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Behaviour observations of the endangered native marble trout (Salmo marmoratus, Cuvier, 1829) and introduced rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) in the laboratory and in a tributary to the Idrijca River in Slovenia were combined to study the movements and dominance relationships between individuals of the two species in an open field test. Under laboratory conditions, no difference between the species was detected for neither time spent actively or distance moved. In species paired tests, rainbow trout initiated more aggressive behaviours towards marble trout than vice versa, and rainbow trout were clearly the dominant individuals. After simultaneous release in the river, marble trout immediately left the release area and spent twice as long time as rainbow trout until they settled in an area of the river; therefore, the release site was immediately occupied exclusively by rainbow trout. Thus, the dominant and aggressive behaviour of rainbow trout seen in the laboratory before release might have influenced marble trout's subsequent behaviour in the river, by marble trout leaving the areas occupied by rainbow trout and moving to locations further away from the release site. In the field, the marble trout occupied sites individually, whereas rainbow aggregated at a few locations. Rainbow trout showed higher movement activity in the morning compared to marble trout. There was a positive correlation between swimming speed in the laboratory and movement in the field for marble trout but not for rainbow trout. In conclusion, the results in this study support the need to end stocking of rainbow trout in rivers with native marble trout. To better understand the interaction between the species, and to develop efficient management plans to protect the native marble trout, reference behaviours should first be understood, and future research in sites where the two species do not co‐exist is needed. This is especially important for marble trout for which behavioural research and data are lacking. [ABSTRACT FROM AUTHOR]

Stocking of native and non-native fish species is a widespread practice commonly used to enhance inland recreational fisheries, appropriate when intense harvesting and the degradation or lack of suitable habitat decrease the abundance of the managed species below carrying capacity. In spite of its popularity, this practice is often poorly informed by scientific information. Salmonids are arguably the most popular and commercially valuable freshwater fishes being managed for recreational fisheries. Stocking of both native and non-native taxa has been practiced for almost two centuries in Europe, dramatically altering the structure and function of riverine and lacustrine ecosystems. In the Verbano-Cusio Ossola Province, northern Italy, within the Lake Maggiore catchment, we measured large numerical mismatches between stocking of cultured native (Salmo marmoratus) plus non-native trout taxa (S. trutta, S. ghigii, Oncorhynchus mykiss) and the number of fishes captured by local anglers. These observations highlight the need for future studies to estimate the stocking effectiveness of S. marmoratus, a critically endangered species of significant cultural and economic value.

Stocking of native and non-native fish species is a widespread practice commonly used to enhance inland recreational fisheries, appropriate when intense harvesting and the degradation or lack of suitable habitat decrease the abundance of the managed species below carrying capacity. In spite of its popularity, this practice is often poorly informed by scientific information. Salmonids are arguably the most popular and commercially valuable freshwater fishes being managed for recreational fisheries. Stocking of both native and non-native taxa has been practiced for almost two centuries in Europe, dramatically altering the structure and function of riverine and lacustrine ecosystems. In the Verbano-Cusio Ossola Province, northern Italy, within the Lake Maggiore catchment, we measured large numerical mismatches between stocking of cultured native (Salmo marmoratus) plus non-native trout taxa (S. trutta, S. ghigii, Oncorhynchus mykiss) and the number of fishes captured by local anglers. These observations highlight the need for future studies to estimate the stocking effectiveness of S. marmoratus, a critically endangered species of significant cultural and economic value. [ABSTRACT FROM AUTHOR]

