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 variations of activity pattern of brown trout (Salmo trutta) in a small stream, as determined by radio-telemetry

Nine brown trout (27.9–42.8 cm FL, 234–995 g) from the Aisne stream (Belgian Ardenne) were tagged with surgically implanted radio transmitters, and their activity patterns were studied at 10-min intervals during twenty-six 24-h cycles from October 1995 to May 1997. The duration and intensity of activity were mainly proportional to water temperature and day length but some differences appeared between individuals. The daily home range in summer was also 3.2 times longer than in winter, and was exploited 2.5 times more intensively. At all seasons, trout were most active at dusk, and seasonal variations in diel patterns consisted mainly in a progressive shift from more crepuscular activity in autumn and winter to more homogenous and intensive activity during spring and summer, but still with a predominance of activity at dusk. This relative consistency of activity rhythm throughout the year was interpreted within the context of foraging risk and efficiency, in view of the scarcity of predators in the Aisne stream, and of drifting macroinvertebrates being consistently more abundant at dusk at all times of the year. However, activity patterns varied substantially between individuals living in different microhabitats, including when fish were investigated on the same day. This suggests that activity patterns are also subordinated to local factors such as habitat structure or abundance of drifting prey.     

Recruitment as a driver of production dynamics in stream-resident brown trout (Salmo trutta)

1. The objective was to assess the role of recruitment as a determinant of the production dynamics of stream-resident brown trout ( Salmo trutta ) across replicate habitats of contrasting quality and population attributes. A total of 128-year-classes (YC) at 12 stream sites were examined along four tributaries of the Rio Esva drainage (northwestern Spain).
2. A meta-comparison revealed that growth, density, mortality and production were essentially site-specific. However, when all data were pooled, recruitment (as a delayed density-dependent process) affected both growth and mortality in a way such that individuals in YC with high recruitment grew less and had higher mortality.
3. The value of total YC production recorded covered the global range of variation in the production of stream salmonids reported in the literature. Linear regressions of log-transformed data revealed that 89.0%, 58.9% and 70.7% of the variation in YC density, biomass and production, respectively, were explained by variations in recruitment.
4. The inclusion of growth and mortality, together with recruitment, into a multiple regression increased the variance explained of the total YC production by 13.3%, from 70.7% to 84.0%.
5. The functional relationships between recruitment and the population attributes elucidated in this study appear to provide a useful tool for management applications, including forecasting population status.     

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.     

Thermoregulatory behavior of brown trout,Salmo trutta

Brown trout, Salmo trutta, were allowed to thermoregulate individually in an electronic shuttlebox. Pooled data for 6 fish showed a diel pattern of preferred temperature, with a diurnal minimum of 10.3°C, an early nocturnal maximum of 13.7°C, a less pronounced mid-scotophase minimum of 11.7°C, and a secondary dawn maximum of 12.8°C, in a somewhat crepuscular pattern. The 24-hour mean preferendum was 12.2°C.     

New biochemical genetic markers in the brown trout (Salmo trutta L.)

Three 'new' polymorphic loci in the brown trout (Salmo trutta L.) was detected using starch gel electrophoresis. Evidence for simple Mendelian inheritance at all three loci are presented.     

Mitochondrial DNA variation in River Usk brown trout, Salmo trutta

Mitochondrial DNA (mtDNA) polymorphism in the essentially non-anadromous River Usk brown trout Salmo trutta population was investigated by restriction analysis. Following mtDNA extraction and purification on caesium chloride density gradients, monomorphic restriction profiles were obtained with Hae III , Hind III , Sau 3AI and Xbal . However, the restriction endonucleases Ava II and HinfIl proved informative. The distribution of four composite genotypes found within the Usk system was heterogeneous, and a fifth genotype appeared exclusively in an outgroup sample from the adjacent River Wye drainage. The source of the observed genetic variation is discussed in relation to estimated divergence times for Usk mtDNA genotypes and the stocking history of the catchment.     

Population genetic structure of brown trout (Salmo trutta L.) within a northern boreal forest stream

Brown trout (Salmo trutta L.) population genetic structure and its temporal stability were studied within a small forest stream in central Sweden using five microsatellite loci. Both resident and migrating brown trout are present in the watershed. Tissue samples were collected from seven sections of the stream during two consecutive years. No differences were found in multilocus FST estimates between years within sections except in one case. Moreover, differences between age cohorts within sample sections were rare. The low interannual variation and the low heterogeny between cohorts is interpreted as indications of temporal stability. Pairwise multilocus FST estimates increased with increasing geographic distance, indicating isolation by distance. It is argued that the brown trout of the F?rs?n stream represent a population complex. The structure is probably maintained by precise natal homing and a limited, but important, amount of gene flow between closely situated sections within the stream supporting a stepping-stone model of gene flow.     

