Susceptibility of Baltic and East Atlantic salmon Salmo salar stocks to Gyrodactylus salaris (Monogenea)

The susceptibility of a Baltic salmon stock Salmo salar (Indals?lv, central Sweden) to Norwegian Gyrodactylus salaris (Figga strain, central Norway) was experimentally tested and compared with previously obtained results on East Atlantic salmon (Lierelva, SE Norway). Contrary to expectation, the Baltic salmon, which had no prior exposure to this parasite strain, appeared almost as susceptible as the Norwegian salmon parr that naturally experience G. salaris-induced mortality. Individually isolated salmon of both stocks sustained G. salaris infections with little evidence of innate resistance. A few individuals of the Indals?lv stock controlled their infection from the beginning, but overall there was considerable heterogeneity in the course of infection in both stocks. On individual hosts, G. salaris growth rates declined steadily throughout the infection, a trend which was particularly marked amongst the Lierelva stock. On shoaling Lierelva fish, there was some evidence of reduced parasite population growth towards the end of the infection; this was not apparent in Indals?lv fishes. These results reflect a growing awareness that not all Baltic salmon may be resistant to Norwegian G. salaris, and that Norwegian and Baltic G. salaris strains may differ in virulence. Consequently, management decisions concerning this parasite-host system should be based upon the actual, and tested, susceptibility of stocks under consideration and not upon identification of stocks as either Atlantic or Baltic.     

Differences in the host resistance of Atlantic salmon, Salmo salar L., stocks to the monogenean Gyrodactylussalaris Malmberg, 1957

The susceptibility and resistance of hatchery-reared salmon parr, native to the rivers Neva (U.S.S.R. Baltic Sea), Alta (northern Norway) and Lone (western Norway) (both eastern Atlantic Ocean), to Gyrodactylus salaris from Norway, was examined. The level of resistance to the parasite was assessed from counts, made on anaesthetized salmon, ofthe numbers of G. salaris after an initial experimental exposure (2 weeks) to G. salaris-infected salmon. Three experiments, all in water at c. 12° C, were carried out: (1) 50 Alta and 50 Neva salmon, initial mean parasite intensity c. 12; (2) 50 Lone and 50 Neva salmon, initial mean parasite intensity c. 60; (3) 10 Lone and 10 Neva salmon individually isolated, initial intensity one gravid G. salaris . In both the Norwegian salmon stocks, the G. salaris infrapopulations steadily increased during the experimental period of 5 weeks, in contrast to a prominent decline in the Neva salmon stock, after, respectively: (Exp. 1) week 3, average peak intensity 32.6; (Exp. 2) week 2, average peak intensity 58.7; and (Exp. 3) week 3, average peak intensity 6.3. The hatchery-reared Baltic Neva stock demonstrated both an innate and an acquired resistance towards G. salaris , in contrast to the highly susceptible, Norwegian Alta and Lone salmon stocks.     

Qualitative analysis of the risk of introducing Gyrodactylus salaris into the United Kingdom

Gyrodactylus salaris is a freshwater, monogenean ectoparasite of Baltic strains of Atlantic salmon Salmo salar on which it generally causes no clinical disease. Infection of other strains of Atlantic salmon in Norway has resulted in high levels of juvenile salmon mortality and highly significant reductions in the population. The parasite is a major exotic disease threat to wild Atlantic salmon in the UK. This paper qualitatively assesses the risk of introduction and establishment of G. salaris into the UK. The current UK fish health regime prevents the importation of live salmonids from freshwater in territories that have not substantiated freedom from G. salaris. The importation of other species, e.g. eels Anguilla anguilla and non-salmonid fish, represents a low risk because the likelihood of infection is very low and the parasite can only survive on these hosts for less than 50 d. Importation of salmon carcasses presents a negligible risk because harvested fish originate from seawater sites and the parasite cannot survive full strength salinity. The importation of rainbow trout Oncorhynchus mykiss carcasses from G. salaris infected freshwater sites might introduce the parasite, but establishment is only likely if carcasses are processed on a salmonid farm in the UK. A number of mechanical transmission routes were considered (e.g. angling equipment, canoes, ballast water) and the most important was judged to be the movement of live fish transporters from farms on mainland Europe direct to UK fish farms. In the future, territories may have to substantiate freedom from G. salaris and economic drivers for live salmonid imports may strengthen. Under these circumstances, legal or illegal live salmonid imports would become the most significant risk of introduction.     

