For Salmo salar, there is a lack of information on the morphology of the first blastomeres formed during embryonic development and which could be used as a diagnostic tool for the first stages of development. The purpose of this investigation, therefore, was to characterize morphometrically the first blastomeres of S. salar. From a pool of eggs incubated at 7.5°C, 100 microphotographs of blastodiscs were extracted and analyzed at different incubation periods: 12, 14, 16, 20 or 24 h. Blastodiscs were characterized morphologically after 16, 20 or 24 h incubation, and classified into symmetric or asymmetric groups according to their morphology. The ratio of length (L) versus width (W) of each blastomere was determined, to establish its symmetry. In addition, 20 microphotographs of blastodiscs of normal appearance were analysed morphologically (control blastodisc: CB) for comparison (20 or 24 h). Results show that the first cleavage ends after 16 h of development. Seven categories were established during blastomere characterization: 47% normal (G1); 27% with dispersed margins (G2); 10% unequal (G3); 9% ‘pie-shaped’ (G4); 3% amorphous (G5); 2% three equal blastomeres and one different one (G6); and 2% with eccentric cleavage (G7). Although the incidence of abnormal cleavage in S. salar is uncertain, there is a potential for some asymmetries to be corrected during embryogenesis to generate viable individuals. More studies are necessary to correlate these abnormal cleavage patterns with indicators of quality in the later stages of embryogenesis in this species, to establish a quality assessment tool for gametes and/or embryos in salmonid species.

The present study investigates vertical distribution of fish size in three 12 m  ×  12 mwide and 14 m deep sea cages stocked with Atlantic salmon of average weight from 3.5 to3.7 kg, at commercial densities between 15.6 to 16.2 kg m-3, in Norway. Foreach cage, individual fish weight were estimated by three 0.6 m  ×  0.6 m measuring framesas fish swam through. The frames were positioned at 3, 6 and 9 m depth. Recordings werecarried out over five days per cage in succession, and during relatively stableenvironmental conditions in late autumn 2010. In all cages, measured fish were 15−25%smaller at 3 m compared to the average weights at 6 and 9 m depth. Largest average weightdifference between depths within one cage was 0.995 kg. The average weight at 6 and 9 mwas higher at night-time compared to daytime. Fish at the lower end of the weight spectrumwere predominantly registered at 3 m, while fish at the higher end of the spectrum weremostly registered at the greater depths. Fish of average size were well represented at allthree depths. The fact that smaller fish swam shallower may be ascribed to naturalbehavioural traits and introduce an important consideration in representative samplingwithin commercial sea cages. In biomass estimations and sea lice counts size-dependentvertical stratification needs to be accounted for.

Phylogenetic analyses of SSU rDNA sequences have previously revealed the existence of 2 Ichthyobodo species able to infect Atlantic salmon (Salmo salar L.). Ichthyobodo necator sensu stricto (s.s.) is assumed to be a freshwater parasite, while a genetically distinct but undescribed species, Ichthyobodo sp. II sensu Todal et al. (2004) have been detected on Atlantic salmon in both fresh- and seawater. In the present study a morphological description of Ichthyobodo sp. II from the gills of salmon reared in fresh-, brackish- and seawater is presented, using both light- and electron microscopy. Comparative morphometry show that Ichthyobodo sp. II from both freshwater and seawater displays a different cell shape, and is significantly smaller than I. necator s.s. Also, ultrastructural characteristics distinguish these two species, notably differences in the attachment region and the presence of spine-like surface projections in Ichthyobodo sp. II. Based on both unique SSU rDNA sequences and morphological characteristics, we conclude that Ichthyobodo sp. II. represents a novel species for which we propose the name Ichthyobodo salmonis sp. n.

The relationship of survival and reproduction of Gyrodactylus salaris Malmberg on the Atlantic salmon (Salmo salar) to water temperature (2·5–19·0 °C), was studied on the basis of temporal sequence of births and age at death of individual parasites on isolated salmon, and of infrapopulation growth on isolated and grouped salmon. Mean life-span of the parasite was negatively correlated with water temperature: 33·7 days at 2·5 °C and 4·5 days at 19·0 °C. The average number of offspring per parasite peaked between 6·5 and 13·0 °C, and was approximately 2·4 at these two temperatures. Both the period between the successive births of the offspring (max 4) and the estimated generation time were negatively correlated with temperature. The innate capacity for increase (rm) was positively correlated with temperature: from 0·02 (/parasite/day) at 2·5 °C to 0·22 (/parasite/day) at 19·0 °C. Growth of the infrapopulations was positively correlated with water temperature and was higher on isolated fish than on grouped fish, though less than the potential parasite population growth estimated from rm. In the infrapopulations the mean intensity of parasites continued to increase throughout all the experiments on both isolated fish and on grouped fish.

