Brown trout (Salmo trutta) stocking impact assessment using microsatelliteDNA markers

The genetic integrity of many salmonid fish populations is threatened by st ocking of domesticated conspecifics. The purpose of this study was to asses s the utility of microsatellite DNA markers for detecting loss of genetic d iversity in hatchery strains, for estimating their genetic relationships, a nd for monitoring the genetic impact of stocking activity on wild populatio ns of salmonid fishes. Brown trout from ten hatchery strains, one supportiv e breeding "strain," and five wild populations were screened for variation at eight loci. In most hatchery strains, genetic variation was comparable t o that of wild populations, but three strains showed loss of allelic variat ion. In six of the hatchery strains, significant differentiation was observ ed between age classes. Genetic differentiation among all populations was m oderate (F-ST = 0.065, p(ST) = 0.076), and only a minor part of genetic div ersity was distributed between the wild and hatchery populations. We assess ed whether wild populations were introgressed by stocked hatchery trout by performing assignment tests to determine population of origin and estimatin g maximum potential introgression rates. The results suggested that genetic introgression by hatchery trout had occurred for only two of the five popu lations potentially influenced by stocking. In one of these two rivers, mic rosatellite data obtained from a limited number of old scale samples indica ted that individuals from the original population were genetically divergen t from these of the present population, suggesting that extinction of the o riginal population had taken place. The study demonstrates that microsatell ite analysis provides a useful tool for distinguishing heavily introgressed populations from those unaffected by stocking. The information can be used to assist in (1) prioritizing populations for conservation and (2) the cho ice of wild populations to be used as sources for the reintroduction of sal monids in areas where local, indigenous gene pools have been extirpated.