios and their standard errors for each of the three vessels participating in

the 1987 triennial bottom trawl survey of the Gulf of Alaska. The scope ratio is

the ratio of trawl wire deployed to depth. Means and their standard errors were

computed only from tows that were randomly chosen and of acceptable

performance.
56

1984 - First Analysis: Adjusting to the Most Efficient Trawl

The goal of the first analysis of the 1984 triennial data was to produce an estimate of

absolute abundance of each species in the Gulf of Alaska that were considered

representatively sampled by a bottom trawl. Since we did not know the true value of q for

any of these species we were forced to assume q = 1. We strongly doubted the validity of

this assumption. However, following the methods of previous surveys (Gunderson and

Sample 1980. Wakabyashi et al. 1985). we assumed that these observed CPUEs

underrepresented true fish density, which is expressed as q less than 1.0. From this weaker

assumption, it follows that the vessel/net combination with the greatest relative fishing

power comes closest to representing that true density. To estimate a mean CPUE with the

least negative bias, we applied an FPC to adjust all catches made by less efficient vessel/net

combinations upwards to an estimate of what the catches would have been had the most

efficient vessel made them. Henceforth, we shah refer to this strategy as “adjusting to the

most efficient trawl.”

An FPC was estimated and catches were adjusted for those species that were

abundant and commonly encountered throughout the survey area. No adjustment was

made for species which had mostly zero catches. The FPC was estimated using the ratio of

arithmetic means. No variance of the FPC estimate was computed or incorporated into the

variance of the estimate of mean CPUE.

Because we wanted to correct for differences between nylon and polyethylene

Noreastern trawls, between U.S. vessels, and between the U.S. vessel/net combination and

the Japanese vessel/net combination, a series of comparisons and adjustments were done.

Every application of an FPC followed the most efficient trawl strategy.
57

First, differences between nylon and polyethylene Noreasterns were resolved. Table

24 gives numbers of tows collected with the two nets in each stratum sampled by the

Morning Star. FPCs were computed for those species that were reasonably well represented

in these alternating tow data (Table 25). FPCs were computed after pooling over strata

within each net type. All of the above computations were predicated on a mean trawl path

of 18.3 m for a Noreastern trawl.

No data exists to calculate or adjust relative fishing power differences between the

Ocean Spray and the Morning Star because there was little spatiotemporal overlap in their 

operations. Indeed, we had assumed, implicitly, that there was no difference between these

two vessels when we originally assigned stations.

Differences between the two U.S. vessels and the Japanese vessel were investigated

and resolved after correcting for nylon/polyethylene differences within the U.S. data. The

"paired” subsets of tows by the Morning Star and the Daikichi Maru No. 37 were mapped

through the FPC estimator, again with no variances being estimated (Table 26).

1984 - Second Analysis: Adjusting to the Standard Trawl

The second major analysis of the 1984 triennial data arose from the need to make

comparisons with the results of the 1987 survey. Our goals in this analysis were different

from those guiding the first:

1. We wanted to assess change in abundance between the two surveys. Our abundance

estimators had to be consistent.

2. We wanted to incorporate a refined understanding of the role of FPCs in the overall

estimation of our index of abundance.
58

Table 24.--Numbers of tows executed by the FV Morning Star using nylon and polyethylene

Noreastern trawls, by stratum.
59

Table 25.--original estimates of fishing power corrections, estimated as ratios of simple

means of CPUEs observed in tows made by the FV Morning Star alternating use

of nylon and polyethylene trawls. To adjust nylon catches to those that would

have been made by a polyethylene net, multiply them by the quantity in the

column called "Nylon to Polyethylene.” To adjust polyethylene catches to those

that would have been made by a nylon net, multiply them by the quantity in the

column called ‘Polyethylene to Nylon.’
Table 26.--First-round estimates of FPCs for the 1964 triennial bottom trawl survey of the

Gulf of Alaska. Estimates are to be used to correct for fishing power differences

between the Daikichi Maru No. 37 and the U.S. standard vessel. These estimates

were derived from CPUE data that had already been corrected for differences

between nylon and polyethylene Noreastern trawls. Data were ‘pairs. of tows

taken by the Morning Star and the Daikichi Maru No. 37.
3. We wanted to incorporate improvements in strata definitions and improved estimates of

net width that had resulted from ongoing net mensuration studies.

In the second round of analysis, we assumed that the fishing power differences

between nylon and polyethylene nets were negligible: that is, qnylon = qpoly. It seemed that

the difference in their designs was slight enough that to correct for small fishing power

differences would provide little gain in accuracy while degrading many valid observations to

estimates. We based this assumption of negligibility not on statistical tests but on the

fundamental similarity between the two nets and the subjective assessment that their

differences were slight.

The validity of this assumption was examined using the alternating tow data

collected aboard the Morning Star. We computed mean CPUEs for each net type within

each stratum and plotted nylon against polyethylene. We did this for six representative

species: arrowtooth flounder, Pacific halibut, rock sole, Pacific cod, walleye pollock. and

Pacific ocean perch. Medians, means, and coefficients of variation estimated from CPUEs in

the Morning Star alternating-tow data are listed for these species in Appendix B by net type

and stratum. These particular species were chosen to provide a variety of frequency

distributions and because all six are important commercially or ecologically. Arrowtooth

flounder was examined because it may have the largest biomass of all species vulnerable to

our sampling gear. has large numbers in a wide range of sizes, is found in a wider depth

range than most groundfish species, and favors bottom types where the Noreastern trawl

performs reasonably well Pacific halibut was examined because it is a very large flounder, a

powerful swimmer, and capable of reacting strongly to the gear: it is also found in less

hostile bottom conditions. Rock sole was examined because it is a small flounder and. in the

Gulf of Alaska, favors slightly harder bottoms.. It was also caught regularly even though its
62 

abundance didn’t seem to be as great. Its CPUE frequency distribution didn’t seem to be as

skewed to the right as other species. Pacific cod was chosen because it is a roundfish that is

vulnerable to our gear, though it has a somewhat contagious distribution due to a degree of

schooling behavior. Both walleye pollock and Pacific ocean perch were examined specifically

because of their highly contagious distributions and the extreme skewness to the right

associated with such patchiness.

Results of this plotting exercise are given in Figures 6 - 11. Figure 6 shows a scatter

plot of mean CPUE for Pacific halibut. Figure 7 shows scatter plots of mean CPUE for rock

sole. Figure 7-A is a plot of means based on the full complement of data. Figure 7-B is a

plot of means that were calculated after the four highest CPUEs had been removed. The

latter plot indicates the influence of extreme observations. Figures 8 - 11 are scatter plots of

mean CPUE for arrowtooth flounder, Pacific cod, walleye pollock. and Pacific ocean perch.

For each of these species, three plots were required to show the influence of large catches.

The first is a plot of means estimated from the full complement of data. The second is also

from the full complement of data but the axes of the plot have been restricted. We did this

to take a closer look at the cloud of points that was crowded into the lower left comer by

including the largest mean CPUEs. The third plot is of mean CPUEs that were calculated

after the four largest observed CPUEs had been culled.

Each plot has a line running through it with an intercept of zero and a slope of 1.0. It

represents the relationship between the two nets were they perfectly devoid of fishing power

differences and observed without random error. If there was no true fishing power difference

between the net types, then the pairs of estimates should scatter evenly around this line.

Had there been a discernable fishing power difference, then the majority of the plotted points

would fall on one side of the line or the other, depending on which net was more efficient.
63

There was no clear case of this. Rather, it appeared that for all of these species the pairs of

means were quite evenly scattered about the “equal efficiency line.’ This appeared to be the

case even for those species that had a large FPC for nylon and polyethylene in the first

analysis, such as arrowtooth flounder, Pacific cod. walleye pollock, and Pacific ocean perch

(Table 25). In each of these cases, rare large CPUEs had levered the ratio of means in the

first analysis well away from 1.0. We deduce this because these very large mean CPUEs had

dramatic shifts of position (reductions of magnitude) when the four highest individual CPUE

observations were eliminated from the estimation. This last exercise, eliminating the

largest tows, was done only to illustrate the sensitivity of means and ratios of means to rare,

large observations. Generally, it resulted in a marked contraction of the point cloud toward

the equal efficiency line and an even more symmetric distribution around it.

While visual inspection of summary statistics hardly constitutes a definitive test for

an effect, these plots of means convinced us that the relative fishing power differences that

were apparent in the first analysis were likely the result of a small number of large CPUEs.

