Abstract
The endemic marble trout,Salmo truttamarmoratus, suffered massive stocking with browntrout in Slovenia and Italy. In the Socˇa River(Slovenia) long-term surveys have evidenced Danu-bian and Atlantic introgressions. The observation ofdiscrepancies between introgression frequencies whenusing morphology, mitochondrial sequences, allo-zymes, or microsatellites markers led to reanalyzethe hybrid population of the Socˇa River. New analyseson diachronic samples of the Socˇa hybrid populationswas performed, genotyping partly the same individu-als on one diagnostic allozyme (LDH-C1*), eightmicrosatellite, and one mitochondrial loci. The con-firmed discrepancies led to interpret the anomaly as the consequence of ancient natural immigration ofDanubian trout by rivers captures together withstocking with Atlantic domestic trout. Managementconsequences are discussed because the natural Socˇainhabitant which was thought to be a pure marble troutshould be now changed into a natural hybrid lineageaccepting few Danubian alleles.