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.     

Characterization of transferrin‐linked microsatellites in brown trout (Salmo trutta) and Atlantic salmon (Salmo salar)

The transferrin (TF) gene has recently received increased interest in fish given its fitness relevance as a resistance gene against pathogenic bacteria. In this study, we characterized five TF‐linked microsatellites in brown trout (Salmo trutta) and Atlantic salmon (Salmo salar). Interestingly, each marker amplified duplicated loci and linkage analysis revealed that the TF gene has most likely experienced a tandem duplication during salmonid evolution. In addition, the amplification of all five markers across a wide range of salmonid species suggests that they may be of general interest for the genetic analysis of the TF gene(s) in this teleost family.     

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.     

High incidence of Atlantic salmon × brown trout hybrids in a Lake District stream

Hypervariable minisatellite DNA single-locus profiling and mitochondrial DNA analysis revealed that 18.48% of juvenile Atlantic salmon Salmo salar in Troutbeck, a stream in the R. Leven catchment of the English Lake District, were hybrids between Atlantic salmon and brown trout S. trutta , and that hybridization was bidirectional.     

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.     

Reproduction of interspecific hybrids of Atlantic salmon and brown trout in a stream environment

1. Reproduction between Atlantic salmon males and interspecific hybrid Salmo salar × Salmo trutta females was monitored in a controlled flow channel diverted from a south European river located at the edge of Atlantic salmon natural geographic distribution in Europe. 2. Post‐F1 hybrids were viable and survived in the wild, at least until dispersal from redds. After transfer to hatchery conditions, 67% survived into the second year. 3. The hybrids possessed 98 chromosomes: two sets of Atlantic salmon(2n = 58) and one set of brown trout (n = 40) chromosomes. 4. The existence of a low proportion of allotriploid individuals can be expected in rivers where Atlantic salmon and brown trout populations coexist.     

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.     

Otolith shape discriminates between juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

Otoliths of Atlantic salmon, Salmo salar L., are more slender than the otoliths of brown trout, Salmo trutta L. Discriminant analysis on otolith measurements of juvenile Atlantic salmon and brown trout from four river systems revealed a discriminant function which distinguished more than 94% of the cases. This function was tested by using data from a fifth river with cohabiting Atlantic salmon and brown trout: all Atlantic salmon and 91 % of the brown trout were correctly classified.     

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.     

Net ground speed of downstream migrating radio-tagged Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts in relation to environmental factors

The downstream migration of Atlantic salmon (Salmo salarL.) and sea trout smolt (S. trutta L.) was investigated using radio telemetry in the spring of 1999 and 2000. Forty wild sea trout smolts, 20 F1 sea trout smolts, 20 hatchery salmon smolts and 20 salmon smolts from river stockings were radio tagged and released in the Danish River Lilleaa. The downstream migration of the different groups of fish was monitored by manual tracking and by three automatic listening stations. The downstream migration of radio tagged smolts of both species occurred concurrently with their untagged counterparts. The diel migration pattern of the radio tagged smolts was predominantly nocturnal in both species. Wild sea trout smolt migrated significantly faster than both the F1 trout and the introduced salmon. There was no correlation between net ground speed, gill Na⁺,K⁺-ATPase activity or fish length in any of the different groups. The migration speed of wild sea trout smolts was positively correlated with water discharge in both years. In F1 sea trout smolts, migration speed was positively correlated with temperature in 1999. The migration speed of salmon smolts did not correlate to any of the investigated parameters.     