The high leverage of rare, huge observations would very easily be heightened in an estimator

as unstable as a ratio of arithmetic means. In turn, rare, large tows seem much ‘more likely

the result of chance and not actually a function of fishing power differences. We found that,

though we could not show there to be no fishing power differences between nylon and

polyethylene nets, the differences appeared small enough to be negligible, validating our

assumption.
64

Figure 6.--Mean CPUEs for both nylon and polyethelene Noreastern trawls for Pacific halibut

in a variety of sampling strata.
Figure 7.--Mean CPUEs for both nylon and polyethelene Noreastern trawls for rock sole in a variety of sampling strata. Plot

A shows means estimated from all observed catches. Plot B shows means estimated after the four largest catches

had been removed from the data.
Figure 8--Mean CPUEs for both nylon and polyethelene Noreastern trawls for arrowtooth flounder in a variety of sampling

strata. Plot A shows means estimated from all observed catches: axes are unrestricted. Plot B shows means

estimated from all observed catches: restricted axes allow examination of details of the point cloud but three sets

of estimates were out-of-bounds. Plot C shows means estimated after the four largest catches had been removed

from the data: axes are unrestricted.
Figure 9.--Mean CPUEs for both nylon and polyethelene Noreastern trawls for Pacific cod in a variety of sampling strata.

Plot A shows means estimated from all observed catches; axes are unrestricted. Plot B shows means estimated

from all observed catches; restricted axes allow examination of details of the point cloud but three sets of

estimates were out-of-bounds. Plot C shows means estimated after the four largest catches had been removed

from the data: axes are unrestricted.

Figure 1l.--Mean CPUEs for both nylon and polyethelene Noreastern trawls for Pacific ocean perch in a variety of sampling

strata. Plot A shows means estimated from all observed catches: axes are unrestricted.. Plot B shows means

estimated from all observed catches: restricted axes allow examination of details of the point cloud but three sets

of estimates were out-of-bounds. Plot C shows means estimated after the four largest catches had been removed

from the data; axes are unrestricted.
70

Another argument against correcting catches by the nylon Noreastern to catches by

the polyethylene Noreastern rises from the number of corrections that would be necessary.

Note that because only the Morning Star used a polyethylene net and then only every other

tow, just 126 of the U.S. observations were made with that net. Thus, because nylon

catches were adjusted to polyethylene catches, 70% of the U.S. CPUEs were degraded from

observations to estimates.

We could not directly estimate an FPC to correct for differences between the Morning

Star and the Ocean Spray. Even if we had the data. we would have chosen not to estimate

this quantity. Our reasoning was that nominally identical procedures, even if not strictly

adhering to ideals, should result in vessel effects that were negligible: Again, by “negligible,”

we mean that the variation introduced by correcting the bias in the data would produce a

more severe en-or than the bias itself. We consider these U.S. vessels to represent the

“standard” vessel/net system.

We assumed that the fishing power difference between the Daikichi Maru No. 37 and

the standard vessel was not negligible and chose to correct for it with an estimated FPC. We

did so because it appeared that the dissimilarity between the Daikichi Maru No. 37 and the

U.S. standard was great. Again, this was a subjective judgment made not by’ testing the

CPUE data but based on our perceptions of how the two vessel types operated and how trawl

design varied. Tables 7- 10 show that there were only minor differences in distances fished

and trawling speeds, but the radical difference in the size and configuration of the Japanese

net and doors did result in much different areas swept. While area swept is standardized in

the CPUE, differences in herding effects with longer dandylines and bigger doors, differences

in the way fish interact with different footropes, and differences in vertical opening of the net

are not. Furthermore, the Japanese net was capable of fishing on much rougher grounds
71

than the Noreastern trawl. Any consequent habitat effects would result in an apparent

fishing power difference. Also, such intangibles as differences in fishing style and experience

between captains and crews may have been great enough to make a difference to fishing

efficiency.

FPCs were calculated with the same estimator as in the first analysis, a ratio of

arithmetic means. We abandoned the most efficient trawl strategy. To estimate abundance

indices that were comparable to those of the 1987 survey, we adjusted Japanese catches to

those of the US. standard regardless of relative fishing power. For most species this meant

a reduction in the magnitude of Japanese catches, Table 27 gives the FPC estimates.

Other important differences between the first and second analysis were the splitting

of a sampling stratum and an improvement in the mean net width estimate used to compute

the area swept. Stratum 30, near-shore Kodiak and Afognak Islands, became strata 35 and

30, with 35 being nearshore on the northwest side, 30 being the remainder of the old

stratum, (Fig. 5 and Table 1). This division enabled us to discern concentrations of fish,

particularly walleye pollock, associated with Shelikof Strait from concentrations associated

with the Pacific side of Kodiak and Afognak Islands. All computations of CPUE in the

second analysis were predicated on a net width of 16.43 m. as described earlier.

1984 - Third Analysis: The Kappenman Fishing Power Correction Estimator

The goals, assumptions, and strategy of the third analysis were the same as those of

the second. The difference is in the FPC estimator, as well as the specific data to which it

was applied. Using the Kappenman estimator, FPCs were estimated directly from the whole

of the survey data instead of from the subsets of “paired tows.” The Kappenman estimator
72

Table 27.--Second round estimates of FPCs for 1984. Data were “pairs” of tows taken by the

Morning Star and the Dalkichi Maru No. 37. It was assumed that there was no

fishing power difference between nylon and polyethylene Noreastern trawls and no

fishing power difference between the two U.S. vessels. Catches made by the

Daikichi Maru No. 37 were adjusted to those made by the U.S. vessels.
73

uses only nonzero tows. The ‘paired tow’ subsets simply did not provide an adequate

sample size under this constraint.

To prevent strata effects being confounded with vessel differences, a resampling

scheme was employed to ensure a balance between the representation of the Daikichi Maru

No. 37 data and data collected by the US. vessels. As the data in Table 3 shows, the

Japanese vessel generally produced more observations than its U.S. counterpart in any

stratum that it sampled. U.S. and Japanese tows were sufficiently interspersed to balance

area effects within most strata. In each stratum all of the nonzero tows made by the least

represented boat went into the data set that would be r-napped through the Kappenman

estimator. The same number of tows was randomly selected, with replacement, from the

nonzero tows of the better represented vessel. The data were filtered in this way for every

stratum. Once balanced subsets were assembled for each vessel type, the FPC was

estimated using the Kappenman estimator. This whole procedure was done 50 times,

producing as many estimates of FPC. The mean of these estimates was taken as the final

estimate of FPC. (Note that the estimator itself ignored stratitfication.) Table 28 gives FPC

values estimated with the Kappenman estimator. Those species for which estimates were

made were represented by 60 to 100 nonzero tows for each vessel or vessel type. Fewer

nonzero observations leads to instability in the Kappenman estimator. FPCs were not

estimated for yellowfin sole, starry flounder (Platichthys stellatus), giant grenadiers,

shortspine thomyhead, dusky rockfish (Sebustes ciliatus), harlequin rockfish (Sebastes

uartegatus), or shortraker rockfish due to inadequate numbers of nonzero tows.
74

Table 28.--Third round estimates of FPCs for 1984. The Kappenman estimator was applied

to data from the entire survey. It was assumed that there was no fishing power

difference between nylon and polyethylene Noreastern trawls and no fishing power

difference between the two U.S. vessels. Catches made by the Daikichi Maru No.

37 were adjusted to those made by the U.S. vessels.
75

1987-First Analysis 

The goals, assumptions, and guiding strategies of the first analysis of the 1987 survey

were the same as those in the first analysis of the 1984 survey. The focus was, again, on

estimating actual abundance: hence, the most efficient trawl strategy was followed. Since

the 1987 survey was not an exact replica of its predecessor, implementation of those

strategies took a slightly different approach. The primary difference was that the

comparison study provided independent data for estimating FPCs. Also, only the

polyethylene version of the Noreastern trawl was employed by the U.S. boats.

From the comparison tows, mean CPUEs were computed for each vessel, compared,

and combined in an FPC estimator. Arithmetic means were calculated for each’ vessel/area

combination. These means were then summed over the strata of the study for each vessel.

Table 29 compares these summed means for species that were commonly encountered. The

comparison was done by taking the highest summed mean and using it as the denominator

in a ratio of summed means. These ratios were aids in deciding whether or not to apply an

FPC. Each decision was subjective; for most species, it was based on the magnitude of these

ratios.