The promotion of sustainable tourism and outdoor sports can represent an important way to couple environmental conservation strategies and economic enhancement in marginal and Alpine areas. In this context catch and release fly fishing zones can represent an interesting tool, although no data is available on the effectiveness of these practices on Alpine salmonid population dynamics. Salmonids are the main group of fish in alpine rivers and they are the only actively targeted by anglers. Aim of this work is filling this gap, with a pilot study on two no-kill zones (Po and Pellice rivers, NW Italy). We conducted a temporal and spatial comparison between freefishing and catch and release management river sections, with a detailed analysis on the Po River site. Our results support the hypothesis that catch and release management allows a numerical increase in wild trout populations. In particular, we detected a massive and rapid increase in younger individuals, possibly linked to a stop on the removal of large-sized reproducers. Protecting trout by the implementation of this practice can at the same time allow the increase of sustainable economic development and sport in marginal areas. [ABSTRACT FROM AUTHOR]

The promotion of sustainable tourism and outdoor sports can represent an important way to couple environmental conservation strategies and economic enhancement in marginal and Alpine areas. In this context catch and release fly fishing zones can represent an interesting tool, although no data is available on the effectiveness of these practices on Alpine salmonid population dynamics. Salmonids are the main group of fish in alpine rivers and they are the only actively targeted by anglers. Aim of this work is filling this gap, with a pilot study on two no-kill zones (Po and Pellice rivers, NW Italy). We conducted a temporal and spatial comparison between free-fishing and catch and release management river sections, with a detailed analysis on the Po River site. Our results support the hypothesis that catch and release management allows a numerical increase in wild trout populations. In particular, we detected a massive and rapid increase in younger individuals, possibly linked to a stop on the removal of large-sized reproducers. Protecting trout by the implementation of this practice can at the same time allow the increase of sustainable economic development and sport in marginal areas.

Abstract Background Enormous variability in skin colour and patterning is a characteristic of teleost fish, including Salmonidae fishes, which present themselves as a suitable model for studying mechanisms of pigment patterning. In order to screen for candidate genes potentially involved in the specific skin pigment pattern in marble trout (labyrinthine skin pattern) and brown trout (spotted skin pattern), we conducted comparative transcriptome analysis between differently pigmented dermis sections of the adult skin of the two species. Results Differentially expressed genes (DEGs) possibly associated with skin pigment pattern were identified. The expression profile of 27 DEGs was further tested with quantitative real-time PCR on a larger number of samples. Expression of a subset of ten of these genes was analysed in hybrid (marble x brown) trout individuals and compared with the complexity of their skin pigment pattern. A correlation between the phenotype and the expression profile assessed for hybrid individuals was detected for four (gja5, clcn2, cdkn1a and tjp1) of the ten candidate genes tested. The potential role of these genes in skin pigment pattern maintenance is discussed. Conclusions Our results indicate that the maintenance of different pigment patterns in trout is dependent upon specific communication—involving gap junctions, tight junctions and ion channels—between chromatophores present in differentially pigmented skin regions.

Saprolegnia infections are among the main parasitic diseases affecting farmed salmonids. The distribution and potential transfer of Saprolegnia spp. between farms and the natural environment has been scarcely investigated. Therefore, this work aimed to study the diversity and abundance of oomycete species in salmonid farms, tributary water, and effluent water systems. Four trout farms in Italy and two Atlantic salmon farms in Scotland were considered. In Italian farms, 532 isolates of oomycetes were obtained from fish and water, at upstream, inside, and downstream the farms. In Scottish farms, 201 oomycetes isolates were obtained from water outside the farm and from fish and water inside the farming units. Isolates were identified to the species level through amplification and sequencing of the ITS rDNA region. In Italy, S. parasitica was significantly more present in farmed than in wild fish, while in water it was more frequently isolated from the wild, particularly in effluent systems, not associated with more frequent isolation of S. parasitica in wild fish downstream the farm. In Scotland, S. parasitica was the most prevalent species isolated from fish, while isolates from water were mostly Pythium spp. with few S. parasitica isolates from upstream and downstream the farms.