Gyrodactylus salaris is a serious pest of wild pre-smolt Atlantic salmon (Salmo salar) in Norway. The closely related G. thymalli, originally described from grayling (Thymallus thymallus), is assumed harmless to both grayling and salmon. The 2 species are difficult to distinguish using traditional, morphometric methods or molecular approaches. The aim of this study was to explore whether there is a consistent pattern of morphometrical variation between G. salaris and G. thymalli and to analyse the morphometric variation in the context of ‘diagnostic realism’ (in natural populations). Specimens from the type-material for the 2 species are also included. In total, 27 point-to-point measurements from the opisthaptoral hard parts were used and analysed by digital image processing and uni- and multivariate morphometry. All populations most closely resembled its respective type material, as expected from host species, with the exception of G. thymalli from the Norwegian river Trysilelva. We, therefore, did not find clear support in the morphometrical variation among G. salaris and G. thymalli for an a priori species delineation based on host. The present study also indicates an urgent need for more detailed knowledge on the influence of environmental factors on the phenotype of gyrodactylid populations.

This study performed in the Patapédia River (Québec, Gaspésie) was designed to examine the space utilization and mobility patterns of individual Atlantic salmon parr over short time scales with frequent detections of position. In August 2004, n = 7 radio-tagged Atlantic salmon parr were located every 20 min during 10 tracking periods chosen to take into account the dawn, day, dusk and night periods. Parr showed important inter individual variability in space utilization and mobility patterns, occupying large home ranges (123 to 5602 m2) and sometimes migrating 2 km upstream in the river. No differences were observed in the mean distance travelled among the four daily periods by individual fish, but the estimated home ranges utilized during the four daily periods overlapped only partially, demonstrating that parr partly occupy different areas of the river during different parts of the daily cycle. The results suggest that an intense survey on a small number of fish may contribute to a complementary comprehension of space utilization and mobility patterns of Atlantic salmon parr when viewed in association with results from studies surveying larger number of fish at lower frequency.

The monogenean ectoparasite, Gyrodactylus salaris Malmberg, 1957, has had a devastating effect on wild Atlantic salmon (Salmo salar) since its introduction to Norway in the mid-1970s. In Lake Pålsbufjorden, southern Norway, upstream of the stretches of the River Numedalslågen with anadromous Atlantic salmon, a resident Arctic charr (Salvelinus alpinus) population has been reported to be infected with G. salaris which is viable in the absence of its normal host, the Atlantic salmon. Currently, there is no record of G. salaris infecting Atlantic salmon in the downstream sections of the River Numedalslågen. We studied experimentally the infectivity and reproductive capacity of G. salaris from Lake Pålsbufjorden on wild and hatchery-reared Atlantic salmon as well as on Arctic charr and rainbow trout (Oncorhynchus mykiss). Arctic charr and rainbow trout were moderately susceptible, whereas the Atlantic salmon stocks from River Numedalslågen and River Drammenselva were innately resistant to only slightly susceptible. Thus, the G. salaris from Arctic charr in Lake Pålsbufjorden is considered non-pathogenic to Atlantic salmon. This is the first observation of variation in host preference among Norwegian G. salaris populations. The observed differences in virulence between G. salaris populations could have important consequences for the international legislation and management of Atlantic salmon.

In the present study, we describe the complete mitochondrial (mt) genome of the Atlantic salmon parasite Gyrodactylus salaris, the first for any monogenean species. The circular genome is 14 790 bp in size. All of the 35 genes recognized from other flatworm mitochondrial genomes were identified, and they are transcribed from the same strand. The protein-coding and ribosomal RNA (rRNA) genes share the same gene arrangement as those published previously for neodermatan mt genomes (representing cestodes and digeneans only), and the genome has an overall A+T content of 65%. Three transfer RNA (tRNA) genes overlap with other genes, whereas the secondary structure of 3 tRNA genes lack the DHU arm and 1 tRNA gene lacks the TΨC arm. Eighteen regions of non-coding DNA ranging from 4 to 112 bp in length, totalling 278 bp, were identified as well as 2 large non-coding regions (799 bp and 768 bp) that were almost identical to each other. The completion of the mt genome offers the opportunity of defining new molecular markers for studying evolutionary relationships within and among gyrodactylid species.

