Detection of hybridization and introgression in wild populations that have been supplemented by hatchery fish is necessary during development of conservation and management strategies. Initially, allozyme data and more recently highly polymorphic microsatellite markers have been used to obtain this information. We used both markers to assess the effectiveness of four assignment methods (STRUCTURE, NEWHYBRIDS, BAPS and GeneClass) to detect hatchery introgression in wild stocked populations. Simulations of hybrid genotypes from real parental data revealed that the number and type of markers used with STRUCTURE, NEWHYBRIDS and BAPS can identify as admixed most first and second generation hybrids as well as first generation backcrosses. In wild populations, introgression rates estimated from different markers and methods were correlated. However, slight disagreements were observed at both population and individual levels. Overall, the fully Bayesian (STRUCTURE, NEWHYBRIDS and BAPS) performed better than partially Bayesian (GeneClass) assignment tests. In wild collections, BAPS analyses were limited because of the lack of a native baseline. In all cases, the efficiency of methods was reduced as introgression increased.

Evidence of different degrees of genetic interactions of exogenous hatchery fish with native populations was electrophoretically obtained from collections of brown trout from four heavily fished areas and eight adjacent protected and unfished areas in northeastern Spain. Although some degree of hybridization or introgression was detected in each collection based on fixed differences between hatchery and native fish at the LDH-C * locus and less distinct differences were found at three other loci (sMDH-A2 * , IDHP-2 * , IDHP-3 * ), fished populations had significantly lower levels of hatchery genes than protected ones. The greater persistence of native fish in areas that were fished and stocked appeared to reflect a greater susceptibility of hatchery-reared fish to angling. Although it remains uncertain whether this difference is innate or a reflection of hatchery rearing, stocking was ineffective in enhancing the recipient population in the exploited river. These data suggest that different strategies may be needed for protection of native populations in fished and protected areas and that more intense measures may be required in the latter instance. Proporciones de Trucha Común Nativa e Introducida en Ríos Españoles Adyacentes Sometidos o No a Explotación PesqueraResumen: A partir del análisis electroforético de muestras de trucha común de cuatro áreas sometidas a un régimen intensivo de pesca y ocho áreas adyacentes protegidas y sin actividad pesquera, se han obtenido evidencias de diferentes grados de interacción genética entre los peces producidos en los centros piscícolas y las poblaciones nativas del noreste de España. En cada muestra se constató algún grado de hibridación o introgresión en base a las diferencias conocidas entre los peces de piscifactoría y los nativos, fijadas en el locus LDH-C * y menores en otros tres loci (sMDH-A2 * , IDHP-2 * , IDHP-3 * ). Sin embargo, los niveles de genes típicos de los centros piscícolas fueron significativamente inferiores en las poblaciones sometidas a explotación pesquera respecto a aquellos observados en las áreas protegidas. Esta mayor persistencia de los genes nativos en las áreas sometidas a explotación, y por ello más intensamente repobladas, parece reflejar una mayor susceptibilidad de los peces criados en los centros piscícolas a ser capturados. A pesar de que es aún incierto si esta diferencia es innata o un reflejo del manejo en el centro piscícola, sí resulta claro que la repoblación se mostró ineficaz para aumentar las poblaciones en los ríos explotados. Los resultados sugieren la necesidad de utilizar diferentes estrategias para la protección de las poblaciones nativas en los zonas pesqueras y en las protegidas, y que en estas últimas serían necesarias medidas más intensas.

Genetic changes in the population structure of brown trout Salmo trutta in the eastern Pyrenees were monitored during the 1990s. Stocking with cultivated exogenous fish has resulted in introgressed populations where those with higher introgression coefficients also have the highest values of heterozygosity. Nevertheless, this increase of local diversity reflects a decrease of genetic differentiation between populations and loss of native alleles. These changes obscure native gene pools and modify evolutionary distinctions among native brown trout populations. # 2004 The Fisheries Society of the British Isles

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