Bull.Eur. Ass. Fish pathot. g (2).23. I9tt8 NATURALINFECTION OF FJARMEDATLANTIC ,salmo sarar L., PARRBy GLOCHIDIAoF THE FRE pEA sHWATE R RL Mu ssE L, M arg a r i tife ra marg a r i tife ra L.

Bv D.W.BRUNO', A.H. McVIcARI ANDI.F. WADDELL,

During December1986 we examined a infections and fluid loss followins high prevalenceof whiteraised spots on the sloughingof the glochidiafrom the eil[ gill lamellae of a population of farmed during their natural migration Atlantic from-the salmon, Salmo salar L., parr fish. reared in tanks. A moderatelv heavv invasion of the glochidial ,iug. oi In addition a variety of methods of margaritifera L., was removingthe glochodiafrom the gills were identified. This is the first reported examined.Due to the sizeof the farm and occurenceof this condition from Scottish the large number of fish involved fish farms. any treatment methods would (a) need to have practicalapplications on the fish farm (b) The freshwater , M. haveto achievea high degreeof successto margaritifera lives in fast flowing cool justify the effort and (c) usechemicals and waterswhich are relativelylow in calcium, methodsrecognized for useon fish farms. thusrestricting its rangein Britain to rivers The histoparhologyof this host-glochidia and streamsto the west and north of the interactionis poorly describedfor Atlantic country (Young & Wiliiams, 19g3). A salmon and therefore was also included hookless, in subovate glochidial stage is this study. releasedannually in largenumbers and is parasiticon thegills of theSalmonidae and Tiials wereeither carried out within the occasionallyother fish and this maintains affected fish farm or on a population of the upstream adult populations (Bjork, fish moved from the farm to an 1962). The reproducrivebiology of the experimental site. The following freshwater pearl mussel is reviewed by treatmentswere carried out on at least20 Young& Williams(1984). parr in eachexperiment: (a) Bathing in salt water at concentrationsof 5.4, 10.1and The non-swimming glochidia 24.20/nfor l, 3 and 9 hours and in 33.30/* undoubtedlyreached the gill lamellae of for 24 hours, (b) bathing in 0.5 mg theparr passivelyin theventilating current. lr CuSO+for lh, (c) bathing in 5 mg l_l The consequence prolonged of Nuvan for th followed by I mg l-, Roical attachementof the glochidia on the parr for th. Fish were also held in water raised was unknown, but it was believed the from 3-5'C ambient at the time of glochidiawould remain on rhe gills and sampling to l0'C for three weeks in an continue their development until the attemptto stimulatenatural detachment of temperaturesrose during the following the glochidiaand to gain early evidenceof year.This studywas designed to investigate any associatedproblems. Entire gill arches whether there was a significant risk of (hemibranch)were carefully dissected from growtl.rbeing contprornised or significant treated and control groups at l, 2 and 3 mortalities resulting from secondary weekspost treatmentand fixeclin bufferecl Bull.Fur. Ass.Fish Pathol.8 (2),24, 19t38

smooth,opaque-white, l09o l'ornralin.The total nrtnlbet of glochicliaappeared number of glochidiaon the ventraland dorsalsurface and slightlyovoid. A maximum glochidia per fish (calculated by was recorded using low Power 736 number of glochidia / gill rnagnification. Both right and left gili multiplying the the number of gill arches) was archeswere examined. No variation in the arch by studY. number of glochidia was foutrd between recordedin this Millemann (1977)reported that the left or right sideso only the right side Meyers& salmon fry is 2300 was processed during subsequent the LDm for Atlantic glochidia, henceit was concluded sampling.Adjacent gill archesfrom these attached lack of significant mortalities iish were sectioned and stained with that the among the farmed fish was due to haemotoxylin and eosin (H&E) and the found levelof infection and the larger numberof glochidiapresent in a mid-tissue the lower the farmed fish. By January the sectionrecorciecl' The developmentof the size of glochidia on the gill lamellae glochidia and the host response was number of by over 50V0, although in examined histologically. Monthly had declined sampling through to May there samplingof the farmed stock was carried subsequent further significant decline' ln June out between December and May' Growth was no 1986 Sl stock had completed of the farmed fish was continuallY when the themean number of glochidiaper monitored. smolting gill arch had declinedby 8490.There wasno difference in the number of Bathingin seawater, Nuvan and Roccal, iignificant glochidia on the ventral or dorsal gill arch CuSOoor raisingthe watertemperature did potential Sl or 52 parr (Thble l)' not kill or dislodgea significantnumber of of the no glochidia on the 51 post- glochidia from the parr when compared There were in September (Thble 1). This data with control groups as determined by smolts wasconfirmed by the counts carried out on counting cysts on whole gill arches and stained gill arch sections' The 52 examining histological sections' No the population were killed in the summer so mortalities occured following these data on this population is not treatnlents.During regular monitoring of further Although the adult musselswere the farmed stock therewere no significant available. prior to July from the water mortalitiesattributed to the glochidia, no translocated intake of the farm and from 500m of the secondaryinfections and growth was not river upstream the 1987 hatch became compromisedin comparisonto uninfected by glochidia in September'It was parr reared in tanks on the experimental infectecl the 1986hatch had beeninfected at site. believed a similar time. A comparison of the two years data suggests that a significant The mean number of glochidia on the of glochidia are lost during the gill arch of the parr at the start of this number first few months post-attachement (Thble outbreakthrough smolting in the following 1). The loss of the glochidia during,this May/June,is rePortedin Thble1' time was considered to result from incomplete attachement and may have The following year class(1987 hatch) was been aided by respiratorymovements' similarlysampied beginning in September'

