NOAA Technical Report NMFS 132 January 1998

Stock Composition of Some

Sockeye Salmon, Oncorhynchus nerka,

Catches in Southeast Alaska,

Based on Incidence of

AIlozyme Variants,

Freshwater Ages, and

a Brain-Tissue Parasite

Jerome Pella

Michele Masuda

Charles Guthrie III

Christine Kondzela

Anthony Gharrett

Adam Moles

Gary Winans

u.s. Department of Commerce
u.s. DEPARTMENT

OF COMMERCE

WILLIAM M. DALEY

SECRETARY

National Oceanic and

Atmospheric Administration

D. James Baker

Under Secretary for

Oceans and Atmosphere

National Marine

Fisheries Service

Rolland A. Schmitten

Assistant Administrator

for Fisheries

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NOAA Technical Report NMFS 132

A Technical Report of the Fishery Bulletin

Stock Composition of Some Sockeye Salmon,

Oncorhynchus nerka, Catches in Southeast Alaska,

Based on Incidence ofAllozyme Variants,

Freshwater Ages, and a Brain-Tissue Parasite

Jerome Pella

Michele Masuda

Charles Guthrie III

Christine Kondzela

Anthony Gharrett

Adam Moles

Gary Winans

January 1998

U.S. Department of Commerce

Seattle, Washington
Suggested reference

Pella,Jerome, Michele Masuda, Charles Guthrie III, Christine

Kondzela, Anthony Gharrett, Adam Moles, and Gary Winans. 1998.

Stock composition of some sockeye salmon, Oncorhynchus nerka, catches

in southeast Alaska, based on incidence of allozyme variants, freshwater ages, and a brain-tissue parasite. U.S. Dep. Commer., NOAA

Tech. Rep. NMFS 132, 23 p.

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Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka,

Catches in Southeast Alaska, Based on Incidence of Allozyme Variants,

Freshwater Ages, and a Brain-Tissue Parasite

JEROME PELLA, MICHELE MASUDA, CHARLES GUTHRIE III,

CHRISTINE KONDZELA, ANTHONY GHARRETT, ADAM MOLES

Auke Bay Laboratory, Alaska Fisheries Science Center

11305 Glacier Highway, juneau, Alaska 99801-8626

GARYWINANS

Coastal Zone and Estuarine Studies

Northwest Fisheries Science Center

2725 Montlake Boulevard East, Seattle, Washington 98112

ABSTRACT

The incidence of four discrete characters of individual sockeye salmon -two genetically inherited proteins (PGM-1*and PGM-2*) , freshwater age at migration, and the presence of the brain-tissue parasite Myxobolus arcticus-in weekly samples from two Alaskan

fisheries (Noyes Island in 1986 and Sumner Strait in 1987) were used to infer stock

composition of the catches based on corresponding character samples from 73 Alaskan and

Canadian stocks. Estimated contributions of 13 stock groups, formed on the basis of

character similarity of their members, were roughly consistent with expectations from

tagging experiments, knowledge of stock magnitudes, and similar assessments from scales.

Imprecision of the estimated contributions by the 13 stock groups limited their practical

value; but variability was much reduced for combined estimated contributions by two

inclusive categories, namely stock groups whose members had either high or low brainparasite prevalence. Noyes Island catches consisted predominantly of unparasitized fish,

most of which were probably of Canadian origin. The majority of Sumner Strait catches

consisted of parasitized fish, whose freshwater origins may have been in Alaska or Canada.

Introduction

Salmon, Oncorhynchus spp., caught in fisheries of the

northern boundary area (NBA) of southern Southeast

Alaska and northern British Columbia (Fig. I) comprise stocks of both U.S. and Canadian origin. In order

to study the population dynamics of large individual

stocks or stock groups and to assess the catches of

either country's stocks taken by the other country's

fisheries, the origins ofsalmon caught in NBA fisheries

must be determined. This study examines the use of

multiple (four) traits of individual sockeye salmon, O.

nerka-two allozymes, freshwater age, and a brain-tissue

parasite-with mixture model analysis (Fournier et al.,

1984; Pella and Milner, 1987) in order to estimate the

stock composition of catches for two Alaskan fisheries

and compares these results with those from traditional

methods, including tagging and scale-pattern analysis.

One of the guiding principles of the Pacific Salmon

Treaty of 1985 between the United States and Canada

was that each country should receive benefits from its

fisheries equivalent to its own salmon production. Future

agreements on salmon harvesting between the two countries will probably retain this concept. Because both U.S.-

and Canadian-origin salmon are commonly caught together in coastal fisheries of each country, origins of

salmon in coastal fisheries must be determined so that the

salmon production by each country can be assessed. Determining the stock origins of salmon catches has been

the focus ofcoastwide research supported under the treaty.
2 NOAA Technical Report NMFS 132

Sumner Strait

(Area 106-41)

Prince of

Wales Island

Noyes Island

Area 104

Dixon

L

Entr/

angara ...

Island

Figure I

Boundary area of southern Southeast Alaska (dark) and northern British Columbia (dots)_

(Fisheries considered are indicated by striped areas.)

From 1982 to 1985, the Auke Bay Laboratory (ABL)

of the U.S. National Marine Fisheries Service (NMFS)

conducted joint research with the Alaska Department

of Fish and Game (ADF&G) and the Pacific Biological

Station (PBS) of the Canada Department of Fisheries

and Oceans (CDFO) to determine stock composition

ofsalmon catches in fisheries of the NBA. The goal was

to evaluate the numbers of fish produced in Canada

that are intercepted by U.S. fishermen and the numbers offish produced in the U.S. that are intercepted by

Canadian fishermen and also to develop further information for improved management of stocks that contribute to NBA fisheries. Initially, large-scale tagging

experiments on adult sockeye salmon and pink salmon,

O. gorbuscha, in NBA fisheries provided estimates of

stock contributions (Pella et aI., 1993; Gazey et aJ.l;

English et aI.2). However, stock composition in BA

fisheries varies among years owing to natural fluctuaI Gazey, W.].,]. Taylor, K K English, T. Webb, and D. A. Birdsall.

1883. Part B: 1982 North coast tagging study. In W.]. Gazey and D.

A. Birdsall (eds.), Design and execution of a stock interception

study. Unpubl. rep. by LGL Limited and ESSA Environmental

Social Systems Analysts Ltd., for Fisheries and Oceans, Canada, 107

p. [Available from LGL Limited, No. 333, 2453 Beacon Ave., Sidney,

B.C., Canada.]

2 English, K K, w.]. Gazey, and]. A. Taylor. 1984. Part C: The 1983

North coast tagging study. In W.]. Gazey and D. A. Birdsall (eds.),

Design and execution of a stock interception study. Unpubl. rep.

by LGL Limited and ESSA Environmental Social Systems Analysts

Ltd., for Fisheries and Oceans, Canada, 115 p. [Available from

LGL Limited, No. 333,2453 Beacon Ave., Sidney, B.C., Canada.]
___ Pella et al.: Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka, Catches in Southeast Alaska 3

tions in the relative magnitudes of contributing stocks.

Furthermore, changes in oceanographic conditions

probably affect migration routes of returning adult

salmon (Blackbourn, 1987). If so, the phenomenon of

el Nino, which occurs with varying intensity at 2-7 yr

intervals and affects ocean temperatures over much of

the Pacific coast, could cause irregular and varying

disturbances of annual stock composition in NBA fisheries (Hamilton and Mysak, 1986). Therefore, estimates

ofstock contributions to NBA fisheries, if based primarily on information from the few years of adult tagging,

are less reliable for nontagging years than for tagging

years. Tagging experiments are prohibitively expensive

for routine evaluation of stock composition in NBA

fisheries if the annually changing stock mixtures are to

be monitored; therefore, other methods are required.

To this end, research has been directed toward determining stock composition of catches from attributes of

sampled individuals.

Knowledge of attributes useful for determining stock

composition of mixed sockeye salmon stocks in NBA

fisheries has grown considerably since the initial tagging experiments. Researchers of ADF&G discovered

that certain growth features on scales ofsockeye salmon

from coastal systems of Southeast Alaska differed from

those on scales of sockeye salmon from large river systems-Nass, Skeena, and Stikine Rivers-where spawning grounds are in the interior of northern British

Columbia (Marshall et aI., 1984). As a result of this

finding, use of scales to assess composition of sockeye

salmon catches in Southeast Alaskan NBA fisheries has

been routine since 1982.

These mixed-stock analyses based on scales have identified contributions of, at most, five major groups of

stocks: 1) Southeast Alaska; 2) Nass River; 3) Skeena

River; 4) Stikine River; and 5) south coast stocks, including those of the large Canadian Fraser River. Estimation of contributions by important componentstocks

within Southeast Alaska or from the Stikine River was

attempted in some years. Which particular groups, or

important component stocks, are included in an analysis depend on the fishery and year in which mixed-stock

samples are assessed and are chosen with the intent of

separating the main contributors. However, substantial

Canadian stocks of the central coast of British Columbia, as well as numerous lesser populations of both

Southeast Alaska and British Columbia that may occur

in NBA fisheries, are not included.

Furthermore, growth patterns on scales used to identify stock origin vary annually because growth ofsalmon,

recorded in the scale patterns, is affected by environmental conditions. Such variations in growth patterns

can bias estimates of stock composition if the samples

used for developing classification rules come from years

different from those when the catch samples were taken.

Consequently, scales of contributing stocks must be

monitored annually Uensen and Frank, 1988; Wood et

aI., 1989) as is currently done by ADF&G. To reduce

costs due to annual sampling and thereby permit

broader coverage of potentially contributing stocks,

more time-stable characters, which do not require frequent resampling of contributing stocks, continue to

be sought even though scales remain at present the

practical basis of assessment.

Genetic characters are quite stable over periods of

repeated sampling of salmon populations and can in

principle be used for the assessment of mixed-stock

composition (Milner et aI., 1985). The basis for estimating stock composition is the variation in relative frequencies of inherited protein types. Such genetic differences among stocks of salmon are determined by

electrophoretic assay of enzymatic proteins and have

been useful in assessment of mixed stocks (Grant et aI.,

1980; Okazaki, 1981; Miller et aI., 1983; Beacham et aI.,

1985; Utter et aI., 1987; Shaklee et aI., 1990a; Wishard3).

