The effect of trout stocking was evaluated in two headstreams located in northern Portugal in order to assess the impact on wild trout (Salmo trutta L.) and to analyze the success of this operation. The results obtained exhibited the limitation of stocking: 1) the clumped character of the release fishes created a high mortality and limited the increasing of salmonid population to a few weeks; 2) because density-dependent factors seem to prevail in the regulation of salmonid populations, stocking is beneficial only if a population has became scarce, otherwise, the autochthonous fish may be strongly impacted; 3) the relative vulnerability of each age class of the native trout may vary according with the site