Appendix C shows the strategies used for estimating and applying FPCs. Though we started

out by assuming that there was a non-negligible fishing power difference between the two

U.S. vessels, we abandoned that assumption for every species except Pacific halibut and

shortraker rockfish. Table 30 gives the estimates for the various species. No variance was

estimated for the estimates of FPC, so no notion of its variability could be incorporated in the

estimator of the stratified mean CPUE.
76

Table 29.--Ratios of summed mean CPUEs to compare for vessel effects. Means were

computed within each vessel/area cell, summed across areas but within vessels, and then

placed in ratios where the denominator was the largest of the three summed means.
77

1987 - Second Analysis: Adjusting to the Standard Trawl

The goals, concerns, and strategies embodied in the second analysis of the 1984

survey also informed this analysis. As before, we assumed that the difference between U.S.

vessels was negligible. Because we wanted to assess change in abundance rather than

abundance itself, we used FPCs to adjust to the standard vessel/net combination regardless

of whether or not the standard had higher or lower fishing efficiency. Data from the

comparison study were analyzed in a way that accounted for area/depth effects: this analysis

will be discussed later.

In this analysis the strategies for estimating and applying FPCs were the same as

those in the first analysis except for Pacific halibut and shortraker rockfish. For these two

species the comparison study data were pooled for the Lets Go and the Nore-Dick, a new

mean CPUE estimated for the U.S. standard vessel, and a new FPC ratio calculated. These

‘new quantities are included in the final column of Table 30. For all species, catches made

by the Taisei Maru No. 35 were adjusted to the fishing efficiency of the U.S. standard. This

changed the direction of adjustment from that of the first analysis for only one species,

shortraker rockfish.

this round were the same as those of the second. The difference is in the FPC estimator and

the data mapped through it.

The ratio of arithmetic means-was abandoned as an estimator of FPC in favor of the

Kappenman estimator. FPCs were estimated directly from survey data rather. than from the

side-by-side tow study. Again, this was because there were too few nonzero tows in that
78

study to permit use of the Kappenman estimator. Balanced representation of Taisei Maru

No. 35 data and U.S. standard data was ensured with the same resampling scheme described

previously. This was necessary because of larger sample sizes for the Japanese vessel in

those strata that it cosampled with a U.S. vessel (Table 12).

FPC estimates generated with the Kappenman estimator are given in Table 31. FPCs

were not estimated for yellowfin sole, starry flounder. grenadiers, shortspine thomyhead,

harlequin rockfish. or shortraker rockfish due to inadequate numbers of nonzero tows.’ FPC

were computed for Dover sole, rougheye rockfish, and dusky rockfish but not applied

because low numbers of nonzero tows gave these estimates questionable certainty. The rest

of the estimates were based on 60 to 100 nonzero observations for each vessel type or vessel,

(the U.S. standard or the Taisei Maru No. 35).

Second Analysis of the Fishing Power Experiment

In this analysis we used ANOVAs to test the hypotheses that there were no vessel

effects, no effects due to depth stratification within the comparison study, and no interaction

effects between vessel and depth. We limited the analysis to those species that were well

represented by nonzero tows: we considered a preponderance of zero catches to imply that a

particular species was simply not present in the study area and thus it was not appropriate

for us to draw inference regarding it. Some species were well represented in only one

stratum of the comparison study area and only a one-way ANOVA could be performed.

Species that were well represented in two or more depths were pollock, Pacific cod,

rock sole, arrowtooth flounder, Dover sole, Pacific halibut,. rex sole, flathead sole, rougheye

rockfish, Pacific ocean perch, and dusky rockfish. The two-way ANOVAs followed an
79

additive, mixed model (Joseph and Calkins 1969, Hoff 1989). The CPUE data were

transformed with the natural log, resulting in the following model:

The log transform was deemed appropriate because it seemed most likely that differences in

fishing power would show as differences in catch rates since CPUE is. in fact, the rate of

catch per unit of effort. Additive effects in the transformed data would imply multiplicative

effects in the untransformed data, which is precisely what a rate change is. The log

transform also had the effect of producing more equal within-cell variances,

This model was used three different ways. The first was with Japanese data

excluded, causing there to be only two levels of the vessel effect. Rejecting the hypothesis

that there is no vessel effect in this case would suggest that it was invalid to assume the

fishing power difference between the two US. vessels was negligible. The second application

was with all three of the vessels for three levels of the vessel effect. The third application

was with data from the U.S. vessels pooled, resulting, again, in only two levels of the vessel

effect.
80

Table 30.--First and second-round estimates of FPCs for the 1987 triennial bottom trawl

survey of the Gulf of Alaska. Data were collected in the fishing power experiment.

First-round estimates were derived according to strategies and with estimators

described in Appendix C.
81

Table 31.--Third-round estimates of FPCs for 1987. The Kappenman estimator was applied

to data from the entire survey. It was assumed that there was no fishing power

difference between the two U.S. vessels. Catches made by the Taisei Maru No. 35

were adjusted to those made by the U.S. vessels.
82

We performed one-way ANOVAs for species represented in only one depth with the

following model:

These species were northern rockfish, shortraker rockfish, shortspine thomyhead,

sablefish, starry flounder, yellowfin sole, and butter sole (Isopsetta isolepis). This model was

used three different ways, just as in the two-way analyses: with two levels of vessel effect,

U.S. data only: with each of the three vessels representing a different level of the vessel

effect: and pooling the U.S. data for only two levels of the vessel effect.

There were a number of species which had too few nonzero tows to support these

analyses. They were English sole, Alaska plaice (Pleuronectes quadritubercufatus),

harlequin rockfish, sharpchin rockfrsh (Sebastes zacentrus), popeye grenadier

(Coryphaenoides cinereus), giant grenadier, and starry flounder.

In the following discussion of the ANOVA results we give a number of tables in which

we have listed a p-value for the significance of the F-test appropriate for each effect. A pvalue is itself a statistic that estimates the probability of seeing a given value of the Fstatistic. Throughout, the discussion we define statistical significance to be the conventional

alpha = 0.05, which corresponds to a p-value of 0.05.

This analysis was performed to assess the validity of assumptions of negligibility

made in the second and third rounds of analysis. Though we estimated various F-statistics
83

and associated p-values, we did not base our assumptions of negligibility on the results of

the hypothesis tests. Such assumptions are about the cost of adjusting for effects that are

either assumed or known to exist while the hypotheses tested here are merely concerned

with the presence or absence of effects, They shed light only on the first Part of the

negligibility assumptions-- that an effect exists or not. Of course, if a test fails to reject the

hypothesis that there is no difference between vessels, it supports the assumption of a

negligible fishing power difference: either there is no difference to correct or there was a

difference but the test failed to detect it because its power was low, the sample size was too

low, or the variance was large enough to obscure it. If a true difference was not detected for

reasons of sample size or variance, then it was also quite likely that the correction factor

would be burdened with a high variance and thus too costly to use.

By failing to reject the hypothesis that there is no difference in log-transformed

CPUEs due to vessel effect, the results in Table 32 support the assumption that there was a

negligible fishing power difference between the two U.S. vessels for all species except

arrowtooth flounder, flathead sole, and Pacific ocean perch. Those species had statistically

significant differences in mean CPUE due to vessel effects. Hypothesis test results in Table

33 show significance for vessel effects for those three species again, as well as for walleye

pollock. rock sole, and Dover sole. For our purposes, the most important result is Table 34.

the case where U.S. data were pooled into the standard category and that vessel type and

the Taisei Maru No. 35 were the only two levels of vessel effect. It describes statistically

significant vessel effects for walleye pollock, arrowtooth flounder, rock sole, Dover sole,

flathead sole and Pacific ocean perch. These results make it more difficult to defend the

assumption that fishing power differences between the U.S. standard vessel and the Taisei

Maru No. 37 were not negligible for Pacific cod, Pacific halibut, rougheye rockfish. and dusky

rockfish. Nonsignificance of vessel effects for these species is not consistent. with the
84

magnitude of the FPCs estimated for them in Table 22. It is consistent with the questionable

certainty hypothesized for the Kappenman FPC estimates for rougheye and dusky rockfish.

The depth effect was statistically significant for all species in each of the ways the ANOVA

model was applied for every species except Pacific halibut and dusky rockfish. We cannot

explain the lack of significance for the depth effect in these two species other than low power

of the test due to small sample size.

Column one of Table 35 shows statistical significance for a vessel effect when the US.

vessels were considered to be different and Japanese data were excluded from the analysis.

Only starry flounder showed significance at the conventional level, supporting the

assumption of negligible fishing power differences between U.S. vessels for the other species.

