Allozyme diversity in brown trout (Salmo trutta) from Central Spain: Genetic consequences of restocking

1. The brown trout (Salmo trutta) represents one of the main freshwater res ources in Spain, but habitat alterations and overharvesting have contribute d to the decline or disappearance of numerous natural populations. In addit ion, reinforcement programs of wild populations based on releases of hatche ry reared fish of exogenous origin compromise the conservation of remnant n ative trout resources. 2. We present allozymic data from Central Spain trout populations including stocked and unstocked populations. Although the levels of genetic variatio n observed were low and affected by hatchery releases ((p) over bar = 18.23 %, (H) over bar o = 3.39%), they were within the range observed in other Eu ropean areas. 3. The effective introduction of hatchery reared fish is genetically homoge nising the populations in the studied area and disturbing the ancestral pat tern of genetic variation that distinguishes the Tajo and Duero basins. Wit hin the eight natural populations analysed, seven had alleles assigned to t he foreign trout. The introgression in these populations, following the LDH -5*90 allele frequency, ranged between 2% and 29.4%, but those values are n ot in concordance with the respective stocking effort undertaken in each po pulation. Moreover, the release of hatchery-reared fish does not solve the problems related to the reduced size of wild populations and their recruitm ent instability.