Production of juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., within different sections of a small enriched Norwegian river

Growth, density and production of juvenile Atlantic salmon and brown trout were studied in three different sections of the Kvassheimsåna River in south-western Norway from 1979 to 1983. Section 1. in the upper part of the river, is located above a waterfall impassable for migratory salmonids and is surrounded by grazing land. Sections 2 and 3, in the middle and lower parts of the river, are influenced by agricultural activity. Total nitrogen concentration varied between 250 and 1000 μg l ^?1 in section 1 and 1500 and 2500 μg l^?1 in sections 2 and 3. Total phosphorus (Tot-P) concentrations also increased with decreasing altitude: 19–46 μg l^?1 in section I and 31–101 μg l ^?1 in sections 2 and 3. The number of 0 + salmon in sections 2 and 3 varied between 30.1 and 167.8 specimens 100 m ^?2, with means 90.2 and 95.2 specimens 100 m ^?2:, respectively; the density of 1 + salmon, with mean values of 16.3 and 51.0 specimens 100m^?2 was significantly correlated with the original fry density. The growth rate of 0+ salmon was not inversely related to cohort density, but was significantly so for 1 + salmon. Mean annual salmon production in section 2 was 1595 g 100 m^?2 year ¹, and in section 3 was 841 g 100m^?2 year ¹. A logarithmic function gave the best curve fit between salmon production and mean annual biomass. Thus, production levelled off for the highest values recorded in section 2, and perhaps approached the carrying capacity of the stream. A multiple regression analysis showed that yearly variation in 1 + salmon density was the single factor accounting for most of the total variability in production (60%). Variation in water temperature and nutrient content were not significantly related to variation in fish production. Densities of brown trout were low in all sections (<20 specimens 100m ^?2). Fry density was highest in section 3 and parr density in section 1. All age groups of sympatric brown trout grew significantly faster in sections 2 and 3 compared with allopatric brown trout in section 1.     

Hybridization between Atlantic salmon Salmo salar L. and brown trout S. trutta l. upon artificial propagation

Samples of Salmo salar and S. trutta were examined in 12 Russian fish hatcheries. With protein markers, hybrids of the two species were found in three hatcheries of the Baltic Sea basin. Some fishes had a phenotype intermediate between the S. salar and S. trutta phenotypes by morphological traits, but did not differ genetically from one of the parental species. Possible consequences of hybridization and ways to prevent it are discussed.     

Seasonal and spatial microhabitat selection and segregation in young Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a Norwegian river

Seasonal microhabitat selection by sympatric young Atlantic salmon and brown trout was studied by diving. Both species, especially Atlantic salmon, showed seasonal variation with respect to surface and mean water velocities and depth. This variation is partly attributed to varying water flows and water temperatures. In winter the fish sought shelter in the substratum. A spatial variation in habitat use along the river due to different habitat availabilities was observed. Both species occupied habitats within the ranges of the microhabitat variables, rather than selecting narrow optima. It is hypothesized that the genetic basis allows a certain range to the behavioural response. Microhabitat segregation between the two species was pronounced, with brown trout inhabiting the more slow-flowing and partly more shallow stream areas. Atlantic salmon tolerated a wider range of water velocities and depths. Habitat suitability curves were produced from both species. It is suggested that habitat suitability curves that are based on observations of fish occupancy of habitat at median or base flow may not be suitable in habitat simulation models, where available habitat is projected at substantially greater water flows.     

The growth of juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a Scottish river system subject to cooling-water discharge

The River Fiddich, a tributary of the R. Spey in north-east Scotland, is a spawning river for both Atlantic salmon and brown trout. Warm cooling water effluent is discharged from several distilleries at different points in the lower reaches and raises the temperature of the river 1–3°C above ambient for most of the year. Salmon and trout grow more rapidly in this region than further upstream, and juvenile salmon generally migrate a year earlier, as 2 + smolts. Available data were too few to determine whether there was a similar difference for trout. Similar studies on the R. Dullan, a tributary of the Fiddich, and on the Cromdale Burn in the same area, confirmed that the growth rate of fish is faster downstream from distillery discharge points. It is suggested that increased invertebrate production may influence the growth rate.     

The effect of the rate of temperature increase on the critical thermal maximum for parr of Atlantic salmon and brown trout

The critical thermal maximum for salmon and trout parr was not affected significantly by age or acclimation temperature, and increased asymptotically with the rate of temperature increase. Mean thermal maxima were estimated with poor precision at high and low rates of temperature increase, and high precision at rates of 1 and 2°Ch⁻¹.     

