Species of Gyrodactylus (Platyhelminthes, Monogenea) on salmonids in Sweden

Fisheries Research, 17 ( 1993 ) 5 9 - 6 8

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Species of Gyrodactylus ( Platyhelminthes, Monogenea) on salmonids in Sweden G. Malmberg*, M. Malmberg Swedish Museum of Natural History and Department of Zoology, University of Stockholm, S- 106 91 Stockholm, Sweden

Abstract Judging by our investigations hitherto of Swedish natural waters, the only GyrodacO,lus species infecting salmonids are: Gyrodactylus salaris Malmberg, 1957, Gyrodactylus derjavini Mikailov, 1975, Gyrodactylus lavareti Malmberg, 1957 and one other unidentified Gyrodactylus species. G. lavareti was found on Coregonus lavaretus (L.) in Lake Ottsj6n (J~imtland, 1955) and the unidentified species (one specimen only) on Thymallus thymallus (L.) in Lake Satisjaure (Lappland, 1971 ). Gyrodactylus salaris was found on Salmo salar L. in two northern rivers (in one of them only on migrating smolts) and in one southern river in the Baltic region (nine rivers investigated), and in two river systems in the Atlantic region (eight rivers investigated). G. derjavini was found on Sahno trutta in three rivers in each region. Our results from Swedish fish farms are: G. salaris was found in five out of 12 farms in the Baltic region and two of six farms in the Atlantic region; G. derjavini was found in two of 12 farms in the Baltic region and one of six farms in the Atlantic region. Thus, in natural waters as well as in fish farms, G. salaris and G. derjavini seem to have an uneven distribution. Apart from the limited number of investigations, this may be due to geographic distribution of the species, anthropochoric spreading, macroenvironment (type of waters, other physical parameters), seasonal variations, or life cycle dependent variations of infestation intensity. The absence of G. derjavini in rivers north of the Dal~ilven may depend on the geographical distribution. Migrating smolts orS. salar (three rivers) and precocious males (one river) seem to be infected more than smaller salmon. Fish farm (or ~culturing') dependent changes in Gyrodactylus species populations (genetic drift) are possible. G. salaris and G. derjavini are secondarily adapted to parasitism on Oncorhynchus mykiss (Walbaum ), which may lead to a spread to new areas and problems in fish farms. Taxonomically, G. salari,~ and probably G. derjavini also, cause problems. For example, are G. salaris and Gyrodaco'lus thymalli Zitnan, 1960 really different species, and how large is the range of variation in G. salaris and G. derjavini? With regard to similar species, complementary morphometric, experimental infection investigations and electrophoretic ( DNA ) analysis are recommended.

Introduction The Swedish investigations into Gyrodactylus infections in fish began in 1951 and included fish species in fresh, brackish and salt water. Before 1971, however, the investigations had no special reference to salmonids, and thus *Corresponding author.

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G. Malmherg, M. Malmherg/Fisheries Research 17 (1993) 59-68

resulted in a rather limited knowledge about Gyrodactylus species on salmonids (Malmberg, 1957, 1970). In 1957 Gyrodactylus salaris was described from salmon at the Hrlle Laboratory, River Indals~ilven, J~imtland. In 1952, in connection with parasitological treatment in the laboratory (fish farm), a sample of G. salaris had been taken from salmon parr specimens. Also in 1952, Gyrodactylus lavareti was found in Lake Ottsjrn, J~imtland, but the species was not described until 1957. In the course of investigations in 1958 and 1972 of specimens of Salmo trutta L. from a small stream (K~illefallsb~icken) near Tidaholm, V/istergrtland, Gyrodactylus derjavini was discovered but it would be a long time before this species was described (Mikailov, 1975; see Ergens, 1983 ). In 1971, a special salmonid investigation was performed in the Stora Sjrfallets National Park, Lappland. One specimen of a Gyrodactylus sp. belonging to the G. salaris group was found on Thymallus thymallus (L.). The effects of G. salaris on the numbers of salmon in Norwegian rivers since 1975 has aroused increased interest in the Gyrodactylus situation in Swedish salmon rivers. In Sweden the reproduction of wild salmonids had traditionally been followed by electro-fishing (e.g. Karlstr/Sm, 1977, 1988 ), without any indications of a 'Norwegian situation'. In September 1986, a Norwegian investigation of salmon parr specimens from three northern Swedish rivers (Kalix, Byske and Aby~ilv; Mo, 1987 ) was performed. No Gyrodactylus specimens were found. Swedish salmonid investigations started in 1988, including northern Sweden (Malmberg and Malmberg, 1991 ). The present paper concentrates on the Swedish investigations of Salmo salar, Salmo trutta and Oncorhynchus mykiss (Walbaum). New data on G. salaris and G. derjavini are presented and, following Malmberg and Malmberg ( 1991 ), geographical distribution, spreading and taxonomic problems of these two species will be discussed. Materials and methods