The hypothesis that the effects of heterozygosity vary with environmental conditions was tested using six populations of marble trout Salmo marmoratus from western Slovenia as a model system. The hypotheses tested were: stronger effects of heterozygosity on survival in populations characterized by low average survival; no effects of heterozygosity on probability of surviving flash floods owing to their largely non‐selective effects across traits; stronger effects of heterozygosity on survival for fish born after floods than fish born before. A significant effect of heterozygosity on survival was found in populations characterized by low average survival. There were no effects of heterozygosity on probability of surviving flash floods, but in one population a positive correlation between heterozygosity and survival for fish born after the extreme events was found, probably because crowding in a small section of the stream caused more intense competition for resources. [ABSTRACT FROM AUTHOR]

In order to determine the function of pigment cell types in different pigment pattern of skin, we have applied the laser capture microdissection method (LCM) for isolation of different pigment cell types from skin sections of marble trout (Salmo marmoratus) and brown trout (S. trutta), characterized with labyrinthine and spotted skin pattern, respectively. RNA was isolated from dissected pigment cell populations and used for expression analysis. Expression profiles of candidate genes, potentially involved in pigmentation, in LCM isolated pigment cell population, were more specific compared to differently pigmented skin regions, containing a heterogeneous mixture of cells, including more types of chromatophores. Moreover, they could provide important information about particular pigment cell type function and its contribution to different pigment pattern. Protocol of combined LCM and genomic approach to study single pigment cell populations can be applied in the future, but needs to be further optimized in order to yield RNA of sufficient quantity and quality required for i.e. transcriptomic analyses. [ABSTRACT FROM AUTHOR]

The reproductive cycle of female wild marble trout Salmo marmoratus was studied in a prealpine watercourse (Northeast Italy). Gonadosomatic index, oocyte growth and plasma levels of 17β-oestradiol and testosterone were bimonthly measured during one year to obtain a detailed dataset and to gain useful tools for the identification of the reproductive stage without impact for the species. Monitored features showed significant variations during the study period: gonadosomatic index and oocyte size increased slightly but significantly during the first part of the cycle (from February to June), while steroid levels remained quite constant; for all parameters, major increases were highlighted between August and the spawning season (which occurred from the middle of November to the middle of December). Ranges and trends observed for gonadosomatic index (from 0.87 ± 0.41% to 10.91 ± 3.37%) and oocyte diameter (from 0.692 ± 0.031 mm to 4.624 ± 0.208 mm) were generally in agreement with literature regarding salmonids, while plasma levels of 17β-oestradiol (from 0.214 ± 0.015 ng ml to 78.090 ± 23.882 ng ml) and testosterone (from 0.327 ± 0.086 ng ml to 71.800 ± 29.406 ng ml) showed wider ranges. A strong non-linear relationship was found between oocyte size and 17β-oestradiol plasma concentration ( r = 0.890) and especially between oocyte size and testosterone plasma concentration ( r = 0.947). This last relationship could likely be used to obtain information relative to the gonadal development, especially in the case of an endangered species like Salmo marmoratus, which needs non-invasive tools for management. [ABSTRACT FROM AUTHOR]

Understanding the causes of within- and among-population differences in vital rates, life histories, and population dynamics is a central topic in ecology. To understand how within- and among-population variation emerges, we need long-term studies that include episodic events and contrasting environmental conditions, data to characterize individual and shared variation, and statistical models that can tease apart shared and individual contribution to the observed variation. We used long-term tag-recapture data to investigate and estimate within- and among-population differences in vital rates, life histories, and population dynamics of marble trout Salmo marmoratus, an endemic freshwater salmonid with a narrow range. Only ten populations of pure marble trout persist in headwaters of Alpine rivers in western Slovenia. Marble trout populations are also threatened by floods and landslides, which have already caused the extinction of two populations in recent years. We estimated and determined causes of variation in growth, survival, and recruitment both within and among populations, and evaluated trade-offs between them. Specifically, we estimated the responses of these traits to variation in water temperature, density, sex, early life conditions, and extreme events. We found that the effects of population density on traits were mostly limited to the early stages of life and that growth trajectories were established early in life. We found no clear effects of water temperature on vital rates. Population density varied over time, with flash floods and debris flows causing massive mortalities (>55% decrease in survival with respect to years with no floods) and threatening population persistence. Apart from flood events, variation in population density within streams was largely determined by variation in recruitment, with survival of older fish being relatively constant over time within populations, but substantially different among populations. Marble trout show a fast to slow continuum of life histories, with slow growth associated with higher survival at the population level, possibly determined by food conditions and age at maturity. Our work provides unprecedented insight into the causes of variation in vital rates, life histories, and population dynamics in an endemic species that is teetering on the edge of extinction. [ABSTRACT FROM AUTHOR]

