Effects of releasing hatchery-reared brown trout to wild trout populations

Brown trout is a species showing extreme genetic differentiation among local populations, which to some degree may reflect local adaptation of the stocks. In many countries pollution and destruction of spawning places have led to a significant decline in population sizes. To counteract this decline the traditional solution has been to stock large numbers of hatchery-reared trout. Hatchery-rearing may lead to domestication and loss of genetic variation, and if massive numbers of hatchery fish are stocked these may replace wild populations. Interbreeding of wild and domesticated populations or formerly reproductively isolated wild populations may result in a breakdown of the local genetic population structure, loss of genetic variation among stocks and loss of specific genotype combinations of potential adaptive value. Population genetic monitoring of stocking programs strongly suggests that releases of trout from exogenous hatchery strains cannot be recommended. Conservation and management efforts should primarily be aimed at individual stocks and priority should be given to improving environmental conditions. If stocking is inevitable the preferred option is to release offspring of local wild fish, and only in already heavily stocked populations is the use of exogenous hatchery fish justified.