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.     

Primers for sequence characterization and polymorphism detection in the Atlantic salmon (Salmo salar) growth hormone 1 (GH1) gene

We report the identification of intraspecific sequence variation in the Atlantic salmon (Salmo salar) growth hormone 1 gene. Rapid and inexpensive assays for polymorphism detection were developed for 10 sites. Five of the assays detected single nucleotide polymorphisms (SNPs) using polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) analyses, and five were indel polymorphisms, detected using fragment length analyses. The average within population frequency of the most common allele varied from 0.52 to 0.90, and the average within population heterozygosity varied from 0.02 to 0.37 in seven European salmon populations.     

Liver ultrastructure of juvenile Atlantic salmon (Salmo salar)

Light and transmission electron microscopy of the liver of juvenile Atlantic salmon (Salmo salar) reveals a tubular arrangement of parenchymal cells, with biliary passages typically located at the center of tubules. Hepatocytes generally contain a single nucleus surrounded by a cuff of rough endoplasmic reticulum (RER), with many round to elongate mitochondria associated with the perinuclear RER. Whereas glycogen deposits are common and usually lie at the cell periphery, parenchymal cells seldom contain lipid droplets. Golgi complexes and heterogeneous dense bodies also occur in many hepatocytes, often in close proximity to bile canaliculi. Numerous microvilli from hepatocytes extend into the subendothelial space of Disse, which is also the location of stellate fat-storing cells. Interhepatocytic macrophages, sometimes containing prominent phagolysosomes and residual bodies, are common in the liver. The intrahepatic biliary system consists of intercellular canaliculi, bile pre-ductules, ductules, and ducts. In contrast to some other teleosts, the liver of the Atlantic salmon contains no intracellular bile canaliculi or Kupffer cells. The hepatic endothelium, arterioles, and perivenous regions are also described.     

Physiological seawater adaptation in juvenile Atlantic salmon (Salmo salar) autumn migrants

Summary

• 1 About 25 % of juvenile Atlantic salmon (Salmo salar) migrating downstream in the River Frome in southern England do so in the autumn rather than in the spring. Here, we examine the physiological status of these fish with regard to those features that adapt them to sea water during the parr–smolt transformation (i.e. gill Na⁺K⁺ ATPase activity; the number, size and type of chloride cells on the gill lamellae; salinity tolerance and relative plasma thyroid levels).
• 2 Autumn migrants, and those fish which subsequently reside in the tidal reaches during the winter, are not sufficiently physiologically adapted to permit permanent or early, entry into the marine environment.
• 3 It is not known what proportion of autumn migrating fish survive and return to spawn as adults. If significant numbers do return, however, the production from tidal reach habitats must be taken into account in the development of salmon stock management strategies, especially monitoring and assessment programmes, and in the evaluation of factors affecting stocks.

     

Parental influence on growth rate, smolting rate and survival in hatchery reared juvenile Atlantic salmon, Salmo salar

The proportion of potential 1-year smolts, their mean length, the mean length of potential 2-year smolts, and the mortality rate in four half-sib families of Atlantic salmon, reared under four contrasted conditions of overhead cover, is shown to be inffuenced primarily by genetic factors (89.9%, 86.1 %, 82.7% and 80.2% of total variance respectively). Variation between families in smolting rate and mortality rate is influenced by both parents, but more by the male than the female. Variation in mean length is influenced almost entirely by the female parent. These results are discussed in relation to previous findings on bimodality of size distribution and inheritance of growth and mortality characteristics in Atlantic salmon.     

Transport-associated proteins in Atlantic salmon (Salmo salar)

 The major histocompatibility complex (Mhc) regions of mice, rats, and humans all contain a pair of related genes, TAP1 and TAP2, which encode members of a large superfamily of proteins of similar structure and function. A functional TAP1/TAP2 heterodimer is probably required for efficient presentation of antigens to CD8⁺ T cells. This heterodimer resides in the membrane of the endoplasmic reticulum, and transports peptides from the cytoplasm into the endoplasmic reticulum lumen for binding to Mhc class I molecules. The TAP transporter demonstrates specificity for both peptide sequence and length, and in rats, allelic variation in the sequence of the transporter molecules results in differential ability to transport particular peptides. Here we report two expressed Sasa-TAP2 loci, both of which are polymorphic, as well as an expressed Sasa-TAP1 locus from Atlantic salmon. The Sasa-TAP2A locus has a genomic organization similar to the human TAP2 equivalent. Received: 23 December 1996 / Revised: 26 February 1997     

