High genetic introgression in alpine brown trout (Salmo trutta L.) populations from Hardangervidda, Norway

Abstract –  We studied the effect of stocking brown trout in three alpine lakes (Skavatn, Midtre Grøndalsvatn and Nordmannslågen) of the Hardangervidda mountain plateau by comparing microsatellite data from historical (1933 and 1967) and contemporary (2003) population samples. Historical and contemporary samples from a control lake (Krokavatn) and a contemporary sample from Gjuvsjå (suspected donor lake) were included as well. A total of 331 brown trout were genotyped with 11 microsatellites. Very low level of genetic diversity is observed in the historical samples from Nordmannslågen and Midtre Grøndalsvatn but not in the contemporary samples. In Skavatn historical and contemporary samples show similar genetic diversity as in the other contemporary samples. Admixture analyses with the structure program indicate high levels of introgression by nonnative brown trout in all three lakes. The stocking source for two lakes (i.e. from Gjuvsjå) is partially verifiable via our analyses. The high introgression in these lakes is discussed and the management issues are briefly addressed.     

Genetic structure of brown trout,Salmo trutta,populations from differently sized tributaries of Lake Mjøsa in south‐east Norway

This study was based on genotyping eight microsatellite loci of 463 brown trout, Salmo trutta L., sampled in nine differently sized tributaries in three areas on the eastern shore of Lake Mjøsa, south‐eastern Norway. The populations were genetically structured, and Mantel's test showed that genetic distance correlated positively with geographical distance. Temporal differentiation F_ST over a 2‐year period was estimated in five streams and was non‐significant after Bonferroni correction. Effective population size N_e was positively correlated with the habitat length available from the lake (0.3–22 km) and negatively with the number of full sib pairs in the sample. There was no correlation between N_e and genetic diversity, and private alleles were recorded in three medium‐sized streams, but not in the largest two. The importance of small spawning and nursery streams for the maintenance of genetic diversity of brown trout was demonstrated.     

Importance of life‐history and landscape characteristics for genetic structure and genetic diversity of brown trout (Salmo trutta L.)

Abstract – Investigating the influence of evolutionary forces on the genetic structure and genetic diversity remains a major challenge. Yet, it is of considerable interest for conservation and management of a species. This study investigates the influence of life‐history and landscape features, such as altitude, connectivity and habitat size, on genetic diversity and genetic structure of brown trout (Salmo trutta L.) with stream‐resident, lake‐dwelling and sea‐migrating life‐history in two river systems in northern Sweden. Using regression tree analysis including ecological and landscape characteristics, we show that life history is the most important variable explaining genetic diversity and population differentiation. Sea‐migrating populations show high diversity and low differentiation, and lake‐ and stream‐resident populations show low diversity and high population differentiation, among all samples. No overall genetic correlation with geographical distance was noted; however, among sea‐migrating populations within the River Vindelälven drainage, this pattern was observed. This study illustrates that life‐history and landscape features help to explain genetic structure and genetic variation. The information is important for conservation and management actions, such as fisheries regulations, habitat restorations, stocking of hatchery fish, defining management units and introducing genetic monitoring programmes.     

Effect of supplementary stocking of juvenile brown trout,Salmo trutta,on yield in a Norwegian mountain reservoir

Abstract The effect of supplementary stocking of juvenile (age 0+), hatchery‐reared, brown trout, Salmo trutta L., on annual yields was assessed in a Norwegian mountain reservoir between 1979 and 2007. Fishing was mainly carried out by local fishermen with benthic gillnets. During the study period, annual stocking ranged from 0 to 52 500 fish (19.8 ha^?1). No stocking has been carried out since 1997. Annual yield varied from 1650 to 5653 kg, corresponding to 0.62–2.13 kg ha^?1. Exploitation rate in terms of number of gillnets and mean weight of 6+ fish (age when catchable size was reached) explained 64% of the variability in catches. Stocked fish contributed very little to the yield or catch‐per‐unit‐effort, exhibiting no positive correlation with stocking density. The lack of contribution from stocked fish was probably caused by a competitive bottleneck in the eroded epibenthic zone, causing high juvenile mortality. If stocking continues, it is recommended that fish with body lengths >15–20 cm are used.     