Marble trout ( Salmo marmoratus ) is an extensively managed salmonid taxon threatened by historical introductions of non-native brown trout ( Salmo trutta ) of different origins into habitats in the northern part of its distribution, namely the Po river system and the Slovenian part of the Adriatic watershed. Hybridization between marble trout and brown trout results in fertile offspring, creating an extensive zone of genetic mixing, with varying amount of introgression. A rehabilitation programme for marble trout has been undertaken in Slovenia for two decades, dependent upon eight non-introgressed marble trout populations inhabiting isolated streams in the upper Soča river valley. MtDNA and a new marker system based on nuclear DNA, designed for identification of trouts and their hybrids in the genus Salmo , were used to determine the extent of introgressive hybridization between marble and brown trout across a large area of the Adriatic watershed in Slovenia. Individual admixture coefficients, the posterior probability of each individual belonging to one or other of the parental species and user-defined categories of admixture were determined along with basic population genetic parameters, revealing variable intensity and patterns of individual introgression throughout the zone of hybridization. In most of the populations analysed, hybridization of native marble trout with the introduced Atlantic lineage of brown trout was observed. In the upper Soča river system a high proportion of genetically non-introgressed (pure) marble trout was found, enabling selection of these fish for supplementary material in the reintroduction programme. Conclusions regarding management of marble trout in Slovenia can be inferred from the patterns observed in the present study: (1) transfer of pure populations to fishless or fished-out streams is recommended, (2) to sustain genetic diversity of the species, non-introgressed individuals sorted out from the zone of hybridization should be used to produce juveniles for supplementation stocking in fish farms, and (3) extension of the marble trout repopulation programme to highly introgressed populations in small unmanaged streams, which represent a potential source for downstream genetic ‘contamination’. [ABSTRACT FROM AUTHOR]

The marble trout Salmo marmoratus (Cuvier, 1829) is an endemic species of great conservation concern, given its geographical restricted distribution and the high risk of hybridization with brown trout Salmo trutta L. Viral hemorrhagic septicemia (VHS) and infectious hematopoietic necrosis (IHN) are predominantly diseases of freshwater salmonids, which may trigger off severe disease outbreaks causing high mortality. Marble trout has not been inserted in the vector species list for those pathogens, but at present, little information on its susceptibility and on the possible carrier status is available. The aim of the present study was to assess the ability of European strains of VHSV and IHNV to cause disease and associated mortality in experimentally infected marble trout, and to determine whether a carrier status can result after experimental infection. Three genetically different strains of marble trout were challenged with VHSV and IHNV by immersion. Rainbow trouts were used as positive controls. Fish were checked twice a day, and dead fish were removed and sampled for subsequent virological analyses. With respect to rainbow trout, results demonstrated a very low susceptibility of marble trout to both VHSV and IHNV under simulated natural conditions. The presence of a certain number of chronically infected marble trout confirms that some individuals can survive the infection and eventually act as carrier of the disease, suggesting that marble trout should be definitely added to the list of susceptible species according to the European legislation. [ABSTRACT FROM AUTHOR]

