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.     

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).     

Failure of a stocking policy, of hatchery reared brown trout, Salmo trutta L., in Asturias, Spain, detected using LDH-5* as a genetic marker

A useful genetic marker exists through the apparent fixation of the LDH-5 * 100 allele in wild populations of brown trout in rivers from Asturias, Spain, contrasted with the near fixation of the LDH-5 * 90 allele in hatchery populations used to stock these rivers. In sampling locations where natural reproduction occurred, the * 100 allele was found exclusively in all areas having no record of hatchery stocking. The * 100 allele also predominated in three stocked areas having natural reproduction; in two of these areas a few individuals of the 0 + age class were homozygous for the * 90 allele. These data indicated that all catchable and reproductive fish originated from indigenous populations and thus the policy of hatchery supplementation was a failure in these areas.     

High level of population genetic structuring in lake-run brown trout, Salmo trutta, of the Inari Basin, northern Finland

Rivers draining into (Lake) Inarijärvi, northern Finland, sustain a number of lake‐run brown trout, Salmo trutta, populations but, as with most lake‐run S. trutta systems, the level of population genetic structuring among populations is unknown. To address this and to assist fish stock management in the region, the population genetic structure of S. trutta collected from 28 sampling sites in rivers flowing into Inarijärvi was studied using 13 microsatellite loci. Populations were clustered into three separate groups, largely corresponding to geographic regions, with between‐region F_ST values ranging from 0·11 to 0·16. The significant differentiation observed between most populations within each region also implies that individual populations should be recognized as separate management units and actions to improve, and subsequently maintain, conditions for natural spawning should be prioritized. The results of this study further indicate that the trout from each of these regions may have different biological characteristics, such as local‐lake feeding behaviour among the western populations and strong isolation among the northern stocks. As a consequence, further research is warranted to better understand the level of ecological uniqueness of lake‐run S. trutta populations.     

Effects of stocking on the genetic diversity of brown trout populations of the Adriatic and Danubian drainages in Switzerland

This study focuses on genetic variation of brown trout Salmo trutta populations of the Adriatic and Danubian drainages in Switzerland. The allozyme and other protein loci data show a major replacement of native stocks from the Adriatic drainages by introduced hatchery trout of Atlantic basin origin. In most samples, diagnostic alleles for the Adriatic form of Salmo trutta f. fario and for the marbled trout Salmo trutta marmoratus are found at very low frequencies (f<0.15). Taking into account previous genetic studies on brown trout of this basin, the Danubian samples are not heavily contaminated with foreign alleles. The results are consistent with records of local stocking activities which account in part for the high introgression rates of Atlantic alleles into local populations of the Adriatic drainages. In addition, introgression is enhanced by a decrease of natural reproduction which is caused by a deterioration of trout habitats through human activities. Furthermore, a third mechanism is proposed that may contribute to the high introgression rates observed: if Atlantic trout are introduced, the reproductive barriers between the two native forms, marbled trout and Adriatic fario respectively, break down. Atlantic trout apparently hybridize with both native forms and generate gene flow between them. In some parts of Adriatic drainages in Switzerland, the patterns of introgression and hybridization are further complicated by introduction of trout from the Danubian system. Alleles of the marbled trout are also found in the samples of the Danubian drainage system. These are due to stocking activities across the watershed.     

Allozyme diversity in brown trout (Salmo trutta) from Central Spain: Genetic consequences of restocking

1.  The brown trout ( Salmo trutta ) represents one of the main freshwater resources in Spain, but habitat alterations and overharvesting have contributed to the decline or disappearance of numerous natural populations. In addition, reinforcement programs of wild populations based on releases of hatchery reared fish of exogenous origin compromise the conservation of remnant native trout resources.
2.  We present allozymic data from Central Spain trout populations including stocked and unstocked populations. Although the levels of genetic variation observed were low and affected by hatchery releases (p = 18.23%, Ho= 3.39%), they were within the range observed in other European areas.
3.  The effective introduction of hatchery reared fish is genetically homogenising the populations in the studied area and disturbing the ancestral pattern of genetic variation that distinguishes the Tajo and Duero basins. Within the eight natural populations analysed, seven had alleles assigned to the foreign trout. The introgression in these populations, following the LDH-5 * 90 allele frequency, ranged between 2% and 29.4%, but those values are not in concordance with the respective stocking effort undertaken in each population. Moreover, the release of hatchery-reared fish does not solve the problems related to the reduced size of wild populations and their recruitment instability.     

