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.     

Regular, cyclic oscillations in cohort strength in an allopatric population of brown trout. Salmo trutta L.

Abstract– Dominant year-classes of brown trout occurred at regular time-intervals in the alpine Lake Skavatn, Norway. In samples obtained by beach scining, electrofishing and gill-netting during 1989–1992, yearclasses 1979, 1982, 1985, 1988 and 1991 were much more abundant than their neighbouring year-classes. Correspondingly, in a sample of gillnetted fish from 1979, the year-classes 1973 and 1976 were dominant. Spawning areas in the outlet are virtually absent, and lotic rearing areas for juveniles very restricted. Young-of-the-year immigrated to the lake during the autumn, and juveniles inhabited the restricted littoral cobble areas until they reached a length of about 10–12 cm and an age of 3+. Competitive exclusion by dominant year-classes may cause the regular, cyclic oscillations in cohort strength. An abundant year-class of juveniles occupying the restricted suitable lacustrine rearing areas may exclude younger fish by inter-cohort interference. The smaller fish are forced to unsheltered marginal rearing areas where they presumably suffer increased mortality. A new strong year-class can arise every 3 years when the dominant fish leave the rearing areas.     

Habitat suitability analysis for lacustrine brown trout (Salmo trutta) in Lake Walchensee,Germany: implications for the conservation of an endangered flagship species

• 1. The lacustrine brown trout (Salmo trutta) is endangered and of high conservation importance. In the only spawning habitat of the population in the Bavarian Lake Walchensee, the River Obernach, a substantial decrease in spawning runs has been reported. In this study, the present ecological state of the spawning stream was analysed with the objective of identifying life‐stage specific limitations to successful recruitment attributable to deficiencies in (i) spawning migration, (ii) spawning habitat quality, and (iii) habitat quality for juveniles.
• 2. Structural stream analysis showed that discharge and several migration barriers — particularly near the river outlet into the lake — prevent successful spawning migrations at normal water levels. Migration barriers are probably the main limiting factor for reproduction of lacustrine brown trout, whereas structural variability of the Obernach meets the habitat requirements of both spawners and juveniles.
• 3. Spawning site quality was suitable for trout, as indicated by stream substratum texture and high exchange rates between free‐flowing water and the interstitial zone in physico‐chemical parameters (redox potential, dissolved oxygen, pH, temperature and conductivity).
• 4. Analyses of fish community structure revealed dominance of lithophilic species, in particular of riverine brown trout (Salmo trutta). Its density and intact demographic population structure suggest that spawning and juvenile habitat quality for salmonids is not limiting. Recapture of stocked lacustrine trout juveniles also indicates habitat suitability for the juvenile stage.
• 5. In conclusion, the results show that the methodology used in this study is suitable for the identification of life‐stage specific habitat deficiencies in lacustrine brown trout and other fish species. Availability of habitat data throughout the species' distribution range is a first crucial step for the development of an effective recovery plan. Copyright © 2009 John Wiley & Sons, Ltd.

     

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.     

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.     

A review of ecological models for brown trout: towards a new demogenetic model

Abstract – Ecological models for stream fish range in scale from individual fish to entire populations. They have been used to assess habitat quality and to predict the demographic and genetic responses to management or disturbance. In this paper, we conduct the first comprehensive review and synthesis of the vast body of modelling literature on the brown trout, Salmo trutta L., with the aim of developing the framework for a demogenetic model, i.e., a model integrating both population dynamics and genetics. We use a bibliometric literature review to identify two main categories of models: population ecology (including population dynamics and population genetics) and population distribution (including habitat–hydraulic and spatial distribution). We assess how these models have previously been applied to stream fish, particularly brown trout, and how recent models have begun to integrate them to address two key management and conservation questions: (i) How can we predict fish population responses to management intervention? and (ii) How is the genetic structure of fish populations influenced by landscape characteristics? Because salmonid populations tend to show watershed scale variation in both demographic and genetic traits, we propose that models combining demographic, genetic and spatial data are promising tools for improving their management and conservation. We conclude with a framework for an individual‐based, spatially explicit demogenetic model that we will apply to stream‐dwelling brown trout populations in the near future.     

