‘31

Genetic protein variation in Atlantic salmon in Scotland: comparison of w ild and farmed fish

ABSTRACT

Youngson. .A.F.. Martin. S..A.M.. Jordan. W.C. and Vcrspoor. E.. 199 I I+!e!~c prslc;n \arUlron in .%ltantic salmon in Scotland: ro!nparison of HIid dl1t.i ~JI II& ftsh. It/rtUc’id/llrfl*. OK: 13 I -2-t:.

Generic variation at six enzyme loci was ckammcd in named lmes elf Atlantic salmon IP culture In Scotland. Twelve lines were exammed i.1 each of two years of stud!. Slgnifirant drffcrcncc\ irl allclc frequencies were observed htween the yar classes of named lutes and hctcrogcnetl> ~3s prcscnt among lines in both years. Allele frequencies in farmed 11~s founded on fish taken from Scottkh rivers or from Norwegian farmed populations dlffcrcd s!Lnllkantl> 31 mou 1~x1. In both ~3~5 allt~lc iqzncicsd~ffered from those in samples of wild Lsh taken from 1 X rt\t’rs In Scotland Xfo51 larmcrl !IXL based on the fi:h of ;inglc rtvcrs also diffcrcd from aild fish In the \p~~~t’ic rI\c’rs from ~h.11 they had been derived. Genetic distances between farmed lutes and their \r~ld sour;~s were of the same order as those between the salmon of diffcrrnt rttcrs rn Srotland. So o.tclaii ~ctiu~twr! rn nct- c-r,;;positv was detected in fumed lines. Most differences lxtwrcn the farmed lanes and thcrr UIILI LWICPS appcarcd stochartlc i.1 origin.

INTRODUCTION

Little is known of the genetrc differences which may exist betw,ecn wild and farmed Atlantic salmon but it has been suggested that fish which escape or are released from farms may alter the genetic constituticn of local. wild pop- ulations. It has been suggested that this may be detrimental to the populations and to the fisheries they support (Maitland. 19891 The magnitude of any effect will be determ ined in part by the extent and nature of the differences between the farmed and wild fish: the greater the adaptively relevaut genetrc differences the greater the likely effect.

In Scotland the culture of salmon is based on many independently founded liner of fish bearing the name of the farm on which they have been developed OI- the river from which founding hroodstock was obrainrd. The lines have been based on weld fish oorained from Scottish rivers or on farmed ppula- ?ions obtained from Norway. Among accessible sources of ova, those derived

0044-8486/9 I /SOL!. 50 XI I9Y 1 Elscv~et Science Publ~sners B.V. alI rrghts reserved.

232 A F YOUNGSON t : AL.

from adults of particularly favoured phenotypes have tended to be exploited most.

Wild salmon in ditf’erent rivers form genetically differentiated populations (Stahl. 198 1, 1987: Verspoor, 1988a) and some differences appear to be: as- socidted with adaptation (Saunders, 198 1; Verspoor and Jordan, 1989). Giver the range of sources from which they have been derived, farmed lines in ! zot- land might be expected to differ from each other in the same way. i yowever. changes may have occurred in culture as a result of founder effects, genetic drift or selection. Evidence for the occurrence of the fiist two of these cate- gorics of change ir: farmed salmon outside Scotland has been presented by Crossand King ( 1983), Stahl (1983) and Verspoor ( 1988b).

The present study examines the genetic constitut;G; 6f nr;mc? 5~s & farmed salmen in Scotland at six enzyme ioci which are commonly poly- morphic in Atlantic salmon. Samples of named lines are compared between year classes and with samples of v.ild fish from the rivers on which the lines were based. This extends ;I prcvtous. prelimiuar, J ret,drt of a single year’s data . (Youngson et al.. 1989).

No centralised salmon brtcding programme, comparable to that which has operated in Norway. has been conducted in Scotland. rns;ead many commer- cial companies have established named lines of farmed salmon indepen- dent:j. Tile iines chosen m tne present study were all stated to have originated from wild adult salmon captured in rivers in Scotland or Norway. In all, 16 named lines were exammed and eight of these were examined in both years of study. Eleven of the 16 lines had been foundeo C;I the progeny of salmon captured in single identified rivers.

O-group iuveniles were sampled in July or August in 1988 or 1989. Each !ine was sampled at its original Scottish source farm. Fifty individuals were sampled pro rata from all the tank groups representing the line, according to the advice of the hatchery manager. Samples were placed immediately on dry- ice and stored subsequently at - 5O’C.