Abstract: Salmonids are known for their variety in skin color and color patterning, which depends on reaction–diffusion mechanism or/and cell–cell interaction. One of the visually most prominent characteristics found in the genus Salmo is the marble color pattern, distinctive for marble trout (Salmo marmoratus). In order to identify and characterize genes potentially involved in skin coloration and color pattern formation in marble trout, a salmonid 32K cDNA microarray was applied to compare skin transcriptome profiles from marble trout, brown trout (Salmo trutta), and marble trout×brown trout hybrids exhibiting the marble color pattern. Microarray results were validated by qRT-PCR and revealed four differentially expressed informative genes (hdac1, vps18, dct and scg2a) involved in animal pigmentation. hdac1 and dct were associated with the wnt signaling pathway, vps18 was involved in melanosome biogenesis and scg2a was observed to act as physiological factor in skin pigmentation and thus could be regarded as a helper protein in prohormone Pomc packaging. We propose that the formation of the marble color pattern is at least partially based upon a reaction–diffusion mechanism and depends upon wnt signaling pathway. [Copyright &y& Elsevier]

We present a set of nuclear DNA SNPs enabling discrimination between Salmo marmoratus, S. obtusirostris and different evolutionary lineages of S. trutta. The loci containing SNPs were either isolated from marble trout nuclear DNA or inferred from literature. By comparing the joint sequences (ca. 5,830 bp) among 48 tested individuals, 127 substitutions and 8 indels were found. Forty-one of them, distributed across 16 loci, turned out as species or lineages specific. This set of SNP markers may be applied as molecular tool for genetic management purposes as well as in resolving phylogenetic and taxonomic uncertainties of different trout stocks. [ABSTRACT FROM AUTHOR]

Abstract: Single nucleotide polymorphisms (SNPs) have become popular markers for population genetics and conservation applications. The use of SNPs is increasing in studies on salmonid fishes, particularly in Salmo salar and several Oncorhynchus species, but they are used only occasionally in S. trutta and not at all in marble trout (Salmo marmoratus). Marble trout is native to the northern Adriatic basin, but the continued existence of native parental populations has been compromised by the introduction of brown trout (S. trutta) and subsequent introgressive hybridization. On the basis of more than a decade of experience of the marble trout action plan undertaken in Slovenia, it has become clear that for effective population restoration it is of paramount importance to be able to discriminate reliably between members of either species and marble trout-brown trout hybrids. By undertaking sequence analysis of five nuclear genes in the marble and brown trout genomes, we found several species-specific SNPs that appear to be a useful tool for assessing the genetic purity of populations of marble trout. No linkage disequilibrium was detected in any allelic combinations among the five genes examined. Genotyping of species-specific SNPs in these five genes may provide a valuable diagnostic test for molecular assessment of marble trout purity and thus be suitable for management of marble trout populations, especially when combined with initial phenotype-based sorting. [Copyright &y& Elsevier]

The conditions experienced early in life can strongly influence life-history trajectories in a variety of animal species. Here, we use data from four isolated populations of the endangered stream-dwelling salmonid marble trout ( Salmo marmoratus Cuvier 1817) living in the Soca and Idrijca river basins (Slovenia) to explore the influence of the total density experienced during and after the first year of life by marble trout year-classes on body length of marble trout through the lifetime. Analyses were performed by pooling together the stream-specific datasets to cover a wider range of densities. Mean body length of marble trout year-classes through the lifetime (from age 1+ to 5+) was negatively related to total density of marble trout during the first year of life. The relationship between density during the first growth period and body length through the lifetime was well described by negative power curves. Total population density after the first year of life was not correlated with body length, thus suggesting that body growth trajectories are heavily determined early in life. Given size-dependent sexual maturity and egg production in marble trout, the relationship between density early in life and lifetime individual growth may have strong implications in terms of population dynamics and regulation of population size. [ABSTRACT FROM AUTHOR]