Comparative susceptibility of two races of Salmo salar (Baltic Lule river and Atlantic Conon river strains) to infection with Gyrodactylus salaris

The susceptibility of various races of salmonids towards infections with the skin parasitic monogenean Gyrodactylus salaris Malmberg, 1957, differs markedly. Norwegian and Scottish salmon strains are known as extremely susceptible to infection, whereas Baltic salmon races such as the Neva strain (Russian origin) and the Indals river (Swedish origin) salmon have been characterized as relatively resistant. However, the status of the many other Baltic strains has remained unknown. The present study reports on the susceptibility of the Baltic salmon from the Swedish river Lule. It was shown that this strain is susceptible to infection but to a lesser extent than the Scottish salmon. Further studies showed that injection of immuno-suppressants (dexamethasone) greatly increased population growth of G. salaris on Scottish salmon but not on the Baltic salmon. Mucous cell density on fins differed between strains, and a general trend to decreased cell density on infected fish 8 wk post-infection, compared to uninfected fish, was observed. The largest decrease in mucous cell density following infection was seen in the most resistant fish. After administration of immuno-suppressants, this decrease in mucous cell density was inhibited in the Scottish salmon but not in the Baltic salmon. Thus, there seems to be a relationship between the fishes' ability to discard mucous cells and the ability to resist infections with Gyrodactylus salaris. Although the Lule salmon seems more susceptible to infection compared to previous reports on the Neva salmon, the results support the notion that Baltic salmon strains are generally more resistant than East Atlantic salmon.     

Host specificity dynamics: observations on gyrodactylid monogeneans

The directly transmitted viviparous gyrodactylids have high species richness but low morphological and biological diversity, and many species are recorded from only a single host. They therefore constitute a guild of species ideal for studies of the evolutionary significance of host specificity. The group has the widest host range of any monogenean family, being found on 19 orders of bony fish. However, individual species range from narrowly specific (71% of 402 described species recorded from a single host) to extremely catholic (Gyrodactylus alviga recorded from 16 hosts). Gyrodactylid-host interactions extend from 60 mya (G. lotae, G. lucii) down to 150 years (G. derjavini on Oncorhynchus mykiss). Co-evolution with the host is comparatively rare within the gyrodactylids, but host switching or ecological transfer is common, and has been facilitated by the mixing of fish strains that followed glaciation. In this review, we consider the factors responsible for gyrodactylid specificity patterns, using examples from our work on salmonid gyrodactylids including G. salaris, responsible for major epidemics on wild Atlantic salmon (Salmo salar) in Norway since 1975, and G. thymalli from grayling and G. derjavini from trout.G. salaris has a wide host range with highest population growth rates on Norwegian salmon strains. However, growth rates are variable on both host strains and species, because of the multitude of micro- and macro-environmental factors influencing parasite mortality and fecundity. A better predictor of performance is the proportion of fishes of a strain which are innately resistant to the parasite, a measure which is negatively correlated with the time to peak infection in a host strain. Population growth rate is also negatively correlated with age of infection; the initial rate, therefore, predicts best the suitability of a fish as host for G. salaris. The host response to gyrodactylids appears to be the same mechanism in all salmonids with innate resistance as one end of a spectrum, but influenced by stress and probably under polygenic control. Hybrid experiments show that performance of G. salaris on a host is heritable, and usually intermediate between that of the parents. This host response mechanism, coupled with the initial parasite population growth on a fish, determines the host specificity, i.e. whether the fish will be susceptible, a responder or innately resistant. The use of population growth rate parameters allows comparison of different hosts as a resource for a gyrodactylid. In the case of G. salaris, East Atlantic and Baltic strains of Atlantic salmon are core hosts, but other salmonids can physiologically sustain infections for considerable periods, and may be important in parasite dispersal and transmission. A further group of non-salmonid fishes are unable to sustain G. salaris reproduction, but can act as transport hosts.Population growth parameters are very labile to stressors and environmental factors, particularly temperature and salinity, and also other aspects of host ecology and water quality. These factors may also influence the spectrum of hosts that can be infected under particular conditions, and probably favoured ecological transfer of gyrodactylids between host species in periglacial conditions. G. salaris may still be undergoing post-glacial range expansion (aided by anthropogenic spread) as shown by the increase in the species range over the last 25 years. The origin of G. salaris, G. teuchis and G. thymalli is discussed in relation to glacial refugiums during the last ice age.     