As part of another study (Jordan et al., unpubl. results) wild juvenile sbrlmon were sampled from I8 Scottish rivers distributed around the Scottish coast. Some of these rivers had been the source of founding broodstock for some ,,f the named lines sampled. O-group juveniles were sampled from single rivers between June arid October in 1987 or 1988. A number of rivers were sampled at the same location in consecutive years. Groups of 50 fish were sampled singly at fixed intervals, in areas of intence rratural spawning, over I- or 2-km lengths of river. Samples were placed immediately on dry-ice and stored later at -50°C.

Elwtruphoresis The enzymes amino-aspartate transferase (AAT), sorbitol dehydrogenase

(IDDH), malic enzyme (MEP). isocitrate dehydrogenase (IDHP) and mal- ate dehydrogenase (MDH) were examined by specific staining (Harris and Hopkinson. 1976) of zynograms of tissue extracts run according to the meth- ods of Vet-spoor ( 1988b ). Variant alleles were detected at the UT-J*. IDDH- I*. IDDH-2*, MEf-2*, ?DHf-3* and ll1DII-3,~* loci. using the terminology and nomenclature recently adopted by the American Fisheries Society (Shak- lee et al., 1989). Two variant alleles were detected at L&4T-3*. All the variant alleles detected appeared identical to those reported previously, elsewhere in the species’ range (e.g. Cross and Nard, 1980).

Allele frequency data for Norwegian river populations used to found fairned lines of salmon were derived from Vuorinen and Ekrg ( 1989 ).

Statistical ana1y.m Although some sampies were of different year classes or different represen-

tations of the same named line, all 24 samples of the 16 lines examined have been treated as independently founded. This was considered appropriate be- cause most lines have been propagated in culture for only a few generations ( c 4 ) and genetic mixing of separately founded components of the lines is correspondingly unlikely to have occurred. Lines have not been identitied by their trivial names, for reasons of commercial confidentiality. Instead, each line has been alphabetically coded.

Departures of genotype proportions from Castle-Hardy-Weinberg expec- tations were tested using the BIOSYS-I program of Swofford and Selander ( 1981 ). Differences among and between samples were tested for individual loci using either a G-test or. where appropriate, a Fishe:‘s exact test. Probe- bilities tar different loci were combined using Fish&s method. A paired t- test was used to test the significance of differences between source and farmed populations and Bartlett’s test was; used tcb examine the homogeneity of vari- ances. Allele frequency differences between groups of farmed lines were com- pared using the non-parametric Mann-Whitney U-test. since variances of al- lele frequencies were heterogeneous among groups. These methods are described by Sokal and Rohlf ( I98 I ). rJenetic distances were calculated ac- cording to Nei ( 1972) and the principal component analysis was performed using SYSTAT (Wilkinson, 1988).

R ESU LTS

Fariation within samples The genotype proportions observed in samples did not differ from th: Cas-

tle-Hardy-We&erg distributiolt expected on the basis of random mating. For samples obtained in 1988 and 1989 respectively, only three of 58 and two

234 A.F. YOUNGSON ET AL.

TABLE 1

Frequencies of variant alleles at six polymorphic loci and mean hctcrozygosity per locus in 16 named lines of farmed saln:on ;n culttire in Scotland. The lines arc identified alphabettcally rather than by trivial name

Locus/Allele

Scottish origin A 1988 50

1989 50 B 1988 50

I’189 50 C’ IV88 50

1989 50 i, iY88 50

1969 50 Ii 1988 50

I’)89 50 F I988 50

I989 50 Ci 1988 50 11 lY8S 5t.l I I989 so

Nowegtan orrgin J 1988 49

1989 50 Ii lY88 50

1989 50 L tY88 50 hl 1988 50 N lYR9 sn 0 1Y8Y 50 I’ 1989 50

0.030 0.080 0.060 0.020 0.580 o.szo 0.000 0.010 0.180 0.040 0.000 0.500 0.350 0.1 IO 0.000 0.030 0.000 o.coo 0.490 0.630 0.010 0.001) 0.150 o.ono 0.000 0.430 0.351 0.011 0.000 O.ObO 0.000 0.000 0.760 0.750 0.000 0.000 0.290 o.oon 0.000 0.34cl 0.280 0.040 V.040 0. i40 0.‘1! !I n.x 5450 t-i.dlO 0.030 0.000 0.230 0.020 0.000 0.450 0.133 0.020 0.010 0.290 0.000 3.000 0.550 0.310 0.060 0.000 0.520 3.000 0.000 0.300 0.420 0.000 0.040 0. I40 0.050 d.010 0.510 0.420 O.WJ 0.130 0.240 woo 0.000 0.660 0.230 0.030 o.wMl 0.060 0.020 0.000 0.750 0.150 0.000 O.OlO 0.ltd.i c.2::: S.t.JOO G.t%!C 0.160 0.100 wxo OJRJ 0. I IO 0.000 0.550 0.198 0.000

