Natural hybridization between Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, in northern Spain

Hybrids between Atlantic salmon and brown trout were detected in two of four watersheds studied in northern Spain. The proportions of hybrids in samples of 'salmon'ranged from 0 to 7–7% but they were not significantly heterogeneous among locations, resulting in a mean hybridization rate of 2–3%. This is the highest rate of natural hybridization so far reported and is significantly greater than rates observed elsewhere in Europe.     

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.     

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.     

Effects of hydropeaking on the spawning behaviour of Atlantic salmon Salmo salar and brown trout Salmo trutta

An in situ camera set‐up was used to study the spawning activity of Atlantic salmon Salmo salar and brown trout Salmo trutta throughout two consecutive seasons in a spawning area affected by hydropower‐related pulse flows due to hydropeaking. The purpose was to test whether the flow variation discouraged spawning in shallow areas or motivated spawning into areas with elevated risk of incubation mortality. There were more S. salar observed on the spawning ground during days with high discharge. The presence of S. salar in the spawning grounds was not affected by the hydropeaking cycles of the preceding night. Female S. salar were observed preparing nests within the first hour after water discharge had increased to levels suitable for spawning. In contrast, the number of S. trutta was not correlated with flow and nest preparation was also observed at a discharge corresponding to the lowest discharge levels during a hydropeaking cycle. Survival was generally high in nests excavated the following winter, with only 5·4% suffering mortality due to dewatering. The results suggest that S. salar may respond rapidly to variable‐flow conditions and utilize short windows with suitable flows for spawning. Smaller S. trutta may utilize low‐flow conditions to spawn in areas that are not habitable by larger S. salar during low flow.     

Home range of juvenile Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, in a Norwegian stream

SUMMARY. 1. The sizes of home ranges of juvenile Atlantic salmon (age 1 +) and brown trout (age 2+ to 9+) in a Norwegian coastal stream were estimated by local movements of batch-marked fish from 12.5 and 25 m long sections. Only recoveries made in the release section and in up-and downstream neighbouring sections were considered.
2. There was no significant difference in the average percentage of recaptures of salmon and trout between 12.5 and 25 m sections; a stream area of about 40–50 m² defines the size of home range for stocks of both species.
3. The fraction of brown trout recaptured in release sections increased with increasing fish densities, indicating a smaller home range under these conditions.     

Lack of species-specific primer effects of odours from female Atlantic salmon, Salmo salar, and brown trout, Salmo trutta

We exposed, in two successive spawning seasons, individually placed precocious male Atlantic salmon ( Salmo salar ) and brown trout ( Salmo trutta ) parr to odour stimuli (ovarian fluid and urine mix) from ovulated conspecific or heterospecific anadromous females. Atlantic salmon parr had significantly higher plasma concentrations of the hormones 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P), 11-ketotestosterone (11-KT) and testosterone (T) after exposure to odours from conspecific females or from brown trout females compared to parr exposed to a control solution (0.9% NaCl). We did not observe any significant differences between the hormone levels in salmon parr exposed to the two female odours. The salmon parr exposed to conspecific odours had significantly higher volumes of strippable milt compared to the controls, but we did not find any significant differences when comparing the effect of the two female odours. Brown trout parr had significantly higher plasma 17,20β-P levels following exposure to heterospecific female odours compared to control males, but there was no significant difference between males exposed to the different female odours. We did not observe any significant differences in plasma levels of T and 11-KT and in milt volumes between exposed and control trout. Taken together, the results from both tested species indicate that the potency of heterospecific stimuli in stimulating increased plasma sex steroid hormone levels in male parr was as strong as stimuli from conspecific females. The results are discussed in connection to observed hybridisation between the two sympatric species.     