Utter and Hodgins (1970) first described phosphoglucomutase polymorphism in sockeye salmon; and an

initial survey of sockeye salmon stocks occurring in

NBA fisheries (Utter et aI. 4) revealed that the two coding loci-phosphoglucomutase-l and -2 (PGM-l* and

PGM-2* in the nomenclature ofShaklee et aI., 1990b)-

had variation within and among populations.

Just as protein genetic characters can be effective for

assessing mixed-stock composition, other biological

characters observable in individuals can be useful, especially if the characters are fairly stable over time. Parasites were used by Konovalov (1975) to identify stock

origins of sockeye salmon; and the brain parasite

Myxobolus arcticus (previously identified as M. neurobius),

first reported in sockeye salmon by BykhovskayaPavlovskaya et aI. (1962), was important for Konovalov's

classification key. The presence of the brain parasite in

an individual fish is determined by microscopic examination of prepared brain tissue (Quinn et aI., 1987).

The percentage of individuals of any sockeye salmon

stock infected by this brain parasite (prevalence of the

parasite [after Wood et aI., 1988]) is quite stable over

time and varies from 0% to 100% among samples from

stocks in NBA fisheries (Wood et aI., 1988; Moles et aI.,

1990).

3 Wishard, L. 1981. Stock identification of Pacific salmon in western

Washington using biochemical genetics. Final unpub!. rep., Washington Dep. Fish. Wild!., 300 Desmond Dr., Olympia, WA 98504-

7600. Contract nos. 1176 and 1276.

4 Utter, F., P. Aebersold, J. Helle, and G. Winans. 1984. Genetic

characterization of populations in the southeastern range ofsockeye salmon. InJ. M. Walton and D. B. Houston (eds.), Proc. of the

Olympic Wild Fish Conference, March 23-25, 1983. [Available

from J. M. Walton, Fish. Techno!. Prog., Peninsula College, 1502

E. Lauridsen Blvd., Port Angeles, WA 98362.]
4 NOAA Technical Report NMFS 132

Another potentially useful biological character for

assessing stock composition of mixed stocks is freshwater age, the number of years young sockeye salmon

spend in fresh water before migrating to the ocean.

Freshwater age is determined from growth patterns on

scales. Individuals of most stocks migrate seaward after

spending 1 or 2 yr in fresh water, but freshwater residency can range from 0 to 3 yr (Wood et aI., 1988;

Rutherford et aI., 1994). Freshwater age of individuals

of many stocks may be influenced by their growth rates,

which vary with changing environmental conditions

(Ricker, 1972). Furthermore, spawners of many stocks

comprise several ages ofmaturity (freshwater plus ocean

years), each with a distinctive freshwater age composition. As a result, variation in numbers of parent spawners and their (typically) highly variable reproductive

success disturb apparent freshwater age composition of

their progeny as spawners in annual samples. Therefore, freshwater age composition from annual samples

of spawners probably varies more than type composition based on genetic or parasite characters. Wood et

al. (1988) suggested that although freshwater age composition of stocks varied among years, the large differences in averages among stocks might yet prove useful

for stock composition estimation.

This report describes our assessment ofstock composition ofsockeye salmon based on four characters: PGM1 *, PGM-2*, freshwater age, and the presence of

Myxobolus arcticus. The assessment was applied to catch

samples from two Southeast Alaskan fisheries: the purseseine fishery of area 104 (commonly known as the

Noyes Island fishery) in 1986 and a gill-net test fishery

in area 106-41 (commonly known as the Sumner Strait

fishery) at the northern entrance to Clarence Strait in

1987 (see Fig. 1). During tagging years of 1982 and

1983, significant percentages of catches of both fisheries were Canadian-origin sockeye salmon (Pella et aI.,

1993). This study describes the utility of the four characters for estimating contributions of U.S.- and Canadian-origin sockeye salmon to these fisheries in years

without tagging data and compares the estimates with

those from concurrent scale-pattern analysis.

Methods _

Samples from the Fisheries

Catches from the two fisheries were sampled weekly

during the fishing season. In 1986, a series of eight

weekly samples (6 July-30 August) were taken from

commercial catches of the Noyes Island purse-seine

fishery. In 1987, a series of eight weekly samples (21

June-IS August) were taken from a test gill-net fishery

in Sumner Strait that was conducted by personnel of

ADF&G to monitor the timing and magnitude ofsalmon

returns for management of the commercial fishery. In

both cases, weekly sample size was about 300 fish. Tissue samples (muscle, heart, liver, and eyeball), several

lateral scales, and the head were collected from each fish.

According to procedures outlined in Aebersold et al.

(1987), fish were typed for PGM-l* and PGM-2* from

tissue samples. Tissues from the 1986 samples were

analyzed at the genetics laboratory of the Coastal Zone

and Estuarine Studies Division (CZES) of the NMFS

Northwest Fisheries Science Center, and tissues from

the 1987 samples were analyzed by the genetics laboratory of ABL. The freshwater age of individuals was

determined from examination of scales by staff of the

Southeast Alaska Region, Commercial Fisheries Management and Development Division of ADF&G. Brain

tissue from the heads was examined for the presence of

the parasite Myxobolus arcticus at ABL (Moles et aI.,

1990). Not all characters were measured for each fish

sampled. Furthermore, identification of some samples

was incomplete, so that collation of all genetic, freshwater age, and parasite information for each individual

fish in the sample (as required by the method used to

estimate stock composition) was not possible. For fish

with incomplete records, only the observed characters

identifiable to individual fish were retained for stock

composition analysis. Resulting numbers of observations used in assessing stock composition of the catches

were substantially reduced, for certain characters and

weeks, from the original sample sizes for Noyes Island

but not for Sumner Strait (Table 1, 2).

Samples from the Contributing Stocks

Baseline samples (i.e. samples obtained on the spawning grounds of stocks potentially contributing to the

catches) of the same four characters were obtained by

U.S. and Canadian agencies for 73 sockeye salmon

stocks (55 Canadian and 18 Alaskan, Fig. 2). The number of years in which samples were obtained from these

baseline stocks was limited, often a single year for a

particular character. Most sampling occurred from 1982

to 1986. Collection of baseline samples of all four characters for any stock was not simultaneous, i.e. tissues

(two genetic characters), scales (freshwater age), and

brain tissue (parasite) were obtained for many stocks at

different sampling times. However, tissues for the two

genetic characters usually were obtained and assayed, if

possible, for each fish in a baseline sample. Genetic

analyses were conducted for U.S. stocks at the CZES

and ABL genetics laboratories (Helle5); and for Cana5 Helle,]. 1992. Auke Bay Laboratory, Alaska Fish. Sci. Center, Nat!.

Mar. Fish. Serv., NOAA, 11305 Glacier Highway,Juneau, AK 99801.

Personal commun.
___ Pella et al.: Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka, Catches in Southeast Alaska 5

Table I

Numbers ofsockeye salmon, Oncorhynchus nerka, from catch samples by week and character that were used to assess stock

composition at Noyes Island in 1986, total fish sampled, and total fishery catches.

Number of fish

Myxobolus Freshwater Total Total

First day of week PGM-l* PGM-2* arcticus age samples catch

July 6 230 222 63 45 256 7,192

13 304 299 129 140 313 30,307

20 77 70 184 227 283 53,715

27 280 277 228 219 300 104,325

August 3 258 240 150 201 300 154,415

10 269 275 143 155 298 63,099

17 276 272 107 103 299 15,872

24 274 272 238 240 296 11,989

Totals 1,968 1,927 1,242 1,330 2,345 440,914

Table 2

Numbers ofsockeye salmon, Oncorhynchus nerka, from catch samples by week and character that were used to assess stock

composition at Sumner Strait in 1987, total fish sampled, and total fishery catches.

Number of fish

Myxobolus Freshwater Total Total

First day of week PGM-l* PGM-2* arcticus age samples catch

June 21 297 295 298 298 298 3,845

28 293 288 291 294 294 7,514

July 5 299 294 297 299 299 15,175

12 293 290 296 297 297 13,761

19 299 293 300 300 300 14,031

26 294 296 299 299 299 10,232

August 2 292 295 297 297 297 11,075

9 298 295 296 298 298 3,532 1

Totals 2,365 2,346 2,374 2,382 2,382 79,165

1 Includes catches for weeks 9 August through 13 September.

dian stocks, at Aqua Life Diagnostics, Ltd., Helix-Biotech,

Ltd., and the PBS (Wood6). Procedures used were those

recommended byAebersold et aI. (1987). Genetic baseline

information was reported by Guthrie et al. 7 and Wood et

al. (1994). Freshwater ages offish were determined from

scales by staff of ADF&G, PBS, and the Pacific Salmon

Commission (PSG). Brain samples were anaIyzed for pres6 Wood, C. 1992. Pacific Biological Station, Canada Dep. Fish. and

Oceans, Hammond Bay Road, Nanaimo, B.C. Canada V9R 5K6.

Personal commun.

7 Guthrie, C. M., Ill,J. H. Helle, and A. J. Gharretl. 1994. Genetic

relationships among populations of sockeye salmon in Southeast

Alaska and northern British Columbia. Auke Bay Laboratory, Alaska

Fish. Sci. Center, Nat!. Mar. Fish. Serv., NOAA, 11305 Glacier

Highway,Juneau, AK 99801. In prep.

ence of Myxobolus arcticus by staff of PBS and ABL. Most

parasite and freshwater age baseline data from Southeast

Alaska and Canada have been published (Wood et al.,

1987a, 1988; Moles et aI., 1990; Rutherford et aI., 1992,

1994). Baseline information used for subsequent analyses

is available from the authors upon request.

Estimation of Stock Composition

Maximum-Likelihood Method and Allocate-Sum Procedure-The conditional maximum-likelihood method

of estimation of stock composition (Fournier et al.,

1984; Millar, 1987; Pella and Milner, 1987) was used to
6 NOAA Technical Report NMFS 132

Vancouver

Island

Canada

"

Figure 2

Geographic locations of the 73 sockeye salmon, Oncorhynchus nerka, baseline stocks and some of

the major rivers (stock reference numbers are defined in Table 3).

estimate percentages of catch samples from each

baseline stock. The composite character types offish in

the catch samples were the basis ofsuch an estimation.

Conditional stock composition estimates were computed

under the assumption that characteristics of baseline

stocks were known without error. In addition, the unconditional maximum-likelihood method (Pella and

Milner, 1987) was employed for comparison. In unconditional estimation, both stock composItIOn and parameters describing characteristics of baseline stocks

were estimated with the optimization begun from the

conditional maximum-likelihood estimate ofstock composition and observed parameter values for baseline

characteristics.