Generally, Norwegian and Swedish rivers belong to different 'drainage regions' separated by the Scandinavian Mountains. Swedish rivers, in turn, belong to two drainage regions, the Baltic and the Atlantic, separated by a northsouth watershed (Fig. 1 ). Rivers of the Baltic region flow into the Baltic and those of Atlantic region flow into the Kattegat and the Skagerrak (parts of the Atlantic Ocean ). Genetically each region has its own form of salmon, Baltic and the Atlantic salmon, respectively. In total, our investigations included wild salmonids from 24 (Fig. 1, 1-15, A1-9;) Swedish rivers: 15 within the Baltic and nine within the Atlantic region. Priority was given to rivers where reproduction of wild salmon occurs: nine within the Baltic region ( 1-4, 6-9, 13, 14) and six within the Atlantic region (A3-7, A9). Five of the 24 rivers have lost the reproduction of wild salmon ( 5, 8, 10, 11, 12 ). Salmonids of 18 Swedish fish farms (Fig. 1 ) were investigated: 12 within the Baltic region, six within the Atlantic region. Results from two Finnish and three Danish fish farms are also presented. To reveal seasonal variations of Gyrodactylus spe-

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G. Malmberg, M. Malmberg/Fisheries Research 17 (1993) 59-68

SWEDEN Length: about 1 600 km Medium width: 30(1 km

• ~i~:,~:¸ /:.

Investigated river "~'River with (;. salaris [] Investigated fish farm • Fish farm with G. salaris Watershed

:#

iiii!!!!il '

ii~i~iiiii!ililili~ :'11

'~12

, \tic g

A4 iii( :

15

Fig. 1. Species of Gyrodactylus on salmonids in Sweden.

cies, certain rivers were investigated twice per year (spring and autumn). The ages of investigated salmon specimens were: 0* (yearlings, when available), 1*, 2* and 3* (if present). In one river (A6) precocious males and in three rivers (2, A4, A6) migrating smolts were subjected to special investigations. Generally, for catching wild salmonids, electro-fishing was used. Slides of Gyrodactylus specimens were made in accordance with Malmberg (1970).

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Table 1 Gyrodactylus species found on Salmo salar L. and S. trutta L. in Swedish rivers (Baltic region) River

Date of collection

Host

S. salar 1. 2

3. 4.

5.

6. 7.

8. 9. 10. 11. 12. 13. 14. 15.

4/5/1988 X 15/6/1991 X 16/6/1991 28/9/1991 29/9/1991 28-29/9/1991 X Kalix~lv 11-12/10/1988 X Kaltum~lv 10/10/1988 × 3/5/1988 X 3-4/5/1988 12/10/1988 X Lule~lv (Hedenslaxodling) Langas 5/8/1971 Kaker 6/8/1971 Byske alv 20/4/1988 X Vindel~lven 18-19/4/1988 X 18-19/4/1988 19/4/1988 14/9/1990 X 14/9/1990 Ume ~lv (Norrforslaxodling) L6dge~lv 30/10/1989 X 30/10/1989 Angerman~lven (Forsmo laxodling) Indals~lven (H~llelaboratoriet/H611eforsens laxodling) Ljusnan (Ljusnefiskodling) Dal~lven 1/10/1990 Emhn 1/9/1989 X 1/9/1989 M6~ums&n 4-6/8/1989 X 4-6/8/1989 15-17/6/1990 X

No. infested

6 25 2 1 1 32 19 5 20

0 17 0 0 0 0 0 0 0 0 0

Species of Gyrodactylus

S. trutta

Muonio ~lv Torne ~lv

16/6/1990

No. studied

X × X

X 5 X X

X

3 20 5

0 1 0 2 0 1 5 0

X

12 10

0 0

10 11 5 7 4 6

10 1 4 4 3 6

31

10

X x

× X X X

1 1 8 8

G. sa&r~ G. s p . ( o n e )

G. s p . ( o n e )

G, sa~r~ G, s p . ( o n e ) G, sa~r~

G. derjavini G. derjavini G. derjavini G. derjavini G. derjavini G. derjavini + G. salaris (one) G. salaris + G. derjavini

Results G. salaris Of wild salmon in the Baltic region, G. salaris was found in two northern rivers (2, 7) and one southern river (15), i.e. in only three of nine investi-

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63

Table 2 Gyrodactylus species found on Salmo salar L. and S. trutta L. in Swedish rivers (Atlantic region )

River

Date of collection

Host S. salar

AI.