High frequency of natural hybrids between Atlantic salmon, Salmo salar L., and brown trout, S. trutta L., in a Swedish river

Among 332 parr from the Swedish River Grönån examined by electrophoresis, 44 (13%) were hybrids between Atlantic salmon and brown trout. The hybrid frequencies in three sections of Grönån were significantly different (23. 8 and 2%). All hybrids are evidently of natural origin. and possible factors promoting hybridization are irregular overlapping spawning times. lack of separate spawning grounds, and involvement of sneakers.     

Comparing upriver spawning migration of Atlantic salmon Salmo salar and sea trout Salmo trutta

Radio tagged wild Atlantic salmon Salmo salar(n = 30) and sea trout Salmo trutta(n = 19) were simultaneously released from a sea pen outside the mouth of the River Lærdalselva and their migration to spawning areas was recorded. The distance from the river mouth to a position held at spawning ranged from 2 to 24 km and did not differ between the species (mean ± s .d . 15·9 ± 4·3 and 14·9 ± 5·2 km for Atlantic salmon and sea trout, respectively). The duration of the migration phase, however, was significantly shorter for Atlantic salmon than for sea trout (8–12 days, respectively). All Atlantic salmon migrated straight to an area near the spawning ground, whereas 50% of the sea trout had a stepwise progression with one or more periods with erratic movements before reaching the spawning area. After the migration phase, a distinct search phase with repeated movements up‐ and downstream at or close to the position held at spawning was identified for the majority of the fishes (75%, both species). This search phase was significantly shorter for Atlantic salmon than for sea trout (mean 13–31 days, respectively). Mean ± s .d . length of the river stretch used during the search phase was larger for sea trout (3·3 ± 2·5 km) than for Atlantic salmon (1·2 ± 0·9 km). A distinct holding phase, with no movements until spawning, was also observed in the majority of the Atlantic salmon (80%, mean duration 22 days) and sea trout (65%, mean duration 12 days). For both species, a weak, non‐significant trend was observed in the relationship between time spent on the migration phase, and time spent on the search (r² = 0·43) and holding phase (r² = 0·24). There was a highly significant decrease, however, in the duration of the holding phase with an increase in the time spent on the search phase (r² = 0·67).     

Movements of brown trout, Salmo trutta, and juvenile Atlantic salmon, Salmo salar, in a coastal stream in northern Norway

Movements of resident brown trout (age 2+ to 9+ years) and young Atlantic salmon (age 1+), stocked as fry, were studied in July, August and September in a coastal stream in northern Norway. Between 85 and 89% of the brown trout were recaptured in the study area (346m, 1326m²) within 45m of their release point throughout the investigation period. Most specimens had moved less than 150m. Trout movements were related to local variation in density. Trout occupying those sections of stream with the lowest fish densities (5.3–10.9 fish 100m^?2) had a significantly lower movement rate than fish from sections with densities between 13.7 and 31.5 fish 100m^?2. Trout that moved from their marking section were significantly larger than specimens that did not leave their original site. There was a significant correlation between permanence of station each month and the mean water level that month. The majority of the trout (47%) were caught at undercut stream banks or at sites in the proximity of these. A decrease in water level during the study period resulted in a high loss (36%) of such habitat, probably forcing some individuals to move. The recapture data indicate that the trout population consists of one small (c. 15–20%) mobile, and one large sedentary component. Young salmon displayed high station permanence in July and August (93% and 96%). However, in the autumn they exhibited a significant downstream movement, and only 73% were recaptured within their release section. This movement was significantly higher for larger specimens, and is thought to occur because of a pre-winter change in habitat, initiated by a decline in stream temperature. In contrast to trout, salmon in sections containing the lowest densities (22.0–25.0 fish 100m^?2) did not show significantly lower movement rates when compared with salmon at higher densities (32.2–46.3 and 51.8–60.6 fish 100m^?2). The spatial distribution of young salmon indicated the formation of territorial mosaics over the stream bed, which are thought to reduce intraspecific competition.     

