Genetic effects of introduced hatchery stocks on indigenous brown trout (Salmo trutta L.) populations in Spain

To assess the levels of gene introgression from cultured to wild brown trout populations, four officially stocked locations and four non-stocked locations were sampled for one to three consecutive years and compared to the hatchery strain used for stocking. Allozyme analysis for 25 loci included those previously described as providing allelic markers distinguishing hatchery stocks and native populations. Different levels of hybridization and introgression with hatchery individuals were detected in stocked drainages as well as in protected locations. These findings indicate that new policies for stocking and monitoring hatchery fish are needed if gene pools of wild Spanish brown trout populations are to be preserved.