Identification of Atlantic salmon × brown trout hybrids with a nuclear marker useful for evolutionary studies

A new nuclear DNA marker is described to identify Atlantic salmon, brown trout, and their hybrids. Species-specific banding patterns are obtained by digestion of genomic DNA with the restriction endonuclease Sac I and hybridization with a histone DNA probe. FISH analyses in hybrid metaphases illustrate the potential of interspecific hybrids for evolutionary studies.     

BAC-based upgrading and physical integration of a genetic SNP map in Atlantic salmon

A better understanding of the genotype–phenotype correlation of Atlantic salmon is of key importance for a whole range of production, life history and conservation biology issues attached to this species. High-density linkage maps integrated with physical maps and covering the complete genome are needed to identify economically important genes and to study the genome architecture. Linkage maps of moderate density and a physical bacterial artificial chromosome (BAC) fingerprint map for the Atlantic salmon have already been generated. Here, we describe a strategy to combine the linkage mapping with the physical integration of newly identified single nucleotide polymorphisms (SNPs). We resequenced 284 BAC-ends by PCR in 14 individuals and detected 180 putative SNPs. After successful validation of 152 sequence variations, genotyping and genetic mapping were performed in eight salmon families comprising 376 individuals. Among these, 110 SNPs were positioned on a previously constructed linkage map containing SNPs derived from expressed sequence tag (EST) sequences. Tracing the SNP markers back to the BACs enabled the integration of the genetic and physical maps by assigning 73 BAC contigs to Atlantic salmon linkage groups.     

Effect of a major QTL affecting IPN resistance on production traits in Atlantic salmon

This study investigated the effect of a major QTL for resistance to IPN in salmon on performance and production traits. The traits studied were related to growth, fillet and gutted yields, and fat content. Two different analyses were performed: (1) regression of the phenotypic data of the production traits on the predicted number of resistant IPN‐QTL alleles in individuals and (2) a variance component analysis using the (co)variance matrix calculated at the putative location of the QTL. No significant effect of the QTL was detected on any of the traits investigated by either method. The result has important practical implications in that it encourages the use of MAS to reduce the risks and impact of IPN mortality.     

The effect of pineal removal on circulating melatonin levels in Atlantic salmon parr

Nocturnal melatonin levels showed a significant decrease in pinealectomized Atlantic salmon parr in comparison with intact control and sham-operated fish.     

Recombinant genotypes in backcrosses of male Atlantic salmon × brown trout hybrids to female Atlantic salmon

When male hybrids of Atlantic salmon × brown trout were backcrossed to female Atlantic salmon, approximately 1% of diploid progeny hatched. These were shown to exhibit recombinant genotypes when examined electrophoreticalty at five enzyme loci. This is the first confirmation of genie recombination in backcrosses of these species. Triploidization greatly increases the proportion of backcross progeny which hatch.     

Mature male parr buried among eggs in an Atlantic salmon redd

A mature mate parr of Atlantic salmon was found buried among eggs 28 cm beneath the gravel surface in a salmon redd. Egg mortality in this egg-pocket was considerably higher than in neighbouring egg-pockets (52 v. 3%), presumably due to decay.     

Endurance swimming of diploid and triploid Atlantic salmon

When groups of diploid (mean ±  s . e . fork length, L _F) 33·0 ± 1·4 cm and triploid (35·3 ± 0·5 cm) Atlantic salmon Salmo salar were forced to swim at controlled speeds in a carefully monitored 10 m diameter 'annular' tank no significant difference was found between the maximum sustained swimming speeds ( U _ms, maintainable for 200 min) where the fish swam at the limit of their aerobic capability. Diploids achieved 2·99 body lengths per second (bl s⁻¹)(0·96 m s⁻¹) and triploids sustained 2·91 bl s⁻¹(1·02 m s⁻¹). The selection of fish for the trials was based on their ability to swim with a moving pattern projected from a gantry rotating at the radius of the tank and the selection procedure did not prove to be significant by ploidy. A significant difference was found between the anaerobic capabilities of the fish measured as endurance times at their prolonged swimming speeds. During the course of the experimentation the voluntary swimming speed selected by the fish increased and the schooling behaviour improved. The effect of the curvature of the tank on the fish speeds was calculated (removing the curved effect of the tank increased the speed in either ploidy by 5·5%). Implications of the endurance times and speeds are discussed with reference to the aquaculture of triploid Atlantic salmon.     