Metabolic rate, growth and aggressiveness in three Atlantic salmon Salmo salar populations

Standard metabolic rate ( R _S), specific growth rate ( G ) and aggressiveness were investigated in three Finnish populations of Atlantic salmon Salmo salar (Neva, Saimaa and Teno), which were reared in identical hatchery conditions. The populations differed in their geographical origin and native habitat. There was a significant difference between populations in R _S: the southernmost Neva population had higher values in R _S than the northernmost Teno population. No difference was found in G or aggressiveness between the populations. G was found to have a significant positive association with aggressiveness and R _S among the three populations, however, these results were not statistically significant after correction for multiple tests. There was no significant association between R _S and aggressiveness. Higher metabolic rate of the most southern population Neva is suggested to be an adaptation to the more abundant food sources of the southern stream.     

Seasonal variation in the thermal performance of juvenile Atlantic salmon (Salmo salar)

1. Experimental data on the maximum growth and food consumption of winter‐acclimatised Atlantic salmon (Salmo salar) juveniles from three Norwegian rivers situated at 59 and 70°N were compared with predictions from published models of growth and food consumption of summer‐acclimatised fish from the same populations. 2. All winter‐acclimatised fish maintained positive growth and a substantial energy intake over the whole range of experimental temperature (1–6 °C). This contrasted with predictions from growth models based on summer acclimatised Atlantic salmon, where growth and energy intake ceased at approximately 5 °C. 3. Growth and food consumption varied significantly among populations. Winter‐acclimatised fish from a Northern population had a higher mass‐specific growth rate, higher energy intake and higher growth efficiency than southern populations, which is contrary to predictions from models developed using summer‐acclimatised salmon, where fish from the Northern population had the lowest growth efficiency. 4. The experiment provides evidence that thermal performance varies seasonally and suggests adaptation to the annual thermal regime.     

The effects of prior residence on behavior and growth rates in juvenile Atlantic salmon (Salmo salar)

It is well documented that prior residence confers advantagesin territorial disputes, but its impact on other aspects ofbehavior and fitness is less understood. We tested how priorresidence influences the subsequent feeding behavior and growthperformance of dispersing Atlantic salmon fry (Salmo salar)using experimental manipulations of residence in a seminaturalstream tank. In replicated trials, groups of seven "primary"fish were released into the stream tank 3 days ahead of seven"secondary" fish. Standardized behavioral observations were madeon each fish over the following 14 days, after which all fishwere removed and measured. Primaries and secondaries were initiallythe same size and body condition and exhibited the same degreeof site fidelity. However, primaries darted higher into thewater column to intercept prey items, fed at a higher rate,and subsequently grew faster. Larger fish (in terms of body length)tended to be more dominant, and dominants grew faster than subordinates.However, there was no difference in dominance between primaries andsecondaries. These results suggest that the well-documentedadvantage of early-emerging salmon fry over late-emerging frycannot be completely attributed to intrinsic differences andthat the advantage is partly mediated via a prior residenceeffect. Furthermore, prior residents gain foraging advantageswithout necessarily becoming more dominant.     

Cloning of the Atlantic salmon (Salmo salar) estrogen receptor-alpha gene

A cDNA clone encoding most of an Atlantic salmon (Salmo salar) estrogen receptor (ER) was obtained from a liver cDNA library and the remainder of the coding sequence from the gene was isolated from a genomic library. Sequence comparisons showed that the cloned gene represents ER-alpha. Expression of the ER-alpha gene in male and female salmon parr was analysed by RT-PCR. Highest expression was found in brain and liver, with lower levels of ER-alpha mRNA present in all other tissues tested. There was little difference in expression of ER-alpha between male and female.     