The susceptibility of grayling (Thymallus thymallus) to experimental infections with the monogenean Gyrodactylus salaris

The pathogenic monogenean Gyrodactylus salaris infecting Atlantic salmon (Salmo salar) is found to attach and reproduce under laboratory conditions on several species in the subfamily Salmoninae other than the Atlantic salmon. The gyrodactylid species Gyrodactylus thymalli infecting grayling (Thymallus thymallus) in another subfamily, Thymallinae, is previously said to be very similar to G. salaris based on morphometry and genetical analysis which prompted the present laboratory experiments to test the susceptibility and resistance of grayling to G. salaris. All 0+ and 1+ grayling became infected with G. salaris during the experimental infection procedure. However, both innate resistant and susceptible grayling were found. In susceptible individually isolated fish, parasite reproduction lasted for more than 35 days. Parasite reproduction also occurred among grouped grayling as judged from the duration of infection of more than 50 days. However, grayling susceptibility as judged from G. salaris reproduction, was very limited. Hence, the results indicate significant biological differences between the function of Atlantic salmon and grayling as host for G. salaris. The grayling is interpreted as unable to sustain G. salaris in nature which implies that G. thymalli is not conspecific with G. salaris. However, G. salaris dispersal by grayling cannot be excluded.     

Characterisation of a low pathogenic form of Gyrodactylus salaris from rainbow trout

Gyrodactylus salaris was isolated from rainbow trout in a Danish freshwater trout farm, and a laboratory population of this particular parasite form was established on rainbow trout. Challenge infections were performed using different salmonid strains and species, including East Atlantic salmon Salmo salar (from the Danish River Skjern?), Baltic salmon S. salar (from the Swedish River Ume Alv) and rainbow trout Oncorhynchus mykiss (from the Danish rainbow trout farm Fousing). These were compared to infection studies on the Norwegian Laerdalselva parasite form kept under exactly the same conditions in the laboratory. The Danish G. salaris form had low virulence towards both Atlantic and Baltic salmon, whereas rainbow trout proved susceptible to the parasite. The Danish G. salaris form was able to maintain a very low infection on East Atlantic salmon, but not on the Baltic salmon, which eliminated the infection within 2 wk. Rainbow trout developed infection intensities ranging up to several hundred parasites per host. The host colonization patterns of the parasite differed clearly from those of previous studies on microhabitats of the Norwegian form of G. salaris. A comparative study on morphological characters (opisthaptoral hard parts) from the Danish parasite form and Norwegian G. salaris showed no significant differences. Selected genes comprising internal transcribed spacers 1 and 2 (ITS), ribosomal RNA intergenic spacer (IGS) and cytochrome c oxidase subunit I (COI) regions were cloned and sequenced. Five sequenced ITS clones from 5 individuals of the Danish strain consistently revealed a single base substitution compared to ITS sequences from all other known species and strains of Gyrodactylus. Mitochondrial COI gene sequences demonstrated that the Danish G. salaris form is closely similar to the Laerdalselva parasite form found in Norway. The IGS sequences were highly variable, but very similar to those obtained from German isolates of G. salaris.     

High Gyrodactylus salaris infection rate in triploid Atlantic salmon Salmo salar

We describe an unusually high infection rate of Gyrodactylus salaris Malmberg in juvenile Atlantic salmon Salmo salar L. of Baltic Sea origin, which are generally believed to be more resistant to G. salaris than East Atlantic salmon populations. Based on analyses of mitochondrial (complete cytochrome oxidase 1 [CO1] gene, 1548 bp) and nuclear (ADNAM1, 435 bp; internal transcribed spacer [ITS] rDNA region, 1232 bp) DNA fragments, the closest relatives of the characterized Estonian G. salaris strain were parasites found off the Swedish west coast and in Raasakka hatchery, Iijoki (Baltic Sea, Finland). Analyses of 14 microsatellite loci of the host S. salarrevealed that approximately 40% of studied fish were triploids. We subsequently identified triploid Atlantic salmon of Baltic origin as more susceptible to G. salaris infection than their diploid counterparts, possibly due to compromised complement-dependent immune pathways in triploid salmon. This is in accordance with earlier studies that have shown elevated susceptibility of triploids to various viral or bacterial pathogens, and represents one of the first reports of increased susceptibility of triploid salmonid fish to an ectoparasite. However, further experimental work is needed to determine whether triploid Atlantic salmon is generally more susceptible to G. salaris compared to their diploid counterparts, irrespective of the particular triploidization method and population of origin.     

