DIFFERENTIAL STOCKING INCIDENCE IN BROWN TROUT (SALMO-TRUTTA) POPULATIONS FROM NORTHWESTERN SPAIN

An allozyme electrophoretic analysis was carried out to evaluate the impact of stocking, and to assess the amount and distribution of genetic variation among brown trout populations in Northwestern Spain. For this purpose, populations from both non-flowing (lagoon and reservoir) and flowing (rivers) waters, stocked and unstocked, were studied in comparison with the hatchery populations used for stocking in this region. Genetic variation was found at 11 of 35 loci analyzed. Detection of suitable genetic markers, especially the LDH-5* diagnostic locus, permitted us to monitor the incidence of stocking practices. The low viability of stocked individuals within river populations was remarkable, in spite of the long period and great intensity of repopulation. Only four individuals out of 197 analyzed were of hatchery origin. The absence of introgression detected in river populations was accounted for by the extreme Wahlund effect observed at the diagnostic locus LDH-5*. On the contrary, in non-flowing waters a large introgression was revealed. The longer the period of stocking, the larger the introgression detected. Gene diversity analysis demonstrated an important genetic differentiation among natural populations (G(ST) = 0.273), mostly within drainages (86%), which shows an important microgeographical differentiation component. In sharp contrast the stocking hatcheries exhibited genetic homogeneity (G(ST) = 0.014). Genetic distance (D = 0.046) between indigenous and stocking groups apparently shows the presence of two divergent evolutionary lineages of brown trout. The levels of genetic variation were far greater in populations from non-flowing waters and stocking hatcheries as compared with river populations. This fact can be accounted for by the mixed origin of the former populations.