Mitochondrial DNA variation in Atlantic salmon, Salmo, salar L., populations

Mitochondrial DNA (mtDNA) variation was examined in 209 Atlantic salmon from two river systems, the R. Itchen in Hampshire (Southern England) and the R. Conwy (North Wales). Within each system, five spawning sites were sampled. Four enzymes (AVA II, HAE III, HINF I and MBO I) revealed restriction fragment polymorphisms that were informative. Ten clonal lines were observed. These clones were differentially distributed between the two river systems and single clonal types were found to predominate at several spawning sites. MBO I variants were found in salmon from the R. Itchen but not the R. Conwy. A significant heterogeneity in frequency distribution of clonal lines between parr and smolt assemblages of the same year class within the R. Itchen was detected. This heterogeneity suggests that differences in survival or migratory behaviour may be identified by changes in mtDNA clonal frequencies.     

Genetic variation in haemolytic activity in Atlantic salmon (Salmo salar L.)

Spontaneous and antibody-dependent haemolytic activity against sheep red blood cells (SRBC) has been studied in serum from Atlantic salmon, Salmo salar L. Considerable increase in haemolytic activity was detected when SRBC were sensitized with haemolysin produced in Atlantic salmon. Haemolytic activity was sensitive both to divalent cations and to heat treatment. Significant reduction in haemolytic activity was detected after absorption with SRBC, indicating the presence of natural antibody against SRBC in Atlantic salmon serum. Persistent haemolytic activity of unsensitized SRBC in serum absorbed to remove natural antibodies was found, which suggests the activation of the alternative complement pathway, while the observed increased haemolytic activity in the presence of specific antibody against SRBC suggests activation of the classical complement pathway. Spontaneous and antibody-dependent haemolytic activity were both analysed in absorbed and non-absorbed sera in a family material of Atlantic salmon. The material consisted of 574 fish belonging to 57 fullsib groups within 20 paternal halfsib groups. Fish with signs of sexual maturity generally showed reduced haemolytic activity. Statistically significant effect of sire on the spontaneous haemolytic activity of both absorbed and nonabsorbed serum, and on antibody-dependent activity, provides evidence of significant additive genetic variation in both the alternative and the classical complement activation in Atlantic salmon. Neither on a phenotypical nor on a family basis were the two traits statistically correlated. The estimated heritabilities were 0.2–0.3 with a standard error of approximately 0.1.     

Natural hybrids between Atlantic salmon, Salmo salar L., and trout, Salmo trutta L., in juvenile salmonid populations in south-west England

Electrophoretic analysis of juvenile salmonids from south-west England revealed the presence of natural hybrids between Atlantic salmon, Salmo salar L., and brown trout, Salmo rrurra L., within three of five rivers. Amongst 409 juvenile 'salmon', 1.2% were hybrids, and of 150 juvenile 'trout' sampled, 2.0% were natural hybrids. In one river, similar hybridization levels in 'salmon' parr samples were found in two successive years. Levels of hybridization are high when compared with most similar studies, especially others within the United Kingdom. Possible factors influencing this are discussed.     

Genetic variation in Atlantic salmon, Salmo salar L., within the Tamar catchment in south-west England

Electrophoretic analysis of enzymes in 383 juvenile Atlantic salmon, Salmo salar L., within the Tamar catchment (south-west England) revealed significant genetic differences at the IDHP-3* locus between the three tributaries studied. Aspects of temporal and spatial variation, and management policy within catchments, are discussed.     

Cortisol stimulates hypo-osmoregulatory ability in Atlantic salmon, Salmo salar L.

Hypo-osmoregulatory ability in juvenile Atlantic salmon, Sulrno salur L., was improved by cortisol treatment. Implantation of a vegetable shortening pellet containing cortisol (50 mg kg⁻¹) resulted in elevated plasma cortisol titres. Maximum cortisol levels (160–170ngml⁻¹) were observed at days 6 and 12 after the implantation and dropped significantly by day 55. Cortisol-implanted fish in fresh water developed a twofold increase in gill Na⁺/K⁺-ATPase activity at days 6 and 12, and a threefold increase by day 55. Intestinal mucosa Na⁺/K⁺-ATPase activity was not affected by cortisol. Cortisol-implanted fish exposed to 28 ppt sea water for 48 h tended to show an improved ability to regulate their plasma osmolarity and reduce their ionic load. The osmo-regulatory ability attained at days 12 and 55 was further evaluated by exposing fish to 37 ppt sea water for 96 h. While all the control fish died relatively early in these tests, cortisol-implanted fish showed a clear reduction in their mortality rate. These results indicate that cortisol can induce biochemical and organismal changes during winter months that typify preadaptive events normally occurring in the spring.     