The comparative feeding behaviour of brown trout, Salmo trutta L. and Atlantic salmon, Salmo salar L. in Llyn Dwythwch, Wales.

A detailed comparative study of the diets of natural brown trout and stocked Atlantic salmon in Llyn Dwythwch, North Wales, was carried out over a period of 13 months. The annual and seasonal composition of both diets was calculated by number, volume and occurrence methods. Food in relation to fish size was compared by Spearman rank correlation. Interspecific competition was only significant during the summer months when food was abundant. The bottom fauna was classified according to accessibility, and utilization of the fauna and electivity is discussed for each species. The similarity of diet between the salmon stocked into Llyn Dwythwch and those in Welsh rivers implies that the former is the result of inheritance rather than from interactive segregation with the lake trout.     

Temporal and spatial segregation of spawning in sympatric populations of Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

Based on data from Norwegian streams with sympatric populations of Atlantic salmon and brown trout, it is suggested that temporal segregation is the main mechanism segregating Atlantic salmon and brown trout during spawning. Peak spawning of trout was about 15 days earlier than that of salmon. Physical factors, such as water depth, water velocity and distance from the river banks segregate spawning sites of salmon and trout poorly. Gravel sizes of the redds of salmon and trout were significantly different, though with a considerable overlap, and mean egg depth of salmon and trout were 0.18 and 0.12 m, respectively, probably attributable to the different size of spawners of salmon and trout. None of the temporal or spatial parameters analysed segregate spawners of salmon and trout completely. Species determination of eggs and alevins from the redds showed no interspecific superimposition of redds. It is, therefore, concluded that low survival of hybrids after hatching does not explain the low frequency of hybrids observed in sympatric populations of salmon and trout.     

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.     

Genetic introgression between native and introduced brown trout Salmo trutta L. populations in the Rhone River Basin

In the Doubs River (Rhône drainage) two distinct brown trout ( S. trutta ) phenotypes are observed. One phenotype is locally called Doubs trout and is characterized by four black stripes on the sides, similar to perch ( Perca fluviatilis L.) and the other is the common phenotype of the fluviatile ecotype of brown trout, Salmo trutta f. fario . Protein data for three samples from the Doubs show that the Doubs trout belongs to the Mediterranean population group of brown trout, whereas the fario phenotype originates from stocking with hatchery strains of Atlantic basin origin. The two forms, however, do not hybridize freely. This is indicated by considerable gametic phase disequilibrium between alleles of hatchery and Doubs trout at one sampling site, and by lack of intermediate genotypes and phenotypes at another sampling site. The introgression patterns observed at the two sites suggest that differences in local habitat conditions can affect the degree of hybridization and introgression.     

Temperature requirements of Atlantic salmon Salmo salar, brown trout Salmo trutta and Arctic charr Salvelinus alpinus: predicting the effects of climate change

Atlantic salmon Salmo salar, brown trout Salmo trutta (including the anadromous form, sea trout) and Arctic charr Salvelinus alpinus (including anadromous fish) provide important commercial and sports fisheries in Western Europe. As water temperature increases as a result of climate change, quantitative information on the thermal requirements of these three species is essential so that potential problems can be anticipated by those responsible for the conservation and sustainable management of the fisheries and the maintenance of biodiversity in freshwater ecosystems. Part I compares the temperature limits for survival, feeding and growth. Salmo salar has the highest temperature tolerance, followed by S. trutta and finally S. alpinus. For all three species, the temperature tolerance for alevins is slightly lower than that for parr and smolts, and the eggs have the lowest tolerance; this being the most vulnerable life stage to any temperature increase, especially for eggs of S. alpinus in shallow water. There was little evidence to support local thermal adaptation, except in very cold rivers (mean annual temperature <6·5° C). Part II illustrates the importance of developing predictive models, using data from a long-term study (1967-2000) of a juvenile anadromous S. trutta population. Individual-based models predicted the emergence period for the fry. Mean values over 34 years revealed a large variation in the timing of emergence with c. 2 months between extreme values. The emergence time correlated significantly with the North Atlantic Oscillation Index, indicating that interannual variations in emergence were linked to more general changes in climate. Mean stream temperatures increased significantly in winter and spring at a rate of 0·37° C per decade, but not in summer and autumn, and led to an increase in the mean mass of pre-smolts. A growth model for S. trutta was validated by growth data from the long-term study and predicted growth under possible future conditions. Small increases (<2·5° C) in winter and spring would be beneficial for growth with 1 year-old smolts being more common. Water temperatures would have to increase by c. 4° C in winter and spring, and 3° C in summer and autumn before they had a marked negative effect on trout growth.     