To determine the organization of transferrin ( TF) locus in the Salmo trutta genome, partial DNA and cDNA sequencing, fluorescent in situ hybridization (FISH) and Salmo salar BAC analysis were performed. TF expression levels and copy number prediction were assessed using real-time PCR. In addition to two previously reported DNA TF variant sequences of S. trutta and Salmo marmoratus ( TF1), two novel variant sequences ( TF2) were revealed in both species. Variant-specific sequence tags, characterizing two variants for each TF type ( TF1 and TF2), were identified in genomic clones from each of the F1 hybrids between S. trutta and S. marmoratus. These clearly documented double heterozygote status at the TF loci. The real-time PCR data showed that each of the two TF types ( TF1 and TF2) existed in one copy only and that the transcription of TF2 was considerably lower compared with TF1. Using FISH, hybridization signals were observed on two medium-sized acrocentric chromosomes of S. trutta karyotype. A TF type-specific PCR followed by a restriction analysis revealed the presence of two TF loci in the majority of analysed BAC clones. It was concluded that the TF gene is duplicated in the genome of S. trutta, and that the two TF loci are located adjacent to one another on the same chromosome. The differing transcription levels of TF1 and TF2 appear to depend on the corresponding promoter activity, which at least for TF2 seems to vary between different Salmo congeners. [ABSTRACT FROM AUTHOR]

The role of endogenous and exogenous factors in regulating population dynamics of freshwater salmonids is still a matter of debate. The aim of the present work was to assess the relative importance of density-dependent and -independent factors in determining the survival of marble trout ( Salmo marmoratus) yearlings in two populations living in Slovenian streams (Zakojska and Gorska). The investigation was performed by combining a classical life table analysis with Monte Carlo simulation. Size-dependent fecundity was estimated by stripping wild adults in the fish farm. A significant positive relationship was found between length of marble trout females and the number of eggs produced. Egg density was the major determinant of survival from eggs to age 1+ ( σ0) in both streams. Residuals of the relationship between σ0 and egg density were positively correlated with rainfall only in Zakojska, probably because, within a certain range, more intense rainfalls increases stream flow and, consequently, suitable habitat for trout. Our study shows how density-dependent and environmental factors can interact to determine the survival of marble trout during the first year of life. [ABSTRACT FROM AUTHOR]

Populations of the marble trout (Salmo marmoratus) are critically declining due to introgression by brown trout (S. Trutta) strains. Hybrids between the two forms are fertile and presently predominant in most rivers of the species’ range. The involved hybridization mechanisms have been studied through two large scale in-stream experiments (Driselpoh and Stopnikarca) as 50% of each species have been stocked at the age of one year in fishless streams, each fish being individually marked. Diagnostic molecular markers were applied to test a partial reproductive isolation between the two species. Emphasis was put on survival and growth patterns of stocked fish (parental generation) as well as on fish hatched within the stream (F1 generation). No evidence of a partial reproductive barrier between the two species was observed. Survival of the parental generation depended on the year in both streams, as well as on the species in Stopnikarca, but was identical for both species in Driselpoh. In both streams survival of instream hatched individuals measured from 0+ to 2+ was lower for brown trout. In Driselpoh, F1 0+, 1+ and 2+ hybrids were larger than pure individuals. Larger hybrids were only observed in Stopnikarca when analyses focused on individuals in inter-specific competition suggesting that heterosis and stress effects may explain the observed size differences. Our results point out important ecological differences between marble and brown trout and have shown that hybridization can easily take place. The findings indicate that high F1 hybrid presence, survival and heterosis effects may impede marble trout rehabilitation in the area. [Copyright &y& Elsevier]

