Effects of body size on sympatric shelter use in over‐wintering juvenile salmonids

The study tests two hypotheses: (1) the degree of shelter dominance in Atlantic salmon Salmo salar and brown trout Salmo trutta increases progressively with increasing size differential between heterospecific fish in a pair and (2) shelter dominance, standardized to size differential, correlates with aggression. The results support the first but not the second hypothesis, suggesting that the fitness consequences of high growth performance during the summer are likely to become evident during winter. At this time of year, when mortality is high among both Atlantic salmon and brown trout, shelter dominance may increase the chances of survival.     

Wild adult hybrids of Salmo salar L. and Salmo trutta L.

Ovarian development was impaired in three adult Salmo salar L. × S. trutta L. hybrids identified among adult salmonids in Scottish fisheries. Species-specific variation at enzyme loci indicated that the fish were F1 hybrids and mitochondrial DNA analysis showed them to be the progeny of S. salar females.     

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.     

The relationship between smolt density and fry density in salmonids

If the Ricker stock–recruitment model describes the relationship between egg density and survivor density at different stages of the life cycle, then the relationship between smolt density and fry density is not simple. Using data from a long-term study (1966–1990) of a sea trout population Salmo trutta , the relationship between density of potential smolts and fry densities in late M ay/early June or late August/early September is shown to be a reflexed curve with zero origin, so that there are two smolt densities for each fry density. A linear relationship is obtained only when the exponential parameter in the Ricker model is constant for the different life Stages.     

Genetic structure of two Turkish brown trout populations

The genetic structure of two brown trout Salmo trutta populations living in Lake Abant in Bolu and Üzüm River in Antalya was determined by examining 15 enzyme coding loci ( AAT, ADH, LDH, MDH, MEP, GPI, PGM and SOD ) using starch gel electrophoresis. Population specific mobilities were observed for the fixed alleles of LDH-B2, mMEP-2 and SOD-1 loci. Polymorphisms in sAA T-4, GPI-B2 loci were observed within the populations. Average heterozygosity of Abant and Antalya populations was 0.0358 and 0.0224 respectively. For LDH-C which is the post glaciation marker locus, the ancestral allele * 105 was found to be fixed in both of the populations. Nei's genetic distance between the two populations was 0.2507 which is the level of genetic distance often found between different species. This difference seems to be due to the presence of unique alleles in the LDH-B2, mMEP-2 and SOD-1 loci of the Abant population, indicating that the conservation of the Abant population and its heterozygosity is of prime importance.     

Introgression in the genus Salmo via allotriploids

Hybridization between sympatric species is not uncommon in the wild. Wild allotriploids (individuals with two chromosome sets from a species + one chromosome set from another species) are generally the result of a backcross between interspecific hybrids that produce unreduced gametes and one of the parental species. In animals, allotriploids are commonly sterile, except for some vertebrate species complexes in which allotriploids reproduce by parthenogenesis, gynogenesis and/or hybridogenesis, producing generally clonal or hemiclonal gametes; nuclear DNA introgression between hybridizing species is considered to be extremely rare. Employing species-specific molecular markers, we show genetic introgression between the chromosomally well-differentiated salmonids Atlantic salmon (2n = 58) and brown trout (2n = 80) through spontaneous bisexual reproduction of allotriploids leading to salmon-like offspring bearing some brown trout genes. Although introgression between these Salmo species can occur via allotriploids, we hypothesize that extinction of parental species can be discarded based on very low survival of allotriploid offspring.     

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.     

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.