In applying the estimation methods, two common

assumptions were made: 1) the genetic loci were in
___ Pella et al.: Stock Composition of Some Sockeye Salmon, OncQrhynchus nerka, Catches in Southeast Alaska 7

Hardy-Weinberg equilibrium, and 2) the four characters were mutually independent within the stocks. HardyWeinberg equilibrium and independence were expected

to be well approximated for the two genetic characters.

Any deviations from Hardy-Weinberg equilibrium in a

stock would be eliminated in the next generation, provided mate selection was independent of these enzyme

genotypes. Genotypes at the two loci are probably inherited independently because linkage has not often

been found between enzyme-coding loci in other species of salmonids (Johnson et aI., 1987). When such

linkages have been found, they are usually present

among all species of salmonids examined (Johnson et

aI., 1987). In particular, breeding experiments found

no evidence for linkage between PGM-l* and PGM-2*

loci for O. mykiss (May et aI., 1982;Johnson et aI., 1987).

If the two genetic characters were linked, the disequilibria produced by such linkage would probably have

disappeared during the many generations preceding

our study (e.g. see discussion on gametic phase disequilibria by Campton [1987]).

The assumption of mutual independence of all characters, including freshwater age and the parasite, is

untested. Some evidence (Wood et aI., 1987a) indicates

that parasite prevalence may increase with length of

freshwater residence, at least between the first (O-yr

fish) and second summer (1-yr fish); O-yr fish were a

minor component of commercial catches used in this

study; therefore this possible dependence would not

seriously affect the estimates. Nonetheless, because of

concerns regarding the validity of the independence

assumption and because of doubts regarding the temporal stability of freshwater age, the analysis was repeated without freshwater age. Regardless oftheir absolute validity, Hardy-Weinberg equilibrium and mutual

independence allow approximations for variables required by the maximum-likelihood method, namely

the relative frequencies in each baseline stock of the

composite character types observed in catches.

The allocate-sum procedure (Wood et aI., 1987b;

Wood, 1989) was used to estimate percentages contributed by stocks that were similar in the four characters.

Estimated percentages of catch from individual members were summed to obtain the estimated percentages

of the similarity group ofstocks. Similarity groups were

identified from a dendrogram of baseline stock relationships based on Nei's distance (1978) and grouped

by the unweighted pair-group method with arithmetic

averages (UPGMA) (Sneath and Sokal, 1973).

Numerical Studies-Simulation studies have been recommended for demonstrating that estimates of stock

composition are trustworthy because knowledge of the

theoretical properties of the method of maximum likelihood is inadequate for credible applications (Fournier

et aI., 1984; Beacham et aI., 1985). Precision and accuracy of our stock composition estimates were examined

by means of three numerical studies: bootstrap

resampling, local accuracy experiments, and tests offit.

Bootstrap Resampling-Bootstrap resampling (Efron

and Tibshirani, 1986) involved recomputation ofstock

composition estimates from sets of resampled catch

and stock samples. Samples equivalent in size to original samples were drawn with replacement from the

original samples. After conditional maximum-likelihood

estimates of stock composition were recomputed from

each of 100 sets of resampled stock and catch samples,

standard deviations of bootstrapped estimates for similarity groups were computed and reported as estimated

standard errors of stock composition estimates. For

convenience, a stock group is said to be detected if its

estimated percentage contribution exceeds two standard errors; however, detection does not invariably indicate stock group presence because of possible confusion among the different groups.

Local Accuracy Experiments-Local accuracy experiments test the capacity of the maximum-likelihood

method to detect changes in stock composition of the

available mixed-stock samples. Ifthe stock composition of

observed mixed-stock samples is altered in a known way,

an accurate estimation method should produce estimates

that reflect the alteration. Local accuracy engenders credibility in observed stock composition estimates by empirically demonstrating a direct link between underlying stock

composition and corresponding expected estimates.

In the local accuracy experiments, the stock composition of each observed catch sample was altered by

augmenting it with a computer-generated random

sample of equal size and an expected stock composition of equal percentages from the baseline stocks. The

computer-generated sample was created by a two-step

procedure. First, a multinomial sample with equal expected percentages was drawn for the number of individuals from every baseline stock to be added to the

catch sample. Second, the composite type of each individual added to the catch sample was determined by

randomly and independently sampling types of each

character from the appropriate baseline stock sample.

Hardy-Weinberg equilibrium was assumed for relative

frequencies of genetic types.

By the experimental process, the unknown stock composition of each original catch sample, p = (PI' P2, .••,

P73)' was expected to change to that of the augmented

sample, p*, as follows:

(1)

A predicted value for P* was computed from Equation

1 by substituting components of the original condi-
8 NOAA Technical Report NMFS 132

where as and f3s are the intercept and slope, respectively, for the group s line. The simple unweighted

least-squares criterion was used in fitting, with the parameter estimates constrained so that

The constraints were a logical consequence of the requirement that fitted values of changes in percentages

from similarity groups should sum to zero. Ideally, averages of estimated changes should agree with those that

have been predicted, so that expected values of the

estimated cis and {1s should equal zero and one, respectively.

Tests of Fit-Accurate estimation of stock composition from catch and baseline samples required that 1)

stocks present in the catch were included among the

baseline stocks and 2) baseline samples and statistical

methods provided valid estimates of relative frequencies of composite character types in the baseline stocks.

To evaluate the plausibility of these assumptions, the

conditional maximum value of the likelihood function

for observed catch and baseline samples was compared

with an empirical null-reference distribution. The empirical null-reference distribution described variation

in the maximum of the likelihood function due to

tional maximum-likelihood estimate ( p) based on observed catch and stock samples for the unknown stock

composition (P). The baseline samples were resampled,

and the conditional maximum-likelihood estimate of

stock percentages (P*) for the augmented sample was

computed. The process was repeated 100 times. At each

repetition, an estimate of the change in stock percentages for an original catch sample, p*-p, was available

for comparison with the single predicted change,

p *-p. Comparison of estimated and predicted changes

in percentages from similarity groups required summing appropriate components of these vectors. Averages of the 1,00 estimated changes for similarity groups,

denoted as Ll , and the corresponding predicted change,

Ll, were used in further analysis.

Regression analysis was used to compare the average

estimated changes of similarity-group percentages in

t!Ie augmented samples with predicted changes. Let

Ll ws be the estimate of change in percentage ofsimilarity group s in the catch sample during week wand let Ll ws

be the corresponding predicted change. By separate analysis for the two fisheries, the estimated changes were fitted

to the predicted changes by using the linear model

~ws = as + f3s . Ll ws + cws '

13 Las =0 and

s=1

13

L~s' Ll ws = 0, w = 1, ... ,8.

s=1

(2)

(3)

uncertainty in underlying stock composItiOn of the

catch, as well as variation induced by sampling the

catch and baseline stocks. The following conditions

were used in generating the null-reference distribution: 1) catch was composed of baseline stocks only, 2)

characters were independent in stocks, 3) relative frequencies ofcharacter types within stocks remained stable

between times ofsampling baseline stocks and catches,

and 4) genetic characters were in Hardy-Weinberg equilibrium. Therefore, if the observed conditional maximum value of the likelihood function was unusually

small compared with the empirical null distribution, one

or more of these conditions were probably not met.

Ifthe true underlying stock composition ofa sampled

catch was known, the empirical null distribution could

be generated by repeating the following steps many

times. First, draw a multinomial mixed-stock sample

that is equivalent in size to the actual catch sample and

that has an expected stock composition equal to the

true stock composition. Second, for each individual in

the mixed-stock sample, draw a composite character

type from the estimated relative-frequency distribution

(based on the observed baseline samples) ofthe appropriate stock to form the simulated catch sample. Third, draw

simulated baseline samples of types for each character

equivalent in size to and from those available. Fourth, find

the conditional maximum of the likelihood function for

the simulated catch sample by using the simulated baseline

samples to estimate the relative frequencies ofits composite character types in the contributing stocks.

The procedure described for generating the null

distribution depends on knowledge of the stock composition ofthe sampled catch. Although the stock composition is unknown, bootstrapped estimates of stock

composition provide a measure of uncertainty. To account for this uncertainty in generating the null distribution of the conditional maximum of the likelihood

function, the underlying stock composition was set equal

to one of the available bootstrapped estimates when

generating a set ofsimulated catch and baseline samples

and the derived conditional maximum of the likelihood function. The process was repeated for each of

the 100 bootstrapped estimates of stock composition

and provided an empirical distribution of 100 values of

the conditional maximum of the likelihood function.

Results

Similarities and Differences in

Attributes among Baseline Stocks

Observed prevalence of Myxobolus arcticus among

baseline stocks ranged from complete absence (0%) to

complete infection (100%) (Table 3), and the prepon-
___ Pella et al.: Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka, Catches in Southeast Alaska 9

Table 3

Similarity groups of sockeye salmon, Oncorhynchus nerka, stocks identified from four characters (PGM-l *, PGM-2*,

Myxobolus arcticus, and freshwater age). Stock reference numbers are given as superscripts; Alaskan stocks are identified by

italics, and percentage offish parasitized is in parentheses. Cr. =Creek, L. = Lake, R. =River.

