Adaptive morphological plasticity foreshadows local adaptation in an invasive freshwater fish ( <i>Salmo trutta</i> ): evidence from reciprocal transplants in nature

Phenotypic plasticity is a key trait of successful invaders, enabling survival in novel environments during early colonization. This study investigates adaptive morphological plasticity in brown trout ( Salmo trutta), a globally invasive species, established in Newfoundland during the late 1800s using reciprocal transplant experiments. We assessed fitness-linked traits—particularly head and body shape—in both wild and lab-reared F1 fish released into three rivers with contrasting environments. Over 70 days, morphometric analyses revealed substantial shape plasticity. In the small, steep Middle Rocky Brook, fish developed larger heads and streamlined bodies; while in the larger, lower-gradient Rennies and Waterford rivers, fish had smaller heads and deeper bodies. These patterns are consistent with responses to hydrodynamic variation reported in prior work. Reaction norms indicated gene × environment interactions, and transplanted individuals often shifted toward the phenotype of local wild fish. Overall, results support a role for adaptive plasticity in enhancing survival during initial stages of invasion and suggest it may act as a precursor to local genetic adaptation.