0.223 0.259 0. I 74 0.206 0.142 0.226 0.227 0.208 0.244 0 ‘37 .- U.‘34 0.234 0.123 0.221 0. ! Y2

0.082 0.1 t 2 0.071 0.0 IO 0.581 0.316 0.04 I v.247 0.04-l 0.140 0.010 0.010 0.510 0.380 0.040 t-t.235 o.ooo 0.200 0.040 0.030 0.690 0.480 0.05 1 0.246 o.ooo 0.000 0.050 0.000 0.320 0.574 0.000 0. I72 0.010 0.030 0.030 0.050 0.620 0.490 0.070 0.222 0.000 o.ooo 0. I67 noI0 0.630 0.650 0.030 0.213 0.050 0.200 o.oso 0.040 0.500 0.540 0.010 0.267 o.ow 0.120 0.030 0.000 0.640 0.311 0.009 0.195 l.moo 0.010 0.204 0.000 0.370 0.150 0.310 0.251

‘Samole six.

of 54 tests were rignifkantly different from expectation (2 test; P~0.05 ), a frequency of significant departures expected by chance alone. Further anal- yses were therefore carried out on allele frequencies (Table 1 ), since these were considered to represent the sampled grtiups adeqtaately.

C briariotl uithin liws Over all the polymorphic loci examined signifirnnt heterogeneity was pres-

ent within seven UI’ tight pairs of samples obtained from tht same lines in both years of study (Table 2). In the same way, significant overall heteroge

GENETIC PRO1 EIN VARITION: COMPARISOS OF WILD AND FARMED FISH

TABLE 2

Slgmficances of allele frequency dlfferervf ki cigh’ jzzir~~~’ isc romwisons of turned lines of farmed salmon sampled in both years of study

IDHP- .%iDH- .\fEP-‘*’ IDDW I+’ IDDH-1*’ Combined’ j+: 3.4.:

A 0.114 0.374 0.249 0.258 0.01 s 3 x lo.‘4 3x10-4 B 0.002 l.ooo l.oon 0.396 I x lo-’ 0.736 0.00 i C 1x10-’ 1.ooo l.ooo < IO-6 i 1il-* ’ I 1160 --IO * D 0.384 1.000 IIKJCI U-158 6x IO-’ 0.249 0 0’7 E <lo-’ G.500 o.o;o t x IO-’ I.ooo I .Ch 0.S’: F 0.002 1.000 l .OckJ ‘! 113 0.204 0.249 -z IO-4 ? 0.433 0.030 0.746 ‘J 513 0.349 0.628 0.‘6? - - K < !o-6 o.mo 0.123 < 10-b 0.189 O.Vj* CliJ

-

‘I--‘est. ‘Fidler’s exact test. ‘Combmed probab,!i;i.

TABLE 3

Significaroces of G-tests for heterogeneity between farmed Ilnes of salmon wirhin each of the IWO years of study

Ycat 4-U-P IDMY-ml* .VDII-3.4. ML-P-2* IDDtt. lb IDDII-2’ Combined

1988 < IO-h <!n-* 2x lW5 < IO-h i 10-h IX IO-‘ *= Ii)-* 1987 < 10-6 c 1o-6 0.236 . IO-” < IO-h < lo-” 6: 10-e

neity w irs also present w ithin all three pairs of samples obtained in the same year from independent reprerentations of lines bearing the same name (G- test; P < 0.05 ).

Variation berween l ines w ithin pars Significant heterogeneity was present among the 12 line5 camplrd in I %d

at ezch of the six loci examined and ar&long ihe 12 lines WY@P~ in I989 for all loci except MDH-3,4* (Table 3 ). Mean allele frequencies in samples from ?he nine lines founded on Norwegian populations differed from those in the 15 lines founded on Scottish rivers. Significant differences were present at the k1DH-3,4* and IDHP-.P loci (Taole 4).

