Liberation of trout (Salmo trutta L.) in Danish streams

During the past century pond-reared torut have been liberated into national Danish fresh waters in order to compensate for decreasing salmonid production caused by the increasing deterioration of the physical environment. At the beginning of the centrury such stocking was perfromed in a very uncritical way without any regard to the biological requrements of the fish and no beneficial effect of theses stockings has been demnonstrated. From the middle of the thirties, however, stocking of trout fry was systematized with regard to the habitat requirement of this developmental stage. Today, modern stocking schemes operate with fry, half-yearlings, yearlings and two-year old fish (smolts). At present about 56 percent of Danish catchment areas are managed according to such modern schemes, according to which about 1 600 000 fry, 350 000 half-yearlings, 200 000 yearlings and 130 000 two-year old fish are liberated per annum.

The paper discusses the biological and technical principles for these schemes and gives an example of ther effects on the trout stocks. The paper finally discusses the actual problems and outlines the work to be done in the future.