The effects of intraperitoneally implanted dummy acoustic transmitters on the behaviour and physiology of juvenile Atlantic salmon, Salmo salar L

The behavioural and physiological effects of surgical implantation of dummy miniature acoustic transmitters into the peritonealcavities ofjuvenile Atlantic salmon, Salmo salar L., were assessed. lntraperitoneal implantations had no significant effect on growth, feeding or swimming behaviour in either parr or smolts. Recovery from the surgical implantation was both rapid and total; infection was absent; and physiological processes such as smoltification and maturation of testes in precocious parr were unaffected. Expulsion of the transmitter through the body wall, not through the implantation wound, occurred in a number of fish but without adversely affecting the animals. The intraperitoneal implantation technique is discussed in relation to its use during biotelemetry studies.     

A microsatellite linkage map for Atlantic salmon (Salmo salar)

A linkage map of the Atlantic salmon is described here consisting of 15 linkage groups containing 50 microsatellite loci with a 14 additional unlinked markers (including three allozymes). The map shows the largest sex-specific recombination rate differences so far found in any vertebrate species (3.92:1 female:male). Homologies with previous linkage mapping studies of Atlantic salmon and rainbow trout are described. An in silico search of the Genbank database carried out using the microsatellites used in the mapping process identified significant matches between the flanking regions of the microsatellite SS11 and the calcium-binding mitochondrial carrier protein, 'Aralar1'.     

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

It is well documented that prior residence confers advantagesinterritorial disputes, but its impact on other aspects ofbehavior and fitnessis less understood. We tested how priorresidence influences the subsequentfeeding behavior and growthperformance of dispersing Atlantic salmon fry(Salmo salar)using experimental manipulations of residence in aseminaturalstream tank. In replicated trials, groups of sevenprimaryfish were released into the stream tank 3 days ahead ofsevensecondary fish. Standardized behavioral observations weremadeon each fish over the following 14 days, after which all fishwereremoved and measured. Primaries and secondaries were initiallythe same sizeand body condition and exhibited the same degreeof site fidelity. However,primaries darted higher into thewater column to intercept prey items, fed ata higher rate,and subsequently grew faster. Larger fish (in terms of bodylength)tended to be more dominant, and dominants grew faster thansubordinates.However, there was no difference in dominance between primariesandsecondaries. These results suggest that the well-documentedadvantage ofearly-emerging salmon fry over late-emerging frycannot be completelyattributed to intrinsic differences andthat the advantage is partly mediatedvia a prior residenceeffect. Furthermore, prior residents gain foragingadvantageswithout necessarily becoming more dominant.     

An analysis of the distribution of juvenile Atlantic salmon (Salmo salar) in nature as a function of relatedness using microsatellites

The major objective of this study was to test the hypothesis that juvenile Atlantic salmon kin occupy adjacent territories in their natural habitat in order to profit from the benefits associated with kin-biased behaviours, as has been observed under controlled laboratory conditions. Microsatellites were used to establish the relatedness of salmon fry (in their first summer of life) and parr (in their second and third summer of life) captured in adjacent territories. We did not observe a relationship between the proximity and the relatedness of either parr of the same cohort or fry in their natural habitat. Although many pairs of fry were identified as being related when sampled immediately after emergence, most family groups did not occupy adjacent territories. The high dispersal potential in rivers, the low occupation rate of the habitat and the incidence of half-sibs in nature most probably reduce the opportunity and advantage of kin-biased behaviour, in contrast to laboratory studies conducted in artificial, high-density conditions.     

The effect of intragastric transmitters on feeding behaviour of Atlantic salmon, Salmo salar; parr during autumn

The proportion of Atlantic salmon parr (146 265 mm total length) that fed in laboratory tanks during autumn was 0% in fish filled with intragastric transmitters compared with 78 100% in controls.     