Genetic studies on serum transferrins in Atlantic salmon

Published and unpublished data on genetic variation at the transferrin locus ( TF *) in Atlantic salmon from rivers in eastern North America sampled from 1968 to 1970 were reanalysed and compared with data for samples collected in 1998 from nine of the same rivers. Genetic differentiation among rivers was highly significant as was spatial differentiation among tributary samples within the Miramichi River system, the largest rivers studied. Comparison of allele frequencies in rivers also sampled in 1998 show no overall evidence of significant genetic change after 30 years, spanning 9 generations. The results strongly support the stability of the patterns of spatial genetic differentiation and support the occurrence in Atlantic salmon of reproductive isolation among rivers and among tributaries within large river systems.     

Feeding and prey-selection of wild Atlantic salmon post-smolts

The diet of post-smolt Atlantic salmon Salmo salar caught in the Trondheimsfjord and Frohavet in central Norway, based on stomach contents analysis, showed a gradual change during migration from the river to the estuary, fjord and coastal areas. Post-smolts caught in the estuary had eaten intertidal gammarid amphipods, while post-smolts caught further seawards preyed upon available marine prey such as Calanus spp., adult euphausiids and fish larvae. The frequency of adult insects was high in all post-smolt stomachs. The gradual change in diet suggested that feeding conditions in the early marine phase were important for post-smolt survival and growth. With the exception of the copepods, there was no overall similarity between species composition of the plankton samples and the stomach contents. Although the hypothesis that the post-smolts are opportunistic feeders cannot be rejected, the composition of the stomach contents suggests a possible selectivity of advantageous prey.     

Movements of adult Atlantic salmon in relation to a hydroelectric dam and fish ladder

The movements of adult Atlantic salmon were recorded as they approached, entered and ascended the pool-and-orifice fish ladder at Pitlochry Dam, Scotland. Thirty-nine returning salmon were captured in the River Tummel by rod-and-line angling, radio-tagged and released near where they were caught. The subsequent movements of each fish were then monitored. An electronic fish counter collected additional data on movements of untagged fish past a fixed point in the ladder. Of the 39 fish that were radio-tagged, 29 individuals were recorded approaching and ascended the ladder. The remaining fish either did not approach the dam (three fish), approached the dam after detailed tracking had ended (two fish), were recaptured by anglers (three fish), or the radio tags failed (two fish). Salmon released earlier in the year delayed longer before first approaching the dam. Delays between first approaching the dam and ascent of the ladder were greater for fish that approached the dam earlier. The majority of salmon visited the ladder entrance more than once (maximum 10 visits) before ascending. Having entered, all but four salmon ascended the fish ladder successfully on their first attempt. The four individuals that failed to do so succeeded on their second attempt. The rate at which salmon ascended the ladder was related directly to temperature. The shortest ascent time of a radio-tagged salmon was 5·25 h. Movements of eight of 11 tagged fish through the ladder ceased with the onset of darkness but continued on the following morning. No radio-tagged fish entered the ladder at temperatures below 9) C. Similarly, few untagged fish were recorded ascending the ladder by the electronic fish counter at water temperatures below 8·5) C. Records from the fish counter indicated that 92% of upstream movements were made during daylight.     

The feeding behaviour of juvenile Atlantic salmon in relation to turbulent flow

The feeding behaviour of juvenile Atlantic salmon Salmo salar in the Sainte‐Marguerite River, Quebec, Canada, varied with the characteristics of turbulent flow. Simulations indicated that juveniles would decrease their swimming costs during attacks by 19·8% in low and by 31·1% in high turbulent conditions by initiating movements in low‐speed flow events. The real swimming costs did not differ from the swimming costs estimated for a situation where fish initiate their movements at randomly selected flow velocities. The juvenile Atlantic salmon did not seem to prefer low‐speed flow events when initiating their movements. The proportion of time used for movements by fish decreased with an increase in the mean and the s . d . of the flow velocity.     

A new species-specific nuclear DNA marker for identification of hybrids between Atlantic salmon and brown trout

The banding pattern of Alu I-digested prepro-gonadotropin releasing hormone gene is species-specific in Atlantic salmon and brown trout and can be applied for identification of their hybrids.     