Similarity

group

A

B

C

D

E

F

G

H

J

K

L

M

Stocks

Mikado L.26 (99)

Mercer L.30 (93)

Shipley [,65 (98)

Kuthai L.54 (100), Speel R70 (87), Damdochax L.47 (94), Tankeeah L19 (100), Hugh Smith L.56 (94), Chuck [,61 (89),

Salmon Bay [,69 (96), Eden L.31 (100), Owikeno L.J7 (99), Banks L.25 (100), McDonald [,64 (94), Crescent [,71 (98), Karta

R,62 (95), Bonilla L.2B (100), Luck [,66 (98), Helm [,63 (98), Koeye L.IB (98), Hobiton L.6 (100), Kennedy L.3 (99), Swan

L.41 (99), Henderson L.4 (99), Devon L.29 (100)

Red Bay [,6B (100), Klakas [,5B (100), Kegan [,59 (93)

Thoms [,67 (99) ,johnson [,57 (100)

Canoona L.21 (100), Cheewhat L7 (98), Sproat L.5 (100), Sakinaw L.1O (81), Tenas L.20 (96), Horsefly RI5 (62)

Gingut Cr.39 (31)

Nanika R.24 (0), Chilkat [,72 (0)

Kimsquit L.23 (0), Motass L.44 (1), Stikine glaciallakes49 (7), Tatsamenie L.51 (0), Bowser L.46 (0), Meziadin L.42 (0)

Woss L1 (55), Mathers L.33 (30), Lowe L.27 (41), Naha R,60 (30), Stikine sea/river-type4B (27)

Alastair L.34 (1), Lakelse L.35 (3), Hackett R.53 (3), McDonnell L.36 (1), Great Central L.2 (5), Kitlope L.22 (0),

Birkenhead Rll (0), Chilkoot L73 (0), Babine LY (0), Weaver channel9 (0), Bear L.45 (0), Trapper L.52 (0), Yakoun L.32

(0), Fred Wright L.43 (2), Cultus L.B (0), Tahltan L.50 (0)

K1ukshu L.55 (1), Long L.16 (0), Takla L.40 (3), Adams R12 (0), Lower Shuswap R13 (0), Stuart L.38 (0), Chilko L14 (0)

derance of these stocks (63 of 73 stocks) had either

prevalence in excess of 90% (32 stocks, 19 of Canada

and 13 of Alaska) or less than 10% (31 stocks, 29 of

Canada and 2 of Alaska). The other 10 baseline stocks

(7 of Canada and 3 of Alaska) had prevalence ranging

from 27% to 89%.

Freshwater age composition of baseline samples varied substantially among stocks (Wood et aI., 1987a,

1988; Rutherford et aI., 1992, 1994). Many stocks were

composed of individuals differing in freshwater residency, most commonly 1- and 2-yr individuals. In most

stocks, l-yr individuals were most frequent, although 2-

yr fish predominated in several stocks. Few stocks contained 0- or 3-yr individuals, and the percentages of

individuals with these freshwater residence periods were

small, with just two exceptions.

The numbers of alleles at the enzyme-coding loci

that were detected among the baseline stocks were as

follows: PGM-1*, two alleles; and PGM-2*, three alleles.

PGM-1*had only two observable phenotypes (one ofits

two alleles, unlike a normal allele, does not produce a

gene product that can be stained and seen), and both

occurred commonly in nearly all baseline stocks, albeit

in a wide range ofrelative frequencies among the stocks

(Wood et aI., 1988; Rutherford et aI., 1994). PGM-2*

had six observable genotypes, and each was observed in

at least one baseline stock. However, one of the three

alleles of PGM-2* was rare or absent in all stocks. As a

result, only three genotypes were common in the

baseline stocks for either locus. Nei's (1973, 1977) measure of stock differentiation (GST) was 0.25 for PGM1*,0.11 for PGM-2*, and 0.19 for both loci.

A dendrogram of baseline stock relationships from

the composite characters (PGM-1*, PGM-2*, freshwater

age, and Myxobolus arcticus) (see Fig. 3; Table 3) was

truncated on the right side to 13 similarity stock groups

labeled A-M. The two major branches of the dendrogram corresponded to high (A-G) and low (H-M)

parasite prevalence. The high-prevalence branch (AG) consisted of 21 Canadian stocks and 15 from Southeast Alaska. The low-prevalence branch consisted of 34

Canadian stocks and 3 from Southeast Alaska.

Stock Composition from Catch Samples

Contributions to the Noyes Island and Sumner Strait

fisheries were estimated for each of the 13 stock groups

(A, B, ..., M), as well as their consolidations into the

high (A-G) and low (H-M) parasite-prevalence

branches of the dendrogram.

Catch Samples from Noyes Island, 1986-Combined

stocks of the low parasite-prevalence branch (H-M)
10 NOAA Technical Report NMFS 132

Stock distance

0.48 0.40 0.32 0.24 0.16 0.08 0.00

Lf r--

I

I

I

I

M""

L """

K"""

J ""

I"""

H""

G""

F "

E"

0"""

C"

B""

A""

Group

Figure 3

Relationships among sockeye salmon, Oncorhynchus nerka, stocks based on four characters: PGM-l*, PGM-2*, freshwater age, and the parasite Myxobolus arcticus (see Table

3 for stocks within each group). Stocks composing each group A through Mare

indicated as being Alaskan only (*), Canadian only (**), or a mixture of both (***).

were predominant and were detected (detection means

the estimated percentage was greater than two standard errors) each week at Noyes Island (Table 4). These

stocks were estimated to have contributed 78-92% of

weekly catches. The major contributing groups were J,

detected in four of eight weeks, and L, detected in

seven of eight weeks. In all weeks, either J or L was

estimated as providing the greatest percentage of the

catches among all groups; in five of eight weeks, estimated percentages from J and L ranked first and second. Other detected groups of the branch were I and

M. Group I was detected in two weeks of mid-August.

~~M~~~~~~~~~ili~~~~

sampl~gwhen itslargest estimated contributions occurred.

Combined stocks ofthe high parasite-prevalence branch

(A-G) also were detected each week and were estimated

to have contributed 8-22% ofilie weekly catches. Each of

iliree branch members--groups A, F, and G-was detected in eiilier one or two weeks ofilie season (Table 4).

Estimated contributions by combined stocks of either parasite-prevalence branch were usually fairly precise with standard errors equaling 3-4% for six of eight

weeks (Table 4). Standard errors for estimated contributions by many oftheir component groups were larger.

Unconditional maximum-likelihood estimates of

weekly similarity-group composition at Noyes Island

were nearly identical to conditional maximum-likelihood estimates. The largest difference for any week and

group was 1.6%. Allowance for change in baseline characteristics only slightly altered perceived stock composition.

Finally, although changes in conditional maximumlikelihood estimates, with omission of freshwater age,

were larger among similarity groups within each parasite-prevalence branch, weekly branch totals changed

at most by 9.0% and averaged only 2.7% (increase of

high parasite-prevalence stocks) over the eight weeks of

sampling. For stocks that were detected at Noyes Island

from four characters, corresponding estimates from

three characters were reasonably well correlated (p=0.77,

n=17) .

Catch samples from Sumner Strait, 1987-ln contrast

to Noyes Island, Sumner Strait catches were composed

more of stocks of the high parasite-prevalence branch

(A-G) (weekly estimates, 50-77%, with average of 66%

of catches) than of the low parasite-prevalence branch

(H-M) (Table 5). Among groups of the high parasiteprevalence branch, D was consistently abundant (weekly
Pella et al.: Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka, Catches in Southeast Alaska II

Table 4

Point estimates (PE) of stock composition (%) of sockeye salmon, Oncorhynchus nerka, catches from Noyes Island in 1986

based on two genetic characters (PGM-I *, PGM-2*) , the parasite Myxobolus arcticus, and freshwater age. Values are reported

by similarity groups from Table 3. Standard errors (5E) are in parentheses. An asterisk(*) indicates group presence was

detected (point estimate was greater than two standard errors from zero).

First day of week

6July 13July 20 July 27 July

Similarity

group PE (SE) PE (SE) PE (SE) PE (SE)

A 0.0 (0.00) 1.5 (2.62) 0.0 (1.40) 3.1 (1.73)

B 0.0 (0.00) 5.5 (3.81) 0.0 (1.37) 0.0 (0.00)

C 0.0 (0.00) 0.0 (0.00) 0.0 (0.00) 0.0 (0.00)

D 4.9 (9.08) 1.0 (3.40) 0.1 (2.04) 0.3 (4.34)

E 13.6 (7.75) 1.3 (2.34) 0.1 (3.76) 3.2 (2.40)

F 0.0 (0.00) 9.0* (4.02) 10.0* (4.79) 0.0 (0.89)

G 3.9 (2.85) 0.0 (0.88) 9.3 (6.24) 1.3 (1.77)

H 0.0 (0.00) 0.0 (0.00) 0.0 (0.00) 0.0 (0.00)

I 18.8 (10.38) 0.0 (4.07) 1.0 (4.47) 6.5 (4.81)

J 21.6 (13.55) 28.0* (10.54) 34.8* (12.65) 19.7 (10.45)

K 17.3 (11.86) 12.8 (9.10) 0.0 (3.03) 15.4 (8.37)

L 14.6 (10.15) 37.5* (10.59) 44.8* (9.64) 50.4* (8.88)

M 5.2 (7.38) 3.4 (4.20) 0.0 (3.51) 0.1 (2.21)

A-G 22.4* (8.20) 18.2* (3.79) 19.4* (3.98) 7.8* (3.19)

H-M 77.6* (8.20) 81.8* (3.79) 80.6* (3.98) 92.2* (3.19)

First day of week

3 August 10 August 17 August 24 August

Similarity

group PE (SE) PE (SE) PE (SE) PE (SE)

A 3.9 (2.46) 0.5 (1.56) 3.8* (1.86) 2.1 (1.76)

B 0.0 (0.31) 4.6 (2.58) 0.0 (0.16) 0.0 (0.48)

C 0.0 (0.00) 0.0 (0.00) 0.0 (0.00) 0.0 (0.00)

D 2.9 (3.12) 2.0 (4.90) 0.0 (0.31) 4.0 (3.96)

E 0.0 (2.22) 3.2 (2.80) 6.3 (3.51) 1.0 (1.43)

F 0.0 (0.99) 0.0 (1.93) 6.3 (3.47) 0.0 (0.48)

G 3.8 (2.86) 5.4 (2.92) 0.0 (1.73) 11.0* (3.62)

H 0.0 (0.00) 0.0 (0.00) 0.0 (0.00) 0.0 (0.00)

I 0.7 (6.01) 16.5* (6.18) 15.9* (7.00) 0.7 (2.79)

J 36.9* (11.93) 13.9 (11.51) 5.8 (11.74) 28.9* (8.81)

K 1.5 (3.75) 10.4 (7.59) 0.0 (0.02) 10.1 (5.16)

L 50.3* (8.37) 27.6* (9.95) 55.4* (8.08) 23.2* (9.48)

M 0.0 (3.12) 15.7* (5.13) 6.5 (5.16) 19.0* (6.94)

A-G 10.6* (3.01) 15.7* (4.62) 16.4* (3.03) 18.1 * (3.81)

H-M 89.4* (3.01) 84.3* (4.62) 83.6* (3.03) 81.9* (3.81)

estimates, 11.2-51.6% of catches). Five of its seven

groups were detected during the season: A, the last

three weeks; D, each week; E, three of the first four

weeks; F, five of eight weeks including the first three

and last weeks; and G, three weeks of 28 June-4July, 5

July-ll July, and 26 July-l August.