Population genetic structure of Atlantic salmon,Salmo salar L., in the River Tamar,southwest England

Abstract Population genetic studies can be useful for informing conservation and management. In Atlantic salmon, Salmo salar L., population structuring frequently occurs between river systems, but contrasting patterns occur within rivers, highlighting the need for catchment‐specific studies to inform management. Here, population structure of Atlantic salmon was examined in the River Tamar, United Kingdom, using 12 microsatellite loci. Gene diversity and allelic richness ranged from 0.80 to 0.84 and from 8.96 to 10.24, respectively. Some evidence of genetic structure was found, including significant genetic differentiation between samples in different subcatchments (pairwise θ and tests of genic differentiation), results from assignment tests and a pattern of isolation by distance. Conversely, structure revealed only one population cluster, and an analysis of molecular variance showed no significant variation between subcatchments. Evidence of population bottlenecks depended on the mutation model assumed and is discussed with reference to catchment‐specific studies of stock abundance. Implications for implementing management actions are considered.     

Response of resident brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), to the stocking of hatchery-reared brown trout

Two strains of hatchery-reared adult brown trout, Salmo trutta L., [208–334 mm total length (TL); n =  591] were individually marked and released into a limestone stream. The estimated survival after one month (86%; n =  508) was comparable to that for resident brown trout and rainbow trout, Oncorhynchus mykiss (Walbaum), (89%; n =  771), but declined to 14% ( n =  83) after 8 months compared with 52% ( n =  451) for resident trout. The movement of resident trout out of stocked stretches was higher (14%) than from control sites (5%), but the population size in both individual sites and the overall study area were unaffected. The growth of resident brown trout was unaffected by stocking, but rainbow trout showed lower growth rates in stocked versus unstocked stretches both one and 8 months after stocking ( P <  0.002).     

Microsatellite diversity assessment of brown trout (Salmo trutta) population structure indicate limited genetic impact of stocking in a Norwegian alpine lake

Abstract – Releases of non-native trout often result in introgression into natural populations and negative genetic effects. The causal ecological mechanisms for a wide range of reported outcomes are poorly understood. Brown trout population structure in an alpine lake with three major recruitment streams was assessed by analysis for eight DNA microsatellite markers and compared with the non-native population. The lake is subject to a 40-year recorded history of stocking with exogenous trout. No certain deviations from Hardy–Weinberg equilibrium were found. Tests for population differentiation and genetic distance indicated separate populations for all the sampled areas, and with the exogenous population as a cluster quite different from the others. Assignment tests indicated that only a small fraction of the fish sampled from the lake originated from the introduced trout strain (<3%). Wild discriminate, naturally reproducing populations characterize this alpine lake ecosystem, in spite of 40 years of stocking, which appears to have had a limited impact. It is unlikely that this population structure can ultimately be explained by trout movement patterns. Genetic analysis needs to be supplemented by studies of local life history strategies, to evaluate the relative importance of local adaptation versus random genetic differentiation, because implications for conservation and management are different.     

Effects of stocking of piscivorous brown trout, Salmo trutta L., on stunted Arctic charr, Salvelinus alpinus (L.)

To study the effects on a stunted freshwater population of Arctic charr, Salvelinus alpinus (L.), two groups of large (26–45 cm) individually tagged brown trout, Salmo trutta L., were released and recaptured with gillnets after 1, 7, 11 and 63 weeks. One group of trout was trained on a fish diet before release, and the other, reared on commercial dry pellets, served as a control. Specific growth rates in both groups were negative 1 week after release and approached zero after 63 weeks. Condition factor and internal fat content decreased during the experiment. Although only 11% of the trout stomachs examined contained fish prey, charr represented 79% of the total stomach weight content. Gillnet samples of charr before and 63 weeks after the release of trout indicated a decreasing population size of charr. Individual growth and mean length of charr increased after release of trout, especially for charr at age 4 years. After the release of trout, 35% of the charr were longer than 20 cm as compared with 6% before the release.     

Long-term variation in the population structure of Arctic charr, Salvelinus alpinus, and brown trout, Salmo trutta

The effects of induced water level fluctuations and introduction of the mysid Mysis relicta Lovén on population structure of brown trout, Salmo trutta L., and Arctic charr, Salvelinus alpinus (L.), were studied during 1953–1995 in Limingen hydroelectric reservoir, Norway. The main response was a marked reduction in catch‐per‐unit‐effort (CPUE) for trout and charr, probably caused by reduced recruitment following increased variation in water level. For both species, mean length decreased until 1967 and increased thereafter, whereas mean mass‐at‐length increased for the whole period. Both length and mass‐at‐length were negatively correlated with CPUE. The increases in mean length and mass‐at‐length were probably because of reduced competition following the reduced recruitment. Mysis relicta has become an important food item for charr but not for brown trout, but the increases in mean length and mass‐at‐length of charr started prior to the appearance of M. relicta in the charr diet.     

