Morphological differentiation among local trout (Salmo trutta) populations

The trout (Salmo trutta) has been divided into three forms: sea-run trout, lake-run brown trout, and resident brown trout. They differ in their living environment, migratory behaviour, growth and appearance. As local trout po pulations are often isolated, and gene now between them is minimal, differe ntiation between populations can be expected. The morphology of 1-year-old trout from ten populations representing all three forms was studied in a co mmon-garden experiment. The fish were reared under similar environmental co nditions, and 20 morphometric characters were measured from each individual fish. Marked morphological differentiation was found, and differences betw een populations were greater than differences between forms. The results su ggest that the differences have a genetic basis, and they are likely to ind icate adaptation to local environmental conditions in the native habitat of the trout. (C) 2001 The Linnean Society of London.