Differences in growth between offspring of anadromous and freshwater brown trout Salmo trutta

In this study, individual growth of juvenile offspring of anadromous and freshwater resident brown trout Salmo trutta and crosses between the two from the River Imsa, Norway, was estimated. The juveniles were incubated until hatching at two temperatures (±S.D. ), either 4.4 ± 1.5°C or 7.1 ± 0.6°C. Growth rate was estimated for 22 days in August–September when the fish on average were c. 8 g in wet mass, and the estimates were standardized to 1 g fish dry mass. Offspring of anadromous S. trutta grew better at both 15 and 18°C than offspring of freshwater resident S. trutta or offspring of crosses between the two S. trutta types. This difference appears not to result from a maternal effect because anadromous S. trutta grew better than the hybrids with anadromous mothers. Instead, this appears to be an inherited difference between the anadromous and the freshwater resident fish lending support to the hypothesis that anadromous and freshwater resident S. trutta in this river differ in genetic expression. Egg incubation temperature of S. trutta appeared not to influence the later growth as reported earlier from the studies of Atlantic salmon Salmo salar.     

The importance of being familiar: individual recognition and social behavior in sea trout (Salmo trutta)

This study addressed two hypotheses: (1) that individual recognitionis used to reduce the cost of contesting resources in sea troutand (2) that the hatchery environment selects against individualrecognition. Predictions from these hypotheses were tested inlaboratory experiments where behavior and growth were studiedin juvenile sea trout. In tact groups of familiar fish werecompared with groups that contained both familiar and unfamiliarindividuals. In general, the results were in agreement withthe first hypothesis. Familiar fish in intact groups had morestable dominance ranks, higher food intake, and better utilizationof food. Familiarity also reduced the distance to the nearestneighbor. Furthermore, initiators of conflicts were more likelyto win against familiar fish than against strangers. These resultssupport game-theory-based hypotheses explaining the dear-enemyphenomenon as an effect of familiarity A picture emerges inwhich familiarity stabilizes the hierarchical structure of agroup and governs behavioral modifications that will promotefeeding and growth, in turn leading to higher fitness. The secondhypothesis, which predicted a reduced effect of individual recognitionin sea-ranched trout, was not strongly supported because familiarityaffected sea-ranched and wild trout similarly in most respects.However, familiarity was not beneficial for growth in sea-ranchedtrout, whereas it increased growth rate in wild fish. In addition,sea-ranched trout tended to maintain larger distances to theirnearest neighbors than did wild trout.     

Phylogeographical lineages in brown trout (Salmo trutta): investigating microgeographical differentiation between native populations from Northern Spain

SUMMARY 1. The large microgeographical differentiation revealed by allozyme studies in brown trout ( Salmo trutta) populations is one of the most striking features of this species. Additionally, allozymes showed great genetic differences between Atlantic and Mediterranean populations on a macrogeographical scale.
2. This study was carried out in order to assess whether the great differences observed between Atlantic and Mediterranean populations persisted where the two are geographically close (the 'microgeographical scale'). Sixteen populations of brown trout, S. trutta , were screened for genetic variation at 25 allozyme loci. The sampling sites, which occupied a relatively small geographical area, were distributed across Cantabrian (Atlantic) and Mediterranean drainages in Northern Spain.
3. The neighbour-joining tree, inferred from Nei's genetic distance, showed that brown trout populations clustered into two different groups. These groups corresponded to the Cantabrian and the Mediterranean groups of populations, although no clear geographical pattern emerged within each of the groups. This geographical pattern is basically caused by significant differences in the frequency distribution of the CK-A1 * locus, with a higher frequency of * 115 in Cantabrian samples (0.586 ± 0.091) while allele * 100 was more frequent in Mediterranean samples (0.931 ± 0.038). In addition, this study revealed alleles exclusive to the Mediterranean and Cantabrian populations, agreeing with previous findings.
4. Genetic differentiation between Cantabrian and Mediterranean regions (14.19%) was similar to that estimated in Spain at a larger scale (13%), showing that most of the differences between the regions can be observed even in a small geographical area.     

An intensive study of allozyme variation in freshwater resident and anadromous trout,Salmo trutta L., in western Ireland*

Ten polymorphic enzyme coding loci were assayed using starch gel electrophoresis in anadromous (sea trout) and freshwater-resident trout, Salmo trutta, from two neighbouring river systems draining into Clew Bay in western Ireland. Samples were collected from the Burrishoole and from the neighbouring Newport river system. In the Burrishoole system, two samples were from above waterfalls impassable to migratory trout, two consisted of anadromous smolts or post-smolts and the remainder were freshwater-resident trout from freely accessible areas. There were large differences in gene frequency between trout from above impassable barriers and all others sampled, and these two groups also differed substantially from each other. More minor differences were evident between some other samples, particularly between trout from the upper lake and streams entering it and a stream associated with the lower lake. Two anadromous populations therefore occur in the Burrishoole system. In contrast, no differences were evident in two sampling years between anadromous trout and cohabiting freshwater-resident trout from the upper part of the Burrishoole system. Thus, in the upper part of this system, anadromous trout do not appear to comprise a separate population from non-migratory trout and interbreeding may occur.     

The post‐spawning movements of migratory brown trout Salmo trutta L.