Dispersion of stocked brown trout, Salmo trutta L., and landlocked salmon, Salmo salar L., from stocking sites in Lake Oulujärvi, Finland

Movement and recaptures of two hatchery-reared brown trout, Salmo trutta L., stocks and landlocked salmon, Salmo salar L., released at different sites in regulated Lake Oulujärvi, were studied in relation to release site. Five groups of fish from each stock were released in approximately equal numbers. Most of the fish released in June and July were recaptured within 3 months, whereas the majority of the fish released in early winter (October and November) were caught the following spring, about 7–9 months after stocking. The release site had a significant effect on recapture rate. The results showed that fishing restrictions targeted mainly at gill net fishing are needed to preserve the stocked fish from overfishing. Significantly fewer recaptures were observed from the landlocked salmon stocking compared with brown trout. The recaptures from the landlocked salmon stocking indicated more active movement and less clumping compared with the two brown trout stocks.     

Direct and indirect effects of riparian canopy on juvenile Atlantic salmon,Salmo salar,and brown trout,Salmo trutta,in south‐west Ireland

Abstract Dense riparian tree canopy is generally found to have a negative effect on salmonid populations. Canopy can affect fish both directly via trophic impacts and its effects as cover, and indirectly via its effects on the distribution of instream vegetation. This study examined the impact of riparian canopy on the ecology of juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in south‐west Ireland. Riparian canopy reduced the density, length and gut contents of juvenile salmon, but not brown trout. The negative relationship between canopy cover and fish size and feeding was strongly influenced by the abundance of instream macrophytes, which in turn varied as a function of catchment water chemistry. As a management strategy, the reduction of dense riparian canopy is unlikely to have the same effect on juvenile salmonids on all streams within an ecoregion because of differences in catchment‐wide factors.     

Genetic variation of body size, condition and pyloric caeca number in juvenile brown trout, Salmo trutta L.

Parental and individual variance components of body length, weight, condition (estimated as the second principal component of the length–weight relationship) and pyloric caeca number were investigated in 6‐month‐old brown trout (Salmo trutta L.) by the mean of two sib analyses, which provided consistent results. The average heritabilities (±SE) were 0.12 (±0.08) for length, 0.16 (±0.08) for weight, 0.47 (±0.14) for condition and 0.38 (±0.12) for pyloric caeca number. Maternal effects were also observed, although short of significance, in length, weight and caeca number. Correlations between caeca number and body size averaged +0.10 among individuals within lots, but genetic correlations were negative, i.e. about ?0.9 for length and ?0.7 for weight. There was no significant correlation between caeca number and condition. These results lead to question the role that pyloric caeca may play in growth, as well as their usefulness in fish breeding.     

Genetic divergence of kingfish from Japan, Australia and New Zealand inferred by microsatellite DNA and mitochondrial DNA control region markers

ABSTRACT: Genetic polymorphism in kingfish, collected from coastal waters of Japan, Australia and New Zealand, were examined using microsatellite (MS) DNA and mitochondrial DNA (mtDNA) control region markers. Sixteen to 25.7 alleles per locus were observed in three MS markers, while the average observed (and expected) heterozygosities were 0.782 (0.918), 0.750 (0.809) and 0.650 (0.888) for Australian, Japanese and New Zealand kingfish, respectively. Twelve mtDNA haplotypes were detected by the digestion of control region sequences with five endonucleases: Hae III, Hinf I Mbo I, Rsa I and Taq I. Significant genetic divergence was observed between the kingfish population from Japan and those from Australia–New Zealand. There was no significant differentiation among the Australian and New Zealand population samples.     

Effects of size and morphology on swimming performance in juvenile brown trout (Salmo trutta L.)

Abstract –  Our study assesses swimming capacity (speed and stamina) and possible morphometric determinants of locomotor performance of juvenile brown trout ( Salmo trutta L.). We addressed these issues at the individual level to have an approach of the functional significance of intraspecific variation in morphological design. Both swimming speed and endurance time showed significant positive relationships with fish length. Size-corrected values of speed and endurance time were negatively correlated suggesting a phenotypic trade-off between burst and prolonged swimming. Size was also highly correlated with all the morphological variables measured. Therefore, we used the residuals of the regressions of those variables on fish length to remove the effect of body size. A principal components analysis (PCA) summarised the 12 morphological variables into two factors, which accounted for 44.3% of the variance. PC1 combined several measures of body depth and width, whereas PC2 represented mainly postanal length relative to abdomen length. Relationships between the scores of the two factors and size-corrected values of maximum swimming speed and endurance time were weak. PC2 showed a significant positive relationship with endurance time; that is, individuals with longer caudal regions were able to swim against water flow for longer periods of time. Stoutness (PC1) showed a marginally significant negative correlation with endurance time. The lack of stronger relationships could be because of the low morphometric variability among the test individuals, all proceeding from the same population, reared in a common environment, and measured at the same ontogenetic stage.     