K~lleNllsbacken

A2. A3.

R6d~n Orekils~lven

A4.

S~vegn

12-13/12/1958 29-30/5/1972 6/6/1972 6/6/1972 28/8/1989 28/8/1989 29/8/1989 29/8/1989 18/6/1990 18/6/1990 7/9/1990 7/9/1990 30-31/5/1991

A 5.

Fagereds~n

A6.

H6gvads~n

A7.

Fyllefin

No. studied

No. infested

× X X × X X x × x × x x x

2 3 3 I 18 4 15

2 0 3 1 0 0 6

5 24 3 14 5 12

2 7 1 l 0 12 0 0 3 5 5 3 5 6 2 6 8 7

× × x x x x x x × x ×

17 I 3 5 5 4 5 6 8 9 8 7

23-24/4/1992 30/8/1989

× ×

10 12

10 0

×

15

0

A8.

Lagan (Laholms laxodling) Stensfin

30/8/1989

Gyrodactylus

S. trutta

4-5/9/1991 4-5/9/1991 21-22/5/1991 5-6/9/1991 28-29/11/1991 10-11/3/1992 24-25/4/1991 6-7/5/1991 21-22/5/1991 5-6/9/1991 28-29/11/1991 10-11/3/1992

A9.

Species of

×

G. derjavml G. derjavim (;. de(javim

G. (;. (;. (;. (;. (;.

deuavini + salams deuavini salaris deuavini salaris

(;. sa/aris+ (;. derja vini

(;. G. G. G. G. (L G. G. (;. G. (;. G.

salaris salaris smarts sa/aris smarts salaris salaris sa/ari~ sa/arL~ salaris + sp. (one) salari.~

gated rivers. Hitherto, in one of the most northern rivers (2, border river) the parasite had been observed on smolts only. On cultured salmon in the Baltic region, G. salaris was found in six of 12 salmonid farms. Three of the six are situated on former wild salmon reproducing rivers (5, 8, 11 ). On wild salmon in the Atlantic region, G. salaris was found in three (A4, A5, A6) of six reproducing salmon rivers investigated. In the River S~ivehn (A4) and the River H6gvadshn (A6), smolt specimens had a larger infection intensity than smaller parr specimens. Furthermore, the Siivehn yearlings had no G. salaris, while precocious males from the H6gvadshn showed the largest

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G. Malmberg, M. Malmberg/Fisheries Research 17 (1993) 59-68

G. salaris infection intensity hitherto recorded in a Swedish salmon river. G. salaris was found in only one of five salmon farms investigated. The parasite was also observed in two other farms, but on Oncorhynchus mykiss, in spite of the fact that one of the farms (at A1 ) had no salmon parr and the second farm had not been culturing salmon for years. G. salaris was also recorded in two Finnish salmonid farms: one farm situated northeast of the River Muonio ~ilv ( 1, border river) and the other in the Finnish Archipelago, /~bolands sk~irg~rd (west of Turku). The first farm stocked tagged salmon smolts in the Muonio/ilv. The second farm was culturing the salmon in brackish water (approximately 5%0). In May 1972, G. salaris was recorded on O. mykiss in a Danish rainbow trout farm (at Koge ). G. derjavini On wild trout in the Baltic region, G. derjavini was found in three rivers ( 13, 14, 15) but in none of the rivers ( 1-3, 6, 7, 9) north of the River Dal~ilven ( 13 ). In the Rivers Em~n (14) and M6rrums~n ( 15 ), G. derjavini was also present on salmon parr. In the river Dal~ilven (13), on wild Swedish trout (body length 8.6-11.2 cm, G. derjavini numbered from less than 60 to 130) the largest G. derjavini infection intensity hitherto was recorded. On cultured trout specimens in the Baltic region, G. derjavini was found in Forsmo laxodling on the River Angerman~ilven (10) and M6rrums laxodling on the River M6rrums~n ( 15 ). Forsmo is situated north of the River Dal~ilven ( 13 ), the most northern G. derjavini infected Baltic river. On wild trout in the Atlantic region, G. derjavini was found in three (A1, A2, A4) of eight rivers investigated. In the River S~ive~n (A4) the parasite was also present on salmon parr. We found G. derjavini on trout in none of six salmonid farms investigated, but in one of the farms (K~illefalls riskodling; at River A 1 ) the parasite was present on O. rnykiss and on two other salmonid species (Malmberg and Malmberg, 1991 ). In May 1972, G. derjavini was recorded on O. mykiss in three Danish fish farms (at Brons, Koge and Fole). At one of them, rainbow trout (body length about 15 cm) were regularly treated against G. derjavini infection. In Denmark, this species is also found on wild trout parr (M. Koie, personal communication, 1990). Discussion G. salaris Our investigations show that G. salaris is present on wild salmon in the Baltic (two rivers) and Atlantic regions (three rivers). In both regions, cer-