Downstream migration of European lake trout, Salmo trutta f. lacustris L., and resident brown trout, Salmo trutta f. fario L., progeny in a Lake Constance affluent river

Abstract The local migratory behaviour of lake trout, Salmo trutta L., and resident brown trout, S. trutta L., progeny from an inflowing stream of Lake Constance was compared. No differences were detected with respect to emigration rate (3.5% and 4.2% for lake trout and resident trout, respectively), rate of residency (6.8% and 8.2% for lake trout and resident trout, respectively) and rate of survival (12.3% and 12.1% for lake trout and resident trout, respectively) one year after stocking. Some of the resident brown trout offspring became migrants and vice versa . The results indicate that the progeny of riverine brown trout contribute considerable numbers to lake trout stock.     

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.     

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.     

Recaptures and resource use of native and non-native brown trout, Salmo trutta L., released in a Norwegian lake

Abstract Rate of recapture (gill netting), habitat use, and diet of three strains of stocked brown trout, Salmo trutta L., were compared with resident brown trout in a Norwegian lake. The strains originated from an alpine lake, from a boreal lake, and from the native brown trout population in the lake. Overall recapture rate was 5–8% for all strains. The low recapture rate could be due to the relatively small size at stocking; mean fish length varied between 13.1 and 14.5 cm with strain and stocking method. Two years after release, the frequency of the different strains decreased from about 12% in the first year to stabilize at about 1%. The alpine strain showed the highest overall recapture rate, whereas the native strain was recaptured at an intermediate rate. The overall recapture rate of scatter-planted brown trout was higher than that of spot-planted brown trout. Immediately after being stocked, introduced fish ate less and had a less-varied diet than resident trout; however, stocked fish adopted a natural diet within the first summer. The distribution of trout between the pelagic and the upper epibenthic habitat was similar for both the resident and the stocked brown trout. Results indicate that the habitat use of stocked brown trout is adaptive and becomes similar to that of indigenous fish.     

Effects of stocking on the genetic structure of brown trout,Salmo trutta,in Central Europe inferred from mitochondrial and nuclear DNA markers

Abstract Stocking has had a considerable effect on wild brown trout, Salmo trutta L., populations throughout Europe. To elucidate this impact and to outline further management strategies, the genetic structure of 25 wild populations and five hatchery stocks from Czech Republic and Slovakia were analysed using mitochondrial (control region) and nuclear DNA (microsatellites, LDH‐C1*) markers. Stocking practices have caused massive hybridisation between the Atlantic and Danube brown trout strains in the central Danube basin and have lead to a loss of among‐population divergence in Slovakia and the eastern part of Czech Republic. Comparison with studies from neighbouring countries revealed substantial differences in haplotype, allele frequencies and genetic diversity across Central Europe. Differences in stocking management and origin of breeding stocks appear to be crucial factors for the spatial variability of the genetic structure of brown trout.     

Gillnet fishing drives lake‐migrating brown trout to near extinction in the Lake Päijänne region,Finland

Abstract Wild stocks of brown trout, Salmo trutta L., collapsed in Finnish inland waters during the 20th Century because dams prevented upstream migration, and low water quality and stream dredging weakened reproduction. The demise in migratory stocks was coupled with overfishing, mainly by gillnetting on lakes. Consequently, the migratory spawning stocks have diminished to negligible levels. The remaining stocks exhibit restricted immigration and emigration, are supplemented by continuous stocking, and their natural genetic diversity is affected by human activities. In recent years, various recovery actions have been implemented including stream channel restorations, fish passage facilities constructed and stocking of eggs and smolts. Gillnetting has also been regulated by banning certain mesh sizes, and catch‐and‐release of wild trout is spreading amongst sport fishers. However, these measures seem to be inadequate and almost no recovery of migratory populations has been reported. The problem of by‐catch in intensive gillnetting continues to threaten stocks and creates disputes between stakeholders.     

Comparison of population dynamics of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in a small tributary of the River Scorff (Brittany, France)

The population dynamics of Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) were compared in a small tributary of the River Scorff (Brittany, France) from spawning time to the beginning of the third growing season. The spawning and fry emergence of the two species took place at approximately the same time. In the first autumn, the densityof 0+ juveniles and settling rate from the egg stage were much higher in trout than in salmon. The emigration rate from 0+ population was much higher in trout than in salmon. The size of resident and migrating fish was always smaller in salmon than in trout, whatever the age. The low level of salmon production in the brook, compared with trout, was the result of low survival from egg to 0+ stage in autumn, combined with the small proportion of juveniles migrating after the first growing season. This was not compensated by a high number of migrants the next year. The role of physical habitat, inter-and intraspecific competition, predation and migration dependence on size and early sexual maturity is discussed. Indications are that small tributaries of the type studied are of great value for recruitment in trout but not very productive for juvenile salmon.     