Column two of that table shows significance for the case when all three vessels were

considered different. The vessel effect was statistically significant for shortspine thomyhead

as well as for starry flounder. Column three shows results for the case where U.S. data were

pooled into the standard category and that vessel type and the Taisei Maru No. 35 were the

only two levels of vessel effect. Only shortspine thornyhead showed a statistically significant

vessel effect.

The interaction term between vessel and depth effects in Tables 33 and 34 was

significant only for arrowtooth flounder. This implies that relative fishing power differences

between the U.S. standard vessel and the Taisei Maru No. 35 vary as some function of

depth. Herding effects may be a function of fish size (Engas and Godo 1989). Larger fish

may be more easily caught because their superior SWimming ability allows more effective

flight from the bridles and doors which places the fish directly in the path of the net.

Arrowtooth flounder have been seen to increase in length with depth (Wilderbuer et al.

1985). Thus, the larger Japanese trawl may have gained in relative efficiency at deeper
85

Table 32.--Results of two-way ANOVAs on data from the 1987 fishing power experiment.

Data from the Taisei Maru No. 35 were excluded from this analysis. The relative

fishing power difference between the Nore-Dick and the Lets Go was assumed to

be non-negligible. p-values are themselves statistics that estimate the probability

of seeing a value of the F-statistic less than or equal to that derived from these

data: it can be thought of as an estimate of the level of significance of the observed

test statistic.
8 6

Table 33.--Results of two-way ANOVAs on data from the 1987 fishing power experiment.

Data from the Taisei Maru No. 35 were included from this analysis. The relative

fishing power difference between the Nore-Dick and the Lets Go was assumed to

be non-negligible. p-values are themselves statistics that estimate the probability

of seeing a value of the F-statistic less than or equal to that derived from these

data; it can be thought of as an estimate of the level of significance of the observed

test statistic.
87

Table 34.--Results of two-way ANOVAs on data from the 1987 fishing power experiment.

Data from the Taisei Maru No. 35 were excluded from this analysis. The relative

fishing power difference between the Nore-Dick and the Lets Go was assumed to

be negligible and their data were pooled. p-values are themselves statistics that

estimate the probability of seeing a value of the F-statistic less than or equal to

that derived from these data: it can be thought of as an estimate of the level of’

significance of the observed test statistic.
88

Table 35.--Results of one-way ANOVAs on data from the 1987 fishing power experiment.

Column one shows results of tests for a vessel effect between the two U.S. boats.

Column two shows results of tests for vessel effects when all three vessels are

considered different. Column three shows results of tests for a vessel effect when

data for all three vessels are used but the U.S. data were pooled to represent a

standard U.S. vessel. p-values are themselves statistics that estimate the

probability of seeing a value of the F-statistic less than or equal to that derived

from these data; it can be thought of as an estimate of the level of significance of

the observed test statistic.
89

depths: This hypothesis is not supported by the FPC estimates of Table 36 which show that

the Taisei Maru No. 35 had a reduction of efficiency with increased depth. However, we

cannot accept or reject this hypothesis because towing parameters such as speed and scope

ratio also changed with depth and thus were confounded with any possible depth effect

(Tables 14 - 17).

These analyses of variance cannot be taken as definitive tests. They are possibly low

in power due to low sample sizes. More importantly, all three applications of the model were

performed on the same data and thus were not independent. We found this acceptable since

we were really only interested in the last application, where the U.S. data were pooled and

there were only two levels of the vessel effect. This is because we had made the assumption

of negligibility based on the cost of correcting a difference rather than on the existence of a

difference.

Table 36 gives alternate estimates of FPCs derived from the data collected in the

fishing power experiment. Rather than pooling all data, we only used data from depth strata

that were well represented by nonzero tows. We also only estimated FPCs for species that

had statistically significant vessel effects. With the exception of arrowtooth flounder, we did

pool data from different depths for those species that were represented in more than one

depth. This was justified by the nonsignificance of the vessel X area interaction effect. In

 estimating these FPCs, we assumed that the fishing power difference between the two U.S.

vessels was negligible, regardless of the statistical significance of the above F-test. Thus

these estimates are ratios of the mean CPUE for pooled U.S. data and the mean CPUE of the

Taisei Maru No. 35. Because the interaction term was significant for arrowtooth flounder,

we estimated an FPC for each depth in which this species was encountered. These FPC
90

Table 36.--Fishing Power Corrections (FPC) estimated from 1987 fishing power experiment

data, following the conclusions of ANOVAs on that data. The estimator is the ratio

of arithmetic means. Data for each species have been pooled across those strata in

which it was found, with the exception of arrowtooth flounder. For that species,

separate FPCs have been estimated for each stratum of the comparison study.
91

estimates are consistent in direction with the Kappenman estimates made from survey data,

(Table 31), though they were not similar in magnitude.

DISCUSSION

The 1984 and 1987 triennial bottom trawl surveys of the Gulf of Alaska comprise an

exercise in applying standardized trawl procedures to an extremely large area and refining

those applications. The sheer size of the Gulf constrains a survey that is both

comprehensive and synoptic to be executed by more than one vessel. The success of the

first two surveys of this area was related to in a large part to the cooperative efforts of the

Fisheries Agency of Japan. However, unavoidable deviations from standardized methods

resulted from the participation of the large Japanese trawlers. The majority of the analyses

presented here have dealt with these deviations.

Data Review

Primary Features

The most powerful feature of these two data sets is their comprehensive coverage of

the Gulf of Alaska. Because of this, geographic shifts in distribution of centers of abundance

can be discerned from changes in abundance itself. This is possible because the

stratification scheme seems to identify truly unique regions and does a good job of

minimizing within-stratum variation and maximizing among-strata variation. Adequate

sample sizes in most strata permit real confidence in separating change in abundance from

random variation and sampling error.

Observed CPUEs provide a good index of abundance. This sample unit is well

matched to the technology and methodology available for use in a trawl survey. All of its
92

parameters except the probability of capture are directly measurable or estimable. This has

permitted very welldefined standardized sampling procedures to be developed, which in

turn permitted the collection of data in these two surveys that are very comparable. Also,

because definitions of standards are so clear, documentation of deviations from them are also

clear for these two data sets, allowing some chance of correcting for those deviations.

These data consist of CPUE observations in which it is assumed that effort is a known

constant rather than a random variable, and has no error. At present. it is not clear how to

account for error in effort, nor are we able to easily incorporate error in that variable into the

CPUE. For these two surveys, sources of error in effort could have entered in several ways.

For every tow, we depended on the judgment of the skippers for what scope to use. This

may have more effect on fishing efficiency than actual effort, but we don’t really know right

now. For many tows, we also depended on the skippers to judge when the net was in fishing

configuration so that we could start timing the tow. This was often not measurable: that

judgment could not be checked. Such tows may have had shorter or longer fishing times

than we thought. Estimating path width was another source of error in the measure of

effort. For many tows, path width was not observed and had to be estimated. Those

estimates were made with error (Rose and Walters 1990). Error also could have entered the

measure of effort due to inaccuracies in start and end positions that are built into the Loran

technology. These errors are likely to be of greater magnitude for tows that took place near

a Loran sending station.

For many species the sampling gear was not ideal. In particular, the standard trawl

was not suitable for harder grounds. Consequently. we expected the relative fishing power

of the Japanese nets to be higher for species that live on rough bottoms since both those
93

nets used tire gear rather than bobbin roller gear. barge catches of rockfish in tows on

rough grounds were often excluded from the estimation of mean CPUE because the net was

badly tom during the tow and the observation defined as invalid. Unfortunately, catch data

were rarely recorded for such tows: the effect of their exclusion is not estimable though

probably of large influence due to their CPUE distributions being so often heavily skewed to

the right.

Between allocation constraints, the difficulty of sampling continental slope strata with

a trawl, and the high variance of CPUE data for most rockfish species. we were unable to

adequately sample strata preferred by rockfish. Their high variance distributions demand

large allocations of sampling effort. To sample the slope strata appropriately would have

weakened the survey with respect to roundfish and flatfish to an unacceptable level. After

the 1984 survey, we reduced sampling levels in the slope strata, despite the commercial

value of rockfish stocks that inhabit those strata. Table 3 shows that sampling density in

slope strata was equal or greater than that of other strata in 1984. Table 11 shows that

station density in slope strata, though reduced in 1987, was still comparable to many other

strata. Given the high variance of CPUEs for these species, we will probably never be able to

sample their strata adequately within the context of a multispecies survey. Data from these

two surveys do not allow definitive conclusions to be drawn with regard to changes in

abundance for rockfrsh species.