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tain infected rivers were investigated on two or more occasions. The largest infection intensity was found on Atlantic salmon in the H6gvads~n (A5) and the Fagereds:~n (A6). In spite of established differences in infection intensity, however, the salmon populations in all five infected rivers seem to have a normal or balanced G. salaris infection. Thus hitherto in Swedish rivers, there seems to be no unbalanced or problematic G. salaris infection, similar to that described from many Norwegian rivers. Furthermore, in both the Atlantic and Baltic regions, our investigations point to an uneven distribution of infected rivers as well as of infected salmonid farms, and most likely certain fish farms are only temporarily infected. The uneven G. salaris distribution may have many explanations, e.g. different macroenvironmental conditions for the parasite, spreading from a n d / o r into salmonid fish farms by the water supply, the transport of salmon between fish farms, the culturing of rainbow trout in an earlier uninfected area, the developmental stage of wild salmon investigated, salmon stocks of different G. salaris resistance, and the combined effects of some of these factors. An unfavourable macroenvironment or the type of macroenvironment combined with a good G. salaris resistance may explain the absence ofG. salaris in certain rivers (3, 4, 6, 9, 14, A3, A7. A9) and certain fish farms: one farm in a northern, salmon-free area (between Rivers 11 and 12 ), two farms at River Dal~ilven ( 13 ), two farms in an oligotrophic, salmon-free part of the Baltic region (between Rivers 14 and 15 ) and two farms in a salmon-free area of the Atlantic region (north of Rivers AI and A2 ). Temporary infection by G. salaris may be present in a northern salmonid farm (on River 5) and in another farm on the River Dal~ilven ( 13 ) due to stocking of salmon from other G. salaris-infected farms. The absence of the parasite from a Baltic farm (on River 2) may depend on the absence of salmon up river and the culturing of salmon from fertilised eggs of collected, incoming salmon from the river mouth. Presumably, the incoming Baltic salmon are free of G. salaris. Although our investigation showed a single wild or escaped rainbow trout from the S/ivehn (A4) was uninfected, the possibility cannot be excluded that the G. salaris infection of wild salmon in the river originated from escaping cultured rainbow trout. In fact, G. salaris was found on rainbow trout in a salmonid farm situated at a river of another, adjacent river system in the Atlantic region. For years, this farm had no salmon specimens. By 1972, G. salaris was established in another farm at K~illefallsb~icken (A1), in a part of the Atlantic region lacking salmon. Likewise for years this farm had had no salmon, but had imported rainbow trout from Denmark. Both the rainbow trout farms had the taxonomic form of G. salaris, which in 1972 was also found on rainbow trout in a farm at K~ge, Denmark. In the River T o m e ~ilv (2; in 1991 ) G. salaris was found on migrating smolts only. In the S~ive~n (A4; in 1991 ) and the H6gvadshn (A6; in 1991 and 1992 ), likewise on migrating smolts, a larger infection intensity was recorded than

