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.     

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.     

Geographic risk factors for inter-river dispersal of Gyrodactylus salaris in fjord systems in Norway

Gyrodactylus salaris has been recorded in 46 Norwegian rivers since 1975 and is considered a threat to Atlantic salmon stocks. The primary introductions of G. salaris (primary infected rivers) have been accounted for by specific events, as reported in the literature. The parasite has subsequently dispersed to adjacent localities (secondary infected rivers). The objective of this paper is to address the occurrence of secondary infections by examining the hypothesis of inter-river dispersal of G. salaris. A dispersal model for the secondary river infections via migrating infected fish is proposed. Due to the limited tolerance of G. salaris to salinity, both freshwater inflow to dispersal pathways and dispersal distance were expected to influence the probability of inter-river dispersal. Eighteen rivers were categorised as primary infected rivers, 28 as secondary infected rivers, and 54 as rivers at risk. Four risk factors: the log10 freshwater inflow; the dispersal distance; the time at risk; and the salmon harvest were combined in a multi-variable logistic regression model of the probability of secondary infection. The final multi-variable model included log10 freshwater inflow (Wald chi-square = 9.93) and dispersal distance (Wald chi-square = 6.48). Receiver operating characteristic analyses of the final model supported freshwater inflow as a strong predictor of G. salaris infection status. The strong influence of the freshwater inflow on the probability of secondary infection adds further support to the hypothesis of inter-river dispersal of G. salaris through fjords.     

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.     

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.     

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.     

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.     

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.     

Seasonal dynamic of the infestation of young landlocked salmon (Salmo salar morpha sebago, Girard) by the monogenean species Gyrodactylus salaris

Seasonal dymanic of the infestation of young landlocked salmon (Salmo salar morpha sebago, Girard) by the monogenean species Gyrodactylus salaris Malberg, 1957 was studied in the river Lizhma, Karelia. It is established, that the temperature optimal for the development and reproduction of the parasite is about 3-8 degrees C. It is stated, that the abundance of G. salaris in the river Lizhma do not reach so high values, as it was observed in the regions outside the natural area of the parasite.     

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.     

Computing expected reproductive success of female Atlantic salmon as a function of salmon size

A method is described for determining the expected reproductive success (gonadal mass of a returning fish times the probability of surviving to return) and expected fecundity of salmonids as a function of smolt size. Application of the method requires data relating (i) return weight and smolt size; (ii) probability of survival and smolt size; (iii) probability of return after one or two sea winters and smolt size; and (iv) gonads and return weight. Although there exists no published data set that contains all of this information, it is possible to piece together enough information from published sources on female Atlantic salmon to demonstrate the feasibility of the method, with the goal of encouraging the publication of datasets that will allow meaningful calculations for a single river. Thus, one should not expect general predictions about Atlantic salmon, but once local conditions are taken into account, it will be possible to predict the relationships between smolt size and expected fecundity or expected reproductive success.     

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.     

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.     

Infection of Atlantic salmon, Salmo salar L., by Gyrodactylus salaris, Malmberg 1957, in the River Lakselva, Misvær in northern Norway

Gyrodacrylus salaris was most probably introduced to the River Lakselva in 1975 through stocking of Atlantic salmon from an infected hatchery. The parasite population grew rapidly, and the parasite spread throughout the entire watercourse during the summer of 1976. This epidemic situation led to mortality among the young Atlantic salmon, and the density of salmon parr was heavily reduced from 1976 to 1977. The density of salmon parr has remained close to zero since then, while there are no apparent trends toward decrease or increase in the density of brown trout. In spite of the reduced density of young salmon, a new epidemic has developed each year among the few young 0+ and 1+ Atlantic salmon present in the river. Results from successive sampling during the summer of 1987, 1988 and 1989 indicate that most of the presmolt salmon are attacked during their first summer or autumn of life. The infection develops into an epidemic during the first autumn, winter or the next summer. The build-up of the parasite burden on the fish leads in turn to mortality. Norwegian Atlantic salmon probably have no resistance against G. salaris, since the parasite has recently been introduced to Norwegian 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.     