The probability of capture, as we have defined it, was not known for any of the

species. Fish behavior with respect to the net was not known. How each species avoided

the gear or was herded by it no doubt influenced the probability of capture. When working

with length and age data from these surveys it would be wise to keep in mind that

probability of capture may be a function of size (Engas and Coda 1989).
94

Area effects, time effects, and vessel effects are confounded in both surveys. Tables 3

and 11 describe a great many strata for which there was little overlap among the vessels.

This is particularly true for the deeper strata. Consequently, FPCs estimated from the whole

of the data (Tables 28 and 31) may be influenced by an area effect. Because in both years

the Japanese vessels sampled from east to west in the second half of their voyages, double

counting may have occurred for any species that might migrate during the survey period.

Likewise. growth studies will have imposed on them either a de facto interaction term

between time and region or large numbers of empty cells.

FPCs were estimated from the same data to which they were applied. This may

artificially remove sources of variation that more properly should remain in the data since

time and area differences may be confounded with fishing power differences. The

significance of the area effect seen in the two-way ANOVAs affirms that FPCs estimated from

the survey data, where area and vessel effects are confounded, should be treated with

caution.

Fishing Power Corrections and Comparability of Survey Results

Tables 7 - 10 show that trawling speed and distance fished were very similar for all

three boats in 1984. This suggests fairly close adherence to standardized methodology.

Areas swept are different, of course, since the tow path of the Daikichi Maru No. 37 was 1.5

to 2 times greater than the tow path of the standard net. If the effective fishing area is really

between the doors rather than between the wing tips, then area swept differences could be

even greater. We do not know this; if it is true, then we might perceive it in the data as a

fishing power difference. Also, herding and avoidance behavior was likely to have been quite

different for the net of the Daikichi Maru No. 37 due to its size, the door and dandyline

configuration. and the presence of tire gear.
95

Tables 25, 26. and 27 describe estimates of FPCs that seem unrealistically large. A

nylon/polyethylene fishing power correction of 1666.67 for Atka mackerel (Pleurogrommus

monopteryglus) is the most extreme case. A number of estimates in these tables were

greater than 2.3. Estimates produced by the Kappenman estimator were less extreme for all

species. Table 27 has as its most extreme values 0.5 and 1.38. The robustness of the

Kappenman estimator and the use of the whole of the survey data caused this moderation of

the estimates.

The most stunning difference between FPC estimates produced by the ratio of means

and estimates produced the Kappenman estimator was that the estimates changed direction.

Under the ratio of means the Daikichi Mar-u No. 37 and its net appeared to have a higher

fishing power than the U.S. standard for all the common species except rock sole, butter

sole, walleye pollock, dusky rockfish, and harlequin rockfish. The relatively higher efficiency

of this vessel for almost all the flatfishes was surprising since it used tire gear on the

footrope.

Under the Kappenman estimator, the Daikichi Maru No. 37 appeared to have

relatively lower fishing power, or equal, for every species with data suitable to allow

estimation. The lower fishing power of the Daikichi Maru No. 37 for rockfish species also

surprised us since we thought it would have higher efficiency on bottoms favored by

rockfish. However, rockfish have the greatest skewness in CPUE frequency distributions of

all the species encountered by these surveys. This means the CPUE observations were

dominated by very low numbers with a few rare but huge catches. Differences in these

extremes produce the differences in arithmetic means of CPUE that might indicate that the

distributions are different. However, the symmetric distributions of the Kappenman
9 6

estimation process reduced the effect of rare extreme observations to such a degree that the

estimator itself is a function of the scale parameters of two nearly identical distributions.

This is as it should be. Because the tows were made blindly, that is, without

responding to fish sign on the sonar, the probability of getting one of the rare, extreme

catches was nearly equal among boats. Even if the Japanese vessel was a little more likely

to make such a catch because it could fish on rougher bottom, it is still a relatively rare

event. Thus the large tows should not be a driving force in the FPC estimation. We did not

want to base our FPC estimates on rare events and the high variance that goes with them.

Tables 20 - 23 show that trawling speed was quite different for all three vessels in

1987. The Lets Go came closest to standardized tow procedures, though it tended to trawl

about 0.4 km/hr too fast. The Nore-Dick deviated a little farther from standard speed,

trawling for the most part about 1.2 km/h.r too slow. The Taisei Maru No. 35 was at greatest

variance from standards, towing about 2.7 km/hr too fast for the vast majority of its tows.

(The difference in trawling speed between the Nore-Dick and the Taisei Maru No. 35 was

generally about 3.8 km/hr.) Differences in trawling speed translated directly into differences

in distances fished. Markedly different distances fished magnified the difference in area

swept that the much wider net of the Taisei Maru No. 35 imposed.

Though we cannot say just how, fish behavior, particularly gear avoidance, herding,

and interactions with the footrope and headrope, were probably quite different between the

vessel/net system of the Taisei Maru No. 35 and that of the U.S. standard. The effect of the

high trawl speed and the effect of the difference in gear configuration on probability of

capture are totally confounded. The performance parameters and the analyses of variance of
97

the comparison study data strongly support the assumption that the fishing power difference

between the Taisei Maru No. 35 and the U.S. standard was not negligible.

Given the failure of the Nore-Dick and the Taisei Maru No. 35 to follow standard

trawl procedures, the large FPC estimates of the first two rounds of analysis, Table 29, were

not as surprising as those of 1984. Table 11 shows that the Taisei Maru No. 35 trawled

even faster in the comparison study than it did throughout the survey. FPCs estimated from

the comparison study data were probably slight inflations of whatever effect the high trawl

speed produced in the survey catch data.

When the Kappenman estimator was applied to the survey data, &direction of

difference did not change, though the magnitude of the differences did change. The

exception to this was Pacific halibut: under the Kappenman estimator the Japanese catches

of halibut needed to be adjusted downward to the U.S. standard vessel/net system where

they had needed to be adjusted upwards before. This makes some sense in that the fastmoving Japanese net may have been more effective in catching a strong swimmer such as

the Pacific halibut.

Again, we were surprised that the U.S. standard vessel/net outperformed the Taisei

Maru No. 35 for rockfish species (Table 31). But, as before, we cannot really conclude too

much from the notable FPC estimates for rougheye rockfish and dusky rockfrsh since they

suffered poor representation by nonzero tows. Restricting the Kappenman estimation

process to data that were cosampled by a U.S. vessel and the Taisei Maru No. 35 and

resampling from data collected by the over-represented vessel eliminated many of the slope

strata from contributing data to the FPC estimate. Again, we simply cannot conclude from

these data whether or not fishing power differences for most rockfish species were negligible.
98

Table 12 shows that the Taisei Maru No. 35 was the only vessel to sample strata

deeper than 300 m. This is particularly unfortunate since this vessel also deviated so far

from standards. Changes in indices of abundance for these strata are confounded with

fishing power differences due to the different net and due to the higher trawling speed.

The Kappenman estimator was not applied to data from the comparison study

because the sample sizes were too small. Otherwise this would have been the preferred data

set because it had full cells and vessel effects were not confounded with area effects.

Additionally, it was an independent study.

Implicit in a comparison of mean CPUEs from the 1984 and 1987 surveys is the

assumption that a standard vessel/net system is common to both. Though standardized

gear was common to both and though standardized procedures were intended to be common

to both, no vessel participated in both surveys. Comparing vessel performance parameters

in the 1984 survey to those of the 1987 survey (Tables 7 - 10 and Tables 20 - 23) indicates

that this assumption is tenuous. To compare indices of abundance between these surveys

almost certainly confounds change in abundance with change in fishing power. However, if

assumptions of negligibility made within each one of these years are valid, then it may also

be valid to assume the fishing power difference between the U.S. standard in 1984 and the

U.S. standard in 1987 is negligible.

The two Japanese vessels could have been considered candidates for the standard but

they were not adopted as such because 1987 was the last year of involvement in cooperative

bottom trawl surveys with the AFSC in the Gulf of Alaska by the Fisheries Agency of Japan.

Another argument against using these vessels to establish the standard system is that their

nets were not alike. Also the tow performance of the Daikichi Maru No. 37 was more
99

different from the performance of the Taisei Maru No. 35 than were the tow performances of

the U.S. standard vessel between the two surveys. Looking at Tables 27 and 31, it is clear

that much larger FPC estimates were generated for the Taisei Maru No. 35.

