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 trou t populations, four officially stocked locations and four non-stocked locat ions were sampled for one to three consecutive years and compared to the ha tchery strain used for stocking. Allozyme analysis for 25 loci included tho se previously described as providing allelic markers distinguishing hatcher y stocks and native populations. Different levels of hybridization and intr ogression with hatchery individuals were detected in stocked drainages as w ell as in protected locations. These findings indicate that new policies fo r stocking and monitoring hatchery fish are needed if gene pools of wild Sp anish brown trout populations are to be preserved.