Search Results (8)

Brown trout (Salmo trutta) populations at the southern edge of the species’ distribution are increasingly exposed to interacting climatic, biotic, genetic, and anthropogenic pressures. This study provides an integrative assessment of the drivers of variation in brown trout density, biomass, and size structure across six Mediterranean river basins in Catalonia (NE Iberian Peninsula), based on long-term standardized electrofishing surveys (2016–2025; 88 sites). Generalized linear mixed models revealed that mean summer temperature, genetic introgression from hatchery-derived Atlantic lineages, and the density of introduced fish species were the most consistent negative predictors of total density, biomass, and juvenile abundance (<120 mm FL). Hydrological and nutrient variables showed comparatively weak or non-significant effects relative to thermal and biotic predictors. Fishing regulations significantly influenced the biomass and density of larger trout (>220 mm FL), with lower values in harvest-allowed sections, whereas total density was less responsive to regulation. These findings indicate that Mediterranean brown trout populations are primarily constrained by thermal conditions, genetic integrity, and biological invasions, with implications for conservation and fishery management under ongoing climate warming. Full article

The aim of this research was to characterize the genetic structure of brown trout species complex populations in nine river basins in the Aosta Valley and neighbouring regions in northern Italy. We used a combined analysis of nuclear lactate dehydrogenase (LDH-C1*), mitochondrial DNA-CR (control region) (mtDNA-CR) sequences and ddRAD-seq-generated single-nucleotide polymorphism. In this way, we estimated the degree of hybridization of wild populations with the Atlantic-derived hatchery lineage. The results of the genetic analyses showed a complex genetic structure with different levels of introgression at the respective sampling sites. The mitochondrial lineages (Atlantic (AT), Mediterranean (ME), Adriatic (AD), and Marmoratus (MA)) were present with varying percentages across the sampling sites. Data analysis using the Admixture v.1.3.0 software allowed the identification of four distinctive cluster units in the Aosta Valley. For the Vertosan River, we identified a distinct native population and a level of hybridization close to zero. In terms of conservation, this population with a distinct native lineage represents a high priority for protection and serves as a reservoir for the entire western north Italian alpine zone. Some interventions to support conservation actions within the study area can be envisaged. Full article

Brown trout populations living in the limit of the distribution of the species face challenging environmental conditions. In islands, this vulnerable situation is exacerbated by geographical isolation. Sicilian trout persist only in the south-eastern part of the island and, based on their morphological characteristics, they have been recognized as a distinct species named Salmo cettii. We present the most extensive genetic study on Sicilian trout, based on 213 individuals from nine different streams in four basins. Sequencing of the mitochondrial control region and genotyping of the LDH-C* locus and eight microsatellite markers were carried out to evaluate hatchery introgression from past stocking practices in natural populations and to estimate the gene diversity distribution in populations. Results showed that only trout from the Tellesimo River remained free of hatchery introgression. Gene diversity was low in this native population, but increased in the rest of the samples, probably because of the introduction of hatchery genes. Despite the high level of introgression, the distribution of gene diversity depicted a clear natural pattern of population structure related to the hydrographic hierarchy. Because they represent long evolutionary histories, Sicilian trout populations should be considered of high conservation priority and managed according to at least four different genetic units. Full article

Brown trout (Salmo trutta) populations living in Mediterranean peninsulas and the surrounding islands belong to old evolutionary lineages that persisted during Quaternary glaciations. Many of these populations inhabit marginal areas along the south limit of the distribution of the species, where they face hard environmental conditions (drought, high temperatures and sudden shifts in water flow) that can get worse by anthropogenic activities and climate change. In islands, this vulnerable situation is exacerbated by geographical isolation. Sicilian trout remain only in the south-eastern part of the island and, based on their morphological characteristics, they have been recognized as a distinct species named Salmo cetti. Despite their genetic singularity among other Italian trout (a unique Italian native trout of Atlantic origin), the mitochondrial DNA haplotypes that were found in Sicily are clearly related with the brown trout Atlantic lineages from North Africa and the Iberian Peninsula. In the present study, brown trout in four rivers from north-eastern Sicily were genetically analysed. Based on the genotyping of mitochondrial (control region) and nuclear (LDHC, GP1, GP14, GP31, GP34, GP37, GP38, GP42, SS2 and SL) genes, this study aimed to: evaluate the impact of past stocking practices in natural populations; estimate mitochondrial and nuclear gene diversity; and reconstruct the phylogenetic relationships of Sicilian trout. The initial results showed that only trout from the Tellessimo River remain free from hatchery introgression. Gene diversity was low in most populations, and both mitochondrial and nuclear phylogenetic reconstruction related Sicilian trout with old Atlantic lineages. Full article