Migratory behaviour of brown trout, Salmo trutta L: importance of genetic and environmental influences

Lake Storvindeln in northern Sweden supports a population of fast-growing lake-run brown trout. Spawning and early rearing take place in the River Vindelälven, while most growth occurs in the lake (piscivory). A smaller tributary to the lake, Låktabäcken Creek, holds a resident, early maturing, short-lived brown trout population, although no migratory barriers have existed since 1947. To establish a lake-run trout population, fry from the migratory R. Vindelälven stock were introduced into the creek during 1985–1991. Introduced trout descended to the lake prior to maturing. As a result of the introduction, large adult trout returned from L. Storvindeln to the creek to spawn in 1991 and 1992; i. e., the introduced trout were able to complete a migratory life cycle. Genetic factors appeared to have a primary influence on the predisposition to migrate in this case, and it is suggested that migratory populations rarely develop from strictly resident ones.     

Stocking impact,population structure and conservation of wild brown trout populations in inner Galicia (NW Spain), an unstable hydrologic region

1. Brown trout (Salmo trutta) is an important conservation resource in the Iberian Peninsula. The Atlantic is considered the most hydrologically stable region for the species, although inner Galicia (NW Spain) shows Mediterranean (unstable) climatic conditions. The Galician region, threatened by past releases of brown trout individuals from central European origin, harbours two native lineages, one of them endemic to the Iberian Peninsula. These populations are thus highly valuable for conservation, as well as being important for recreational fisheries. 2. In total, 546 individuals from 16 sampling sites (15 natural locations from inner Galicia and one from a central European hatchery stock) were genotyped for 11 nuclear markers (10 microsatellite loci and the LDH‐C* locus) to analyse genetic variability, population structure and introgression impact from stocking in order to assess the conservation status of brown trout in the region. Moreover, correlation among hatchery introgression and environmental variables relevant for species population dynamics was also investigated. 3. Genetic variability was within the range of Iberian brown trout (He = 0.500–0.600). Stocking impact was higher than previously reported values for the Atlantic region and was related to environmental instability. Highly significant native population differentiation was observed in the whole region (F_ST = 0.283), at least four main genetic groups being detected across the geographic distribution studied. 4. Conservation strategies at local level (including the creation of genetic refuges and temporal monitoring of genetic composition) are suggested to agencies and administrations for the sustainable management of brown trout.     

Genetic diversity within sea trout population from an intensively stocked southern Baltic river,based on microsatellite DNA analysis

Sea trout, Salmo trutta L., populations in the Slupia River basin have been affected by mass stocking with smolts and fry. This work is focused on a small‐scale differentiation in sea trout populations from one basin with a strong emphasis on the relationship between wild and hatchery origin fish. A total of 216 sea trout were genotyped at 10 microsatellite loci. Global F_ST obtained by amova was low at 0.0165. Pairwise F_ST were significant for all tests except wild and stocked adults. The highest pairwise difference was found between the hatchery sample and Kwacza (F_ST = 0.038). Analysis of the genetic structure revealed micro‐geographical differentiation with four subpopulations. The quality of the artificial spawning was found not to be adequate with a high risk of adverse effects to the whole population. All future stocking actions in the basin should consider the existing population structures.     

Survival of stocked hatchery-reared brown trout, Salmo trutta L., fry in relation to the carrying capacity of a trout nursery stream

Abstract. During the period June 1982-84 hatchery-reared brown trout, Salmo trutta L., fry were Stocked into stretches of the Owendoher. a trout nursery stream on the east coast of Ireland. These experiments were designed to examine the survival of stocked fry and to estimate the carrying capacity of the system. During the first year fry were stocked into sectors already supptorting wild fish at densities normal for the system. In the following year fry numbers were artificially reduced prior to stocking with the hatchery-reared fish. Mortality of the stocked fry was high after release with less than 33% of the fish surviving beyond the first 3 weeks. No stocked fish survived after October 1982. In the second year, however, 2-9% of the fish survived. The best survival rates were achieved where wild fry numbers were lowest. Regardless of the initial stocking density the various experiments yielded autumn fry densities (0.07-0.7 fish/m²) similar to those at unstocked sites (0.1-0.62 fish/m²).
Stocking did not increase recruitment to the 1+ group and again 1+ densities (0.15-0.35 fish/m²) similar to unstocked sites (0.07-0.39 fish/m²) were obtained at the end of each year. These results suggest that spawning and recruitment in the Owendoher yield population densities approaching the maximum carrying capacity of the stream. The system appears to support a maximum summer fry density in the region of 1 fish/m² and a maximum autumn density of 0.7 fish/m².     