The viviparous monogenean Gyrodactylus salaris continues to devastate Norwegian Atlantic salmon populations despite the extreme measures taken to control this pathogen. Increased understanding of parasite biology is needed to develop alternative control and management strategies of wild Atlantic salmon. We have examined temperature-dependent survival of G. salaris, both on and off the host. At 18 °C, survival off the host was 1 day, but at 3 °C parasites survived for 4 days. However, in contrast to assumptions made by earlier authors, many parasites remained with their host following its death. Ultrastructural evidence indicated that G. salaris individuals can feed on a dead host, and laboratory tests demonstrated that worms on their hosts more than double their life-span compared with individuals maintained off the host. Experimental infections also demonstrated that establishment and subsequent population growth of parasites previously maintained on dead hosts for 3 days, was similar to that of parasites transferred directly between living hosts. Hence, for G. salaris, dead infected hosts may increase the chances of successful transmission and be a potential important infection source in rivers and hatcheries.

Atlantic salmon (Salmo salar) parr (age 0+), infected by the ectoparasite Gyrodactylus salaris, were exposed to aqueous aluminium (Al), copper (Cu), zinc (Zn), iron (Fe) and manganese (Mn), at 4 different concentrations. There was a negative correlation between G. salaris infections and metal concentrations in both Zn- and Al-exposed salmon. In the Zn-experiment, all 4 concentrations tested caused a decrease in the G. salaris infections, while in the Al-experiment the G. salaris infection did not decline at the lowest concentration. The number of G. salaris increased continuously during the experiments in all control groups, and in all groups exposed to Cu, Fe and Mn. At the highest concentration, however, copper seemed to impair the growth of G. salaris infection. The results show that aqueous Al and Zn are environmental factors of importance controlling the distribution and abundance of the pathogen G. salaris. Other pollutants might also have an influence on the occurrence of G. salaris. Finally, the results demonstrate that aqueous Al and Zn have a stronger effect on the parasite than on the salmonid host, suggesting that both metals may be used as a pesticide to control ectoparasites such as G. salaris.

A laboratory population of a Danish Gyrodactylus salaris variant founded by 1 single specimen was established and infection studies performed. Rainbow trout as well as Atlantic salmon of 3 different stocks were infected both in cohabitation systems and as single-parasite infections on isolated hosts. Both infection systems revealed that this particular morphotype exhibits low virulence towards Atlantic salmon. Thus, in isolated hosts, the parasites could either not establish or only reproduce to a limited degree on salmon. Rainbow trout, in contrast, proved to be rather susceptible to infection with this G. salaris variant and abundances on this host species always attained significantly higher values in cohabitation systems compared to salmon. Detailed morphological examination confirmed the very high resemblance to G. salaris (sensu stricto), as the range of variation in sclerite characters of the morphotype is almost fully covered by the total range of variation reported for reference G. salaris. Morphological similarities to the closely related congeneric species G. bohemicus were noted. Molecular studies, however, showed that the morphotype most likely represents a G. salaris variant, as it differed only slightly from G. salaris sensu Malmberg, 1957, which is also known to inhabit Danish watercourses. The genomic target region investigated does not allow us to rule out the possibility that it represents a variant form of G. thymalli. Sequences of the ribosomal RNA internal transcribed spacer (ITS) revealed that single individuals contained 2 different ITS sequences, one identical to reference sequence of G. salaris while the other differed at 3 positions. This can be interpreted as an example of a hybrid or, more likely, as intra-individual variation of ITS within single individuals. As one of the nucleotide changes in the variant ITS affects an Hae III restriction site, the current G. salaris variant can be distinguished from G. salaris sensu Malmberg by RFLP diagnosis.