Trophic relationships of nonnative brown trout, Salmo trutta, and native Bonneville cutthroat trout, Oncorhynchus clarkii utah, in a northern Utah, USA river

Nonnative trout invasions have caused the widespread decline of cutthroat trout populations in western North America. In contrast to other nonnative salmonids, the role of nonnative brown trout in native cutthroat trout decline is poorly understood. Specifically, the level of ecological similarity that occurs between these species and the importance of other trophic mechanisms (e.g., predation) in their interactions are key uncertainties. We evaluated the trophic relationships of brown trout and cutthroat trout in a northern Utah river using a combination of diet and stable isotope analyses. We compared the dietary habits of these two species using multiple and complementary measures. Based on both stomach contents and δ¹³C signatures, we found that these species consumed a similar and opportunistic diet (i.e., they were nonselective in their foraging patterns). However, at most sizes, brown trout ingested larger prey—including fishes—and occupied a higher relative trophic position (i.e., δ¹⁵N) than cutthroat trout. Overall, these results demonstrate a high degree of dietary similarity and therefore strengthen earlier conclusions regarding interspecific competition between these two species. Our study, when considered alongside the work of others, suggests there is potential for predatory interactions between these species (i.e., brown trout preying on small cutthroat trout). We believe that future research on brown trout–cutthroat trout interactions should consider predatory effects in greater detail.

Effects of angling on population structure of brown trout,Salmo trutta L., in mountain streams of Northern Spain

Effects of angling exploitation on brown trout populations were assessed by comparing fished sections with close ones unfished for at least 20 years, in mountain streams of Asturias (Northern Spain). Both the fish size and age structure significantly differed among sections in the expected direction according to their exploitation status. The main effects were a significant decrease in age structure complexity (diversity), life span, and percent individuals above the legal limit size in the exploited stocks versus the unexploited ones. Trout above the minimum length limit for fishing (18 cm) averaged 19.47% of the fish caught in the unfished sections (sd = 4.01; n = 5), and 4.72% (sd = 3.46; n = 4) in those subjected to angling. Furthermore, fish older than 4 years represented 39.84% (sd = 8.53) and 1.19% (sd = 1.60) of the catch, respectively. Effects on recruitment (density of young fishes) and growth rates (length at age 1 + to 3 +) were not absolutely consistent, though maximum values were associated with fished sections.     

MHC-mediated spatial distribution in brown trout (Salmo trutta) fry

Major histocompatibility complex (MHC) class I-linked microsatellite data and parental assignment data for a group of wild brown trout (Salmo trutta L.) provide evidence of closer spatial aggregation among fry sharing greater numbers of MHC class I alleles under natural conditions. This result confirms predictions from laboratory experiments demonstrating a hierarchical preference for association of fry sharing MHC alleles. Full-siblings emerge from the same nest (redd), and a passive kin association pattern arising from limited dispersal from the nest (redd effect) would predict that all such pairs would have a similar distribution. However, this study demonstrates a strong, significant trend for reduced distance between pairs of full-sibling fry sharing more MHC class I alleles reflecting their closer aggregation (no alleles shared, 311.5 ± (s.e.)21.03 m; one allele shared, 222.2 ± 14.49 m; two alleles shared, 124.9 ± 23.88 m; P<0.0001). A significant trend for closer aggregation among fry sharing more MHC class I alleles was also observed in fry pairs, which were known to have different mothers and were otherwise unrelated (ML-r = 0) (no alleles: 457.6 ± 3.58 m; one allele (422.4 ± 3.86 m); two alleles (381.7 ± 10.72 m); P<0.0001). These pairs are expected to have emerged from different redds and a passive association would then be unlikely. These data suggest that sharing MHC class I alleles has a role in maintaining kin association among full-siblings after emergence. This study demonstrates a pattern consistent with MHC-mediated kin association in the wild for the first time.     

The food of juvenile trout,Salmo trutta L., in Loch Leven,Kinross, Scotland

The food of juvenile brown trout in Loch Leven was studied by the examination of 662 stomachs of fish caught between October 1974 and December 1976. The food consumption was estimated only on the basis of the analysis of the stomach contents.The main food items of juvenile trout in the loch are chironomids, crustaceans and nematodes: these are abundant in the offshore mud zone of the loch.     

Endocrine responses of brown trout, Salmo trutta L., to acid, aluminium and lime dosing in a Welsh hill stream

Brown trout caged in an acidic, softwater, Welsh hill stream running through close-canopy conifer forest and in an adjacent, circumneutral, moorland stream were studied after a natural rainfall event and after experimental acid and aluminium dosing of the forest stream. Endocrine (cortisol and thyroxine) and metabolic (glucose) stress responses occurred in fish held in the forest stream. Liming downstream of the acid zones mitigated these responses.     

Aggressiveness and kinship in brown trout (Salmo trutta) parr

In a series of experiments, kin-biased behavior of young browntrout (Salmo trutta) was observed. The aggressiveness shownby groups of familiar siblings (siblings reared together sincefertilization) and groups of unfamiliar siblings (siblings rearedapart since fertilization) was significantly lower comparedto that of mixed groups of two unrelated sibling groups (offspringof two different pairs of parents). The evolution of kin-biasedbehavior, as shown by a reduction in aggressiveness, is assumedto have evolved through a kin-selective mechanism.[Behav Ecol7: 445-450 (1996)]     

Effects of sea trout stocking on the population genetics of landlocked brown trout, Salmo trutta L., in the Conwy River system, North Wales, U.K.