The histopathology of the glochidia ln December,at least 90Voof the 1986 infection in parr appears harchwere carrying a moderateburden of be similar to the observationsmade by glochidiawhich had encystedon the gills' to & Millemann (1978)who reported preclominantlyon the middle section ot Karna on experimentallyinfected alevin chinook lach hemibranch(Fig' 1),but occasionally salmon. OncorYnchus tshqw)'/scha thev were attachedto the gill rakers'The Bull. Eur..Ass. Fish pattrol.8 (2).25. lgSg

Walbar-rrn. In the Atlantic salmon Summary examinedhere, each glochidia was covered by a hosr epithelial layer which rapidly A high prevalenceand moderatelyheavy invasion by mussel glochidia of the developedinto an fresh water pearl mussei extensivehyperplasia, Margaritifera margaritifera on the gilis of farmed graduallyprolif'erating to involveadjacent Atlantic salmon, Salmo salar pair is reporte<.i. Attempts lamellae,parricularly when the glochidia to. remove the glochidia by raising the salinity,bathing in Nuvan fbllowed continued by Roccal;CirSOr their developmentto juvenile bath treatment; or raising temperature, had no musselsduring the spring (Fig. 2). success.There was no significant mortality, reduction 11growlh, or seriousparhology In contrast,Fustish & Millemann of the gilli of infected 097g) fish in fresh water or after smolt transfir to seawater. found in experimentally infected coho salmonO. kistrtchWalbaum that sloughing Rel'erences ol theglochidia wasnearly complete by l4 Bior.k S. (1962). Investigations on Margatiti.fera dayspost infection.Meyers, Millemann & Margarilifera and Unio crassa.r.Acta Lintnolos. 4, t-t09. Fustish (1980) suggestedthat apparent resistanceto rhe glochidia in coho salmon Fustish,C.A. & Millemann, R.E. (197g).Glochidiosis may be due,in part, to humoral factors.In of salmonid fishes. II. Comparison or tissue response of coho and the presentwork, where encystment chinook salmon to had experimental infection with Margaritifera occuredat the tips of the Atlantic salmon margaritifera (L.) (Pelecupoda:Margaritanidae). J. gill lamellaethey were often clubbed and Parasitol.64, 155-157. bent, prolif'eration the cellular fusing with Karna, DW. & Millemann, R.E. (1978).Glochicliosis other adjacent lamellae. L)espitea well oI salmonid fishes. IIl. Comparative susceptibility developedhyperplasia the giochidia ro natural infecrion wirh only Margaritifera margaritifera (L.) (pelecypoda: appeared to have sloughecl from the Margaritanidae).J. Parasitol.64, 529-537. filament tips, and this \.\rils recorded Meyers,T.R. & Millemann (1977). histologicallyas empty R.E. Glochidiosis cystsrvith ruptured of salmonid fishesI. Comparativesusccptibilirv walls associatedwith the presence of ro experimental infecrion with Margaritifeia aneurysms.Haemorrhaging from the giils margaritifera (L.) (Pelecypoda:Margariianidae). J. Parasitol.63. 728-'733. and lifting of the epitheliumon secondary lamellaewas noted in the presentstudy, Meyers,T.R., Millemann. R.E. &Fusrish,C.A. (19g0). Glochidiosisof salmonid particularlyin those fish where a large fishes.IV. Humoral and tissueresponses of coho and chinook salmon to numberol-glochidia had attached. experimental infection with Margaritiferu morgori t i|bra (L.) (Pelecypoda:Margaritanidae). J. Parasitol.66, 274-281. It is concluded that although the appearanceof the glochidia on the gills is Young, M. & Williams, J. (1983).The starusand spectacular and a localized pathoiogy conservation of the freshwater pearl mussel Margaritdero morplariti"fera Linn. in Crear occurs,the survival and growth of theparr Britain. Biol. Conserv.25, 35-"12. in fresh water and

,A,uthor'saddresses: rDepartment of Agriculture& Fisheriesfol scotland,Marine laboratory,Victoria Road, Torry, Aberdeen AB9 8DB, Scotland. : SeaFhrrn (Polly) t.tct., 9 ArdrossTerrace, Inverness, IV3 5 Ne, Scotland. l}till.i:ur. i\ss lrishl)rtthol. 8 (:). 26, 1988 otr lt waterpcall ntusselMctgariti't'eru mttrguritifbrz' couutc:ci litirlr,1: \learr nlillrt)eI ol grochicliaoi thc tlcsh saltnon'Salnto salat part 'iiillr. rriltatch rrl tirtmer'i Atlantic of glochidia Number lvleanuumber of- gtochidia Nu.mber Mean number Month pore.tialSi\"- ._"#i'"10potenrials2's !j"Trii".o gitl Dorsal gill Ventralgiil Dorsalgill Ventral arclt atch arcil atch 4NSNS Dccctnbcr 50 45 135204 Jan uary 22 20 829269 Ftbruary 2t 2l ")R 620186 March o ls 9 4 Apfll 30 3l 618126 Nlay 7t 15 109144 .lurte 88 * 5NSNS Septclnbcr O0

S!'ptclnbcr**A 11^ 604 **B 41 4 Septcntber )u * Postsmolts NS = not sarnPled = ** 198?hatch A = 6mm grade B Tmmgrade

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't"ij:rected auachecrmussel gtochidia o1 Atlantic salr.on, sa/nro sa/ar wirh the g'l arch ir.r>ma farmed the glochidia. ii::T '.*ti;;i^;;;;.d) can be seencoverilrg Margaritifertt morg0ritifera.A lryperplastic or a rarnrcd Atlantic salnron' gro:l]t: to the secondarvIamellae 2. MaryaritiJerq ntargarittfera ti.ig. i::""::"1(t{&E' x145)' Solmo sqtarpatrs"ampled inJanuery