Microsatellite and mitochondrial DNA polymorphism reveals life-history dependent interbreeding between hatchery and wild brown trout (Salmo trutta L.)

The effects of stocking hatchery trout into wild populations were studied i n a Danish river, using microsatellite and mitochondrial DNA (mtDNA) marker s. Baseline samples were taken from hatchery trout and wild trout assumed t o be unaffected by previous stocking. Also, samples were taken from residen t and sea trout from a stocked section of the river. Genetic differentiatio n between the hatchery strain and the local wild population was modest (mic rosatellite F-ST = 0.06). Using assignment tests, more than 90% of individu als from the baseline samples were classified correctly. Assignment tests i nvolving samples from the stocked river section suggested that the contribu tion by hatchery trout was low among sea trout (< 7%), but high (46%) among resident trout. Hybrid index analysis and a high percentage of mtDNA haplo types specific to indigenous trout observed among resident trout that were assigned to the hatchery strain suggested that interbreeding took place bet ween hatchery and wild trout. The latter result also indicated that male ha tchery trout contributed more to interbreeding than females. We suggest tha t stronger selection acts against stocked hatchery trout that become anadro mous compared to hatchery trout that become resident. As most resident trou t are males this could also explain why gene flow from hatchery to wild tro ut appeared to be male biased. The results show that even despite modest di fferentiation at neutral loci domesticated trout may still perform worse th an local populations and it is important to be aware of differential surviv al and reproductive success both between life-history types and between sex es.