The estimated weekly contributions at Sumner Strait

from stocks of the low parasite-prevalence branch (HM) were 24-50% and averaged 34%. Group H was

detected each week in small percentages averaging

slightly less than 4%. Group I was detected in three

weeks including the first and last;J, only in week 19-25

July; K, only in week 26 July-l August; L, in six of eight

weeks; and M, in only the first week (Table 5).

As for Noyes Island, estimated contributions by combined stocks of either parasite-prevalence branch were

fairly precise, with standard errors equaling 3-4% for

seven of eight weeks (Table 5). Standard errors for

estimated contributions by many of their component

groups were larger.
The largest difference between unconditional maximum-likelihood estimates of similarity-group percentages and corresponding conditional estimates of any

week and group was 3.8%. As with the data from Noyes

Island, allowance for change in baseline characteristics

only slightly altered perceived stock composition.

Finally, although changes in conditional maximumlikelihood estimates, with omission of freshwater age,

were larger among similarity groups within parasite-prevalence branches, weekly branch totals changed at most by

4.6% and averaged 0.6% (increase of high parasite-prevalence stocks) over the eight weeks ofsampling.

Local Accuracy Experiments

The purposeful alteration of the stock composition of

the Noyes Island and Sumner Strait catch samples was
___ Pella et aI.: Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka, Catches in Southeast Alaska 13

-5 0 5 10

Experimental Change (6)

5

o

-1 0 IL..-~_ _'_______'__ __'_____'__ _'__~_......J

-10

Q)

0)

c

CO

..c

o

"0

Q)

-CO

E

<<J

.~

en

W

-5-

CO.....

Q)

10 ,-----------------,.

10

•

•

-5 0 5

Experimental Change (6)

-1 0 "'------'-_--'------'_-'-_...L....-----'-_--L-------'

-10

Q)

g' 5

CO

..c

o

"0

.$ 0 CO

E

.~

en

W

-5

CO.....

Q)

<<J

10 ,----------------~

Figure 4

Experimentally induced changes in percentage for similarity group F versus estimated changes by the maximum likelihood method for the eight weekly samples

of Noyes Island, 1986.

Figure 5

Experimentally induced changes in percentage for similarity group I versus estimated changes by the maximum likelihood method for the eight weekly samples

of Noyes Island, 1986.

Discussion

approximately described by the conditional maximumlikelihood method. Average changes in estimated similarity-group percentages varied roughly in direct relation

with our experimentally induced changes (Figs. 4, 5).

Most estimated intercepts and slopes for the similarity

groups were near zero and unity (Table 6), respectively.

Tests of Fit

Maxima of the logarithm of the likelihood function

(support function) computed for weekly Noyes Island

samples on the basis of the four characters were far

smaller than expected ifrandomly drawn from the nullreference distribution. However, when freshwater age

was omitted, weekly maxima fell nearer to the center of

the reference distribution (Table 7). Compared with

Noyes Island samples, maxima of the support function

for weekly Sumner Strait samples were larger in relation to their null-reference distribution when four characters were included, and omission of freshwater age

had a lesser effect on the values (Table 7). All maxima

for both sets of weekly samples fell below the 50th

percentile when four characters were included, but six

of eight maxima (Noyes Island) and eight of eight

maxima (Sumner Strait) also fell below the 50th percentile when freshwater age was omitted.

Coverage of Contributing Stocks

by Baseline Samples

The 73 baseline stocks were distributed from southern

British Columbia to northern Southeast Alaska (Fig. 2)

and probably accounted for most fish in NBA catches.

Most large Canadian and Alaskan stocks of sockeye

salmon that contributed to NBA fisheries were included

in the baseline. The baseline comprised stocks of the

major Canadian rivers: Nass River, Skeena River, and

Fraser River. Two of these Canadian stocks were that

from Babine Lake in the Skeena River (Group L, Table

3) and that from Meziadin Lake of the Nass River

(Group J), both known from tagging studies to occur

abundantly at Noyes Island and to a lesser degree at

Sumner Strait (Pella et aI., 1993; Gazey et al. l ; English

et al.2). The tagging studies also showed that stocks of

the Fraser River, some of which were very abundant,

contributed measurably to catches at Noyes Island in

late season (Pella et aI., 1993; English et aI.2). Nine

Fraser River stocks were in the baseline, including four

of its largest populations during the catch sampling

years ofI986 and 1987: Birkenhead River stock (Group

L); and Adams River, Chilko Lake, and Lower Shuswap

Riverstocks (Group M). Finally, the largestsockeye salmon

stocks of the British Columbia central coast were three
14 NOAA Technical Report NMFS 132

Table 6 Table 7

Estimates of similarity group intercept (as) and slope Percentiles of the null-reference distributions for con-

({3s) and their estimated standard errors (SE) from ditional maxima of the logarithm of the likelihood

local accuracy experiments for Noyes Island and Sum- function computed from Noyes Island and Sumner

ner Strait samples (NA =not available). Strait weekly samples of sockeye salmon, Oncorhynchus

nerka, based on 1) all four characters: PGM-I *, PGM-2*,

Similarity A

S£(ffi5)

Myxobolus arcticus, and freshwater age, and 2) three of

group as 5£(0.5) f3s the four characters (freshwater age was omitted).

Noyes Island Percentile of

A 0.2 0.76 1.1 0.89 null-reference distribution

B -0.0 0.68 1.2 0.54

C NA NA NA NA First day D 11.3 10.02 0.2 0.69

£ -0.5 0.67 1.8 0.31 of week 4 characters 3 characters

F -0.3 0.67 1.5 0.39

G 0.4 0.75 1.4 0.30 Noyes Island

H NA NA NA A July 6 2 34

1 0.7 0.70 1.4 0.21 13 1 38

J 3.9 1.31 1.6 0.11 20 1 33

K 2.0 0.67 1.3 0.20 27 3 58

L 0.7 0.92 1.4 0.09 August 3 1 50

M 1.1 0.72 1.1 0.19 10 1 39

17 1 36

Sumner Strait 24 1 49

A 0.1 0.58 1.1 0.08

B -0.1 0.47 0.9 0.47 Sumner Strait

C 0.0 1.02 1.0 1.98 June 21 33 38

D 0.5 0.47 1.1 0.08 28 11 25

£ 0.1 0.51 1.1 0.08 July 5 14 21

F -0.9 0.72 0.9 0.14 12 18 24

G 0.5 0.51 1.1 0.21 19 28 45

H 0.0 0.96 0.9 0.70 26 22 44

1 -0.3 0.50 1.1 0.26 August 2 19 41

J 0.2 0.74 1.3 0.30 9 31 43

K -0.7 0.47 1.5 0.14

L -2.5 0.73 1.3 0.12

M 0.8 1.41 1.0 0.47

baseline stocks: Owikeno Lake stock (Group D), Tenas

Lake stock (Group G), and Long Lake stock (Group M).

Baseline stocks accounted for a substantial portion of

sockeye salmon from Southeast Alaska. Most (14 of 18

stocks) Southeast Alaskan baseline stocks were from

southern Southeast Alaska because tagging studies supported conjectures that northern Southeast Alaskan

stocks ordinarily contributed only minor numbers to

NBA fisheries (Pella et aI., 1993). Nonetheless, four

stocks of northern Southeast Alaska were included in

the baseline. Two northern Southeast Alaskan baseline

stocks, those ofChilkat (Group I, Table 3) and Chilkoot

(Group L) Lakes, are among the largest sockeye salmon

populations in all of Southeast Alaska (McPherson,

1990). Only about one-fourth of the known number of

southern Southeast Alaskan stocks (14 of 59) were

baseline stocks; however, these baseline stocks composed a much larger portion of the combined populations from this region. Nine of the 12 largest stocks of

southern Southeast Alaska were included in the baseline:

Hugh Smith Lake, McDonald Lake, Salmon Bay Lake,

Karta River, and Luck Lake stocks (Group D); Klakas

Lake and Kegan Lake stocks (Group E); Thoms Lake

stock (Group F); and Naha River stock (Group K).

Combined weir counts and peak foot-survey counts of

southern Southeast Alaskan baseline stocks during the

tagging years of 1982 and 1983 equaled 42% (1982)

and 47% (1983) of this region's estimated total escapements (325,000-350,000 fish, Pella et aI., 1988). Although many Southeast Alaskan stocks were not included in the baseline, one of their attributes-parasite

prevalence-could be inferred from the similarities and

differences of available baseline samples and was valuable in assessing contributions of Alaskan and Canadian stocks to NBA fisheries.

Similarities and Differences in

Attributes among Baseline Stocks

Percentages of NBA catches from stock groups (H-M)

of the low parasite-prevalence branch could be consid-
___ Pella et al.: Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka, Catches in Southeast Alaska 15

ered almost entirely of Canadian origin if northern

Southeast Alaskan stocks were always negligible contributors as they were in the tagging years of 1982 and

1983. Basdine stocks ofthe low-prevalence branch were

predominantly from Canada; 34 stocks were Canadian

and only 3 were Alaskan. Among the latter, 2 were from

Chilkat and Chilkoot Lakes in northern Southeast

Alaska; the third stock was from Naha River in southern

Southeast Alaska. All 15 other baseline stocks of Southeast Alaska (2 of 4 baseline stocks from northern Southeast Alaska and 13 of 14 baseline stocks from southern

Southeast Alaska) had parasite prevalence of 87-100%

and belonged to the high parasite-prevalence branch

(Table 3).

Although individuals in NBA catches from stocks

belonging to the low parasite-prevalence branch were

probably mainly of Canadian origin, the country of

origin of fish from stocks of the high parasite-prevalence branch was unclear: 21 of these baseline stocks

were Canadian and 15 were Alaskan. Although stocks

with high parasite prevalence were preponderant in

Southeast Alaska, they were common to Canada as well,

making it difficult to identity the country of origin for

individuals in NBA catches belonging to this branch.

On the other hand, such estimates reasonably provide

upper bounds for highly parasitized Alaskan stocks in

catches.

Stock Composition from Catch Samples

Notwithstanding statistical imprecision, estimated contributions to the Noyes Island and Sumner Strait fisheries by certain groups comprising major stocks were

roughly consistent with background knowledge ofpopulation magnitudes and migration routes. However, apparent presence (indicated by statistical detection) of

some groups composed of lesser stocks disagreed with

expectations at times. Additional aggregation of the 13

stock groups into consolidations of low and high parasite-prevalence branches was of practical interest because of the apparent predominance of Canadian fish

among stock groups of the low-prevalence branch and

because of the improved precision of such estimates.

