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.     

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.     

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.     

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.     

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.     

Variation in mitochondrial DNA and post-glacial colonization of north western Europe by brown trout

A purified mitochondrial DNA (mtDNA) probe was used to examine restriction fragment length polymorphisms produced by six restriction enzymes ( Xba I, Eco RV, Ava II, Hinf I, Hae III, Mbo I) in 915 brown trout from western Europe. A total of 20 composite haplotypes were found with one to seven haplotypes in individual populations. Icelandic trout samples from north, south, east, and west coast drainages showed only a single common haplotype in contrast to the high level of polymorphism found in Irish and Scottish populations. The phylogeny of mtDNA haplotypes and the pattern of haplotype distribution suggests that post-glacial colonization of brown trout in NW Europe was more complex than the dual colonization model which has been proposed on the basis of differential LDH-5* allele distribution. For example, Lough Melvin (Ireland) appears to have been independently