Comparison cffarmed lines w irh w ildfish from Scottish rivers The general extent of genetic differences w ithin and among farmed lines

and w ild T.sh sampled in Sco;land waz visualised using the principal compo- nents (Fig. I ). Farmed lines as a group were distributed difterently from the group of samples from the rivers. Farmed lines were genetically more diverse,

236 9 F YOt~NGSON El‘ AL.

Slgmficancc ofd~f&xewcs in ali& frequencies in three groups of salmon populalions: mild fish from Scottish rlwrs. farmed lmes bawl on Nnrwcgian populationsand farmed hncs bawd on Scottlrh rivers. The .~nal>ws :!r’: bawd on Xlann-U’hitnck I.-statdcs and probahilitrcs (P! are indtcatcd

Locus/:\llelc ..--

Farmed Norwscgidn \ USJS farmed Scottish i’ 64.50 ‘al.50 3ll.ooc 30.030 61.CU-KI 47.50 49.00 I’ 0.84 0.06 0.0x 0.008 0.700 0.23 0.26

H itd \crws farmed Korwcgtan 1’ 9l.W I’.frO 99.500 3 I .5Od 42,5O\J 34.00 46.50 I’ 0.w 0.0’1 O.?dC) 0.00 I 0.050 0.02 0.07

\i’~ld \c’rsu!, thrnk-d S,rlttlsh 1 I57.00 I J4.N~ 2x000 I2.5.500 78..co(’ I 12.50 I 15.50 f’ 0.W (I.73 O.LliJ ! 0.490 0.04 I 0.4’ 0.46

----~ .--- ----- --

4 j

2

. l - *

. .

.* ,*

.; l

l * ’

l .

-4 c~-- -_-. _ . _ . .-

-4 -3 -2 -1 Cl 1 7 Faclor 2

Fig. I. Tiw lcla:i;?n:hin b-twccn crincipal comwnentsofallelr frcquew! data in wild iish from Scotttsit rt\crs ( 3 1 and I‘arrnrd lines in Sctit1ar.d fwnded on Scottish rivers ( 0 j or Norwegian populattons ( 1; i.

particularly those of Norwegian origin. Moieover the mean genetic composi- tion of the groups appeared to be different. A more detailed analysis (Table 4) indicated that the mean aLIe frequencies of farmed lines of Norwegian origin and Scottish wild Gsh were significantly different for four of s-:ven al-

Sigmfkances of dlfferPnces in genotyp frcquenclcs III farmed Iws and m lish r;implcd from the rl\c’r\ or, uhich the imes were founded -- --_ Lane Yrar i. ‘T-j* * , IDlIP-F .\11111-.J. 4.: .IIEf’--‘*’ iDJ111~ I” IUIUI-2.’ C‘ombined ’

.4 1988 0.300 2.020 r> 'j .- 1989 0.580 0.004 I.< .In

6 ; 9x O.,ltl b uI\ I” 3x io- ; .oo 198Y 0. 130 3x10-5 I .dJ

(‘ 1988 2% IO-’ 0.060 I.00 I989 0.530 0.060 1.00

I3 tC)?S 0.210 0.006 1.00 1989 6x1s 4 0.w.l I.00

E 1988 0.002 0. I23 I.00 198C :lO-” 0.113 1.00

F 1988 3. I70 0.36C 0.38 l9R9 U.240 0. I’0 I.&J

ci 1988 Rx IO-J 0.500 I .lW t-l l98U 0.470 O.JO3 : .PI) I 1989 0.007 0. I YO il.42 1 19R8 O.O?G 5x lo-\ ! 46

1989 0.04u O.JlO 0.45 N 1989 6~ IO.-’ 0.010 0.31

----

‘Ci-test. ‘Fisher’s exact test. ‘CombirScd prob,.bilit;..