Fixed location split-beam horizontal echosounding was used to assess the size and timing of the Atlantic salmon (Salmo salar) spawning run in the River Tornionjoki. Four transducers, two on each river bank, were mounted across the river at the study site 4 km upstream from the river mouth. Net weirs were used on both shores to direct the passage of fish through the acoustic beams. Hydroacoustic monitoring covered 40–50% of the river cross-sectional area. Also test fishing and yearly catch statistics of salmon were used as an indication of the size of the spawning run in the river. Altogether, 7700, 5300 and 4300 salmon-sized targets (target strength, TS >= –29 dB) moving upstream were detected in 1997, 1998, and 1999, respectively. The fish migration began in all the years by early June and peaked during the second half of the month; the migration period of large salmon lasted until mid-July. The observations made by the echosounding and catch statistics were similar in this respect. In 1998 and 1999, however, more targets of TS >= –29 dB were detected during late summer than could be expected by the river catches of salmon. It may be that the large targets in late summer were, in fact, whitefish whose run occurred during the same time. Hydroacoustic estimation of the total salmon run at the study site was found difficult. The numbers of salmon-sized targets detected were almost the same as the numbers of salmon caught each year by fishermen. Therefore, only an index of the run timing and the size of the stock can be produced from the data. It was clear that a considerable amount of fish escaped the acoustic monitoring by using areas uncovered by the beam, such as gaps in the bottom and the surface layers of the water column near the shores. Moreover, it was found that species recognition based on TS only is not adequate in multispecies environments. Assessment of spawning runs remains, however, a key issue in the management of Baltic salmon, and with further development, the hydroacoustic monitoring may be the most viable means of doing it.

While the pineal organ in poikilothermic vertebrates consistently influences a number of physiological processes, its precise role, and what controlling signal (hormonal and/or neural) is abolished by its removal, is still uncertain. For this reason, the effect of long-term pinealectomy (PINX) on seasonal growth and reproductive development was investigated in Atlantic salmon, Salmo salar, parr. Mean daytime plasma melatonin level in the control, sham-operated and PINX fish was 35 pg·mL–1 in all groups, while mean night-time levels were 296, 255 and 25 pg·mL–1, respectively, indicating that PINX abolished the natural nocturnal rise in melatonin. Pinealectomy did not influence the incidence or timing of early sexual maturation in the male parr. However, pinealectomy significantly affected growth, its effect being boldly seasonally dependent. Compared to sham-operated and control fish, pinealectomized fish showed significantly lower specific growth rates (SGRs) during the period of lengthening photoperiod up until the summer solstice. Thereafter, corresponding to the season of decreasing photoperiods, the pinealectomized fish exhibited higher SGRs. These results suggest a functional relationship between the pineal organ and somatic growth in the Atlantic salmon, although what controlling signal from the pineal (melatonin or neural) is involved has yet to be determined.

The relationship of size to MEP-2* genotype was investigated in 0+ juvenile Atlantic salmon (Salmo salar) in the River North Esk, Scotland. Significant temperature-associated MEP-2* heterogeneity was observed among tributaries and, in the two tributaries studied, size and MEP-2* genotype were significantly associated. No size association, or heterogeneity among tributaries, was found for the other allozyme polymorphisms screened. The results suggest that the association between size and MEP-2* variation, noted by others, is likely to be widespread in wild populations, and potentially linked to habitat temperature.

Gyrodactylus salaris, an important pathogen of Atlantic salmon Salmo salar, has been shown to be highly sensitive to factors in host serum and mucus, being killed rapidly (50% within 1h) by serum at a dilution of 1[ratio ]200. The time needed for killing was inversely proportional to serum concentration. Similar effects were noted using host mucus, which contained approximately 1/20th of the anti-Gyrodactylus activity of serum. Serum activity was abolished completely by heating at 45°C for 30 min, and by addition of EDTA, but not by EGTA+1 mm magnesium ions. Activity was not dependent on whether the serum was from infected or naive fishes, nor was it species specific. Attempts to pre-coat parasites in salmon anti-Gyrodactylus antibodies also failed to enhance the activity of fresh serum. These observations suggest that killing is due to the complement system of the host, acting via the alternate pathway. G. salaris appears to be exceptionally sensitive to complement, being killed at concentrations which could be experienced in vivo. The role of complement in the protection of fishes against gyrodactylid infection therefore deserves further investigation.

The aim of the present study is to test the extent to which different intensities of continuous additional light affect somatic growth and sexual maturation in Atlantic salmon postsmolts in sea cages. Postsmolts (9 200 individuals) were randomly distributed among four commercial sized sea cages and exposed to either natural light or natural light + continuous additional light of low, medium or high intensity from January to June. In May the high light intensity group had a signiticantly higher mean live body weight than the other groups, and at the end of the experiment in June, all groups differed in mean gutted weight. There was apositive logarithmic regression between level of light intensity exposure during night and mean gutted weight. A small proportion of fish matured sexually (after 1.5 years in seawater) in the natural light group, whereas no maturation was detected in the light groups. These results suggests that to affect growth and proportion or maturation in Atlantic salmon by additional light, there might be different threshold values of light intensity.