Though differences between the two Japanese nets are confounded with differences

between trawling procedures, we are led to speculate that something about the faster

trawling speed of the Taisei Maru No. 35 reduced its effectiveness in catching most of the

species commonly encountered by these surveys. The Taisei Maru No. 35 specialized in

rockfish trawling when fishing commercially, often targeting on Pacific ocean perch,

rougheye rockfish. or shortraker, rockfish. The accustomed strategy of the Fishing Master

aboard the vessel was to hunt for schools or promising habitat with the sonar while flying

the trawl off-bottom. When a school or site was located, the trawl was quickly dropped

down onto it. A fast towing speed was a critical element in this strategy. During the survey

the Fishing Master was highly resistant to slowing his vessel down to the standard speed of

5.56 km/hr (3 knots) The chief scientist aboard the vessel was in constant conflict with the

Fishing Master over this, to no avail as Table 20 shows. It may be that a fast towing speed

increases the catch when targeting on schools of rockfish that have been located by echo

sounder because it allows quicker response by the vessel to fish sign. However. when fishing

blindly for multiple species, as in these surveys, it may be that such fast trawling speed

causes the net to simply pass over many individuals. It may be that a smaller fraction of the

encountered fish are strong enough swimmers to respond to the doors, dandylines,

mudcloud, or wings and be herded into the net.

The estimates of FPCs in Tables 28 and 31 were derived from survey data and then

applied to that same data. Thus estimates of FPC and mean CPUE were not independent,

which in turn invalidates commonly accepted methods of estimating the variance of a
100

function of random variables. The 1987 comparison study was an attempt to avoid this.

Though not large enough to provide adequate certainty in its estimates of FPC, it is

encouraging that the directions of differences indicated by the comparison study agree, for

the most part, with directions of difference indicated by the Kappenman estimates made

from the survey data.

Despite such imperfections, comparable estimates of abundance indices can be

rendered from these two surveys. The survey design and the underlying methodology of

taking the sample remained consistent. Departures from standards in 1987 on the parts of

the Nore-Dick and the Lets Co are very unlikely to obscure changes in abundance. So long

as the data have been calibrated to the U.S. standard vessel/trawl systems, changes in

abundance will be quantifiable from the data of these two surveys.

Considerations for Future Surveys

Invalid Tows and High Catches

The phenomenon of very high catches in tows that were invalid due to damaged gear

is common for species that live in rough terrain, particularly major rockfish species. This

presents an interesting question: What is the effect on the estimate of the mean CPUE of

including or excluding these tows? Unfortunately, few or no data exist to investigate this

question. Since invalid tows have traditionally not been mapped into the estimate of mean

CPUE. it has been common practice to simply not process their catches for data. They are

invalid tows only in that we do not know what the effort was for them and do not know how

to incorporate them into the estimate of the index of abundance. They are still concrete

observations. They may prove critical to figuring out how to cope with the highly skewed

frequency distributions that are the hallmark of CPUE data. We may learn how to use such

tows as minimum CPUE observations and in that way build them into the estimate of mean
101

CPUE. Perhaps such observations can be weighted according to the degree of damage to the

gear. In any case, in future surveys. catches in ‘invalid’ tows should be worked up

completely .

Survey Design and Execution

The most obvious way to improve triennial trawl surveys of the Gulf of Alaska is to

build a common factor into them, namely, a vessel and captain that participates in survey

after survey. This will anchor the notion of a standard vessel/net system in something a

little more concrete and measurable than the abstractions of definitions and standardized

procedures. The Gulf of Alaska is too large to survey adequately with only one vessel within

the time of one season. A multivessel survey is still the only way to ensure adequate sample

size in each stratum and still sample all strata. This makes having a repeat vessel that

much more important as it would provide the standard by which to assess the adherence of

other vessels to standardized procedures or specifications.

The alternative to a standard vessel is to gain such control over the sampling gear

and the execution of a tow to a degree sufficient to assure that there are no systematic

deviations from standards. There must be rigorous control of the speed of the net over the

bottom. We need precise assessment of the distance fished in a tow and precise

measurement of the width of the net. From this, we can more accurately and precisely

estimate the area swept. We must assess the effect on estimation of mean CPUE of effort

being a random variable rather than a known constant. We may need to control the net in

some way that causes it to have a constant area swept regardless of depth or width of the

vessel. Scope ratios must be strictly defined. No gear other than the-polyethylene

Noreastern should ever be used, nor other than the doors and dandylines described earlier.

If a vessel or a gear is to be used in a survey but does not or cannot conform to standards,
102

then it should not contribute to data for estimating indices of abundance. Rather, it should

be assigned the tasks of collecting age and growth data as well as collecting special

specimens and data for studies such as food habit investigations. Such a vessel or gear

could legitimately operate in a nonrandom fashion since the sample unit would no longer be

the standardized catch. It could target on species and areas for which biological data are in

greatest demand.

Fishing power differences appear to us to be much easier to control in the field than

to correct once they have insinuated themselves into the data. The data must be collected in

a way that is as platform-independent as possible. Fishing power differences will always

exist, assuming them to be negligible will not make systematic error in data disappear.

Future surveys must be designed with this in mind.

If fishing power differences were controlled to the point of elimination, then the most

efficient use of vessel time would be to have different vessels work in entirely different

regions of the Gulf. With no vessel effect there would be no confounding of area effect with

fishing power differences. We did, in fact, act on this strategy when we designed the 1984

survey which resulted in little overlap between the Morning Star and the Ocean Spray.

However, we recommend against following such a strategy again. The definition of

negligibility employed in these analyses was not an assumption of no fishing power

differences. It was an assumption about the costs of correcting for fishing power differences

that were assumed to exist, a very different proposition. Based on vessel performance

(Tables 7 - 10, 14 - 17, and 20 - 23) it would a mistake to assume there were no actual

fishing power differences.
103

This being so, it is important that all strata be sampled at least twice by every vessel

participating in each future triennial survey. Whatever the sampling allocation is for each

stratum, it should be divided evenly among the vessels. Furthermore, all vessels should

work in the same stratum at the same or nearly the same time. While this will not reduce

confounding of time and area effects, it will prevent those effects from being confounded

with the vessel effect.

The Role of Fishing Power Corrections

Until more thorough investigations show otherwise. the appropriately conservative

strategy is to estimate FPCs but not apply them. There are two reasons for this. The most

important is that sufficient data for an FPC estimate, which is independent of the survey

data, is far too costly to collect. If the Kappenman estimator remains the estimator of

choice, an independent study of sufficient sample size is quite likely to be larger than either

the 1984 or the 1987 survey.

It is also more conservative to assume negligibility of fishing power differences and

challenge that assumption than to do the reverse. This strategy comes from the assumption

that the cost of bringing the variance of an FPC estimate into the estimate of mean CPUE is

generally higher than the cost of the bias being corrected. We believe that it is better to be

slightly wrong due to bias and not risk being extremely wrong due to variance of the FPC

estimate.

Though we recommend that they not actually be applied to data, FPC estimates may

be used. along with vessel and gear performance evaluations, to assess the rigorousness with

which vessels followed standardized procedures or conformed to specifications. As data from

sequential surveys accrues, this measure may become important in trying to determine how
104

much weight to give an estimated index of abundance for a particular year when charting

long-term trends.

Even if we develop an estimator to correct bias which is low enough in variance, we

still cannot apply it until there is a vessel that is common to several surveys in a sequence.

Until such a de facto standard vessel has contributed to two or more surveys we will have no

way of knowing which vessel/net systems among those participating in a survey is ‘more

standard’ than the others.

Critical Ancillary Work

If a low variance estimator of the probability of capture becomes available, and if the

data it requires are affordable, all of the FPC considerations become moot. Even the need for

a repeat vessel would disappear. Work to identify such an estimator and to develop the

appropriate technology should be strongly supported.

Regardless of the estimator used to estimate FPCs, it is critical that the variance of

the FPC estimate be incorporated in the estimate of the index of abundance. When an FPC

is applied, the estimated mean CPUE is no longer just a function of the data It becomes a

function of the data and an estimate of a random variable, which itself is a function of the

data. The resultant variance of the estimate of the index of abundance cannot help but be

inflated by this. Currently our variance estimates of mean CPUE do not account for this

inflation and are thus false. An estimator that does account for this increased uncertainty

needs to be developed as soon as possible.