Baltic salmon activates immune relevant genes in fin tissue when responding to Gyrodactylus salaris infection

Immune mechanisms in 2 strains of Salmo salar (Baltic salmon from River Ume Alv in Sweden and East Atlantic salmon from River Skjern? in Denmark) infected with the monogenean ectoparasite Gyrodactylus salaris were elucidated by molecular tools (real-time PCR). The gene expression in the fins (the preferred microhabitat of the parasite) of the susceptible but responding Swedish salmon was compared to the expression in the fins of the highly susceptible and nonresponding East Atlantic salmon. Experimental infections confirmed that both the Swedish and the Danish salmon allowed initial propagation of the parasite on the fins for a few weeks. Baltic salmon subsequently activated a response from Day 28 and limited the parasite population to a few parasites per host within the following weeks. In contrast, the Danish salmon did not respond and experienced a continuing increase in the parasite load during the same period, which reached several hundreds of parasites per host. RNA was isolated from fins of the 2 salmon strains during the course of infection and subsequent real-time PCR showed an increased expression of INFgamma, Mx and MHC I genes in Baltic salmon fins during large segments of the response phase. No upregulation of these genes could be detected in susceptible salmon. No increase in immunoglobulin genes was seen in any of the fish strains, which supports the notion that antibodies are not involved in the response. Further, the work suggests that cellular factors could at least partly contribute to the anti-parasitic response in Baltic salmon.     

Analysis of ribosomal RNA intergenic spacer (IGS) sequences in species and populations of Gyrodactylus (Platyhelminthes: Monogenea) from salmonid fish in Northern Europe

The intergenic spacer (IGS) region of ribosomal RNA genes was amplified and sequenced from a variety of Gyrodactylus specimens collected from wild and farmed Atlantic salmon Salmo salar, rainbow trout Oncorhynchus mykiss, and grayling Thymallus thymallus, from various locations in Northern Europe. Phylogenetic analysis of the sequences confirmed the distinction between G. salaris Malmberg, 1957 and G. thymalli Zitnan, 1960, supporting their validity as separate species. G. salaris adapted to rainbow trout are also distinct from the parasites found on Atlantic salmon, supporting the existence of a rainbow-trout form that was initially identified on the basis of morphological differences. Analysis of the IGS did not provide good resolution of different populations of G. salaris sensu stricto, but was consistent with epidemiological evidence which indicates that introduction of the parasite to Norway was recent and limited. The IGS may be helpful in distinguishing forms of G. salaris that are pathogenic to Atlantic salmon from those that are not.     

Differences in susceptibility of anadromous and resident stocks of Arctic charr to infections of Gyrodactylus salaris, under experimental conditions

The ability of Gyrodactylus salaris , an important pathogen of the Atlantic salmon Salmo salar , in Norway, to infect anadromous and resident stocks of the Arctic charr, Salvelinus alpinus , has been examined in the laboratory. Resident charr (Korssjoen stock) exposed to heavily infected salmon, were considered innately resistant as they lost their infections within 21 days when individually isolated. Isolated anadromous harr (Hammerfest stock) remained infected for up to 150 days, although most infections disappeared within 30–50 days. In many cases the parasite population grew initially, but growth was limited after 20–30 days and infections subsequently disappeared. At the same time, shoals of 50 anadromous charr, swimming in the tanks containing the individually isolated fish in floating cages, remained infected for up to 280 days. Charr isolated from these shoals after 115 days and subsequently monitored individually lost their infections within 30 days, although the parasite persisted within the shoals for a further 75–135 days. This suggests that G. salaris , persisted on shoaling charr despite an immune response which led to the elimination of parasites from isolated hosts. The Hammerfest stock of anadromous charr supports G. salaris , in the laboratory, and the extended period of survival on this host suggests that charr may be important in the epidemiology of G. salaris , in northern Norway.     