Population size in stream-living juveniles of lake-migratory brown trout Salmo trutta L.: the importance of stream discharge and temperature

Abstract –  Based on the analysis of 17 successive year-classes, this investigation attempted to identify the factors determining year-to-year variation in population size of the stream-living juveniles of Lake Hald-dwelling brown trout Salmo trutta L. Population size appeared to be influenced chiefly by annual recruitment that in turn, was determined by stream discharge and temperature. These patterns matched those previously highlighted for a resident population located >2500 km apart and emphasised the importance of environmental (climatic) variability as a major regulating agent of population size in stream brown trout. However, distinctly shaped recruitment–discharge relationships between the two populations suggested different mechanisms in response to environmental variability and thus to persist in time.     

Effect of triploidy on paternal and maternal variance components in brown trout, Salmo trutta L.

The covariation between diploid and triploid progenies from common breeders and the effect of triploidy on the parental variances were investigated in brown trout (Salmo trutta L.) using two progeny testing experiments, sampling, sires and dams respectively, from the same population. The traits studied were body weight, growth, condition factor and red spotting of the skin. Triploidization generated some interactions with the parental breeding value, but their effect was minor (less than 20% of the genetic variance, in most cases) as compared with the amount of variation common to both ploidy levels. These interactions were mainly caused by a scale effect, triploidy reducing the variance attributable to sires and increasing the variance attributable to dams. Actual lack of correlation (genetic correlation significantly less than 1) between diploid and triploid familial performances was observed in a single instance out of 18. The modification of respective parental variance components by triploidy, already observed in the rainbow trout Oncorhynchus mykiss (Walbaum), appears as a logical consequence of the genetic make‐up of triploids, and should be taken into account in selective breeding.     

Growth and population structure of brown trout, Salmo trutta fario L., in homeothermous stream conditions from a management point of view

Abstract. A brown trout, Salmo trutta fario L., population in homeothermous stream conditions was investigated in terms of age, length-weight relationship, annual and seasonal growth, changes in seasonal condition and specific growth rates, as management variables. The growth of this species in the stream studied was very good, with the most rapid growth in the first year of its life (15·9cm) when the juvenile trout emerged early (April) from the stream bed. The greatest specific growth rate in length was between birth and the first year of life (182·6%). There was an agreement between the condition factor (K) and the growth rate. The values of K, for the juvenile trout, increased continuously because these mainly resulted from their growth through the year. The growth of the two sexes was similar except for the period from October to February when the growth of males was higher than that of females. Also the growth for both sexes was well described by the Von Bertalanffy growth model. No effect was found on the growth rate of tagged and untagged fish. The density and biomass were low, ranging between 0·008 and 0·035 fish.m^?2 and 1·25 and 5·63 g.m² respectively.     

Paternal variation in juvenile survival and growth of the triploid hybrid between female rainbow trout (Oncorhynchus mykiss Walbaum) and male brown trout (Salmo trutta L.)

Abstract Related sib‐groups of rainbow trout × brown trout triploid hybrid and monospecific brown trout controls were obtained from a common set of brown trout sires. On the basis of hybrid juvenile survival and growth, 10 sib‐groups were selected and the corresponding brown trouts were raised up to adult stage. Males from each group of brown trout were used to produce a second generation of hybrid progeny, the performances of which were analysed for grandpaternal variation and relation with first generation relatives. Results showed that hybrid traits were influenced by paternal and grandpaternal origin, but that there was little relation between hybrids and brown trouts, or between successive generations of hybrids.     

Evaluation of large-scale stocking of early stages of brown trout, Salmo trutta, to angler catches in the French–Swiss part of the River Doubs

Around 500 000 brown trout, Salmo trutta L., alevins are stocked annually in the 24‐km section of the River Doubs under study. All the alevins stocked in the period 1994–1996 were identifiable by fluoromarking their otoliths with tetracycline chlorhydrate. Anglers' catches, between June 1997 and September 1998, comprised trout aged 1+ to 7+ , but most (90% +) were 2+ to 3+ or 4+ , with the majority at 2+ and 3+. There was no significant difference in the size for a given age between marked and unmarked angled trout. The contribution of stocked fish in anglers' catches was around 22% for the 1995 cohort. The contribution of stocking (cohorts 1994 to 1995–1996) to the 1998 catches was around 23% (95% confidence limits: 19–27%). The estimated recapture rate was three to four trout per 1000 alevins stocked for the 1995 cohort. The major contribution (78%) of natural recruitment to anglers' catches suggests that the fishery management based on natural recruitment is still realistic in this part of River Doubs.