A clear problem is that of sample size. In this paper, we have written of ‘adequate

sample size’ without really defining “adequate.” Given the high variances of CPUE data, we
105

have been operating under the assumption that the levels of sampling in 1984 and 1987

were adequate but barely so, and that larger sample sizes could be justified. Since vessel

time is costly,\ it is important to determine if this assumption is valid. We need to know both

ideal and minimum sample sizes, stratum by stratum, over the whole of the survey. Then in 

the face of vessel-time constraints, we would know whether to reduce sample density or

eliminate regions of the survey area. We suggest, as a starting place, simulation studies to

relate sample size to coefficients of variation, super variance. and the ability to detect

changes in the index of abundance among years. We also need to investigate the role of

sample size in FPC estimation and assuming negligibility of fishing power differences.

An important step forward would be the quantification of the notion of negligibility as

it is defined here. It may be possible to construct cost functions that provide a means to

optimize the costs and benefits of controlling each. We would balance the cost of error due

to bias, the cost of error due to uncertainty, the initial costs of controlling bias, and the costs

of collecting data to estimate bias. From such functions, we could derive decision rules to

more rigorously determine whether to control bias, correct bias, put up with bias, or some

combination of these responses. Such work, of course, must be done in the context of

estimating indices of abundance: estimation of an FPC is of itself not important. Costs and

benefits must be defined in terms of reducing or increasing error in the estimate of mean

CPUE and reducing or increasing the ability to detect change in those indices.

It remains to be seen whether or not it is wise to correct for fishing power differences

at all. In this work, we assumed that it was not wise for all but the extreme cases of the

Japanese vessels. The opposite assumption is invoked in the estimation of abundance

indices from other AFSC surveys (Wilson and Armistead 1991, Bakkala et al. 1992). As

usual, it is a question of cost-s. The initial costs of controlling differences through
106

standardization are quite high. If, however, correcting a bias renders an index of abundance

incomparable to another due to obscuringly large variance, that cost may be well worth it.
107

CITATIONS

Alverson. D. L., and W. T. Pereyra. 1969. Demersal fish explorations in the northeast

Pacific Ocean--An evaluation of exploratory fishing methods and analytical

approaches to stock size and yield forecasts. J. Fish. Res. Board Can. 26:1985-2001.

Bakkala, R. G., W. A. Karp, G. F. Walters, T. Sasaki, M. T. Wilson, T. M. Sample. A. M.

Shimada; D. Adams, and C. E. Armistead. 1992. Distribution, abundance, and

biological characteristics of groundfish in the eastern Bering Sea based on results of

U.S. Japan bottom trawl and hydroacoustic surveys during June - September 1988.

US. Dep. Commer., NOAA Tech. Memo. NMFS FINWC-213, 372 p.

Cochran. W. G. 1977. Sampling Techniques, Third Edition. 428 p. John Wiley and Sons,

New York.

Engas A.. and O. R. Godo. 1989. The effect of different sweep lengths on the length

composition of bottom-sampling trawl catches. J. Cons. int. Explor. Mer 45:263-268.

Feldman, G. C.. and C. S. Rose. 1981. Trawl survey of groundfish resources in the Gulf of

Alaska, summer 1978. U.S. Dep. Commer.. NOAA Tech. Memo. NMFS FINWC-13. 44

P.

Gunderson, D. R., and T. M. Sample. 1980. Distribution and abundance of rockfish off

Washington, Oregon, and California during 1977. Mar. Fish. Rev. 42(3-4):2-16.
108

Hoff, R. Z. 1989. Spatial distributions and abundance trends for two species of flatfish in

the eastern Bering Sea. M.S. thesis, University of Washington, Seattle. 154 p.

Hughes, S. E. 1976. System for sampling large trawl catches of research vessels. J. Fish.

Res. Board Can. 33:833-839.

Joseph, J., and T. P. Calkins. 1969. Population dynamics of the skipjack tuna (Katsuwonus

pelamis) of the eastern Pacific Ocean. Inter-American Tropical Tuna Commission

Bulletin 13:26-34.

Kappenman;R. F. 1992. Estimation of the fishing power correction factor. AFSC Processed

Rep. 92-18, 10 p. Alaska Fisheries Science Center, National Marine Fisheries

Service, NOAA. 7606 Sand Point Way NE, Bldg. 4. Bin C 15700. Seattle. WA, 98 115-

0070

Munro. P. T. 1989. Estimating sport catch in Puget Sound using aerial survey and creel

census techniques. M.S. thesis, University of Washington, Seattle. 74 p.

Ricker, W. E. 1975. Computation and interpretation of biological statistics of fish

populations. Bull. Fish. Res. Board Can. 191. 382 p.

Robson. D. S. 1960. An unbiased sampling estimation procedure for creel census of

fishermen. Biometrics 16:261-277.

Robson. D. S. 1961. On the statistical theory of a roving creel’ census of fishermen.

Biometrics 17:414-437.
109

Ronholt. L. L.. H. H. Shippen. and E. S. Brown. 1978. Demersal fish and shellfish resources

of the Gulf of Alaska from Cape Spencer to Unimak Pass, 1948-1976; (a historical

review), Volumes 14. NWAFC Processed Rep., 955 p. Alaska Fisheries Science

Center, National Marine Fisheries Service, NOM, 7600 Sand Point Way NE, Bldg. 4.

Bin C 15700. Seattle, WA 98115-0070

Rose, C. S., and G. E. Walters. 1990. Trawl width variation during bottom trawl surveys:

Causes and consequences, p. 57-67. In Loh-Lee Low [ed.], Proceedings of the

Symposium on Application of Stock Assessment Techniques to Gadids. Int. North

Pac. Fish. Comm. Bull. 50.

Sasaki, T.. D. Rodman, M. Onoda, and J. Rosapepe. 1982. Preliminary report on Japan-U.S.

joint longline survey for sablefish and Pacific cod by Anyo Maru No. 22 in the

Aleutian Region and the Gulf of Alaska in the summer of 1981. 88 p. Far Seas

Fishery Research Laboratory. Shimizu, 424 Japan.

Stark, J.. K. Mito. E. Brown, and T. Yoshimura. 1988. Report to industry: Results of the

1987 U.S. - Japan Cooperative Bottom Trawl Survey of the central and western Gulf

of Alaska. NWAFC Processed Rep. 88-15, 215 p. Alaska Fisheries Science Center,

National Marine Fisheries Service, NOAA, 7600 Sand Point Way NE, Bldg. 4, Bin C

15700, Seattle, WA, 98115-0070

Wakabyashi. K. 1984. Preliminary report on the Japan-U.S. cooperative demersal trawl

survey in the Gulf of Alaska by Daikichi Maru No. 37 in 1984. [Document submitted

to the International North Pacific Fisheries Commission.) 6 p. Fisheries Agency of

Japan, Tokyo, Japan 100.
110

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

trawl surveys, p. 7-29. In R. G. Bakkala and K. Wakabayashi [ed;]. Results of

cooperative U.S.Japan groundfish investigations in the Bering Sea during MayAugust 1979. Int. North Pac. Fish. Comm. Bull. 44.

Wathne. F. 1977. Performance of trawls used in resource assessment. Mar. Fish. Rev.

39(6): 16-23.

Wilderbuer. T. K. 1988. Fishing power standardization in multi-vessel, multi-gear, resource

assessment surveys. M.S. thesis, University of Washington, Seattle. 171 p.

Wilderbuer. T. K.. K. Wakabyashi, L. L. Ronholt, and H. Yamaguchi. 1985. Survey report:

Cooperative U.S. Japan Aleutian Islands groundfish trawl survey - 1980. U.S. Dep.

Commer., NOAA Tech. Memo. NMFS FINWC-93. 356 p.

Wilson, M. T.. and C. E. Armistead. 1991. 1989 bottom trawl survey of the eastern Bering

Sea continental shelf. U.S. Dep. Cornmer., NOAA Tech. Memo. NMFS FINWC-212,

212 p.

Zenger. H. H., and K. Wakabyashi. 1985. Groundfish survey Central and Western Gulf of

Alaska. NOAA, NWAFC. Fishing Log, 165 p.
111

APPENDIX A

SAMPLING GEAR SPECIFICATIONS
113

Table A- 1 .--Differences between nylon and polyethylene Noreastern trawls:
114

Table A-2.--Features common to both polyethylene and nylon Noreastern trawl.
115

Figure A-l .--Diagram contrasting the polyethylene Noreastern trawl and the nylon

Noreastern trawl.
116

Figure A-2.--Diagram of the polyethylene Noreastern trawl used in the 1984 and 1987

triennial demersal trawl surveys of the Gulf of Alaska.
Figure A-3.--Diagram of the nylon Noreastern trawl used in the 1984 triennial demersal trawl

survey of the Gulf of Alaska.
118

Table A-3.--Features of the trawl and rigging used by the Daikichi Maru No. 37 in the 1984

triennial demersal trawl survey of the Gulf of Alaska.
119

Figure A-4.--Diagram of the polyethylene trawl and headrope used by the Daikichi Maru No.

