The Atlantic bluefin tuna (Thunnus thynnus) is an extraordinary fish that has amazed humanity since ancient times. However, the continuous overexploitation of this fishery, particularly in the Mediterranean Sea, could result in a total collapse of this resource. Currently, this species is managed as two stocks: Western Atlantic and Mediterranean-Eastern Atlantic, with a recognized genetic differentiation between them. On the other hand, the population structure within the Mediterranean Sea is still unclear. The biological data supports the idea of two separate populations in the eastern and western Mediterranean basins. However, nuclear microsatellite and mitochondrial DNA (mtDNA) analyses of two samples representative of these two basins result in a lack of heterogeneity. A comparison of these results with previously published studies reveals some discrepancies. We have compared 59 genetic differentiation tests that include samples within the Mediterranean. Of these, about 60% gave significant differentiation while the remaining 40% were non-significant. But, when only nuclear-based loci were considered, genetic differentiation was detected in up to 73% of the cases with an average significant F ST of only 0.018, whereas the average significant F ST of the mtDNA-based studies was significantly higher (0.029). However, in some cases, it is difficult to reconcile the biology of the species with the results suggesting genetic differentiation. In conclusion, although it is not yet possible to reach a definitive conclusion about the population structure, but considering all biological and genetic data, we suggest an independent management approach for each basin to avoid the impact of a type II error that could lead to the possible loss of the regional subpopulations.

Since the end of the 20th century, some headwaters of rivers in the eastern Pyrenees have been designated as genetic refuges to protect remaining native brown trout (Salmo trutta) diversity. The declaration was based on limited or no evidence of genetic impact from released non-native Atlantic hatchery fish. Hatchery releases were completely banned into the genetic refuges, but pre-existing fishing activities were maintained. Specific locations in each refuge have been monitored every 2-3 trout generations to update genetic information to accurately assess the contribution of these reservoirs to the preservation of native brown trout gene pools. This work updates genetic information to year 2014 in three of these locations (in Ter, Freser and Flamisell rivers). Previous studies identified hatchery introgressed populations within refuges and suggested discrepancies between the underlying intention of the genetic refuges and the gene pools detected. Therefore, we also examined genetic divergences among locations inside refuge river segments. Combined information at five microsatellite and the lactate dehydrogenase C (LDH-C*) loci showed reduced but significant temporal native allele frequency fluctuations in some of the above specific locations that did not modify overall levels of local diversity and river divergences. Bayesian clustering analyses confirmed the presence of differentiated native units within each genetic refuge. Some locations of the Freser River within the genetic refuge area showed high hatchery impact of nonnative fish (over 20%). We discuss additional local actions (releases of native fish, selective removals and fishery reinforcement with sterile individuals) to improve the conservation objective of genetic refuges.

The presence of introgressive hybridization in the wild, especially that resulting from human interference, can have negative impacts on biodiversity. Genetic tools provide essential information for species and hybrid identification, facilitating the conservation of natural resources. Here, we tested a set of markers to precisely elucidate introgressive hybridization between Pseudoplatystoma corruscans and Pseudoplatystoma reticulatum, two South American catfishes. New microsatellites showed high interspecific genetic divergence, and simulated data demonstrated the high power of STRUCTURE and NEWHYBRIDS for hybrid identification and classification, especially when all 11 nuclear markers were used. The investigation of real populations suggested that natural hybridization is rare. Otherwise, different hybridization scenarios were observed in two wild populations: one involving advanced backcrosses and the other involving high admixture. Our data represent the first detailed evaluation of genetic introgression between these species in Parana and Paraguay Basins and suggest that genetic contamination is occurring through F1 hybrids from aquaculture facilities. The results also provide a useful set of markers for monitoring escapees to aid in the conservation of the wild population and sustainable aquaculture. Additionally, species genotypic data are freely available to be used in the future as ''parental species reference'' in Bayesian methods assignments.

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The spatial distribution of closely related individuals can be inferred from genetic data and provides valuable information about dispersal patterns and gene flow contributing to the population genetic structure of organisms. Here, we analyzed family distribution of brown trout Salmo trutta in the uppermost reaches of the Mediterranean Son River basin by genotyping nine microsatellite loci. Population structure suggested a model of two units heavily affected by hatchery introgression superimposed on a hydrographic pattern. We observed kin-biased spatial distribution, implying limited dispersal of related fish, probably favored by severe climate conditions in this system. However, downstream dispersal was observed for some large adult fish. Variance in family size defined small estimates of effective population sizes (Ne) in all streams. Comparison of estimates by several methods indicated that factors other than variation of family size are responsible for such low values. The population structure in the Son River basin is, then, represented by interconnected locations in which dispersion of the fish is probably conditioned by oscillations in water discharge, particularly the extremely low water flow during summer. Stream discharge variation could also be related to the flexibility of the mating system. Such characteristics make populations especially vulnerable to stocking and to any other potential effects.