Landscape features determine brown trout population structure and recruitment dynamics

Variation in brown trout (Salmo trutta L.) population recruitment and structure is related to migratory patterns, which should depend on ease of access to habitats providing increased opportunity for growth. We quantified the number of young of year (YOY) as a proportion of the total number of brown trout at 24 locations on 11 streams within the Taieri catchment, New Zealand, including back calculated growth rates and emergence dates from otoliths. Locations with high absolute and relative abundance of YOY fish were related to elevation and distance from the river mainstem (habitat used by migratory fish), fish density, and the interaction between invertebrate food biomass, distance and elevation. Hatch date and growth were not related to the proportion of YOY fish, though growth was negatively correlated to total fish density. We suggest landscape features play a large role in determining recruitment and population structure. Locations at lower elevations have a high YOY density, high competition and lower growth, likely prompting out‐migration. These conditions could be created by successful return migration and spawning of large fecund fish resulting in YOY densities exceeding the habitat carrying capacity. Environmental factors, such as food availability, also played a role in determining population structure. These results provide an example of how population structure and recruitment might be controlled by local conditions and access to high growth environments in wild populations of introduced brown trout across a catchment.     

Post-stocking behaviour of hatchery-reared European grayling, Thymallus thymallus (L.), and brown trout, Salmo trutta L., in a semi-natural stream

Hatchery-reared, juvenile European grayling, Thymallus thymallus (L.), and brown trout, Salmo trutta L., were each stocked six times into an area of a semi-natural stream. The order in which the two species were released was switched after every second experimental stocking. Temporal and spatial post-stocking dispersal, effects of previously stocked species, feeding behaviour and the influences of sex and size were studied. During each 48-h experimental release period, some fish were recaptured in a trap situated 200 m downstream from the stocking site, and fish remaining in the stream after each experimental release were caught by electric fishing. Significantly more grayling than trout moved downstream and left the semi-natural stream. Proportions of stocked grayling recaptured in the trap within 2 h and from 2 to 48 h post stocking in the stream were 36.4% and 10.0%, respectively. Corresponding recapture rates for brown trout were both 1.5%. Most of the grayling and brown trout that did not leave the stream early were recaptured in deep, slow-moving water at low velocities in the release area. The presence of grayling at the time that the brown trout were stocked resulted in significantly fewer brown trout staying in the upper part of the stream. Within 48 h of their release, 33.3% and 22.8% of the grayling and brown trout, respectively, had eaten natural food items. The mean length of brown trout recaptured in the upper part of the stocking area was significantly lower than that of fish recaptured in the lower part and in the trap. Among brown trout, males showed a significantly greater propensity to eat and to stay in the upper part of the stream near the stocking site compared with females . Brown trout with natural food items in their stomachs had significantly lower mean length than trout without such items. No sex- or size-related differences were found in the spatial distribution or feeding activity of grayling.     

Competitive exclusion of brown trout Salmo trutta L., by rainbow trout Oncorhynchus mykiss Walbaum, in lake tributaries, New Zealand

Spawning runs of brown trout Salmo trutta L., and rainbow trout, Oncorhynchus mykiss Walbaum, were compared in three tributaries of two lakes for timing, numbers and redd characteristics. Redd characteristics for the two species in four other streams were examined. No significant differences in redd characteristics between the two species were observed, but in all lake tributaries the possibilities of superimposition of spring spawning rainbow trout on redds of brown trout were examined. A common feature was the high ratio of numbers of spawners to the area of preferred spawning gravel, and it was concluded that in the lake tributaries, superimposition resulted in increased mortality of brown trout eggs, and in one case elimination of the brown trout population. Management possibilities for control of the species proportions in comparable situations are discussed.