Abstract– To assess the levels of gene introgression from cultured to wild brown trout populations, four officially stocked locations and four nonstocked locations were sampled for one to three consecutive years and compared to the hatchery strain used for stocking. Allozyme analysis for 25 loci included those previously described as providing allelic markers distinguishing hatchery stocks and native populations. Different levels of hybridization and introgression with hatchery índividuals were detected in stocked drainages as well as in protected locations. These findings indicate that new policies for stocking and monitoring hatchery fish are needed if gene pools of wild Spanish brown trout populations are to be preserved.

: Genetic variability within and among four Spanish natural populations of Salmo trutta L. was evaluated on the basis of 25 enzyme loci, 3 microsatellite loci, and 9 randomly amplified polymorphic DNAs (RAPDs). A total of 21 allelic markers were found, 12 of which were reported by microsatellites, whereas enzyme and RAPD accounted only for 6 and 3, respectively. Genetic variation within samples was significantly higher for microsatellites and RAPD than for enzyme loci. Although all methods reported a high degree of allelic heterogeneity among samples, also revealing a high degree of gene diversity, genetic relationships depicted by UPGMA dendrograms closely agreed for all kinds of data. Microsatellite loci appeared to be the most feasible technique when searching for specific alleles for a population or an area, owing to the higher number of allelic variants found.

SUMMARY 1. The large microgeographical differentiation revealed by allozyme studies in brown trout (Salmo trutta) populations is one of the most striking features of this species. Additionally, allozymes showed great genetic differences between Atlantic and Mediterranean populations on a macrogeographical scale. 2. This study was carried out in order to assess whether the great differences observed between Atlantic and Mediterranean populations persisted where the two are geographically close (the ‘microgeographical scale’). Sixteen populations of brown trout, S. trutta, were screened for genetic variation at 25 allozyme loci. The sampling sites, which occupied a relatively small geographical area, were distributed across Cantabrian (Atlantic) and Mediterranean drainages in Northern Spain. 3. The neighbour‐joining tree, inferred from Nei's genetic distance, showed that brown trout populations clustered into two different groups. These groups corresponded to the Cantabrian and the Mediterranean groups of populations, although no clear geographical pattern emerged within each of the groups. This geographical pattern is basically caused by significant differences in the frequency distribution of the CK‐A1* locus, with a higher frequency of *115 in Cantabrian samples (0.586 ± 0.091) while allele *100 was more frequent in Mediterranean samples (0.931 ± 0.038). In addition, this study revealed alleles exclusive to the Mediterranean and Cantabrian populations, agreeing with previous findings. 4. Genetic differentiation between Cantabrian and Mediterranean regions (14.19%) was similar to that estimated in Spain at a larger scale (13%), showing that most of the differences between the regions can be observed even in a small geographical area.

The anglerfish species Lophius piscatorius and Lophius budegassa are among the most valuable fishes sought after by bottom fisheries in western and southern European waters. It is currently believed that there are two stocks for each of the two species, north and south, which determine their assessment and management. A genetic analysis using eight polymorphic microsatellite markers was carried out on samples collected from western European waters and the south-western Mediterranean Sea. The results strongly suggest that the boundary between northern and southern stocks is not genetically supported. However, populations were not genetically homogeneous. Besides a pattern of genetic differentiation between Mediterranean and the rest of the samples, the L. budegassa samples taken from the Spain Atlantic zone and from the Portugal Atlantic zone were genetically distinct, whereas the samples taken in the French Atlantic zone for the L. piscatorius species seem to be different from the rest of the samples under study. This can be indicative of a more subtle genetic structure that deserves more study for guaranteeing adequate fishery management of these species.