The accuracy of estimated contributions by combined

groups of major stocks of the large Canadian rivers was

supported by comparison with independent ADF&G

assessments in which scales were used.

Catch Samples from Noyes Island, 1986-Canadian

stocks were chief contributors to catches at Noyes Island. Stocks of the low parasite-prevalence branch

(Groups H-M), composed mainly of Canadian stocks,

preponderated in all weekly catches; and stocks of the

high parasite-prevalence branch (Groups A-G), a mixture ofstocks from Canada and Alaska, contributed the

rest. The major contributing groups, J and L, were

expected to be well represented at Noyes Island throughout the season because of relatively large escapements

among NBA stocks to Meziadin Lake (Group J) and

Babine Lake (Group L) (Pella et aI., 1988; Pacific Salmon

Commission8,9) and the predominance of these two

stocks at Noyes Island during the tagging experiments

of 1982 and 1983 (Pella et aI., 1993; Gazey et al.l;

English et aI.2). Estimates ofstock composition at Noyes

Island from tagging (Pella et aI., 1993) indicated that

over 60% of the 1982 and 1983 catches were from the

Skeena River, mainly the Babine Lake stock, and over

10% were from the Nass River, mainly the Meziadin

Lake stock. During both tagging years, the estimated

percentage contributed by the Babine Lake stock to

Noyes Island catches during the last half ofJuly reached

80-90%. Escapements to the Skeena River declined in

1986 to 0.7 million sockeye salmon from escapements

of 1.2 million and 0.9 million fish in 1982 and 1983,

respectively (Pacific Salmon Commission9). Escapements to the Nass River, another major contributor at

Noyes Island in 1982 and 1983, also declined slightly in

1986 to 0.2 million sockeye salmon from escapements

of 0.3 million and 0.2 million fish in 1982 and 1983,

respectively (Pacific Salmon Commission9).

Independent evidence for presence ofgroup M, comprising large Fraser River stocks, was provided by Pacific Salmon Commission catch samples from Noyes

Island. Scale features of the sampled fish and their

length and age composition indicated that Fraser River

fish were at Noyes Island during the last four weeks and

were most abundant in the last two weeks (WoodeylO).

This time of appearance coincided with our detection

of group M in two of the last three weeks when largest

estimated contributions occurred. Fraser River stocks

were very large compared with other contributors in

1986 with 3.7 million total escapement, including that

of baseline Adams River (1.3 million escapement) and

Lower Shuswap River stocks (0.6 million escapement)

(Pacific Salmon Commission8). Over 50% of the estimated total return of 16 million fish to the Fraser River

in 1986 was from the Adams River and Lower Shuswap

River stocks (Pacific Salmon Commission8).

8 Pacific Salmon Commission. 1988. Report of the Fraser River

Panel to the Pacific Salmon Commission on the 1986 Fraser River

sockeye salmon fishing season, 36 p. Pacific Salmon Commission,

600-1155 Robson St., Vancouver, B.C., Canada V6E IB5.

9 Pacific Salmon Commission. 1992. Northern Boundary Technical

Committee Report. U.S./Canada northern boundary area. 1992

salmon fisheries management report and 1993 preliminary expectations. Report TCNB(92)-I, 160 p. Pacific Salmon Commission,

600-1155 Robson St., Vancouver, B.C., Canada V6E IB5.

10 Woodey, J. 1994. Pacific Salmon Commission, 600-1155 Robson

St., Vancouver, B.C. Canada V6E IB5. Personal commun.
16 NOAA Technical Report NMFS 132

During mid-August, the Alaskan Chilkat Lake and

Canadian Nanika River stocks, composing group I, were

likely mistaken for abundant and similar stock groups

such as J and L. Historically, sockeye salmon of the

Nanika River stock have passed through terminal fishing areas between late June and late July (Jantzll); in

1986, fish of the Chilkat Lake stock continued through

the weir from early July to the end of September

(McPherson, 1990). The 1986 Nanika River escapement count was 3,000 (Peacock12 ), compared with the

Chilkat Lake escapement count (weir count) of about

24,000 (McPherson, 1990). Considering the small size

ofthe Nanika River stock and its migration through the

fisheries before detection of the group during midAugust, the fish at Noyes Island were more likely from

Chilkat Lake if they were from either of the two stocks.

Substantial numbers of Chilkat Lake sockeye salmon

were not expected at Noyes Island in 1986 based on

results of the tagging programs of 1982 and 1983.

Group A of the high parasite-prevalence branch, the

Canadian Mikado Lake stock, was detected as contributing less than 4% in the week 17-23 August. This small

stock (1986 escapement count: 2,000) was expected to

migrate through terminal fishing areas by mid:July

(Jantzll), therefore another contributing group was

presumably mistaken for Group A.

Group G ofthe high parasite-prevalence branch, composed ofCanadian stocks ranging from the Fraser River

to the Canadian central coast, was detected in only the

last week. Catches at Noyes Island during at least the

last three weeks probably contained a broad mixture of

stocks from Alaska southward to the Fraser River.

Catch Samples from Sumner Strait, 1987-In contrast

to Noyes Island, stocks of the high parasite-prevalence

branch (Groups A-G), rather than the low parasiteprevalence branch (Groups H-M) , were more abundant at Sumner Strait. The five detected groups of this

branch (A, D, E, F, and G) included Alaskan and Canadian stocks. Each stock group of the less-abundant, low

parasite-prevalence branch (Groups H-M) , composed

mainly of Canadian stocks, was detected at Sumner

Strait sometime during sampling.

Significant contributions to catches at Sumner Strait

were expected from stocks ofsouthern Southeast Alaska

(included in groups D, E, F, and K), Nass River

(Meziadin Lake stock ofGroupJ), Skeena River (Babine

Lake stock of Group L), and Stikine River (Tahltan

Lake stock of Group L), based on geographical locations of the fishery and spawning grounds, magnitudes

11 Jantz, L. 1992. Canada Dep. Fish. and Oceans. 202-417 2nd Ave.

W., Prince Rupert, B.C., Canada V8J 1G8. Personal commun.

12 Peacock, D. 1992. Canada Dep. Fish. and Oceans, 202-417 2nd

Ave. W., Prince Rupert, B.C., Canada V8] 1G8. Personal commun.

ofstocks, and results of tagging experiments on returning adults (Pella et aI., 1993; Gazey et aLI; English et

al.2). Group D contained many of the largest stocks of

southern Southeast Alaska, including the large

McDonald Lake stock, known from the coded-wire tagging of outmigrant smolts to return as adults predominantly by means of Sumner Strait (Van Alen13).

Adult sockeye salmon were tagged in Sumner Strait

during the 1983 fishing season (Pella et aI., 1993;

English et aI.2), and computed stock composition indicated that Alaskan stocks contributed 49% of the catch;

nearly all the remainder was from Nass River (15%),

Skeena River (20%), and Stikine River stocks (15%).

Escapements to the Skeena River increased to 1.3 million sockeye salmon in 1987 from 0.9 million in 1983

(Pacific Salmon Commission9). Escapements to the Nass

River in 1987 equaled 0.2 million sockeye salmon, essentially unchanged from 1983 (Pacific Salmon Commission9 ). Escapements to southern Southeast Alaska

in 1987 were unknown but were estimated at 0.3 million sockeye salmon in 1983 (Pella et aI., 1988).

Other stock groups, including A, H, I, and M, were

detected at times in Sumner Strait, but their actual

presence is doubtful. More likely, fish of other stock

groups were confused with those ofthese groups. Group

A, the Canadian Mikado Lake stock, was detected in

apparent abundance (14.4-30.0%) during the last three

weeks and after its expected migration through the

NBA fisheries (Jantzi I).

The apparent persistence of Group H, the Gingut

Creek stock, throughout the season was caused by the

small and continual occurrence (1.7-5.8%; average,

3.9%) of age-O fish in the weekly test-fishery samples

and their scarce presence in baseline samples. Other

sockeye salmon populations, composed mainly of age-O

fish, are also known to spawn in the lower Nass River

system, of which Gingut Creek probably supports the

largest numbers (Rutherford et aI., 1994). Nonetheless,

the Gingut Creek stock is usually small and had an

escapement count ofonly 1,000 fish in 1987 (PeacockI2).

Time of return of the lower Nass River stocks through

terminal fisheries probably occurs from early June to

the first week ofJuly (Rutherford et aI., 1994;Jantzll).

Therefore, these stocks were probably not present in

Sumner Strait for at least the last five weeks. More

likely, either the baseline percentages of age-O fish in

some baseline stocks present in Sumner Strait during

1987 were too low because freshwater age composition

changed after the time of baseline sampling or stocks

present at Sumner Strait with higher percentages of

age-O fish were not included in the baseline. In particular, higher percentages (up to 44%) offreshwater age-O

13 Van Alen, B. 1993. Alaska Dep. Fish and Game, P.O. Box 240020,

Douglas, AK 99824-0020. Personal commun.
___ Pella et al.: Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka, Catches in Southeast Alaska 17

fish have been found at some locations within the Stikine

River drainage (Wood et aI., 1987a).

Again, fish ofsimilar stocks present at Sumner Strait

probably were misassigned to Group I (Chilkat Lake

and Nanika River stocks). The largest contribution detected (12.0%) occurred during the last week, 9-15

August, after Nanika River fish had presumably passed

through terminal fisheries (JantzII). Escapement to

Chilkat Lake was about 49,000 fish in 1987 (McPherson,

1990) and occurred from late June to the end of September. The escapement count to Nanika River was

4,000 (Peacock12). The Chilkat Lake stock was not expected to be present in substantial numbers if their

migration was like that during the 1982 and 1983 tagging experiments.

Finally, fish ofsimilar stocks were probably misassigned

to Group M. No sockeye salmon tagged in Sumner Strait

or Clarence Strait (adjoining Sumner Strait) during the

1982 or 1983 studies were recovered from southern Canadian fisheries or escapements. If Fraser River stocks were

present at Sumner Strait in 1987, migration routes must

have changed since the tagging years.

Comparison of Stock Composition Estimates with

Those Based on ADF&G Scale Pattern Analysis

sembled into five groups ofstocks: 1) Alaska I, typified

by Salmon Bay Lake and Hugh Smith Lake patterns; 2)

Alaska II, typified by the McDonald Lake pattern; 3)

Nass River and Skeena River; 4) Stikine River; and the

5) Tahltan Lake stock of the upper Stikine River.