Comparative seawater performance and deformity prevalence in out-of-season diploid and triploid Atlantic salmon (Salmo salar) post-smolts

The use of sterile triploid stock in the Atlantic salmon (Salmo salar, L) farming industry is the only commercially available means to prevent the ecological impact of domesticated escapees. This study compared the seawater (SW) performance and deformity prevalence of diploid and triploid post-smolts from 2 full-sib families produced out-of-season. Triploids completed smoltification 4 weeks earlier and at a significantly higher body-weight. Growth and survival in SW were not significantly affected by ploidy. The incidence of external deformities, dominated by jaw malformation, was ~12% in triploids and below 5% in diploids. Vertebral deformities were more prevalent in the fastest growing triploid family only. Heart morphometry differed between ploidies which may relate to a higher cardiac workload in triploids. No clear alteration of the gill apparatus was detected. The most significant detrimental effect of triploidy was on the rate and severity of cataract that were observed from August onward (50% and 92% of diploids and triploids respectively affected after 1-year in SW). At that time, cataracts were diagnosed by histological examinations as irreversible with a probable osmotic origin which could arise from factors such as water quality, nutritional deficiencies or thermal variations. This study warrants further research aiming at adapting rearing practices to the needs of triploid stocks as to improve their performance and welfare.     

Movement rhythms in juvenile Atlantic salmon, Salmo salar L.

Nocturnal downstrean migration of juvenile Atlantic salmon is usually interpreted as increased locomotor activity. The frequency of downstream passages of 0–1 + salmon in an endless stream channel was greater by night than by day in both smoking and non-smolting fish in autumn and spring. Movement increased at dusk, and decreased after dawn. Mature male 1 + fish moved slightly less than immatures in October, but significantly more in November. Total movement frequency was lower at full moon than at other moon phases, and movement was reduced when the moon was up. Under turbid conditions by day, the threshold water velocity inducing nett downstream movement was 8.2 cm s⁻¹, and the relative velocity of fish swimming downstream was never more than one third that of fish holding station at the normal maximal flow of 25–30 cm s⁻¹.
At the end of their first growing season in October, fish which had been offered food continuously through 24 h did not differ in size from those fed by day only, but the latter were significantly larger than those offered food only at night.
We conclude that: (1) the fish fed actively by day, and not by night; (2) station-holding represented activity, and downstream nocturnal movement represented relative inactivity (displacement) which occurred on loss of visual orientation, hence migration resulted from reduced activity; (3) lack of displacement in early autumn has adaptive value for maturing fish, but not for non-spawners.     

A lymphosarcoma in an Atlantic salmon (Salmo salar)

A lymphosarcoma that appeared to be of thymic origin and of lymphoblastic type was found in a 3.5-yr-old Atlantic salmon (Salmo salar). The fish was from a population of 60 broodfish maintained at a research fish laboratory. A large tumor mass was found under the left operculum. Small tumor nodules were found on the swim bladder and in the abdominal adipose tissue. The location of this neoplasm differed from those of previously described tumors in this fish species.     

Influence of exogenous thyroxine on plasma melatonin in juvenile Atlantic salmon (Salmo salar)

One of the most clearly defined endocrine changes during the parr-smolt transformation of anadromous salmonids is an increase in plasma levels of thyroid hormones. The role of pineal hormone melatonin in timing and synchronisation of smoltification is widely discussed. The effect of administration of exogenous thyroxine (T4) on plasma melatonin was investigated in juvenile Atlantic salmon (Salmo salar) at the early stages of parr-smolt transformation. Fish were kept in fresh water under simulated-natural photoperiod and exposed to exogenous T4. Fish were sampled at 12.00 and 24.00 h from treatment and control tanks, 2 and 14 days after treatment started. Plasma melatonin and L-thyroxine were measured using RIA and competitive enzyme immunoassay, respectively. After 2 days of T4 treatment, marked difference in plasma melatonin concentration measured at 12.00 and 24.00 h was still observed in both groups. However, 2-week exposure to T4 caused a reduction in night-time plasma melatonin level and thus, probably, inhibited melatonin related time-keeping system in juvenile salmon. Additional studies are needed to clarify the mechanism of the described phenomenon.     

Diel activity pattern of juvenile Atlantic salmon (Salmo salar) in early and late winter

Radiotelemetry was used to investigate the diel activity pattern of juvenile Atlantic salmon (Salmo salar) in early and late winter. Fish were active throughout the diel cycle. However, there was significantly less daytime than nighttime movement and movement declined significantly with increasing fork length. Maximizing winter growth rate, through an overall increase in foraging activity, may reduce the risk of starvation in smaller fish. The results of the present study provide evidence that the activity patterns of juvenile salmonids are quite complex and support the suggestion that individual variation in activity patterns are, at least, partially driven by body size.     