T.4BLE 6

Paired comparisons of mean difference of &le frequencies and hrtcroz:gosil: and compailsun cl! ~MISC‘C In g-oups of farmed lmes and wild fish from their source Gcrs. Variation LI \IINf-.l,P ha\ noI hcrn rwwd- ered because of the low freq.!enc) of the less common allclc. hlean alklv fw;li;cnc) (.I 1 standard de* utlon 4s.d. ). the values of the I or f statistics and probability values (PI are’ mdicat:d -.- -- - ---

Locu.s/.4llclr -~- -_-- -_------

:I. 1 l--P IDIll’-3’ .\IFf’- ‘* 1 - lUl)fi- I’ IDI,jl ‘I'

25 53 . /II i.2.i __ -' ‘5 II --

Comparrson of mcar.s ( paired !-ttst ) I988

I 0.005 0.070 O.L45 ‘J. I I(J 0 i 1lJ u.rw II rr2\

s.d 0.060 0.108 O.lW u 1157 U.2l.l 0.05 O.U61 I 0.060 I.600 I .3w J,‘W 1 .f.(H) u. I I c O.V:U P lJS ns n5 O.CtiJJ n5 ns ns

1989

id. OSKI6 0.0'6 0.044 O.Il IO U.&l3 Il.(HlY -(I (Y!O 0 IJ’R 0 II> ..r,:; n I57 (1.01 ‘? ‘1 O-l? n (‘7~

I 0.520 0.550 I.46U b. I ‘I) 0. ! ; (I 'J 5IH) (1. N,'J P ns ns ns ns 115 FI n\

C’om,urison 06 varlanccs ( Bat-M t-5 tc51)

19R8 ? 0.080 0.050 0 140 3. I60 I .JW 0.650 0.190 P 0.780 0.820 O.ilc, O.tP’I cl.230 ‘1.42” f3.c I9

I989 f 0.310 0.410 I.910 %I.150 l.?l( 2.030 0.100 P 0.600 0.540 0.190 CJ.64 0.210 II. I dr! 0.760

--. ---.

A F YtrlJN(;SUN ET AL

. .

Ei - .- . . . . . . . ” .

F’ l . l l . . l .

L. -_ I- -_] 0 ,Ol ,02 ,03 .04 .05 -06

Gencti.: Dastance

Fig. 2. (icnctic distances in pav *A iv con,narisons of groups ofsalwon in Scotland. A: trlbutan ;xqwlaltons bct\rccn !cars: H: lribuldq populations within the Kylcs of Sutherland calchment: \ ‘: trihutan populatwns within the Ri\cr Twcrd: D: river populations wthin Scolland (25 rb I&MN comparisons of 18 r’\ C’TS I: E: fwmcd IIncs and Hild source populations: F: named farmcc! linlr twtx\ccn bcnrs.

lele frtquency comparisons. Farmed lines based on Scottish rivers and sam- ples of k ild fish differed with respect to r-.ean frequencies for two alleles.

In 198;. all nine possibie comparisons of lines founded on salmon drawn from sin& rivers with the salmon of these source rivers indicated overall significant d;fferences compounded of differences at most individual loci. In 1089. eight of .>ine possible comparisons indicated overall differences of the same type i Tabi- 5 ). Exceptionally. mean allele frequency at the MEP-2* locus in farmed lines in 1988 was significantly differen! from the mean fre- quency in samples fror.7 their source rivers (paired I-test, f c 0.01; Table 6). However. no similar diffe;-rice was evident for named lines s mplcd the fill- lowing year (Table 6 ). Meal. ivariances of allele frequencies in farmed lines used in the paired comparison did not differ significantly from mean vari- ances in the matched group of samples from rivers (Table 6 ). Mean hetero-

zygosity also did not differ significantly in I988 or 1989, between lines and samples obtained from their source rivers (Table 6 ).

The genetic distance between named lines of farmed salmon and their wild sources was greater than genetic distances observed between years in wild fish in different rivers in Scotland (Fig. 2). It was greater on average than the genetic distances between salmon in different tributaries of the same river system (Fig. 2 ). Average genetic distances between iines and their sou~c’es appeared somewhat greater than average genetic distance between rivers. Furthermore. and perhaps surprisingly, genetic distances between samples of named lines from tb e two study ! ears were generally greater than those ob- served between lines and their sources.

DISCUSSION

There wa; no evidence in the present study for generally reduced variability in the farmed lines examined. as detected previously in cultlired Atlantic salmon (Cross and King, 1983; Stahl. 1983: Verspoor. 1988b) although a fe- indivtdual lines did show reductions. Given the sampling regime employed. the results obtained are likely to be representative of Scottish farmed lines ctf salmon generally. The genetic characterisation of the lines is also likely to be accurate within the limits of the sample sizes used. Because of this. compari- sons of the genetic character of the lmes with the samples of wild fish from rivers should be indicative of the actual differences between farmed and wild fish in Scotland.