As noted earlier, scales of stocks from spawning

grounds in interior British Columbia within the Nass

River, Skeena River, and Fraser River differ from scales

of coastal stocks. Misclassification ofscales among stocks

of the major Canadian rivers is more likely than between those Canadian stocks and Southeast Alaskan

stocks. Similarly, our method more likely misassigns

contributions among stocks within the high and low

parasite-prevalence branches of the dendrogram than

between these branches. If stocks of the J, L, and M

groups ofthe low parasite-prevalence branch other than

those ofthe Nass, Skeena, and Fraser Rivers were minor

contributors to the catches, classification by scales may

well have assigned most fish of these three groups as

either of Nass or Skeena River origin. If so, the sum of

Nass and Skeena River percentages based on scales

should agree with the sum of percentages from groups

J, L, and M, based on PGM-l*, PGM-2*, freshwater age,

and Myxobolus arcticus (Fig. 6). Agreement was only fair

among weeks ofthe season at Noyes Island (p=0.37 with

100.,.-----------------,.

Figure 6

Comparison of weekly point estimates of percentages

of catches of sockeye salmon, Oncorhynchus nerka, at

Noyes Island in 1986 and Sumner Strait in 1987 from

stock groups], L, and M (likelihood analysis based on

PGM-l *, PGM-2*, Myxobolus arcticus, and freshwater age)

and Nass and Skeena River stocks (classification analysis based on scales).

• Noyes Island 1986 t Sumner Slrail1987

20 40 60 80 100

Percentage by classification analysis

o

(/) 80

'iii

>-

(ij

c:

<1l

" 60 0

0

£:

Qi

>- 40 .0

Q)

0>

<1l

"E

Q) + + 20 + Q)

c..

14 Oliver, G. T., K. A. Jensen, 1. S. Frank, and N. J. Sands. 1987.

Contribution of Alaskan, Canadian, and transboundary sockeye

stocks to catches from Southeast Alaskan districts 101-108, 1986,

based on analysis ofscale patterns. Unpubl. rep., 39 p. Alaska Dep.

Fish and Game, Div. Commer. Fish., Douglas, AK 99824.

Weekly scale samples from the 1986 Noyes Island commercial purse-seine catches and the 1987 Sumner Strait

commercial gill-net catches (recall that our samples

were obtained from the Sumner Strait test gill-net fishery) were classified to stock groups by linear discriminant functions and used to estimate stock group percentages in the weekly catches (Jensen and Frank, 1988;

Oliver et aI. 14). Classification functions were developed

for each of the four significant age groups of sockeye

salmon (1.2, 1.3, 2.2, and 2.3) occurring in either fishery, by using scale characters observed in baseline

samples ofthe appropriate year ofreturn. For the Noyes

Island analysis, baseline scale samples were aggregated

into four stock groups: 1) Nass River, 2) Skeena River,

3) Alaskan stocks typified by McDonald Lake of Southeast Alaska, and 4) other Southeast Alaskan stocks.

Fraser River stocks, now thought to have been present

at Noyes Island late in the season of 1986, were not

included in the scale baseline. The Pacific Salmon Commission8 estimated 12,000 (2.7%) of 445,000 sockeye

salmon caught at Noyes Island were from the approximately 16 million Fraser River return of 1986. For the

Sumner Strait analysis, baseline scale samples were as-
18 NOAA Technical Report NMFS 132

n=8) and Sumner Strait (p=O.69 with n=7). However, by

combining pairs of estimates by the two methods from

Noyes Island and Sumner Strait into a single sample,

good overall agreement was evident (p=O.92 with

n=15). Clearly, both approaches detected fish of the

same stock groups as expected. However, sampling errors and possible contributions from stocks other than

those of the Nass River, Skeena River, and Fraser River

to the j, L, and M similarity groups were disruptive.

When the difference in total percentage contributed by

Canadian interior stocks was large, as between Noyes

Island and Sumner Strait, both approaches discovered

the change.

Although assessments of stock percentages from the

large Canadian rivers agreed fairly well between methods, a persistent discrepancy was evident between the

parasite prevalence in catch samples and the estimates

ofAlaskan contributions to the Noyes Island and Sumner

Strait catches based on classification from scales. The

scale and parasite samples were collected under different sampling schedules; scale samples were larger and

represented the fishing fleets better. In particular, parasite samples at Noyes Island may have emphasized

catches from the southern portion ofthe fishery, owing

to practical constraints (Van Alen13), and the Sumner

Strait parasite samples were from the test fishery rather

than the commercial fishery. On the other hand, if the

parasite samples were representative of the parasite

prevalence in the catches in either fishery, some scales

from high parasite-prevalence Alaskan stocks in the

scale baselines must have been similar to those of low

parasite-prevalence stocks omitted from the scale

baselines. These low parasite-prevalence stocks were

probably from Canada; if not, then they were almost

surely from northern Southeast Alaska because 13 of 14

stocks from southern Southeast Alaska, including most

of the larger stocks, were highly parasitized (Table 3). If

results of the 1982 and 1983 tagging studies apply,

fewer northern Southeast Alaskan sockeye salmon would

be expected at Sumner Strait than at Noyes Island; and in

neither fishery were large numbers of northern Southeast

Alaskan sockeye salmon expected (Pella et al., 1993).

If all the parasitized fish in catches at Noyes Island

and Sumner Strait were from Alaskan stocks, an estimate of the percentage of Alaskan fish that were parasitized could be obtained by dividing the percentage of

parasitized fish in the samples by the estimated percentage of catch from Alaska based on scales (Tables 8, 9).

At Noyes Island (Table 8), only the improbable presence of large numbers of fish from low parasite-prevalence Alaskan stocks, like the large stocks of Chilkat

Lake and Chilkoot Lake from northern Southeast

Alaska, could account for the apparently low percentage of Alaskan fish that were parasitized. Unweighted

Table 8

Comparison of observed parasite, Myxobolus arcticus, prevalence of the 1986 sockeye salmon, Oncorhynchus nerka, samples

from Noyes Island purse-seine catches, percentage of catch judged from scales to be ofAlaskan origin, parasite prevalence

of Alaskan fish if all parasitized fish were Alaskan, and implied minimum percent contribution from low-parasitized

Alaskan stocks such as (1) Chilkat Lake and Chilkoot Lake or (2) Naha River.

First day of week

6Jul 13Jul 20Jul 27 Jul 3Aug 10 Aug 17 Aug 24 Aug

Percent with parasite} 34 25 20 15 13 22 18 23

Percent Alaskan by scales2 36 39 24 21 16 24 27 49

Percent Alaskan with parasite] 96 64 85 70 81 90 66 47

Implied minimum percent from

1) Chilkat Lake and Chilkoot Lake 4 -4 30 8 24 12 2 28 49

2) Naha River5 0 48 17 39 23 9 45 74

} Source: Moles et al. (1990).

2 Source: Oliver et a1. 14 .

] Computed as 100 x percent with Myxobotus arcticus + percent Alaskan by scales, i.e., assuming all parasitized fish were from Alaska.

4 Percent Chilkat Lake and Chilkoot Lake stocks (p x 100%) satisfies the mixture equation relating prevalence of the parasite among

Alaskan sockeye salmon in the catch samples (A) with prevalence in Chilkat Lake and Chilkoot Lake stocks (0%) and average

prevalence among the other Alaskan stocks (92%), i.e. p x 0% + (1- p) x 92% ; A..

5 Percent Naha River stock (p x 100%) satisfies the mixture equation relating prevalence of the parasite among Alaskan sockeye salmon

in the catch samples (A) with prevalence in the Naha River stock (30%) and average prevalence among the other Alaskan stocks (96%),

i.e. p x 30% + (1 - p) x 96% ; A..
___ Pella et al.: Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka, Catches in Southeast Alaska 19

average prevalence ofthe parasite among the 14 baseline

stocks ofsouthern Southeast Alaska was 92%; if parasite

prevalence of stocks could be weighted by magnitude

of return (annual information is not available for most

stocks), the weighted average would probably be even

higher because the largest stocks have high parasite

prevalence. If the parasitized fish of catches were composed of a mixture of Chilkat Lake and Chilkoot Lake

fish (0% prevalence) and fish of other Alaskan stocks

(92% prevalence), the Chilkat Lake and Chilkoot Lake

stocks must have contributed between 0 and 49% ofthe

Noyes Island weekly catches, with average contribution

of 19% (Table 8). Unless migration routes of sockeye

salmon in 1986 were radically different from those during the tagging years of 1982 and 1983, Chilkat Lake

and Chilkoot Lake fish would not be present in such

abundance, and thus the estimates of percentage of

catches from Alaskan stocks based on scales appear too

high if estimated parasite prevalence was representative

of commercial catches. If an abundance-weighted rather

than unweighted average of the parasite prevalence

among Southeast Alaskan fish had been used in the

computations or if some parasitized fish in Noyes Island catches were from Canadian stocks (as seems certain because Canada also has numerous heavily parasitized stocks), the necessary contributions by low parasite-prevalence Alaskan stocks, such as those of Chilkat

Lake and Chilkoot Lake, would be even greater.

As for Noyes Island, estimated weekly percentages of

Alaskan fish in Sumner Strait that were parasitized appeared unreasonably low in most weeks when calculated from parasite prevalence of our test-fishery catch

samples and from stock composition of commercial

gill-net catches determined from scales (Table 9). Average prevalence of the parasite among the 13 baseline

stocks ofsouthern SoutheastAlaska, excluding Naha River

stock, was 96%; Naha River stock had 30% prevalence. If

the parasitized Alaskan fish comprised a mixture of the

Naha River (30% prevalence) and other southern Southeast Alaskan stocks (96% prevalence), the Naha River

stock must have contributed 17-23% ofthe Sumner Strait

catch over the first six weeks (Table 9). Unless abundance

ofthe Naha River stock greatly increased from 1983, when

the bulk of escapement through a weir was roughly 5,000

fish (Pella et aI., 1988), such a significant contribution is

implausible. In 1983, tagging studies indicated that all

stocks of Alaskan district 101 (including the Naha River

stock and larger stocks of McDonald Lake and Hugh

Smith Lake) contributed less than 30% to the Sumner

Strait catch (Pella et aI., 1993). Because the omission from

the baseline ofsignificant, low parasite-prevalence, southern Southeast Alaskan stocks is unlikely, estimates of the

percentage of Sumner Strait catches from Alaskan stocks

based on scales also appear too high without the significant presence of low parasite-prevalence Alaskan stocks

like Chilkat Lake and Chilkoot Lake (Table 9).

