From wild to hatchery and back: How captive Mediterranean brown trout are impacting remnant populations of Salmo ghigii in Italy

Over the past decade, hatchery lineages of the so-called Mediterranean brown trout (Salmo ghigii) have increasingly been used in restocking programs across Italy. However, the geographical origin, phylogenetic composition, and breeding practices of these hatchery stocks remain poorly documented. This study used mitochondrial control-region sequencing and nuclear microsatellite markers to investigate the phylogenetic origin and genetic structure of hatchery Mediterranean trout used for restocking in Italy. Sampling sites were grouped into four categories: i) wild stocked (WS), where wild populations were supplemented with Mediterranean hatchery strains; ii) Mediterranean hatcheries (MH), where “Mediterranean trout” are artificially bred; iii) wild native (WN), representing the last pure or nearly pure native S. ghigii populations; and iv) Atlantic hatcheries (AH), where brown trout of Atlantic Ocean drainage origin are artificially bred. Three main findings emerged: i) MH stocks show widespread Atlantic genetic introgression; ii) hatchery stocks are mixtures of gene pools from multiple rivers and do not reflect local wild populations; and iii) genetic traces of S. ghigii in Lombardy, previously interpreted as evidence of native persistence, are more plausibly explained by recent restocking with non-native Apennine trout. Overall, stocking programs based on MH strains accelerate the erosion of Italy's remaining native trout genetic diversity. The release of admixed stocks promotes genetic homogenization, weakens local adaptation, and threatens endemic lineages. Urgent reform of fisheries regulations and management practices, including mandatory genetic evaluation and catchment-based management, is essential to safeguard the remaining native genetic sanctuaries of S. ghigii in Italy