Reproductive isolation and genetic differentiation of ferox trout from sympatric brown trout in Loch Awe and Loch Laggan, Scotland

Molecular marker studies reported here, involving allozymes, mitochondrial DNA and microsatellites, demonstrate that ferox brown trout Salmo trutta in Lochs Awe and Laggan, Scotland, are reproductively isolated and genetically distinct from co-occurring brown trout. Ferox were shown to spawn primarily, and possibly solely, in a single large river in each lake system making them particularly vulnerable to environmental changes. Although a low level of introgression seems to have occurred with sympatric brown trout, possibly as a result of human-induced habitat alterations and stocking, ferox trout in these two lakes meet the requirements for classification as a distinct biological, phylogenetic and morphological species. It is proposed that the scientific name Salmo ferox Jardine, 1835 , as already applied to Lough Melvin (Ireland) ferox, should be extended to Awe and Laggan ferox.     

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.     

Estimates of gene flow among neighbouring populations of brown trout

Between-population gene flow was estimated in neighbouring freshwater and anadromous Salmo trutta populations from Asturias, Northern Spain. Populations from the same drainage showed a high mean level of gene flow. Gene flow was also found between populations from different drainages and was negatively related to the distance between river mouths. The strength of the homing orientation of migratory individuals (sea trout) is discussed.     

Prey orientation in piscivorous brown trout

Piscivorous brown trout Salmo trutta change their feeding behaviour depending on prey species, prey size and number of prey eaten. In trout which had eaten fish recently, most had one fish in their stomach, but up to 16 prey fish were found. Individuals of the small-sized minnow Phoxinus phoxinus were swallowed chiefly tail first, whereas individuals of the larger Arctic charr Salvelinus alpinus were taken both head and tail first. The largest charr were swallowed head first. In stomachs containing more than one fish prey, prey orientation was likely to be mixed. For all three types of prey orientation (i.e. tail first, head first and mixed), significant and positive correlations existed between prey length and predator length. The maximum prey size eaten tail first or in mixed orientation was about 70–85% of the size of prey eaten head first, indicating morphological advantages in eating the prey head first.     

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     

Initial feeding in migratory brown trout (Salmo trutta L.) alevins

Initial feeding of brown trout was investigated under laboratory conditions. Fifty per cent feeding occurred when yolk constituted approximately 31% of total alevin dry weight, and feeding rate was positively correlated to developmental stage. The possible ecological implications of initial feeding are discussed.     

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.     

Electrofishing results corrected by selectivity functions in stock size estimates of brown trout (Salrno trutta L.) in brooks

To overcome the problems usually met as a consequence of small numbers of fish collected for trout stock size evaluation in small rivers, a selectivity function was developed and tested in two small rivers in western Switzerland. The function takes the form In E = bL ^−k where E is the fishing efficiency, L is the fish length in cm, k is a constant and b is the slope, estimated by regression analysis.
A good fit was obtained for k = 1 (R. Flon de Carrouge) and k = 2 (R. Cerjaule). Individual selectivity functions have to be computed for each survey, because they depend on the operators of the fishing gear and on the physical and biological conditions in the rivers.
The results are discussed in the light of the Petersen mark-recapture method and the De Lury removal method for stock evaluation. The mark-recapture method gives better results when time intervals between successive fish removals are short.     

RNA-DNA ratios in white muscle tissue biopsies reflect recent growth rates of adult brown trout

Recently developed fluorescence techniques were used to quantify RNA-DNA ratios in white epaxial muscle tissue biopsies taken from live adult brown trout. RNA-DNA ratios from small biopsies (4–24 mg wet weight) were measured concurrently with growth over 5 weeks in two groups of brown trout fed different rations, 5% BW day^{– 1} (reference) and 1% BW week^{– 1}(treatment). Reduced rations had significant effects on RNA-DNA ratios in muscle tissue biopsies. The treatment group had significantly lower mean RNA–DNA ratios than the reference group after the first week. After 5 weeks of treatment the mean RNA–DNA ratios of the treatment and reference groups were 1–98 and 4–31, respectively. RNA-DNA ratios from muscle tissue biopsies reflected recent growth rates in the two groups of fish. This technique allows a biochemical measurement of growth rates in adult fish without mortality.     

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.     

Latitudinal clines in life-history variables of anadromous brown trout in Europe

Based on published data, we reviewed clinal variations in life-history characteristics of anadromous brown trout Salmo trutta from 102 European rivers at latitudes between 54 and 70° N. Growth rate in fresh water, mean smolt age, mean sea age at first sexual maturity, proportion of repeal spawners among adults, longevity, and length of adult life span exhibited latitudinal clines. Brown trout grew faster in fresh water, smolted and matured younger, lived fewer years but spawned more times in the south than in the north. The life-history traits studied were often correlated. Longevity, smolt age and sea age at maturity were negatively and smolt length and proportion of repeat spawners among adults were positively correlated with growth rate in fresh water. Longevity was positively correlated with smolt age and sea age at maturity. The latter also increased with increasing smolt age. None of these significant correlations among life history variables, except for those between smolt age and parr growth and proportion of repeat spawners and parr growth, are latitudinal effects. We do not know to what extent the latitudinal variation in life–history traits reflects phenotypic plasticity and to what extent it is caused by inherited differences among populations.     

