Extensive mortality in Atlantic salmon fry was reported in the River Åelva from 2002 to 2004. Dead fish were collected in late summer 2006, and live fish were sampled by electrofishing in September the same year. At autopsy and in histological sections, the fish kidneys were found to be pale and considerably enlarged. Proliferative lesions with characteristic PKX cells were seen in a majority of the fish. DNA from kidney samples of diseased fish was subjected to PCR and sequencing, and the amplified sequences matched those of Tetracapsuloides bryosalmonae. We concluded that this myxozoan transmitted from bryozoans was the main cause of the observed mortality in salmon fry in 2006. Results from quantitative electrofishing in 2005 and 2006, combined with the observed fry mortality from 2002 to 2004, show that the smolt production in the river is severely reduced and that T. bryosalmonae is the most likely explanation for this decline. The present study is the first to report a considerable negative population effect in wild Atlantic salmon due to proliferative kidney disease (PKD). It also represents the northernmost PKD outbreak in wild fish. The river is regulated for hydroelectric power purposes, causing reduced water flow and elevated summer temperatures, and the present PKD outbreak may serve as an example of increased disease vulnerability of northern fish populations in a warmer climate.

To evaluate the potential invasiveness of pumpkinseed Lepomis gibbosus introduced to northwestern European inland waters, growth and reproduction traits were examined in ten populations along a trajectory spanning northwestern Europe (Norway, England, Holland, Belgium and France) and evaluated in light of published dataset from Europe. In the 848 pumpkinseed captured, maximum age was 3-4 years, with a sex ratio near unity in all but one population. Significant variations with increasing latitude were observed in adult growth (age 2-3 increment in total length, TL) and mean age at maturity (A M ), with

Histophagous ciliates caused high mortality among turbot in a commercial fish farm in southern Norway. The ciliates spread systemically in fry (< 0.3 g). In the early stages of infection the ciliates were found in connective tissue in skin and fins, as well as in nervous tissue. In terminal stages the whole organism was infected. In large turbot (500–1000 g), the ciliates were predominantly found in the central nervous system, causing liquefaction of the nervous tissue. The ciliates were not identified to species but resembled species in the genus Uronema (Scuticociliatida).

The validity of Gyrodactylus thymalli has been questioned, based on its morphological and genetic resemblance to G. salaris. This taxonomic problem has practical implications regarding correct diagnosis of G. salaris, which has proved to be highly pathogenic to stocks of wild Norwegian Atlantic salmon. The host specificity and pathogenicity of G. salaris and G. thymalli were experimentally tested on salmon and grayling. Both parasite species were able to infect, live and reproduce on both fish species. G. salaris was highly pathogenic for the experimental stock of salmon, while grayling mounted an effective response against this parasite. Both fish species responded to an infection with G. thymalli. The results did not support the hypothesis of conspecificity between G. thymalli and G. salaris. The ribosomal RNA gene intergenic spacer of both species was sequenced. Variation in sequence was lower than expected for different species. Variation in the sequences of tandemly repeated elements was found and may prove useful in distinguishing G. salaris and G. thymalli.

A hexamitid flagellate Spironucleus burkhanus n. sp., from the lumen of the gut and gall bladder of wild grayling Thymullus thymallus, and from muscle abscesses of farmed Atlantic salmon Salmo salar from Norway, is described by light, scanning and transmission electron microscopy. The flagellate was axenically cultured in trypticase, yeast extract, iron serum medium. Live trophozoites from axenic cultures incubated at 5" C , measure 11-20 X 6-14 p n . The flagellates show a typical bi-radial symmetry. Each recurrent flagellum is almost completely surrounded by a striated lamina. In the posterior end the lamina widens, appearing heart shaped in transverse section. Accompanying each recurrent flagellum are three narrow bands of microtubules, following the longitudinal groove created by the incomplete closure of the striated lamina. The recurrent flagella emerge posterio-medially through cytostome openings halfway surrounded by crescent-shaped ridges, oriented in opposite directions in the two openings. The position and adornment of the cytostome openings, and the arrangement and number of the microtubules accompanying the recurrent flagella, distinguish Spironucleus barkhanus n. sp. from previously described species of Spironucleus.Supplementary key words. Axenic culture, Hexamifa, systemic infection. URING a parasite survey in the autumn of 1989, a hex-Received 11- 5-97, 2-24-97; accepted 4-28-97