Genetic assessment of Atlantic salmon Salmo salar and sea trout Salmo trutta stocking in a Baltic Sea river

Microsatellite DNA variation was used to assess the outcome of stocking Atlantic salmon Salmo salar and migratory trout Salmo trutta in River Sävarå, N Sweden. No information on pre‐stocking genetic composition of S. salar and S. trutta in River Sävarå was available. In 2 year‐classes of S. salar smolt, microsatellite data indicated that post‐stocking genetic composition differed markedly (F_ST= 0·048) from the main donor strain, Byskeälven S. salar, and from other Gulf of Bothnia S. salar stocks (F_ST 0·047 and 0·132). The STRUCTURE programme failed to detect any substructuring within Sävarå salmon. It was concluded that only minor introgression estimated to a proportion of 0·11 (95% CI 0·07–0·16) has occurred in S. salar. Salmo trutta showed overall low differentiation among populations with maximum F_ST of 0·03 making analysis more cumbersome than in S. salar. Still, the SävaråS. trutta deviated significantly from potential donor populations, and STRUCTURE software supported that majority of trout in Sävarå formed a distinct genetic population. Admixture was more extensive in S. trutta and estimated to 0·17 (95% CI 0·10–0·25).     

An acute septicaemic disease of brown trout (Salmo trutta) and Atlantic salmon (Salmo salar) caused by a Pasteurella-like organism

Ulcerations of the skin associated with haemorrhagic petechiae of liver and kidneys, were the main signs of disease affecting salmon and brown trout in Norway. A death rate of 15–20% was estimated for the 5 month period of mid-March-August, although mortalities occurred throughout the year. Bacteriological examinations, involving 36 isolates, suggested the causual organism to be a Pasteurella although the DNA homology examination gave a G.C. ratio of 55.6 % which is high for Pasteurella as a group.     

An aerial method of assessing spawning activity of Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in Norwegian streams

A method using light aircraft to observe spawning activity of Atlantic salmon and brown trout in some Norwegian streams was tested between the years 1981 and 83. From the air, spawning redds of these species appear as light, oval spots in the river bottom. The method was successfully applied to most of the rivers studied, and information about numbers of redds, distribution of redds and spawning times was obtained. However, it has several limitations, the most important being that it proved to be unsuccessful in a deep river with high water turbidity. Also, during periods with high precipitation and high water level, the method cannot be used. Several quantitative and qualitative aspects of the results obtained by this technique are discussed.     

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.     

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.     

Isolation, characterization, and chromosomal location of the tRNA(Met) genes in Atlantic salmon (Salmo salar) and brown trout (Salmo trutta).

This work describes the isolation, characterization, and physical location of the methionine tRNA in the genome of Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.). An Atlantic salmon genomic library was screened using a tRNA(Met) probe from Xenopus laevis. Two cosmid clones containing the Atlantic salmon tRNA(Met) gene were isolated, subcloned and sequenced. The tRNA(Met) was mapped to metaphase chromosomes by fluorescence in situ hybridization (FISH). Chromosomal data indicated that the tDNA of methionine is tandemly repeated in a single locus in both species. Analysis of genomic DNA by Southern hybridization confirmed the tandem organization of this gene.