The effect of the introduction of fry of anadromous sea trout, Salmo trutta L., on the genetic integrity of landlocked brown trout populations was evaluated. Samples were taken from six brown trout populations from streams above impassable waterfalls in the Conwy river system (North Wales, U.K.) in 1989 and 1990. Three of these streams had no known stocking history and three had been stocked with sea trout fry from the lower Conwy system over the last few years. Representatives of these sea trout were collected from two streams in the lower Conwy system and from a hatchery. Allele frequencies at 13 loci, six of which were polymorphic, were determined by starch gel electrophoresis.
The stocked populations were intermediate in their allele frequencies between unstocked brown trout and sea trout samples. A principal component analysis suggested significant numbers of hybrids in all of the stocked streams. This shows that some of the introduced sea trout did not migrate down the falls to the sea, but stayed in fresh water and hybridized with the local population. The significance of this finding for the conservation of the genetic resource of brown trout stocks is discussed.     

Mitochondrial DNA as a genetic marker for brown trout, Salmo trutta L., populations

Mitochondrial DNA was examined in natural and hatchery-reared stocks of brown trout, using different methods of restriction analysis. The methods included the development of a brown trout mt DNA hybridization probe through cloning part of the brown trout mitochondrial genome. In addition, fragments were analysed by ethidium bromide staining and end-labelling. The relative merits of each of these methods in assessing levels of genetic relatedness between the natural and hatchery-reared brown trout stocks were evaluated. In addition, the study revealed a diagnostic mtDNA restriction pattern which could be used as a genetic marker for the discrimination of these two groups of brown trout.     

Interpopulation variation in reproductive traits of anadromous female brown trout, Salmo trutta L.

We studied reproductive traits in nine anadromous brown trout, Salmo trutta L., populations in seven Norwegian rivers. Within populations we found a positive significant correlation between fish length and fecundity in all populations, and between fish length and egg diameter in five populations. There were significant differences in these relationships between populations from different rivers, and between populations from different locations within rivers. When adjusted for variation in fish length, mean fecundity and mean egg diameter showed a negative significant correlation among populations. The ratio of gonadal weight to somatic weight (gonadosomatic index) varied significantly among populations but was not associated with variation in fish length. Comparatively few large eggs were found in brown trout populations co-existing with several other fish species.     

Distribution, growth and movement of River Usk brown trout (Salmo trutta)

The River Usk catchment in South Wales supports mainly freshwater resident brown trout, with few anadromous fish. Electric fishing and netting revealed that age-class distribution differed between main river and tributary habitats, the latter environment acting as a nursery area for young fish. Few fry were found at main river sites. Age-class distribution also differed between tributary systems, and possible reasons are discussed. Trapping experiments indicated that trout move to main river habitat at 2+ yr. Lengths at age (back-calculated from scale reading) were similar for main river and tributary resident fish at 1 and 2 year, but main river fish were larger at 3 and 4 yr. This habitat shift and size difference is discussed with reference to current angling regulations.     

Maintenance of a small anadromous subpopulation of brown trout (Salmo trutta L.) by straying

1. Microsatellite and isozyme loci variation were used to study structure and dynamics of a brown trout (Salmo trutta) population heavily affected by damming. The downstream area accessible for spawning was drastically reduced to a stream 1 km long influenced by regulated discharge. 2. Stocking of hatchery‐reared juveniles failed and the population is entirely supported by anadromous adults from neighbouring populations. 3. Temporal genetic stability is reported here. Some punctual between‐river genetic differences are likely because of different contribution from each neighbouring river through years. 4. High anadromy‐mediated gene flow produces a lack of genetic substructure in the region. The role of anadromous brown trout on maintenance of endangered small populations is emphasised.     

Post-glacial colonization of brown trout, Salmo trutta L.: Ldh-5 as a phylogeographic marker locus

The Ldh-5 locus, which codes for the eye-specific lactate dehydrogenase in brown trout, has been shown to be polymorphic for two codominant alleles, Ldh-5 (100) and Ldh-5 (90). The Ldh-5 (100 ) allele is present in 11 other salmonid species and is therefore likely to be the ancestral one, whereas the unique brown trout Ldh-5 (90 ) allele would seem to be the result of a mutation in that lineage. The Ldh-5 (90 ) allele appears to have arisen in north-west Europe during or after the last glaciation, with allelic substitution taking place under the action of natural selection. The Ldh-5 (90 ) allele can be used as a phylogeographic marker to trace the post-glacial spread of the populations possessing it. Examination of the current distribution of the two alleles suggests that, in the formerly glaciated area of north-west Europe, there have been two post-glacial colonizations by brown trout. The first was by an 'ancestral' race fixed for the Ldh-5(100 ) allele. This was later replaced by, or introgressed with, the later-arriving 'modern' race characterized by the Ldh-5 (90 ) allele, except where physical barriers prevented colonization by this latter form. Artificial stocking has resulted in 'genetic contamination' of many populations of the ancestral race and there is an urgent need to conserve the remaining pristine populations, especially in view of the likely genetic propensity for longevity and ultimate large size exhibited by this race.