Although the main features of salmonid life cycles are currently well known, marble trout ( Salmo marmoratus) populations have been scarcely studied due to the present limited geographical distribution of the species. In this work we tested the hypothesis of density-dependent individual growth of marble trout with data gathered from multi-year on-going monitoring started in 1996 in three streams (Zakojska, Gorska and Gatsnik) in the Soca and Idrijca river basins (Slovenia). As observed for other salmonid species, marble trout exhibits high plasticity in body size and growth in response to environmental conditions. Age-specific mean lengths are significantly different among the three streams. Despite this variability, the statistical analyses outlined some clear patterns. Mean lengths of marble trout cohorts aged ≥1+ are correlated with total population density in the first year of life of the cohorts; the age-specific relationship between mean length and total density is well explained by negative power curves. Length of marble trout at age 1+ is significantly correlated with length at subsequent years up to age 4+. ANCOVAs performed on the stream-specific datasets showed a significant effect of total density of marble trout on annual age-specific individual growth rates. Von Bertalanffy’s body growth models were calibrated on length–age data of Zakojska and Gorska marble trout. Our analysis shows the existence of density-dependent effects on growth of marble trout that might be crucial in regulating population dynamics. [ABSTRACT FROM AUTHOR]

Abstract: In Slovenia, the Adriatic basin inhabited by native marble trout (S. marmoratus), and the Danubian basin inhabited by native Danubian lineage of brown trout (S. trutta) have been intensively affected by stocking with non-native trout strains. In order to assess spread of non-native strains and their introgression with native trout, a population study based on five microsatellite loci was applied across ten marble and ten brown trout populations, ranging from allegedly non-introgressed to heavily managed. On the basis of correspondence analysis, which revealed three clear groupings consisting of the Danubian and Atlantic lineages of brown trout and the marble trout, the alleles, characteristic of each grouping were identified and used for estimating genetic composition of each population according to the three possible origins. Among the wild populations, five marble and one brown trout populations were found to be pure; all the others were introgressed with exotic alleles (Atlantic and marmoratus alleles in the Danubian basin and Atlantic and Danubian in the Adriatic basin) that markedly dominate in intensively managed populations. As revealed by non-significant F IS values, panmixia between native and introduced fishes has for the most part already been reached. Our research showed that it is not only marble trout whose identity is endangered in Slovenia but also the existence of autochthonous Danubian brown trout is critically compromised, which is new information to be taken into account for local trout conservation. [Copyright &y& Elsevier]

Abstract Populations of the marble trout (Salmo marmoratus ) have declined critically due to introgression by brown trout (Salmo trutta ) strains. In order to define strategies for long-term conservation, we examined the genetic structure of the 8 known pure populations using 15 microsatellite loci. The analyses reveal extraordinarily strong genetic differentiation among populations separated by < 15 km, and extremely low levels of intrapopulation genetic variability. As natural recolonization seems highly unlikely, appropriate management and conservation strategies should comprise the reintroduction of pure populations from mixed stocks (translocation) to avoid further loss of genetic diversity. [ABSTRACT FROM AUTHOR]

Saprolegnia infections are among the main parasitic diseases affecting farmed salmonids. The distribution and potential transfer of Saprolegnia spp. between farms and the natural environment has been scarcely investigated. Therefore, this work aimed to study the diversity and abundance of oomycete species in salmonid farms, tributary water, and effluent water systems. Four trout farms in Italy and two Atlantic salmon farms in Scotland were considered. In Italian farms, 532 isolates of oomycetes were obtained from fish and water, at upstream, inside, and downstream the farms. In Scottish farms, 201 oomycetes isolates were obtained from water outside the farm and from fish and water inside the farming units. Isolates were identified to the species level through amplification and sequencing of the ITS rDNA region. In Italy, S. parasitica was significantly more present in farmed than in wild fish, while in water it was more frequently isolated from the wild, particularly in effluent systems, not associated with more frequent isolation of S. parasitica in wild fish downstream the farm. In Scotland, S. parasitica was the most prevalent species isolated from fish, while isolates from water were mostly Pythium spp. with few S. parasitica isolates from upstream and downstream the farms.