The observed heterogeneity among lines is expected becava35 they have beea founded on disparate wild sources and previous studies of protein variation in wild salmon indicate that they form genetically differentiated populations (Stahl. 1987; Verspoor, 1988a,b). The observation that lines founded on wild Scottish salmon differed as a whole from those founded on Norwegian popu lations probably reflects regional differentiation. In the same way the hetero- geneity observed among farmed lines founded on No.wegisn populations and among farmed lines based on Scottish rivers is also expected: the fish of dif- ferent rivers in both countries are genetically differentiated (Stahl and H,o- dar, 1988; Jordan et al.. unpubl. results).

It appears however that some genetic differentiation among lines is attrib- utable to changes which have occurred in culture. This is evident in tne on- servation that most farmed lines showed significant genetic divergence from their specific source. The divergence was probably not attributable to any sampling mismatch; it was greatei !!~an ?hat which might be expecre. from sampling different year classes or at different loeatiolls within a ri\,er and was similar in magnitude to the differentiaticn observed between wild fish in dif- ferent rivers.

There was some evidence that genetic divergence between farmed lines and

thc’r cnllm :es might be partly directional. In those farmed lines examined in 1988, it appeared that directional change had occurred at the MEf-2* locus in favour of the 125 allele, since mean frequency differed significanti, be- tween wild sources and lines. However, no similar difference could be shown to be present ill independent comparisons made in 1989.

Protein loci are b;ually assumed not to be susceptible to selection. although the literature contains many apparent exceptions (e.g. Place and Powers, 1979; Powers et al.. 1979; Koehn et al., 1980: Koehn and Immermann, 198 1, also see review in Kirpichnikov, 198 1). In wild Atlantic salmon, allele frequency at the M‘P-2* locus shows a clir,al relationship to latitude, attributed to the effect of temperature variatitin (Verspoor and Jordan, I989 ). Genotype at the .\IEY-2* locus has been associated with age at sexual maturity: maturity after only one winter at sea tends to be associated with the heterozygous gen- otypr (Jordan et al., 1990 ). Differential growth and survival has been ob- served among ,t-fEP-2* genotypes in juveniles (Jordan and Youngson, I99 1). These findings suggest that the possibility that MEf-2* polymorphism might be susceptible to ac!ive or passive selection under some conditions in culture should not be CdSily discounted.

1 n tereqtingiy . ovel all alleie frequencies were significantly different in al- most every comparison of different representatio;l;. of the same named lines. This difference is prob;lbl) not attributable to differences between fish in the locations chosen in rivers to found farmed lines and those chosen to represent the wild fish of the same rivers genetically. In Eeperal, the genetic distances between different representatic,:~ of the same named lines exceeded those detected in comparisons of wild fish sampled at different locations within single rivers and were generally greater than those which exist between the fish of different catchments. This suggests that the authority conferred on lines by their trivial names is not warranted, at least in the present context.

It appears that allele frequencies differ overall be::l.een farmed and l aiid salmon rn Scotland !n the absence i;f compelling evidence of non-random change at individual loci the differences in mean genetic composition dre probably largely attributable to the presence of fish of Norwegian origin on Scottish farms. Differences exist between Lss and their source but the changes have occurred in the absence of overall reductions of diversity. The magni- tude of the differences which -IOW exist are comparable to the natural differ- ences which exist between wild fish in separate river catchments and are probably attributable to genetic drift, enhanced by practices which rtduce the effective population sizes of farmed lines.

Drawing further conclusions from studies of tissue protein variation is con- strained by the relatively small number of loci which i; is tcchnisaily p&blr: to screen and because protein loci represent only one class of genetic loci in which genetic differentiatitin may arise. The observed level of differentiation may not be representative of differentiation in the genome as a whole. Addi-

tionally. the adaptive significance of genetic differences of Lne magnitude de- tected is not known.

.ACKNOWLEDGEMENTS

We acknowledge the support of the commercial fish farms in granting us access to their stocks. We thank the District Salmon Fishery Boards for their support and the river proprietors for their authority to carry out sampling of wild salmon populations. This work was supported by a grant to W.C. Jordan from the Department of Eductic ‘5~ for Northern Ireland and by a vacation studentship to S.A.M. Mart.in jointly from :hrs rirlaiitic Salmon Trust and the Scottish Salmon Growers Association. The Depa,<yent of Fisheries and Oceans, St. Johns, Nfld. supported E. Verspoor over the course of this study. This paper was improved by the efforts of two. anonymous referees.

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