Atlantic salmon, Salmo salar, and several species of whitefish, Coregonus nasus, C. peled, C. Lavaretus baeri, males were treated with a long-acting medical androgen preparation - "Testoenatum". This preparation contains 24 mg of testosterone enanthate and 111 mg of testosterone propionate in 1 ml peach oil solution. Multiple (2–5) or single intraperitoneal injections of a standard dose (0.1 ml/kg fish body weight) were used. Treatment performed shortly before the beginning or within the breeding season resulted in a significant stimulation of spermiation, improvement of sperm parameters and extention of the gonad functional maturity period for 1–4 months. Low water temperatures (0.5–l.0 °C) seem to be essential to obtain a long-lasting effect after one injection. The histology of the pituitary gland suggests that the gonadotropic cells were activated through a positive feedback mechanism. Testoenatum suppressed Sertoli cells' phagocytary activity.

La gestion de la reproduction du saumon atlantique nécessite généralement un transfert des géniteurs en eau douce avant la phase finale de la maturation sexuelle pour obtenir des gamètes de bonne qualité. Afin d'analyser l'impact de l'environnement marin, des géniteurs de saumon atlantique âgés de 2 ans (2+) ont été, soit maintenus en mer en cage (EM) jusqu'à la période de reproduction, soit transférés en eau douce (ED), en juin (EDP) ou en octobre (EDT). Les dates d'entrée en spermiation et d'ovulations ont été notées grâce à des observations hebdomadaires. La qualité des gamètes a été évaluée par le taux d'embryonnement au stade œillé après insémination de lots de 200 œufs par du sperme dilué à 10−3 avec du DIA 532®. Les femelles EDP et EDT ont ovulé entre novembre et décembre alors que les femelles EM présentaient un retard et un étalement des ovulations : 67 % des femelles EM avaient ovulé à la fin de l'expérience en avril. La fécondité relative des femelles EDP était significativement plus basse que celle des femelles maintenues en mer au moins jusqu'à l'automne. En dépit des cinétiques d'ovulation et des osmolantés des liquides coelomiques plus élevées, les œufs de femelles EM présentaient une qualité similaire à ceux des femelles ED lorsqu'ils sont utilisés moins d'une semaine après l'ovulation.

Les mâles ED et EM présentent des courbes similaires d'entrée en spermiation. Les spermes des mâles EM présentaient une qualité hétérogène et en moyenne médiocre. L'osmolarité des plasmas séminaux des mâles EM était significativement plus élevée (347 ± 13 mOsmol/l pour EM contre 265 ± 23 mOsmol/l pour ED). Les spermes EM de mauvaise qualité donnaient des résultats médiocres quelle que soit l'osmolarité des dilueurs d'insémination utilisés et comprises entre 150 et 450 mOsmol/l. De plus, aucune relation n'a été mise en évidence entre l'osmolarité individuelle des plasmas séminaux et la mauvaise qualité des spermes. D'un point de vue appliqué, l'osmolarité du plasma séminal ne peut être retenue comme critère qualitatif et le vieillissement des ovules après ovulation dans la cavité abdominale doit être strictement évité pour les femelles EM.

Two techniques of stock discrimination both utilizing scales, i. e. termed as classical technique and shape analysis, are compared for discriminating between North American and European Atlantic salmon (Salmo salar L.) caught off West Greenland. Three scales from each of 60 salmon were analyzed: 30 from North American and 30 from European-origin salmon collected from home rivers in 1982. The best results indicated misclassification and error rates of 0% for shape analysis and 5.0% and ± 1.7%, respectively, for the classical technique. A nested ANOVA indicated that variation was greater between scales of salmon of North American and European origin than from several scales from the same salmon. It was concluded that shape analysis with computer-aided devices may be superior to the classical technique and it is recommended that it be examined for use at West Greenland.

The final phase of spawning migration of eight adult male Atlantic salmon, Sulnio solar L., was analysed by radio-tracking in relation to sea age and size in a spawning tributary of the Kiver Sélune. Four grilse and four multi sea-winter fish, one of which as a previous spawner, were radio-tagged and released. Their fork length ranged from 602 to 882 mm. The daily movement in the first two days alier release and the furthest upstream distance recorded from the trap during the study period were independent of fish length. Maximum amplitude of movement observed over the entire study period was inversely correlated with length. Daily movement decreased in all males during the spawning period and was negatively correlated with the size of fish. Surviving males remained in the vicinity of the spawning grounds after spawning. It was concluded that the length of male seemed to influence its behaviour in the last phase of the spawning migration, enabling a much more successful participation of the larger males in spawning.