Fatty acid profiles of wild brown trout and Atlantic salmon juveniles in the Nivelle basin

Samples of Atlantic salmon Salmo salar 0+ year parr and brown trout Salmo trutta 1+ year juveniles, and 2+ and 3+ year sub‐adults were obtained from nine sites of the Nivelle River, France, located at the southern edge of the species distribution area. Fatty acid composition of the different samples were analysed. Within the same river basin, the fatty acid profiles appeared to vary among samples from different locations. Based on cluster analysis, fatty acid signatures of both species appeared to discriminate samples from different locations.     

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.     

Genetic differentiation among natural European populations of Atlantic salmon, Salmo salar L., from drainages of the Atlantic Ocean

Previously published allelic frequencies at four polymorphic protein coding loci were used as a basis for examining genetic relationships among 19 European populations of Atlantic salmon, Salmo salar L.--exclusive of Baltic drainages--from the Barents Sea to Spain. The data did not support a model of distinct ancestral (e.g. Boreal and Celtic) origins, but were consistent with all populations descending from a single ancestral group within this region with genetically diverged populations drawn together through limited local migrations.     

Links between patterns of marine growth and survival of Atlantic salmon Salmo salar, L.

The hypothesis that marine survival of Atlantic salmon Salmo Salar is linked to marine growth was explored by using inter-circuli distances and total numbers of circuli existing on scales from a population monitored over nearly four decades. The results suggest that marine growth controls survival, particularly during the late summer and early winter of the first year at sea. Recruitment is strongly linked to growth, described as the total number of circuli, but not to inter-circuli distances. This highlights the potential of patterns of circuli number to be considered as proxies for growth. Indications that hatchery populations might be subject to other mortality events, in addition to those experienced by wild populations, are also presented.     

Summer stream habitat partitioning by sympatric Arctic charr, Atlantic salmon and brown trout in two sub-arctic rivers

Summer habitat use by sympatric Arctic charr Salvelinus alpinus, young Atlantic salmon Salmo salar and brown trout Salmo trutta was studied by two methods, direct underwater observation and electrofishing, across a range of habitats in two sub-arctic rivers. More Arctic charr and fewer Atlantic salmon parr were observed by electrofishing in comparison to direct underwater observation, perhaps suggesting a more cryptic behaviour by Arctic charr. The three species segregated in habitat use. Arctic charr, as found by direct underwater observation, most frequently used slow (mean ±s .d . water velocity 7·2 ± 16·6 cm s⁻¹) or often stillwater and deep habitats (mean ±s .d . depth 170·1 ± 72·1 cm). The most frequently used mesohabitat type was a pool. Young Atlantic salmon favoured the faster flowing areas (mean ±s .d . water velocity 44·0 ± 16·8 cm s⁻¹ and depth 57·1 ± 19·0 cm), while brown trout occupied intermediate habitats (mean ±s .d . water velocity 33·1 ± 18·6 cm s⁻¹ and depth 50·2 ± 18·0 cm). Niche overlap was considerable. The Arctic charr observed were on average larger (total length) than Atlantic salmon and brown trout (mean ±s .d . 21·9 ± 8·0, 10·2 ± 3·1 and 13·4 ± 4·5 cm). Similar habitat segregation between Atlantic salmon and brown trout was found by electrofishing, but more fishes were observed in shallower habitats. Electrofishing suggested that Arctic charr occupied habitats similar to brown trout. These results, however, are biased because electrofishing was inefficient in the slow-deep habitat favoured by Arctic charr. Habitat use changed between day and night in a similar way for all three species. At night, fishes held positions closer to the bottom than in the day and were more often observed in shallower stream areas mostly with lower water velocities and finer substrata. The observed habitat segregation is probably the result of interference competition, but the influence of innate selective differences needs more study.