Initial steps of speciation by geographic isolation and host switch in salmonid pathogen Gyrodactylus salaris (Monogenea: Gyrodactylidae)

To test the hypothesis that host-switching can be an important step in the speciation of gyrodactylid monogenean flatworms, we inferred the phylogeny within a cluster of parasites morphologically close to Gyrodactylus salaris Malmberg 1957, collected from Atlantic, Baltic and White Sea salmon (Salmo salar), farmed rainbow trout (Oncorhynchus mykiss), and grayling (Thymallus thymallus) from Northern Europe. The internal transcribed spacer region of the nuclear ribosomal gene was sequenced for taxonomic identification. Parasites on grayling from the White Sea Basin differed from the others by one nucleotide (0.08%), the remainder were identical to the sequence published earlier from Norway (G. salaris on salmon), England (Gyrodactylus thymalli on grayling), and the Czech Republic (unidentified salaris/thymalli on trout). For increased resolution, 813 nucleotides of the mitochondrial COI gene of 88 parasites were sequenced and compared with 76 published sequences using phylogenetic analysis. For all tree building algorithms (NJ, MP), the parasites formed a star-like phylogeny of six definite sister clades, indicating nearly simultaneous radiation. Average K2P distances between clades were 1.8-2.6%, and internal mean distances 0.2-1.1%. The genetic distance to the nearest known relative, Gyrodactylus lavareti Malmberg, was 24%. A variable salmon-specific mitochondrial Clade I was observed both in the Baltic Basin and in pathogenic populations introduced to the Atlantic and White Sea coasts. An invariable Clade II was common in rainbow trout farms in Sweden, Denmark and Finland; the same haplotype was also infecting salmon in a landlocked population in Russian Karelia, and in Oslo fjord and Sognefjord in Norway. Four geographically vicariant sister clades were observed on graylings: Clade III in the Baltic Sea Basin; Clade IV in Karelian rivers draining to the White Sea; Clade V in Norwegian river draining to Swedish lake V?nern; and Clade VI in rivers draining to Oslo fjord. The pattern fitted perfectly with the postglacial history of grayling distribution. Widely sampled clades from salmon and Baltic grayling had basal haplotypes in populations, which were isolated early during the postglacial recolonisation. The divergence between the six clades was clear and linked with their hosts, but not wide enough to support a species status for them. Parasites from the Slovakian type population of G. thymalli were not available, so this result does not mean that G. salaris and G. thymalli are synonyms. It is suggested that the plesiomorphic host of the parasite cluster was grayling, and the switch to salmon occurred at least once when the continental ice isolated Baltic salmon in an eastern freshwater refugium, 130,000 years ago. At the same time, parasites on grayling were split geographically and isolated into several allopatric refugia. The divergence among the parasite clades allowed a tentative calibration of the evolutionary rate, leading to an estimate of the divergence of 13.7-20.3% per million years for COI coding mtDNA. The results supported the hypothesis that parallel to the allopatric mode, host switch and instant isolation by host specificity can be operated as a speciation mechanism.     

Susceptibility of brown trout to Gyrodactylus salaris (Monogenea) under experimental conditions

Brown trout ( Salmo trutta ) from anadromous River Lierelva, resident Lake Tunhovd, and resident Nordmarka stocks were exposed to Gyrodactylus salaris -infected salmon parr. The brown trout were fed pellets before the experiments, except for one group of the Nordmarka stock which was starved for 19 days before the experiments. The mean number of parasites declined directly and rapidly post infection for all groups of trout. There were no pronounced differences in resistance between the anadromous and the resident stocks. G. salaris infections tended to persist longer on starved than on fed trout of the Nordmarka stock. The maximum parasite persistence on trout was 50 days, and as parasite numbers increased on some fish parasite reproduction must have occurred on those trout. However, the limited susceptibility and marked innate resistance of trout to G. salaris establishment, development and reproduction, suggest parasite metapopulations will not survive on this species. Nevertheless, trout may still play a role in the dispersal of G. salaris within and between rivers.     

