The introduction of predatory salmonids such as rainbow trout Oncorhynchus mykiss Walbaum 1792, and brown trout Salmo trutta Linnaeus 1758, has been responsible for declining populations of native galaxiids in the Southern Hemisphere. The empirical evidence of mechanisms of this decline are, however, scarce. Currently, trout are the most common species in Chilean Patagonia. Galaxias platei Steindachner 1898 is the most widely distributed native species in Patagonia and can still be found in lakes that are not invaded by salmonids. We studied the trophic ecology of the two most common trout species (rainbow and brown) as well as G. platei in invaded and non-invaded lakes by analysing their diet and isotopic composition of their muscle tissues (carbon and nitrogen). The results indicated that the presence of trout delayed piscivory in G. platei and led to a slower ontogenetic scaling and lower trophic level. In addition, juveniles of G. platei tended to expand their trophic niche toward allochthonous prey in salmonid-invaded lakes. Trout showed high levels of piscivory, preying mainly on G. platei. The protection of the remaining salmonid-free lakes appears to be a crucial measure needed for the conservation of G. platei.