Table 9

Comparison of observed parasite, Myxobolus arcticus, prevalence of the 1987 sockeye salmon, Oncorhynchus nerka, test

fishery samples from Sumner Strait, percentage of fishery samples judged from scales to be of Alaskan origin, parasite

prevalence of Alaskan fish if all parasitized fish were Alaskan, and implied minimum percent contribution from lowparasitized Alaskan stocks such as (I) Chilkat Lake and Chilkoot Lake or (2) Naha River.

First day of week

21Jun 28Jun 5Jul 12Jul 19Jul 26Jul 2 Aug 9 Aug

Percent with parasite} 78 69 54 74 73 73 75 65

Percent Alaskan by scales2 91 86 62 87 90 88 80

Percent Alaskan with parasite 3 85 81 87 85 81 83 94

Implied minimum percent from

1) Chilkat Lake & Chilkoot Lake 4 8 12 5 8 12 10 -2

2) Naha River5 17 23 14 17 23 20 3

} Source: Moles et al. (1990).

2 Source:Jensen and Frank (1988).

3 Computed as 100 x percent with Myxobolus anticus .... percent Alaskan by scales, i.e., assuming all parasitized fish were from Alaska.

4 Percent Chilkat Lake and Chilkoot Lake stocks (p x 100%) satisfies the mixture equation relating prevalence of the parasite among

Alaskan sockeye salmon in the catch samples (A) with prevalence in the Chilkat Lake and Chilkoot Lake stocks (0%) and average

prevalence among the other Alaskan stocks (92%), i.e. px 0% + (1- p) x 92% = A.

5 Percent Naha River stock (p x 100%) satisfies the mixture equation relating prevalence of the parasite among Alaskan sockeye

salmon in the catch samples (A) with prevalence in the Naha River stock (30%) and average prevalence among the other Alaskan

stocks (96%), i.e. px 30% + (1 - p) x 96% = A.
20 NOAA Technical Report NMFS 132

Local Accuracy Experiments

The conditional maximum-likelihood method was capable of quantitatively assessing the experimentally induced changes in stock composition of mixed-stock

samples with available sample sizes from stocks and

weekly catches. Although available sample sizes were

adequate for the purpose of discovering and describing

changes in contributions from similarity groups A-M

under local accuracy experimental conditions, the potential variation in estimated similarity-group percentages from the available samples was often large unless

similarity groups were further consolidated into high

and low parasite-prevalence branches of the dendrogram (Tables 4, 5).

Local accuracy was demonstrated when composite

types of individuals of the computer-generated sample

(appended to the actual catch sample) were assigned

by reference to the baseline samples. However, relative

frequencies of genotypes in the baseline stocks must

have varied among years from genetic drift alone, especially for small populations (Crow and Kimura, 1970).

Relative frequencies oftypes for the nongenetic characters-parasite prevalence and especially freshwater

age-probably varied even more than for genetic characters (Wood et aI., 1988). Occurrence of such variations between times of sampling baseline stocks and

catches was investigated by tests of fit.

Tests of Fit

Variation in relative frequencies of character types within

stocks, especially of freshwater age, between the sampling times of stocks and catches was probably responsible for the discernible lack of fit of the mixed-stock

model. The fit of the model, with freshwater age omitted, was improved: nonetheless, other conditions used

to generate the null distribution may not have been

realized in actual sampling. Certainly, some drift in

relative frequencies of genotypes between baseline and

catch sampling may have occurred, and stocks absent

from the baseline may have been present in the catches.

Summary _

Sockeye salmon caught in NBA fisheries, including

Noyes Island and Sumner Strait, comprise mixtures of

Canadian and Alaskan stocks. Annual variations in n umbers ofsalmon returning to stocks and in the migration

routes of these salmon through the NBA make estimation ofstock composition in fisheries imprecise without

regular monitoring of catches for stock composition. At

present, scales from fish in catch samples are used in

the monitoring ofthese stocks. Scale features vary among

years; therefore baseline samples of scales from groups

of stocks are obtained annually to develop rules for

classifying scales of catch samples to stock groups.

Assessments of stock composition in NBA fisheries

would be improved by better coverage of stocks potentially contributing to fisheries and by increasing the

number of characters useful for distinguishing among

stocks. This feasibility study increased stock coverage

over that in which scale-pattern analysis was used but

was limited to characters known in earlier years to have

shown promise for distinguishing stocks when the catch

samples were obtained. Baseline samples of four characters from 73 stocks (55 Canadian and 18 Alaskan) covered

all major and many lesser stocks potentially contributing

to the fisheries. The four characters ofsockeye salmontwo genetically inherited proteins, freshwater age, and a

parasite-were examined for their utility in realistic assessments of weekly stock composition in two Alaskan fisheries: Noyes Island in 1986 and Sumner Strait in 1987. All

characters were variable among years in their relative

frequencies of types within populations; but among the

four characters examined, the genetic proteins were probably the most stable among years, and freshwater age, the

least stable. Repeated sampling over years indicated that

stability of parasite prevalence was more like that of genetic characters than offreshwater age. Reasonably timestable characters are essential if better baseline coverage

ofstocks is to be economically feasible.

A dendrogram based on the composite four characters was used to delineate the baseline stocks into 13

groups similar in their characters. All 13 similarity groups

are represented in the two major dendrogram branches:

the low parasite-prevalence branch and the high parasite-prevalence branch. Prior knowledge of magnitudes

of Alaskan and Canadian sockeye salmon stocks, their

migration routes, and their estimated contributions to

NBA fisheries based on tagging studies of 1982 and

1983 demonstrated that low parasite-prevalence stocks

occurring at Noyes Island and Sumner Strait were mainly

of Canadian origin. However, high parasite-prevalence

stocks could have been of Canadian or Alaskan origin.

Estimated stock compositions at Noyes Island in 1986

and Sumner Strait in 1987 were disparate, as expected

from previous scale analyses and tagging studies. Differences in stock composition were most evident from the

parasite. The percentage of fish parasitized in weekly

samples at Noyes Island ranged from 13 to 34%, and at

Sumner Strait, from 65 to 78%. Maximum-likelihood

estimates of weekly percentage of Noyes Island catches

from low parasite-prevalence stock groups ranged from

78 to 92%, and at Sumner Strait, from 24 to 50%.

Estimates of percentages from low parasite-prevalence

stock groups represented reasonable minimal estimates

ofCanadian contributionsiflow parasite-prevalence north-
___ Pella et al.: Stock Composition of Some Sockeye Salmon, Oncorhynchus nerka, Catches in Southeast Alaska 21

ern Southeast Alaskan stocks were rare in NBA fisheries in

1986 and 1987, as in the tagging years of 1982 and 1983.

Numerical studies were conducted in which the percentages of the 13 similarity groups in catch samples

from Noyes Island and Sumner Strait were altered by

adding random samples of individuals drawn from the

baseline stocks. A regression analysis of the estimated

changes on the induced changes for the eight weekly

samples of either Noyes Island or Sumner Straitshowed

that the maximum-likelihood method was able to recognize the alteration of the samples. With perfectly

time-stable characters, the composition changes were

fairly accurately, if imprecisely described.

Numerical studies detected lack offit of the mixture

model to samples of the baseline stocks and catches at

both Noyes Island and Sumner Strait. Probably changes

in relative frequencies of characters between sampling

catches and stocks were largely responsible. Freshwater

age appeared responsible for much ofthe lack offit ofthe

mixture model to catch and baseline samples, although

other characters also likely contributed to the lack offit.

Two reassessments ofstock composition at Noyes Island

and Sumner Strait were performed to accommodate the

changes in baseline characters. First, both stock composition and parameters describing baseline stock characteristics were estimated by the unconditional maximum-likelihood method. Differences between conditional and unconditional maximum-likelihood stock composition estimates for any ofthe 13 similarity groups were less than 2%

in any week at Noyes Island and less than 4% in any week

at Sumner Strait. Second, conditional maximum-likelihood estimates, based on only three characters (freshwater age was omitted), showed minor differences in estimated percentages of combined high or low parasiteprevalence stock groups and, hence, in the proposed

minimum estimates ofCanadian stock contributions.

Estimates ofAlaskan contributions to the catches based

on scales were shown to be too high throughout most of

the season at both Noyes Island and Sumner Strait unless

the parasite prevalence observed in catch samples at both

fisheries was not representative ofthat ofthe commercial

catches or migrations were radically different from those

of tagging years. Greater confusion between Alaskan and

coastal Canadian salmon scales was expected than between Alaskan and noncoastal Canadian scales, but direct

evidence ofsuch errorwas notfound. Rather, many salmon

from low parasite-prevalence stocks ofCanada (the largest

of which were noncoastal) were apparently misc1assified

as Alaskan stocks when scales were used.

Conclusion

Stock composition of sockeye salmon in two NBA fisheries was assessed with good precision from the four

characters studied, if stock detail no finer than that

defined by low and high parasite-prevalence branches

was required. However, sampling error was generally

much larger for some subgroups within these branches.

Moreover, contributions of some stock groups were

misidentified, and this confusion was likely among similar stock groups within the branches. Since this study

began, the baseline has grown both in number ofstocks

and characters. Many more polymorphic loci that show

promise in resolving mixed-stock composition of sockeye salmon in NBA fisheries are now available for the

baseline stocks: PEPC* and LDH-Bl* (Utter et aI.4);

sIDHP-l*, SIDHP-2*, and sAAT-l,2* (Wishard3); ALAT*

and sMDH-Bl,2* (Grant et aI., 1980); SAAT-3* (Wood

et aI., 1987a); LDH-B2* (Hodgins et aI., 1969); and

mAAT-l* and ESTD* (Guthrie et aI.7). Feasibility of a

higher resolution of stock contributions by addition of

these genetic characters to the baseline will be examined in future studies.

Acknowledgments _

The authors thank Chris Wood of the Pacific Biological

Station of CDFO for compiling freshwater age samples

from fisheries organizations of the United States and

Canada and for providing a computer file of the four

baseline characters in the analysis. The helpful comments of Norma Jean Sands and Ben Van Alen of

ADF&G, Chris Wood, and an anonymous reviewer on

an earlier draft are much appreciated.

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