Phenotypic differences between non-native populations of rainbow trout Oncorhynchus mykiss in the Argentinean Patagonia

Patagonia offers several examples of local adaptation in the exotic rainbow trout Oncorhynchus mykiss. Specifically, the Valcheta Stream, characterized by hot spring headwaters and temperate lower reaches, was initially stocked in 1928. Subsequently, progressive upstream expansion, predation by O. mykiss on the critically endangered naked characin Gymnocharacinus bergii, and a rapid adaptation to high water temperature were observed. We documented the life history, breeding, growth, stomach content, Fulton condition factor, hepatosomatic and gonadosomatic indexes, haematological parameters, and body shape of Valcheta rainbow trout, comparing it with a cold-water population of the same origin (Guillelmo Stream). The Valcheta population exhibited an earlier acquisition of sexual maturity and smolt characteristics, along with different growth parameters, including a shorter asymptotic length, higher growth rate, and breeding rooted in winter, contrasting with the spring breeding observed in Guillelmo Stream. Body shape variations were evident between populations and among parr, smolt, and adults. Stomach content volume, condition factor, hepatosomatic and gonadosomatic indexes, and haematology displayed differences related to annual cycle and reproductive adjustments. The Valcheta population of rainbow trout serves as an illustrative example of how fish species can adapt to effects of the climate change through local adjustments.