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G. Malmberg, M. Malmberg/Fisheries Research 17 (1993,) 59-68

on earlier stages of salmon. Furthermore, precocious males in the River H/Sgvadshn (November 1991 ) had the largest infection intensity hitherto found on any Swedish salmon specimens. In all cases, smolt and precocious males seemed to be in good condition. The fact that the large infection intensities were correlated with hormonal changes of the salmon may imply that these changes are influencing G. salaris to an increased rate of reproduction, resulting in a considerable number per host specimen. The increased reproduction capacity in turn may influence the worm's unique viviparity which includes both sexual reproduction and formation of clones (Malmberg, 1987 ). During spawning time in the autumn, precocious males are especially active members of a salmon river population and in spring time when a river has its largest number of parr specimens, migrating smolts may pass all along the river. Thus for spreading and maintaining a G. salaris population within a river, an increased infection intensity on precocious males and on smolts might be important. If so, stocking of a river in autumn with cultured parr specimens, instead of releasing smoltified salmon specimens into the river mouth in spring might imply an increased risk of creating an unbalanced G. salaris situation in the river. The fact that the infection intensity of G. salaris was larger in the H~Sgvads~tn (A6) than in the S~ivehn (A4) may imply that the infection of the Hrgvadshn is younger than that of the S~ivehn. It may also support the opinion that Atlantic salmon stocks are less resistant than Baltic salmon to infection by G. salaris (Malmberg, 1988, 1989; Bakke et al., 1990). However, our investigations indicate a balanced G. salaris situation even in the H/Sgvadshn. If this is so, the Atlantic salmon population of this river has a larger resistance to G. salaris than is exhibited by Atlantic salmon in many Norwegian rivers. G. salaris belongs to a species group with an eastern area of distribution, which most likely also includes the Baltic region. Salmon stocks within the region may have developed resistance to the parasite a long time ago (Maimberg, 1989 ). It seems likely that, during certain post-glacial periods of Baltic history, G. salaris may have spread also to Atlantic salmon in western Sweden, which gave rise to the present salmon stocks on the Swedish west coast. Thanks to this spreading the salmon had the opportunity to develop resistance to G. salaris. G. derjavini We found G. derjavini on trout in three Baltic ( 13, 14, 15 ) and three Atlantic rivers (A1, A2, A4). The species was not observed in any of seven (1, 2, 3, 4, 6, 7, 9) rivers and only in one fish farm north of the River Dal/ilven ( 13 ). G. derjavini is not reported from Finnish salmonid farms. Furthermore, the parasite was observed on salmon parr in three southern rivers (14, 15,

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67

A4), in two southern farms (at 15 and at A 1 ), on trout from Danish rivers and in three Danish rainbow trout farms. This indicates that G. derjavini has a southern distribution in Scandinavia. The presence of the parasite on trout in a northern fish farm on the river/kngerman~ilven (11) may depend on anthropochoric spreading. An infected water supply may explain the presence of the parasite in one farm on the K~illefallsb~icken (A 1 ) and another on the M~irrums~in ( 15 ), because the parasite was established in both rivers. The presence of G. derjavini on wild salmon seems not to be an accidental occurrence, but a useful adaptation, increasing its spreading capacity within a river (Malmberg and Malmberg, 1991 ).

Taxonomic considerations Gyrodactylus salaris exhibits a wide range of variation (Tanum, 1983 ). In fish farms, reproductive isolation and extreme fluctuations in size of a G. salaris population may cause genetic drift, resulting in extreme morphological forms of the parasite (Malmberg, 1987 ). Furthermore, G. salaris can propagate and remain for a long time on the rainbow trout (Malmberg and Maimberg, 1987; Mo, 1991 ). Judging by investigations on material collected in Denmark and Sweden (Malmberg, 1973) and in Norway (Mo, 1991 ), the shape and size of G. salaris specimens from rainbow trout differ from that found on specimens from salmon: e.g. specimens from rainbow trout have longer and more robust haptoral anchors than those exhibited by specimens from salmon. Generally, however, natural populations of Gyrodactylus species exhibit a smaller range of variation in size and shape of morphological characters than "cultured populations' of G. salaris. Apart from morphologically different forms, 'culturing' also may result in physiologically divergent Gyrodactylus forms of different pathogenicity (Malmberg, 1987 ). Thus, exchange of material between fish farms and stocking of rivers with cultured, infected fish may cause taxonomic as well as pathological problems in natural waters. The morphological differences between closely related Gyrodactylus species (natural waters ) can be very subtle (Malmberg, 1970 ), e.g. species of the Gyrodactylus wageneri group, including Gyrodactylus truttae Gliiser, 1974 and G. derjavini. Most likely, the taxonomic problems connected with these two species (Malmberg, 1987) have been increased by 'culturing effects'. Such effects, however, underline the importance of complementary morphometric, experimental infection investigations and electrophoretic (DNA) analysis of natural and 'cultured' populations of similar Gyrodactylus species.