The ultrastructure of hypersymbionts on the monogenean Gyrodactylus salaris infecting Atlantic salmon Salmo salar

There is increasing pressure to develop alternative control strategies against the pathogen Gyrodactylus salaris, which has devastated wild Atlantic salmon Salmo salar in Norway. Hyperparasitism is one option for biological control and electron microscopy has revealed two ectosymbionts associated with G. salaris: unidentified rod-shaped bacteria, and the protist, Ichthyobodo necator. No endosymbionts were detected. The flagellate I. necator occurred only occasionally on fish suffering costiosis, whereas bacterial infections on the tegument of G. salaris were observed throughout the year, but at variable densities. Bacteria were seldom observed attached to fish epidermis, even when individuals of G. salaris on the same host were heavily infected. Wounds on salmon epidermis caused by the feeding activity of bacteria-infected G. salaris did not appear to be infected with bacteria. On heavily infected gyrodactylids, bacteria were most abundant anteriorly on the cephalic lobes, including the sensory structures, but no damaged tissue was detected by transmission electron microscopy in the region of bacterial adherence. Furthermore, transmission and survival of infected G. salaris on wild salmon did not appear to be influenced by the bacterial infection. The lack of structural damage and impact on G. salaris biology indicates that these bacteria are not a potential agent for control of gyrodactylosis. However, this may not be the case for all gyrodactylid-bacterial interactions and a review of bacterial infections of platyhelminths is presented.     

Molecular faunistics of accidental infections of <Emphasis Type="Italic">Gyrodactylus</Emphasis> Nordmann, 1832 (Monogenea) parasitic on salmon <Emphasis Type="Italic">Salmo salar</Emphasis> L. and brown trout <Emphasis Type="Italic">Salmo trutta</Emphasis> L. in NW Russia

Salmon Salmo salar L. and brown trout S. trutta L. juveniles were examined for the presence of accidental monogenean ectoparasitic species of Gyrodactylus Nordmann, 1832 in the Baltic and White Sea basins of Russian Karelia in order to estimate the frequency of host-switching attempts on an ecological timescale. To collect phylogeographical information and for exact species identification, the parasites were characterised by nuclear internal transcribed spacer sequences of rDNA (ITS) and, for some species, also by their mitochondrial DNA (CO1 gene) sequences. Four accidental Gyrodactylus species were observed on salmon and brown trout. A few specimens of G. aphyae Malmberg, 1957, the normal host of which is the Eurasian minnow Phoxinus phoxinus (L.), were observed on lake salmon from the Rivers Kurzhma (Lake Kuito, White Sea basin) and Vidlitsa (Lake Ladoga, Baltic basin). G. lucii Kulakovskaya, 1952, a parasite of the northern pike Esox lucius L., was observed on salmon in the Kurzhma. In the River Vidlitsa, two specimens of G. papernai Ergens & Bychowsky, 1967, normally on stone loach Barbatula barbatula (L.), were found on salmon. On anadromous White Sea salmon in the River Pulonga in Chupa Bay, a few salmon parr carried small colonies of G. arcuatus Bychowsky, 1933, which were shown to have originated from the local three-spined stickleback Gasterosteus aculeatus L. consumed as prey. No specimens of Gyrodactylus salaris Malmberg, 1957 were observed, although the Pulonga is the nearest salmon spawning river to the River Keret', which is heavily infected with introduced G. salaris. In the River Satulinoja, Lake Ladoga, three specimens of G. lotae Gusev, 1953, from burbot Lota lota (L.), were collected from a single brown trout S. trutta. All nonspecific gyrodactylid infections on salmonids were judged to be temporary, because only a few specimens were observed on each of the small number of infected fishes. The prevalence of endemic G. salaris was also low, only 1% (Nfish = 296) in Lake Onega and 0.7% (Nfish = 255) in Lake Ladoga, while brown trout specific Gyrodactylus species were not observed on any of the 429 trout examined from the Ladoga basin. The host-specific and unspecific burden of Gyrodactylus spp. on these 'glacial relict' populations of salmon and brown trout was very low, suggesting a generalised resistance against the co-evolved freshwater parasite community, or some kind of 'vaccination' effect. These hypotheses deserve further testing.     

Infestations of Atlantic salmon, Salmo salar, by Gyrodactylus salaris in Norwegian rivers

Over the 6 years 1980–85, 212 Norwegian rivers have been examined for occurrence of Gyrodactylus salaris: it was found in 26 rivers and six salmon hatcheries scattered throughout the country from Troms county in the north to Sogn og Fjordane in western Norway. The distribution of G. salaris is connected with the stocking of fish from infected salmon hatcheries. The populations of salmon parr have been drastically reduced in the infected rivers. In later years catches of ascending salmon in these rivers have also sharply declined: in 1984 salmon fishery losses were estimated at 250–500 t.
Gyrodactylus salaris is most probably a recent introduction to Norwegian rivers. A primary aim is to exterminate this parasite from all infected rivers and hatcheries: so far this has been accomplished in one river and one hatchery.     