The post spawning behaviour of sea trout Salmo trutta was studied over a 2 year period in the river and estuary of the River Fowey, south‐west England. Forty‐five sea trout kelts were trapped immediately after spawning in December and intraperitoneally tagged with miniature acoustic transmitters. The subsequent emigration into coastal waters was monitored using acoustic receivers deployed throughout the river catchment. The levels of gill Na⁺K⁺ATPase activity in sea trout kelts sampled at the same time as the tagged fish were within the range of 2·5 to 4·5 μmol Pi per mg protein per h indicating that the post‐spawning fish were not physiologically adapted to salt water. The tagged kelts were resident in fresh water between 4 and 70 days before entering the estuary. Sixty two per cent of the tagged kelts subsequently migrated successfully into coastal waters, with a higher success rate for male fish (75%) than females (58%). There was a significant size related difference in the run‐timing of the kelts with the larger fish moving more quickly into coastal waters after spawning than smaller fish. Seaward migration within fresh water was predominantly nocturnal and generally occurred in conjunction with increasing river discharge and rising water temperature. Migration through the estuary continued to be predominantly nocturnal and occurred during an ebbing tide. Residency within the estuary varied amongst individuals although it was invariably short, with most fish moving out into coastal waters within one to two tidal cycles. Five tagged kelts returned from the coastal zone and re‐entered fresh water during April and June. Marine residence time varied between 89 and 145 days (mean 118 days) and the minimum estimated marine survival was c. 18%. One of these sea trout was subsequently recaptured after successfully spawning in the vicinity where it had been previously tagged demonstrating a degree of spawning site fidelity.     

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.     

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.     

Morphological variability among seven populations of brown trout, Salmo trutta L., in Greece

Morphological variation in five meristic and 21 morphometric characters was examined in the brown trout, Salmo trutta L., from seven distinct streams and rivers from N and NW Greece. Variation within each sample, estimated as the multivariate generalization of the coefficient of variation, did not vary substantially among the populations. The phenetic relationships of these populations were examined using the stepwise discriminant analysis. The taxonomic implications of these results are discussed.     

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.     

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.     

Continuous outmigration and sequential encountering of environmental cues are important for successful homing of hatchery-reared,anadromous brown trout Salmo trutta

When rehabilitating and reintroducing trout Salmo trutta in rivers, it is a goal that as many as possible survive, home and form self-sustaining populations. Hatchery-reared, anadromous S. trutta have significant lower ability to return to the area where they were raised if (a) transported in a closed tank to sea and released 5 km from the River Imsa, relative to those that were (b) transported when swimming in a partly submerged tank with sea water run-through, while being slowly towed by a boat the same distance or (c) released at the outlet of the River Imsa. Thus, if deprived from environmental cues during part of the way, they lose their ability to home.     

Mitochondrial DNA sequence variation and phylogeography among Salmo trutta L. (Greek brown trout) populations

To investigate the phylogenetic relationships and geographical structure among brown trout S. trutta L. populations from the South Adriatic-Ionian and Aegean sea basins, mitochondrial DNA nucleotide sequence comparisons were used. A 310-base-pair (bp) segment of the control region (D-loop), and an additional 280-bp segment of the cytochrome b gene were sequenced from representatives of 13 brown trout populations. Phylogenetic analyses, conducted after combining the data presented with published data from other Eurasian brown trout, revealed four major phylogenetic groups, three of which were found widely distributed within the southern Balkan region. The phylogeographical patterns revealed by mtDNA represent one of the few cases where phylogenetic discontinuity in a gene tree exists without obvious geographical localization within a species' range and has most likely resulted from the differentiation of the major mtDNA clades during Messinian or early Pleistocene times. Finally, the genetic relationships among the populations suggested by mtDNA were generally not in accordance with either allozyme or morphological data.     

Sexual maturity in sea trout, Salmo trutta L., running up the River Calonne (Normandy, France) at the 'finnock' stage

Sexual maturation in sea trout ( Salmo trutto L.) running up the River Calonne (Normandy, France), at the finnock stage, is characterized. Autopsy of 118 finnock in summer showed that 87 ± 7% aged 2+ and 33 ± 15% aged 1 + were undergoing gametogenesis. In autumn all the fish were maturing. The analysis of plasma levels of 11-ketotestosterone, 17β-oestradiol, maturational gonadotropin and vitellogenin showed that only females in vitellogenesis can be well identified in summer (plasma vitellogenin greater than 0.30 mg ml^-1). The morphological recognition of females among 134 autumn finnock and the ELISA assay of plasma vitellogenin of 814 summer finnock, demonstrated that vitellogenic females represented 12 ± 2%0 of the 1 + individuals and 46 ± 2% of 2+ fish captured during the 1985 run. The question of a possible relationship between the salinity tolerance ofjuvenile trout migrating towards the sea in the spring and their age on first migration in the river is discussed.