Morphological criteria for identification of two forms (river, Sea) of brown trout (Salmo trutta) present in the same river.

The brown trout (Salmo trutta) is living under two forms, river and sea, in many rivers along the Atlantic and Channel coasts. These two forms are con sidered as coming from the same stock because each form can produces the ot her one and they can interbreed between them in sympatric spawning area. Th e present study showed there existed some morphological reliable criteria f or the identification between the two forms during their biological cycle. In resident and migrating juvenile fish, the distinction was only possible at the migrating stage because of the higher silvering rate in sea form (sm olt group) than in river form (parr group). In adult fish, the distinction is possible when size and color criteria were used but it had to check by a scale-reading method. Distinction between the two forms allows to measure the abundance level and their long-term evolution in the same river.