Spawning behaviour of a farmed escaped female Atlantic salmon (Salmo salar)

Excavation of stranded redds revealed differences in spawning behaviour between farmed and wild Atlantic salmon. The redd of a farmed fish contained more egg pockets (nine v . average of two) and fewer eggs per pocket (averages: 459 v . 707). No other pocket measures differed.     

Residence time of the sea louse, Lepeophtheirus salmonis K., on Atlantic salmon, Salmo salar L., after immersion in fresh water

Returning adult salmon caught at the mouth of the River Dee, Aberdeenshire, were transferred to tanks in the laboratory. For fish placed in fresh water, sea lice remained attached for up to 6 days, though most lice were lost in the first 48 hours. Few lice were lost from salmon maintained in sea water. The experiments were conducted in water within a temperature range of 12·8 to 16° C, equivalent to summer river temperatures in the Aberdeenshire Dee.     

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.     

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.     

Mitochondrial DNA and allozyme analysis of Atlantic salmon, Salmo salar L., in England and Wales

Allozyme variation was examined in 429 Atlantic salmon, from seven sampling sites in England and Wales. Contingency tests for genetic homogeneity using three diagnostic loci revealed no significant differences among populations from the north-east and north-west of the U.K., but detected significant aspartate aminotransferase (AAT) differences between these samples and those from the R. Itchen in Hampshire (southern England). Mitochondrial DNA variation was analysed in 40 salmon from five spawning sites in the R. Itchen. Eight restriction endonucleases were used, of which three (AVAII, HAE III and HINFI) revealed polymorphisms. Six matriarchal lineages were discriminated. Two of the six mtDNA types observed were site-specific. These results suggest a reasonable expectation of discriminating spawning aggregations of Atlantic salmon.     

Territory size and distribution of newly-emerged Atlantic salmon (Salmo salar)

Newly emerged Atlantic salmon (Salmo salar) were observed from May to August 1981 at six isolated redds in Washington County, Maine, USA. Territorial size and distribution were measured. At the end of the emergence period (12 to 28 May), fish maintained positions (stations) at redds where water velocity did not exceed 52 cm s^–1 By 12 June, most salmon (80–96%) had moved off the redds of origin and had established territories 1 to 5 m from the redd. The area defended increased substantially after mid-June, but territorial aggression diminished by 15 July, and the fry dispersed downstream. All fish observed were territorial, and the percentage of time during which stations were held decreased from 89 in mid-May to as low as 40% in mid-June.Cooperators are the Maine Department of Inland Fisheries and Wildlife, University of Maine, U.S. Fish and Wildlife Service, and the Wildlife Management Institute     

Vertical distribution of Atlantic salmon (Salmo salar) as a function of illumination

The vertical distribution of cultured Atlantic salmon larger than 1 kg was monitored by hydroacoustics in both a shallow (6 m) and a deep (20 m) cage. Surface light levels and fish distribution were registered for three periods throughout the year. The two fish groups were fed calculated rations twice a day. During the winter and spring (including vernal equinox), the fish were distributed at around 5 m depth in both cages when not feeding. Around summer solstice, the fish preferred deeper waters when not feeding. The light dependency of the vertical migration was pronounced, and indicates clearly that even a 20 m deep cage is not deep enough to meet the depth requirements of large Atlantic salmon during summer.     

Migratory and reproductive behaviour of female adult Atlantic salmon, Salmo salar L., in a spawning stream

Migration and spawning behaviour of eight Atlantic salmon adult females were analysed by radio-tracking in relation to the degree of sexual maturity in a spawning tributary of the R. Sélune. Six of them were grilse and four of them were ripe. All the fish migrated upstream until reaching their spawning site at a distance of 4–12 km from the trap. The daily migration rate up to this site was inversely correlated with the length of the female. Spawning occurred in all fish at the same time when the water temperature increased dramatically. Spawning lasted 1–10 days according to the fish. After spawning, females quickly moved downstream only small distances and then stayed in approximately the same location until death. This study concluded that ripeness did not influence behaviour on the spawning migration and describes certain characteristics of the reproductive phase in a spawning tributary.