Acknowledgements We thank all the people in fish farms and at different institutions for helping us with working areas, transport, salmonid material and electro-fishing in

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connection with our investigations. Financial support was given by The National Swedish Board of Fisheries.

References Bakke, T.A., Jansen, P.A. and Hansen, L.P., 1990. Differences in the host resistance of Atlantic salmon, Salmo salar L., stocks to the monogenean Gyrodactylus salaris Malmberg, 1957. J. Fish Biol., 37: 577-587. Ergens, R., 1983. Gyrodactylus from Eurasian freshwater Salmonidae and Thymallidae. Folia Parasitol., 30:15-26. Karlstr6m, 0., 1977. Habitat selection and population densities of salmon (Salmo salar L.) and trout (Salmo trutta L.) parr in Swedish rivers with some reference to human activities. Acta Univ. Uppsaliensis, Abstr. Uppsala Diss., Faculty Sci., 404, pp. 1-12. Karlstr~m, 0., 1988. Situationen f'6r de naturliga lax~ilvarnas laxbest~nd. Fiskeri styrelsen. Utredningskontoret i Lule~, 1988-02-04, pp. 1-14. Malmberg, G., 1957. Om f6rekomsten av Gyrodactylus ph svenska fiskar. Skr. s6d. Sver. Fisk F~r. Arsskr. 1956, pp. 19-76. (In Swedish, with description of species and a summary in English. ) Malmberg, G., 1970. The excretory systems and the marginal hooks as a basis for the systematics ofGyrodactylus (Trematoda, Monogenea). Ark. Zool. Ser. 2, 23( 1 ): 1-235. Malmberg, G., 1973. Gyrodactylus infestations on species of salmon in Danish and Swedish hatcheries. Norw. J. Zool., 21: 325-326. Malmberg, G., 1987. Increased infraspecific divergence in Gyrodactylus salaris resulting from genetic drift in fish farm populations. Information, 19, Abo, p. 33. Malmberg, G., 1988. Gyrodactylus salaris--infektioner, laxfisktransporter och odling i Norden. Vattenbruk, 2: 22-29. Malmberg, G., 1989. Salmonid transport, culturing and Gyrodactylus infections in Scandinavia. In: O. Bauer (Editor), Parasites of Freshwater Fishes of North-West Europe. Int. Syrup. Soviet-Finnish Coop., 10-14 January 1988, Inst. Biol. Acad. Sci., Karelia Petrozavodsk. Zool. Inst. Acad. Sci., Leningrad, pp. 88-104. Malmberg, G. and Malmberg, M., 1987. Gyrodactylus in salmon and rainbow trout farms. In: A. Stenmark and G. Malmberg (Editors), Parasites and Diseases in Natural Waters and Aquaculture in Nordic Countries. Naturhistoriska Riksmuseets Reprocentral, Stockholm, pp. 199-204. Malmberg, G. and Malmberg, M., 1991. Investigations of Gyrodactylus on salmonids in natural waters and fish farms during the periods 1951-72 and 1986-May 1991. Inf. Inst. Freshwater Res., Drottningholm, 2, pp. 1-30. (In Swedish with legends and a summary in English. ) Mikailov, T.K., 1975. Parasites of fishes in water reservoirs in Azerbaidzhan. Izd. Elm, Baku, 299 pp. (In Russian. ) Mo, T.A., 1987. Taksonomiske og biologiske undersokelser, virksomheten i 1986 og forslag til virksomhet i 1987. Rapp. no 2: Gyrodactylus udersokelsene ved Zoologisk Museum, Universitetet i Oslo, pp. 1-69. Mo, T.A., 1991. Variations of the opisthaptoral hard parts of Gyrodactylus salaris Malmberg, 1957 (Monogenea: Gyrodactylidae) on rainbow trout Oncorhynchus mykiss (Walbaum, 1792) in a fish farm, with comments on the spreading of the parasite in south-eastern Norway. Syst. Parasitol., 20: 1-9. Tanum, K., 1983. Studier av taksonomi og vertsforhold hos Gyrodactylus-arter p~ laksefisk av slektene Salmo og Salvelinus i Norge. Hovedfagsoppgave i spesiell zoologi, parasitologi; Universitetet i Oslo, Zoologisk Museum, Vhren 1983, pp. 1-99.