Effects of stocking with brood fish to manage resident stream dweling brown trouth Salmo cf. trutta STOCK

Abstract

Management with the indigenous stock of resident brown trout Salmo cf. trutta of the Danubi-an strain in the protected area of the River Gradac with the catch-and-release fishing regime since 2000 resulted in the prime fly fishing stream in Serbia. Stocking with the brown trout parr in 2007 in the density issued by fisheries management plan supplemented with the large-sized brood fish beyond the issued fisheries measures in the upper section of the River Gradac was followed by change in structure of fish community and population structure of brown trout there. The time of maturation of brown trout was delayed and their size at maturation increased in 2008, due to the abrupt decrease in the density of their stock. That change did not affect an increase of brown trout mean weight, but only an increase of their average age, with the de-crease in relative abundance, relative biomass and annual natural production, implying the pro-longed recovery period of brown trout stock in the upper section of the River Gradac. In con-trast to that, stocking with brown trout parr only in the density issued by fisheries management plan was beneficial on the second, downstream situated section. Another prominent feature that followed stocking as a management measure was the introgression of brown trout of the Atlan-tic lineage into the gene pool of brown trout of the native, Danubian lineage, as confirmed in wild-bred brown trout.