37 in the 1984 triennial demersal trawl survey of the Gulf of Alaska.
Figure A-5.--Diagram of the footrope. roller gear, codend, codend cover, and doors and dandyline assembly used by the Daikichi Maru

No, 37 in the 1984 demersal trawl survey of the Gulf of Alaska.
121 

Table A-4.--Features of the trawl and rigging used by the Taisel Maru No. 35 in the 1987

triennial demersal trawl survey of the Gulf of Alaska.

Feature Description

Netting: polyethylene

Headrope: 55.6 m

Footrope: 65.0 m

Overall net length: 89.8 m

Codend: triple layer, 100 mm meshes in each layer

Roller gear:

Bobbins: 530 mm diameter

Tires: 600 mm diameter

Dandylines: 156 m

Doors: 2.55 m by 3.85 m. approximately 3,200 kg
122

Figure A-6.--Diagram of the trawl used by the Taisei Maru No. 35 in the 1987 demersal trawl survey

of the Gulf of Alaska.
123

Figure A-7.--Diagram of the footrope, roller gear, and doors and dandyline assembly used by the

Taisei Maru No. 35 in the 1987 demersal trawl survey of the Gulf of Alaska.
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125

APPENDIX B

ESTIMATES FOR THE SCATTERPLOT COMPARISON

OF NYLON AND POLYETHYLENE NOREASTERN TRAWLS

Means, medians, and coefficients of variation

for selected species
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127

Table B-l.--Numbers of nonzero tows, medians, means, and coefficients of variation of CPUE

for arrowtooth flounder estimated from tows made by the FV Morning Star using

nylon and polyethylene trawls. CPUE is in kilograms per square kilometer. CV

is 100% times the sample standard deviation divided by the sample mean.
128

Table B-2.--Numbers of nonzero tows, medians, means, and coefficients of variation of CPUE

for Pacific halibut estimated from tows made by the FV Morning Star using nylon

and polyethylene trawls. CPUE is in kilograms per square kilometer. CV is

100% times the sample standard deviation divided by the sample mean.
129

Table B-S.--Numbers of nonzero tows, medians, means, and coefficients of variation of CPUE

for rock sole estimated from tows made by the FV Morning Star using nylon and

polyethylene trawls, CPUE is in kilograms per square kilometer. CV is 100%

times the sample standard deviation divided by the sample mean.
130

Table B-4.--Numbers of nonzero tows. medians, means, and coefficients of variation of CPUE

for walleye pollock estimated from tows made by the FV Morning Star using

nylon and polyethylene trawls. CPUE is in kilograms per square kilometer. CV

is 100% times the sample standard deviation divided by the sample mean.
131

‘Table B-5.--Numbers of nonzero tows, medians, means, and coefficients of variation of CPUE

for Pacific cod estimated from tows made by the FV Morning Star using nylon

and polyethylene trawls. CPUE is in kilograms per square kilometer. CV is

100% times the sample standard deviation divided by the sample mean.
132

Table B-6.--Numbers of nonzero tows, medians, means, and coefficients of variation of CPUE

for Pacific ocean perch estimated from tows made by the FV Morning Star using

nylon and polypropylene trawls. CPUE is in kilograms per square kilometer. CV

is 100% times the sample standard deviation divided by the sample mean.
133

APPENDIX C

STRATEGIES AND FORMULAE FOR FIRST

ESTIMATES OF FISHING POWER CORRECTIONS IN 1987
135

Data from the comparison study were used to calibrate two of three vessels to the

third vessel, the boat with the greatest fishing power. In these estimations, data from all

depths and areas of the study were pooled for each vessel. The vessel with the highest

fishing power was not the same for every species. For some species there was no apparent

difference between the two U.S. vessels. For other species, one of the boats was so poorly

represented by nonzero tows that its data was not used in the estimate. A few species were

so poorly represented by nonzero tows among all boats that FPC estimates from other

species or species groups were used instead. These considerations led to a variety of

strategies for estimating FPCs, using several versions of the estimators, all slight variants on

the basic ratio of means. The ‘Most Efficient Trawl” principle was always followed in

determining how to apply the estimates to catch data. The strategies are described here.

The actual estimates of the fishing power corrections and the strategy for application are

given in Table 30.

Throughout this appendix three different estimates of mean CPUE are combined in

various ways. Rather than describe them repeatedly in each strategy, they are given here:

Mean CPUE for the Lets Go was estimated with:

Mean CPUE for the Nore-Dick was estimated with:
136

Mean CPUE for the Taisei Maru No. 35 was estimated with:

One last quantity was used in several of the following strategies, a combined mean of CPUEs

for the two U.S. vessels to represent the “standard’ U.S. vessel/net combination. The mean

CPUE for this standard vessel was estimated as the unweighted average of the mean CPUEs

for each of the two U.S. boats:

Strategy 1

In this strategy the fishing power difference between the Lets Go and the Nore-Dick

was considered negligible. Their estimated mean CPUEs were combined in an unweighted

average to represent a U.S. standard vessel. The U.S. vessel appeared to have a higher

fishing power than the Taisei Maru No.. 35. Catches made by the Taisei Maru No. 35 as part

of the survey were multiplied by an estimate of the fishing power difference to adjust them

to those of the U.S. standard vessel.

The estimator was
137

Strategy 2

The fishing power difference between the Lets Co and the Nore-Dick was considered nonnegligible. The Nore-Dick appeared to have the highest fishing efficiency. Separate FPCs

were computed for the Lets Go and Taisei Maru No. 35. Catches by each of those vessels

were multiplied against their FPC estimates to adjust them to catches made by the NoreDick.

The FPC estimator for the Lets Go-was

The FPC estimator for the Taisei Maru No. 35 was

Strategy 3

The difference in fishing power for northern rockfish between the Lets Go and the

Nore-Dick seemed large. However, we were forced to consider it negligible due to poor

representation by nonzero tows in catches by the Nore-Dick. For the same reason only

catches by the Lets Go were used to represent the standard vessel. The fishing power

difference between the Taisei Maru No. 35 and the standard vessel was not considered

negligible. It was corrected in the survey data by multiplying catches made by the Taisei

Maru No, 35 with an FPC estimated by the following:
138

Strategy 4

The fishing power difference between the Lets Go and; the Nore-Dick was considered

non-negligible. The fishing power differences between the Taisei Maru No. 35 and each of

the other two vessels were also considered non-negligible. The most efficient vessel was the

Taisei Maru No. 35. Survey catches made by the Lets Go were multiplied against an FPC

estimated by:

Survey catches made by the Nore-Dick were multiplied against an FPC estimated by:

Strategy 6

For each species of smaller flatfish which was poorly represented by nonzero tows in

the comparison study an FPC was estimated as a combination of mean CPUEs for other

similar species, flathead sole, Dover sole, rex sole, and rock sole. Fishing power differences

between the two U.S. boats were assumed negligible and a CPUE was estimated for a

standard U.S. vessel/net combination. This standard vessel was seen to be the more

efficient. Survey. catches made by the Taisei Maru No. 35 were multiplied against an FPC

estimated by:
Strategy 6

139

Starry flounder, a somewhat larger flatfish, was poorly represented in the comparison

study data. The estimated FPC generated for arrowtooth flounder using Strategy 1 was

applied to survey catches of this species made by the Taisei Maru No. 35. This was done

because this species was so poorly represented by nonzero tows in the comparison study.

Strategy 7

The FPC for Pacific ocean perch estimated from the comparison study data. using

Strategy 1, was also used to correct survey catches made by the Taisei Maru No. 35 of

harlequin, sharpchin. and dusky rockfish. This was done because these species were so

poorly represented by nonzero tows in the comparison study and were thought to be

sufficiently similar to Pacific ocean perch.

Strategy 8

The FPCs to adjust survey catches of giant grenadier and popeye grenadier by the

Taisei Maru No. 35 were estimated as combinations of CPUEs for Pacific cod and walleye

pollock. The estimator was:
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

(web site: www.ntis.gov). Paper and microfiche copies vary in price. 

AFSC49 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.

39 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.

 

38 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.

37 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. PB94-181831.

36 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.

35 PARKS, N. B., and F. R. SHAW. 1994. Relative abundance and size composition of sablefish

(Anoplopoma fimbria) in the coastal waters of California and Southern Oregon, 1984-1991. NTIS

number pending.