Brown trout is considered a complex of incipient species, including several phylogenetic lineages, whose natural distribution in the Mediterranean area has been altered, since the beginning of the 1900s, by massive introductions of domestic strains of Atlantic origin to support fisheries. Introduced trout naturalize in new suitable environments and extensively hybridize with native populations. Here, we characterized putatively neutral and adaptive genetic variability and population structure of Mediterranean brown trout from six river catchments in central peninsular Italy, as revealed by both mitochondrial (Control Region) and nuclear (microsatellites, LDH-C1, major histocompatibility complex) markers. We quantified the admixture of wild populations with hatchery strains and evaluated the effects of domestic trout introductions on shaping population genetics. Our analyses indicated: (1) a composite picture of genetic variability in the area, with the presence of all native Mediterranean trout mitochondrial lineages (“Adriatic”, “Mediterranean”, “marmoratus”), various frequencies of allochthonous genotypes and different rates of introgression among sampling sites; (2) asymmetric mito-nuclear introgression; (3) increasing nuclear marker diversity with increasing levels of admixture across populations; (4) strong population structure coupled with relatively low effective population size. Data allowed the identification of five management units and we propose specific actions to support ongoing and future conservation strategies within the examined area. Full article

Brown trout (Salmo trutta L.) populations have been restocked during recent decades to satisfy angling demand and counterbalance the decline of wild populations. Millions of fertile brown trout individuals were released into Mediterranean and Atlantic rivers from hatcheries with homogeneous central European stocks. Consequently, many native gene pools have become endangered by introgressive hybridization with those hatchery stocks. Different genetic tools have been used to identify and evaluate the degree of introgression starting from pure native and restocking reference populations (e.g., LDH-C* locus, microsatellites). However, due to the high genetic structuring of brown trout, the definition of the "native pool" is hard to achieve. Additionally, although the LDH-C* locus is useful for determining the introgression degree at the population level, its consistency at individual level is far from being accurate, especially after several generations were since releases. Accordingly, the development of a more powerful and cost-effective tool is essential for an appropriate monitoring to recover brown-trout-native gene pools. Here, we used the 2b restriction site-associated DNA sequencing (2b-RADseq) and Stacks 2 with a reference genome to identify single-nucleotide polymorphisms (SNPs) diagnostic for hatchery-native fish discrimination in the Atlantic and Mediterranean drainages of the Iberian Peninsula. A final set of 20 SNPs was validated in a MassARRAY® System genotyping by contrasting data with the whole SNP dataset using samples with different degree of introgression from those previously recorded. Heterogeneous introgression impact was confirmed among and within river basins, and was the highest in the Mediterranean Slope. The SNP tool reported here should be assessed in a broader sample scenario in Southern Europe considering its potential for monitoring recovery plans. Full article

The impacts and interactions between hatchery-bred fish and wild fish populations has been a topic of active investigation in recent decades. In some instances, the benefits of stocking can be overshadowed by negative effects such as genetic introgression with natural populations, loss of genetic diversity, and dilution of local adaptations. Methods that facilitate the identification of stocked fish enable us to estimate not only the effectiveness of stocking but also the level of natural reproduction and the degree of hybridization. The longest Baltic river, the Vistula, also has the second highest discharge. Historically, it hosted numerous populations of the anadromous form of brown trout (sea trout); however, dam construction has since interfered with and reduced spawning migration to a rate that is much lower than before. Reduced spawning has resulted in a population collapse and a negative flow-on effect on commercial catches. In response, Poland (along with many other Baltic countries) initiated an intensive stocking program which continues today and which sees the average annual release of 700,000 smolts. As a consequence, today’s main-river and inshore catches come from stock-enhanced populations. High-throughput single-nucleotide polymorphism (SNP) genotyping was performed on samples of sea trout from southern Baltic populations; results suggest that a significant portion of the sea trout catches in the Vistula mouth region have direct hatchery origin and indicate the presence of Pomeranian specimens. SNP loci identified as outliers indicate a potential selection pressure that may be related with effects of hatchery breeding and mixing with natural populations. The brown trout SNP array applied in this study showed high effectiveness not only for population differentiation, but more importantly, it emerged as a sensitive tool to provide evidence of detection selection. Full article

The biological and anthropogenic (management) factors that may contribute to the expansion of non-native lineages in managed fish have been studied in this work taking brown trout (Salmo trutta) as a model species. The changes of users’ opinion about stocking was studied employing social science methodology (surveys). The evolution of hatchery stocks together with the outcome of stocking were analysed with two genetic tools: the LDH-C1* locus (marker of non-native stocks) and six microsatellite loci (for assignment of wild trout to the natural population or putative hatchery stocks). Consulted stakeholders were convinced of the correctness of releasing only native stocks, although in practice the hatcheries managed by them contained important proportions of non-native gene carriers. Our results suggest that allochthonous individuals perform better and grow faster in hatchery conditions than the native ones. We also find a dilution of the impact of this kind of suplementation in wild conditions. The use of only native individuals as hatchery breeders tested for the presence of non-native alleles previously to the artificial crosses must be a priority. Surveys can help steer policy making toward decisions that will be followed by the public, but they should not be used to justify science. Full article