Genetic differentiation in hatchery and wild sea trout ( Salmo trutta) in the Southern Baltic at microsatellite loci

Sea trout ( Salmo trutta) is a commercially important anadromous salmonid species in the Baltic Sea. Sea trout return for spawning to natal rivers (homing). The number of wild smolts migrating to the sea from rivers in Poland decreased from about 1.5 million ca. 70 years ago to less than 100,000 in recent years. This decrease has been caused by deterioration of the environment, regulation of rivers and dam constructions. Stocking activities have been undertaken to enhance naturally occurring populations. Over 1.5 million smolts are released to Polish rivers each year. Variation at microsatellite loci was used to study genetic differentiation between hatchery-produced smolts and wild-caught spawners returning to rivers. Polymorphism at five loci, Ssa197, Ssa171, Ssa85, Str15 and Str73, was assessed. The number of alleles in hatchery populations was lower, whereas heterozygosity levels were similar across all populations. The genetic composition of adult fish returning to the Vistula River in summer was similar to that of the Rega River population. Populations of hatchery-produced smolts originally derived from Vistula River sea trout were also similar to the Rega River sea trout population. The original Vistula River population may still be represented by some winter spawners and by populations in Drweca or tributaries above the Włocławek dam.