Return migration of one‐sea‐winter Atlantic salmon in the River Tana

Fifty‐three one‐sea‐winter Atlantic salmon Salmo salar (45–63 cm L _T) were radio‐tagged in the Tana fjord, Barents Sea, in 1995. Thirty‐seven fish (70%) entered the freshwater zone of the River Tana in an average of 3 days after release in the fjord. The migration speeds in the lowest river section below the first riffle area were significantly higher than in the subsequent river section below the second riffle area. Similarly, the observed time spent in the first riffle area was significantly lower than in the next riffle area. The majority of Atlantic salmon entered the river during the hours of high tide and the subsequent ebb tide. In addition, most river entries were recorded around midnight. No effects of river flow on the river entry or migration speed were detected, but the migration speed of Atlantic salmon in both river sections examined was greater at lower temperatures. Twenty‐eight fish (72%) were recaptured in the river, 71% of them with weirs and gillnets, and 29% by rod and line. Over half of the Atlantic salmon (54%) were recaptured within 3 weeks following river entry, and within the first 100 km of the river (56%). The results are discussed in relation to earlier studies on multi‐sea‐winter Atlantic salmon in the River Tana.     

Twenty years of hydropower regulation in the River Alta: long-term changes in abundance of juvenile and adult Atlantic salmon

The River Alta, northern Norway (70°N), was regulated for hydropower in 1987. Densities of juveniles and catches of adult Atlantic salmon have been studied since 1980–1981 to examine the effects of regulation. The need to control environmental variables during electrofishing was emphasized, as flow variables explained up to 42% of the variation in estimated juvenile densities. The number of spawning redds was counted along the river from 1996 to 2005. The annual number of spawning redds was correlated with the catch of multi-sea-winter salmon (predominantly females). In the upper 7 km section, just downstream of the power station outlet, juvenile densities were reduced by 80% from pre-regulation levels to minimum levels in 1992–1996. This was followed by partial recovery during 1997–2005, although not entirely back to pre-regulation levels. In contrast, the general trend in the middle part of the river was a linear increase in juvenile densities during 1981–2005. Decreased juvenile densities in the upper section was subsequently followed by reduced catches of adult salmon in this part of the river. The relative catches of smolt year classes migrating to sea in the upper section was reduced by up to 75% from 1991 onwards. Spawning and recruitment in the upper section have increased in recent years, probably back to the introduction of catch-and-release angling and an increase in salmon runs. However, present day smolt production in the upper section is still reduced compared to the middle part of the river, 18 years after regulation. The decreased densities of juvenile salmon in the upper section were probably caused by several factors, of which stranding mortality due to sudden drops in the water level and increased winter mortality due to changed environmental conditions, especially reduced ice-cover, may be the most important. In conclusion, the regulation caused a considerable reduction of the salmon production in the upper 16% of salmon reaches, but did not affect the salmon population negatively further downstream. This study illustrates that apparently small environmental disturbances can cause large changes in Atlantic salmon abundance in high latitude populations. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

The impact of a brominated flame retardant on smoltification and olfactory function in Atlantic salmon (Salmo salar L.) smolts

Despite a reduction in exploitation of salmon stocks throughout the NE Atlantic, there continues to be a decline in many populations. The factors regulating these populations remain poorly understood, although there is evidence that environmental conditions experienced in freshwater can effect survival in the marine environment. The aim of this study was to investigate the impact of a brominated flame retardant (hexabromocyclododecane, HBCD) on the parr-smolt transformation in juvenile salmon. It is during this pre-adaptive period to marine life that olfactory imprinting to the natal river is considered to occur. Fish were exposed to low levels of HBCD for 30 days over the peak smoltification period in freshwater, and then transferred to clean seawater for 20 days. Fish were sampled weekly to assess changes in some of the physiological parameters associated with smoltification, and olfactory response to conspecific smolt urine was measured using an electro-olfactogram (EOG). Exposure to HBCD did not affect seawater adaptability, although there was some disruption of plasma thyroid hormone levels, as well as a reduction in olfactory function to conspecific smolt urine. The results are discussed in relation to the marine survival and successful homing of adult salmon.