Efecte de les repoblacions sobre les poblacions de truita de la conca del riu Ter

Abstract

Releases of foreign specimens to wild populations is deteriorating genetic integrity of native populations. Identifying whether the origin of the individuals in wild populations is native or not, is crucial to improve protection and conservation management strategies. Brown trout (Salmo trutta) is suffering such releases. In this species, it is possible to know the genetic makeup of each individual and estimate the population levels of introgression caused by genetically differentiated foreign fish. Genetic studies allow to identify and to distinguish the different genotypes, the population level of genetic diversity (alleles range and present genotypes) and patterns of distribution of genetic variability among populations. In a large population, where there is random mating and no mutation, neither migration nor selection, allele and genotype frequencies become stable in one generation. This is called the Hardy-Weinberg equilibrium law, which is a crucial genetic model for the conservation of the population diversity and the genetic study of natural populations. By using genotype information at 5 microsatellite loci and at the LDH-C1 locus, the present study characterized the brown trout (Salmo trutta) population in the upper reaches of the Ter River. The level of genetic introgression (caused by releases of trout cultivated at the Bagà hatchery) was estimated. The Bagà hatchery stock is used to reinforce wild populations in Catalonian Rivers. Furthermore, we also studied the genetic differentiation of the Ter River brown trout in relation to another population from the Freser River headwaters (the Núria stream), the most important Ter River tributary. Results showed that there is only a single brown trout population at the studied Ter River location. This population did not present any genetic introgression. In relation to the Freser and Bagà fish, it was concluded that there is a significant genetic differentiation between the different studied trout collections. Thus, results showed three clearly differentiated populations: Ter, Núria and Bagà. In spite of this, some individuals from Freser River have genetic traces from the other populations. This finding indicated that the analysed trout population in the Freser River was affected by released Bagà trout, and, on the other hand, that there is natural contact between Ter and Freser brown trout populations