Reproductive Trade-Offs May Moderate the Impact of Gyrodactylus salaris in Warmer Climates

Gyrodactylus salaris is a notifiable freshwater ectoparasite of salmonids. Its primary host is Atlantic salmon (Salmo salar), upon which infections can cause death, and have led to massive declines in salmon numbers in Norway, where the parasite is widespread. Different strains of S. salar vary in their susceptibility, with Atlantic strains (such as those found in Norway) exhibiting no resistance to the parasite, and Baltic strains demonstrating an innate resistance sufficient to regulate parasite numbers on the host causing it to either die out or persist at a low level. In this study, Leslie matrix and compartmental models were used to generate data that demonstrated the population growth of G. salaris on an individual host is dependent on the total number of offspring per parasite, its longevity and the timing of its births. The data demonstrated that the key factor determining the rate of G. salaris population growth is the time at which the parasite first gives birth, with rapid birth rate giving rise to large population size. Furthermore, it was shown that though the parasite can give birth up to four times, only two births are required for the population to persist as long as the first birth occurs before a parasite is three days old. As temperature is known to influence the timing of the parasite''s first birth, greater impact may be predicted if introduced to countries with warmer climates than Norway, such as the UK and Ireland which are currently recognised to be free of G. salaris. However, the outputs from the models developed in this study suggest that temperature induced trade-offs between the total number of offspring the parasite gives birth to and the first birth timing may prevent increased population growth rates over those observed in Norway.     

Risk of inter-river transmission of Gyrodactylus salaris by migrating Atlantic salmon smolts, estimated by Monte Carlo simulation

The possibility of Gyrodactylus salaris infection of wild North Atlantic salmon Salmo salar spreading to new rivers poses a major threat in Norway. This freshwater parasite can survive for some time in brackish water, and it has been suggested that smolts leaving infected rivers could transport vital parasites to new rivers. A Monte Carlo simulation model was used to estimate the risk that infected smolts would ascend a new river. Data from an infected watercourse in Norway, where the salmon population is maintained constant by cultivation, were used. The model included information on prevalence of infection, hydrographical conditions, survival of G. salaris in brackish water, fish population characteristics, and smolt behaviour during seaward migration. The annual risk was estimated for 3 neighbouring rivers situated at different distances from the index river. For the nearest river, which shares the same brackish water zone with the index river, the model estimated an annual risk of 31% that at least 1 infected smolt would ascend this river. The results of the simulation were highly sensitive to the water salinity along the migration route. For the other rivers, the annual risk was lower than 0.5%. Risk was positively correlated with the number of fish leaving the index river, indicating control of this number as a possible tool in risk management.     

Mitochondrial DNA variation of Gyrodactylus spp (Monogenea, Gyrodactylidae) populations infecting Atlantic salmon, grayling, and rainbow trout in Norway and Sweden

Approximately 800 bp of the mitochondrial cytochrome oxidase I (COI) gene were sequenced from 76 Gyrodactylus specimens of 32 salmonid host populations, i.e. from Salmo salar, Thymallus thymallus, and Oncorhynchus mykiss in Norway, Sweden and Latvia. The COI sequences indicated a substantial intraspecific differentiation of Gyrodactylus salaris and Gyrodactylus thymalli. In total, 12 haplotypes were identified which group into five well supported clades, three clades with parasites from Atlantic salmon and two clades with parasites from grayling. The basal nodes linking the five clades together are only weakly supported. Thus, there is no support for the monophyly of all G. salaris haplotypes and the monophyly of all G. thymalli haplotypes. The lack of monophyly of the mitochondrial haplotypes of G. salaris and G. thymalli may indicate that G. salaris and G. thymalli represent (i). two polytypic species or (ii). one polytypic species, or (iii). refer to a complex of more than two sibling species. The mtDNA data indicate multiple introductions of G. salaris and G. thymalli into Norway. A minimum of three independent introductions of G. salaris and two independent introductions of G. thymalli are supported. This is congruent with earlier hypotheses on the introduction of G. salaris and G. thymalli into Norway.