Size variability of juvenile Atlantic salmon: links to environmental conditions

Climate change models predict a 2 to 6° C increase in air temperature within the next 100 years in the Maritime Provinces of eastern Canada. Higher air temperatures are expected to contribute to increased water temperatures, alterations in stream flow conditions, and ultimately reductions in fish growth. Mean annual size-at-age of juvenile Atlantic salmon Salmo salar decreased in the Northwest Miramichi and Southwest Miramichi Rivers between 1971–1999. Lengths-at-age of juveniles were significantly correlated between the two rivers. For Atlantic salmon parr, stronger associations between inter-cohort fork length ( L _F) than intra-cohort L _F were observed, suggesting that environmental conditions in the current year of growth have the more significant effects on size of age 2 year parr than conditions encountered the previous year by age 1 year parr of the same cohort. Fork lengths of parr were significantly and negatively associated with spring air and water temperatures. In the Miramichi River, increases in air and water temperature as predicted from climate change models may adversely affect growth of juvenile Atlantic salmon parr, reducing the overall productivity of the Atlantic salmon populations in this region.     

Genetic changes in an Atlantic salmon population resulting from escaped juvenile farm salmon

The study was undertaken on three adjacent rivers in NW Ireland, on one of which an Atlantic salmon Salmo salar freshwater juvenile rearing unit is situated. Two markers which distinguished farm and wild populations were used. An Ava II-B RFLP in the ND1 region of mtDNA was at a frequency of 0.58 in the farm strain but absent in the wild populations. Allele E at minisatellite locus Ssa- A45/2/l was at a frequency of 0.91 in farm samples, but at a maximum of 0.41 in the populations in the two rivers adjacent to the one with the juvenile rearing unit. The farm strain showed a significant reduction in mean heterozygosity (0.281 ± 0.057), over three minisatellite loci examined, compared to wild samples (0.532 ± 0.063). The occurrence of farm genotypes and the independent occurrence of mtDNA and minisatellite markers in several parr samples from the river indicated that escaped juvenile salmon completed their life cycle, bred and interbred with native fish, upon their return to the river. Escaped fish homed accurately, as adults, to the site of escape, i.e. the area adjacent to the hatchery outflow in the upstream part of the river. Breeding of males in the lower part of the river was also indicated but this could have been due to mature male parr which had moved downstream. The return of adults of farm origin to the river to breed was indicated by the presence of the Ava II-B haplotype in adults netted in the estuary.     

Reproductive priming in mature male Atlantic salmon parr exposed to the sound of redd cutting

The sounds of a wild female Atlantic salmon cutting a redd were associated with significant increases in the levels of plasma 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) and of expressible milt in mature male parr, comparable with levels of the steroid and milt produced in parr exposed to the priming pheromone, prostaglandin F_2a. Hence auditory cues may have a significant role in synchronising reproductive physiology in Atlantic salmon.     

Chondroid bone and secondary cartilage contribute to apical dentary growth in juvenile Atlantic salmon

The microstructure and tissue composition of the dentary bone in Atlantic salmon salmo salar parr were examined using a variety of histological and whole‐mount techniques. Proximally, the dentary is composed of typical cellular lamellar bone with Sharpey's fibres extending dorsally, proximally and ventrally. Meckel's cartilage is located medially through the entire length of the dentary, and degrades distally resulting in a short transitional zone between hyaline cartilage and connective tissue. At the distal tip of the dentary, isogenic clusters of chondrocytes of periosteal origin were observed secreting small amounts of pericellular cartilage matrix within the bone matrix. These characteristics are highly indicative of secondary chondrogenesis, and suggest that the apical part of the dentary bone in Atlantic salmon does not grow via 'pure' intramembranous ossification, but rather via a modified mode of periosteal ossification involving secondary cartilage and chondroid bone. Furthermore, the unusual mode of gender‐related dentary growth (kype formation) in adult male Atlantic salmon could be the continuation of a general mode of salmonid apical dentary growth.     

Thermal habitat of adult Atlantic salmon Salmo salar in a warming ocean

The year-round thermal habitat at sea for adult Atlantic salmon Salmo salar (n = 49) from northern Norway was investigated using archival tags over a 10 year study period. During their ocean feeding migration, the fish spent 90% of the time in waters with temperatures from 1.6–8.4°C. Daily mean temperatures ranged from −0.5 to 12.9°C, with daily temperature variation up to 9.6°C. Fish experienced the coldest water during winter (November–March) and the greatest thermal range during the first summer at sea (July–August). Trends in sea-surface temperatures influenced the thermal habitat of salmon during late summer and autumn (August–October), with fish experiencing warmer temperatures in warmer years. This pattern was absent during winter (November–March), when daily mean temperatures ranged from 3.4–5.0°C, in both colder and warmer years. The observations of a constant thermal habitat during winter in both warmer and colder years, may suggest that the ocean distribution of salmon is flexible and that individual migration routes could shift as a response to spatiotemporal alterations of favourable prey fields and ocean temperatures.