There are two genotypes of the diplomonad Spironucleus barkhanus. Based on sequence data from the small subunit ribosomal RNA gene the conspecificity of these two genotypes has been questioned. Therefore, we have sampled Spironucleus from 27 fish, representing 14 populations, five species, and four genera. Partial nucleotide sequences from the three genes; small subunit ribosomal DNA, glutamate dehydrogenase 1 and alpha-tubulin were compared. The pathogenic isolates of S. barkhanus, which causes systemic spironucleosis in Atlantic salmon, Chinook salmon, and Arctic charr, all farmed in sea water, were genetically very different from the commensal isolate found in wild freshwater populations of Arctic charr and grayling. The genetic distances between the genotypes were of the same magnitude as those separating species of Giardia. Based on these genetic and ecological data, the pathogenic genotype from farmed salmonids is described as a new species, Spironucleus salmonicida n. sp. Scanning and transmission electron microscopy showed no specific morphological or ultrastructural features distinguishing S. salmonicida n. sp. from S. barkhanus. The present study clearly demonstrates the value of applying genetics in identification of Spironucleus species. Phylogenetic analyses that included the isolates of S. salmonicida n. sp. did not change the phylogenetic relationship within the genus Spironucleus.

Gyrodactylus salaris Malmberg, 1957 is a freshwater monogenean ectoparasite of salmonids, first recorded in Norway in 1975 and responsible for extensive epizootics in wild Atlantic salmon Salmo salar L. The susceptibility of different populations of Atlantic salmon to G. salaris infection differs markedly, with fish from the Baltic being characterised as relatively resistant whereas those from Norway or Scotland are known to be (extremely) susceptible. Resistance to Gyrodactylus infection in salmonids has been found to be heritable and a polygenic mechanism of control has been hypothesised. The current study utilises a 'Quantitative trait loci' (QTL) screening approach in order to identify molecular markers linked to QTL influencing G. salaris resistance in B1 backcrosses of Baltic and Scottish salmon. Infection patterns in these fish exhibited 3 distinct types; susceptible (exponential parasite growth), responding (parasite load builds before dropping) and resistant (parasite load never increases). B1 backcross fish were screened at 39 microsatellite markers and single marker-trait associations were examined using general linear modelling. We identified 10 genomic regions associated with heterogeneity in both innate and acquired resistance, explaining up to 27.3% of the total variation in parasite loads. We found that both innate and acquired parasite resistance in Atlantic salmon are under polygenic control, and that salmon would be well suited to a selection programme designed to quickly increase resistance to G. salaris in wild or farmed stocks. KEY WORDS: Gyrodactylus salaris · Atlantic salmon · Resistance · Linkage mapping · Quantitative trait loci Resale or republication not permitted without written consent of the publisherDis Aquat Org 71: [119][120][121][122][123][124][125][126][127][128][129] 2006 Western Atlantic Ocean, the Eastern Atlantic Ocean and the Baltic (Stahl 1987, Bakke et al. 1990. Within these groups the species comprise multiple, genetically differentiated and, to a large extent, reproductively isolated river populations (Stahl & Hindar 1988). There is virtually no migration of fish from the Baltic into the Eastern Atlantic, or vice versa (Christensen & Larsson 1979). Population level genetic heterogeneity in resistance within and between salmon populations may thus be important in determining whether an epizootic takes place in a particular river (Pickering 1987), and for the future development of stocks resistant to the parasite (Bakke et al. 1999).Most species of freshwater fish seem to be more susceptible to attack from parasites to which they have not been previously exposed (Dobson & May 1987, Bakke et al. 1990). Heterogeneity in susceptibility of different salmon stocks, and of individuals of the same stock, to Gyrodactylus salaris has been noted in a number of studies, with fish from the Baltic being less susceptible than those of the Eastern Atlantic (Bakke et al. 1990, Bakke & MacKenzie 1993, Jansen & Bakke 1993a,b, Rintamäki-Kinnunen & Valtonen 1996, Cable et al....

A hexamitid flagellate was found in the gall bladder and intestine of Arctic char Salvelinus alpinus in northern Norway. Scanning and transmission electron microscopy showed this flagellate to be identical to Spironucleus barkhanus from grayling Thj~mallus thymallus and farmed Atlantic salmon Salmo salar. It is hypothesised that systemic spironucleosis in sea-caged Atlantic salmon was due to transmission of flagellates from feral char to the salmon.