Stocked and wild trout diet was assessed in two north-eastern Portugal headstreams during the summer season of three successive years (2000 to 2002). Significant differences were detected in the diet composition between stocked (age 1+) trout and distinct size-class of wild trout. Stocked fish showed preference, almost exclusively, for food items captured near the surface (primarily terrestrial adult insects), emergent pupae and subimagos. In contrast, young-of-year (YOY) wild trout fed predominantly on the most available aquatic prey taxa such as Chironomidae (Diptera) larvae and Baetidae (Ephemeroptera) nymphs; however, Trichoptera larvae were not taken. Wild trout preference changed from benthic aquatic invertebrates to terrestrial origin organisms and this ratio increased with size, suggesting an ontogenetic diet variation. Significant diet overlap was only detected between stocked and dominant wild trout, which were not displaced from the energetically profitable areas. Therefore, this study showed the absence of an evident competition for food in both streams, even when trout density was largely augmented by stocking activity. However, since no obvious benefits on space and time were obtained, stocking must be carefully evaluated and alternative approaches considered, like habitat improvement and adequate fisheries management, in order to increase the natural productivity of these systems.
The level of introgression between wild and stocked individuals in Portuguese brown trout Salmo trutta L. populations was evaluated. Fish were sampled from rivers of the Douro Basin and compared with fish used in restocking programme. This study, based on microsatellite and allozyme loci, showed, thanks to the most discriminant loci, a low percentage of heterozygote individuals. This happened due to absence or low levels of intercrossing between stocked and native brown trouts. It was confirmed the inefficiency of stocking practices in Portugal, as suggested by mtDNA haplotype distribution. From the management and conservation points of view, any kind of stocking with exogenous specimens should be avoided to allow preservation of the autochthonous genetic pools. As well as the stocking techniques and released fish density should be considered. Therefore new policies for stocking and monitoring hatchery fish are needed to preserve the gene pools of wild Portuguese trout populations.