Influence of photoperiod on growth changes in juvenile Atlantic salmon, Salmo salar L.

During their first 6 months sibling Atlantic salmon parr, Salmo salar L., grew larger under constant light than under natural photoperiod or simulated natural photoperiod (control). When rate of change of photoperiod was accelerated after midsummer, ×2, ×3 and ×4, there were no growth differences between the three groups, but all were smaller than the control population. Under constant autumn photoperiod of 8 h light: 16 h dark growth was less than under all other experimental photoperiod conditions. Mean length was directly correlated with total hours of daylight experienced, excluding those fish kept under constant light. Fish reared from first feeding under photoperiod regimes delayed 6 and 9 months out of phase with the natural light cycle were smaller than the controls, whereas those under a regime 3 months out of phase did not differ from the controls. The clear segregation of modal length groups within the 3, 6 and 9 months out-of-phase populations occurred 1, 4 and 4 months, respectively, after the segregation in the control group. Under constant light, and under constant 12 h light: 12 h dark (12 LD), the segregation was delayed 3 and 4 months, respectively. The proportion of the population which maintained growth (upper modal group) was significantly less in the 9 and 6 months out-of-phase and 12 LD groups (39, 40 and 42%, respectively) than in the other three groups (82.5-85%). Acceleration of photoperiod change also resulted in decreased growth. The results support a model of salmon development in which, 2-3 months after first feeding, growth is maintained if feeding opportunities at that time are above a threshold level, and in which this critical timing is influenced by photoperiod. It is suggested that the delays reflect a synchronizing effect of photoperiod on an endogenous rhythm of appetite and growth. The differences in upper modal group proportions observed in the present experiments, reflect the relative feeding opportunities available at the critical period in July-August.     

Effects of discharge and local density on the growth of juvenile Atlantic salmon Salmo salar

The study explored the combined effects of density, physical habitat and different discharge levels on the growth of juvenile Atlantic salmon Salmo salar in artificial streams, by manipulating flow during both summer and winter conditions. Growth was high during all four summer trials and increased linearly with discharge and mean velocity. Differences in fish densities (fish m^?3) due to differences in stream volume explained a similar proportion of the variation in mean growth among discharge treatments. Within streams, the fish aggregated in areas of larger sediment size, where shelters were probably abundant, while growth decreased with increasing densities. Fish appeared to favour the availability of shelter over maximization of growth. Mean growth was negative during all winter trials and did not vary among discharge treatments. These results suggest that increased fish densities are a major cause of reduced summer growth at low discharge, and that habitat‐mediated density differences explain the majority of the growth variation across habitat conditions both during summer and winter.     

Distribution of genetic variation in the growth hormone 1 gene in Atlantic salmon (Salmo salar) populations from Europe and North America

The level and hierarchical distribution of genetic variation in complete sequences of the Atlantic salmon (Salmo salar) growth hormone (GH1) gene were investigated in populations from Europe and North America with a view to inferring the major evolutionary forces affecting genetic variation at this locus. Seventeen polymorphic sites were identified in complete sequences from nine populations, with levels of noncoding (intron and untranslated region sequences) nucleotide diversity being similar to those observed in other species. No variation, however, was observed in exonic sequences, indicating that nucleotide diversity in the Atlantic salmon GH1 gene is three and 25 times less than that estimated for human and Drosophila coding sequences, respectively. This suggests that purifying selection is the predominant contemporary force controlling the molecular evolution of GH1 coding sequences. Comparison of haplotype relationships within and between populations indicated that differentiation between populations from Europe and North America was greater than within-continent comparisons. However, several haplotypes observed in the northernmost European populations were more similar to those observed in North American than to any other haplotypes observed in Europe. This is most likely to be a result of historical, rather than contemporary, gene flow. Neutrality test statistics, such as Tajima's D, were significantly positive in the European populations in which North American-like haplotypes were observed. Although a positive Tajima's D is commonly interpreted as the signal of balancing selection, a more likely explanation in this case is that either historical migration or ascertainment bias, rather than within population local adaptation, has given rise to an excess of intermediate frequency alleles.