A rapid method for detecting mitochondrial DNA variation in the brown trout, Salmo trutta

A method is described for the amplification and restriction digestion of two variable regions of mitochondrial DNA (mtDNA) in the brown trout ( Salmo trutta ) as a non-invasive sampling and rapid screening technique for measuring genetic variation in populations.     

Effects of low pH on the chorion of rainbow trout, Oncorhynchus mykiss, and brown trout, Salmo trutta f. fario

Morphological studies on the effect of low pH on egg shells (chorion) of rainbow trout, Oncorhynrhus mykiss Walbaum (formerly Salmo gairdneri ), and brown trout, Salmo trutta f. fario L., showed that the plugs of the outer layer of the chorion, which block the channels of the underlying layer, developed distinctive vacuolization at low pH exposure. This may lead to loss of chorional semi-permeability. Chorions were incompletely degraded at low pH inducing hatching problems. Partially hatched alevins showed only a small local zone of dissolution of the chorion around the head of the embryo preventing the larvae from being freed.     

Impact of habitat fragmentation on the genetics of populations in dendritic landscapes

1. The effect of habitat fragmentation on freshwater species has been addressed using brown trout Salmo trutta L. as a model species with a dendritic population structure. 2. Microsatellite loci were employed as molecular markers. Levels of gene flow and population subdivision were determined in more than 1200 brown trout individuals inhabiting four south European rivers with contrasting patterns of fragmentation, defined by the presence of barriers. 3. The genetic units in the four rivers were restricted by artificial barriers, and gene flow among samples within each river was associated with the level of fragmentation of the river. 4. Loss of genetic diversity and dislocation of the dendritic model have been detected in fragmented rivers. These results emphasise the importance of mitigating the impact of dams by constructing passages to restore gene flow along the river, for fish and other migratory species, as well as the need for caution in relation to stocking in isolated areas to avoid problems of inbreeding.     

A comparison of AFLPs and microsatellites to identify the population structure of brown trout (Salmo trutta L.) populations from Hardangervidda, Norway

A total of 495 fish from 11 Hardangervidda lakes were genotyped in order to compare amplified fragment length polymorphisms (AFLP) and microsatellites in terms of their capacity to infer population genetic structure. The 11 microsatellites used in this study gave a greater polymorphism information content and greater gene diversity, with an average of 14.8 alleles per locus, than the six AFLP primer combinations used. However, the AFLPs resulted in 178 polymorphic loci and a 3.1 times larger marker index (effective multiplex ratio multiplied with the gene diversity). Comparable population structuring, for example in terms of distinguishing fish from the different river systems, was obtained with both marker systems. An AFLP and microsatellite multilocus Bayesian assignment test with the structure program divided the fish into six groups largely concurrent with main branches on a population neighbour-joining tree. Yet, the admixture status of individuals is mostly contradictory in the AFLP and the microsatellite analyses. The results are discussed concerning migration between lake populations.     

Movement and site fidelity in young brown trout Salmo trutta populations in a southern Irish stream

Populations of brown trout Salmo trutta were monitored at a number of sites within a single stream, using an individual marking technique and recapturing uniquely marked fish repeatedly over a period of 12 months. Individual 1 + and 2 + resident brown trout in the Glenfinish River were found to consist of stationary and mobile component populations. The latter population consisted of a number of individuals observed moving mostly in an upstream direction, within a range of 0.03–2.24 km. On a large spatial scale, individuals in the stationary component population exhibited some degree of home site fidelity within the stream, over a period of 3–4 months, after which the fish tended to move from the site. Within sites, fidelity to either riffle or pool habitats, mostly the latter, was apparent in a proportion of the population. On a smaller scale, fidelity to the exact position with respect to boulders in the stream was also evident in a number of individuals. Home range size was calculated amongst these individuals, with ranges of up to 20 m recorded.     

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 problem of sampling families rather than populations: relatedness among individuals in samples of juvenile brown trout Salmo trutta L.

In species exhibiting a nonrandom distribution of closely related individuals, sampling of a few families may lead to biased estimates of allele frequencies in populations. This problem was studied in two brown trout populations, based on analysis of mtDNA and microsatellites. In both samples mtDNA haplotype frequencies differed significantly between age classes, and in one sample 17 out of 18 individuals less than 1 year of age shared one particular mtDNA haplotype. Estimates of relatedness showed that these individuals most likely represented only three full-sib families. Older trout exhibiting the same haplotypes generally were not closely related.     

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.     

Catecholamine release and interrenal response of brown trout, Salmo trutta, exposed to aluminium in acidic water

Cannulated brown trout, Salmo trutta , were exposed for 36 h to synthetic water with a low calcium content of pH 5 and similar synthetic water dosed with aluminium to raise the filterable A1 from 5 to 290 μg 1⁻¹ over the 36-h period. There were no significant disturbances of plasma concentrations of glucose, cortisol or catecholamine (adrenaline and noradrenaline) in fish held in water of pH 5. The addition of aluminium to this acidic synthetic water resulted in a generalized endocrine stress response with a four-fold increase in plasma glucose concentration after 18 h and a significant increase in plasma cortisol concentration from 24 h onwards when filterable A1 exceeded 200 μg 1⁻¹. Plasma catecholamine concentration indicated an adrenergic stress response in aluminium-exposed brown trout. A transient doubling in noradrenaline after 6 h in A1 was followed by a larger increase in both plasma adrenaline and noradrenaline concentrations in fish surviving the 36-h exposure to A1.