EFFECTS OF STOCKING ON THE GENETIC DIVERSITY OF BROWN TROUT POPULATIONS OF THE ADRIATIC AND DANUBIAN DRAINAGES IN SWITZERLAND

This study focuses on genetic variation of brown trout Sa(mo trutta po pulations of the Adriatic and Danubian drainages in Switzerland. The a llozyme and other protein loci data show a major replacement of native stocks from the Adriatic drainages by introduced hatchery trout of At lantic basin origin. In most samples, diagnostic alleles for the Adria tic form of Salmo trutta f. fario and for the marbled trout Salmo trut ta marmoratus are found at very low frequencies (f<0.15). Taking into account previous genetic studies on brown trout of this basin, the Dan ubian samples are not heavily contaminated with foreign alleles. The r esults are consistent with records of local stocking activities which account in part for the high introgression rates of Atlantic alleles i nto local populations of the Adriatic drainages. In addition, introgre ssion is enhanced by a decrease of natural reproduction which is cause d by a deterioration of trout habitats through human activities. Furth ermore, a third mechanism is proposed that may contribute to the high introgression rates observed: if Atlantic trout are introduced, the re productive barriers between the two native forms, marbled trout and Ad riatic fario respectively, break down. Atlantic trout apparently hybri dize with both native forms and generate gene Row between them. In som e parts of Adriatic drainages in Switzerland, the patterns of introgre ssion and hybridization are further complicated by introduction of tro ut from the Danubian system. Alleles of the marbled trout are also fou nd in the samples of the Danubian drainage system. These are due to st ocking activities across the watershed. (C) 1995 The Fisheries Society of the British Isles