EXPERIMENTAL-STUDY OF INTERSPECIFIC COMPE TITION BETWEEN COHO SALMON,ONCORHYNCHUS-KISUTCH, AND ATLANTIC SALMON, SALMO-SALAR, JUVENILES IN FRESH-WATER

The risk of accidental introduction of coho salmon in rivers inhabited by Atlantic salmon induced research on competition mechanisms between fry and juveniles of the two species under experimental conditions. P ossible consequences of the competition were evaluated in terms of sur vival, growth, predation, downstream movement and habitat use. Results showed that under good trophic conditions, the presence of coho salmo n had no influence on Atlantic salmon fry and juveniles. Microdistribu tion was different, Atlantic salmon holding position in riffles and co ho salmon preferring pools. The pelagic and gregarious behaviour of co ho salmon juveniles, that are much larger than Atlantic salmon at the same stage after emergence, contrasted with the benthic substrate-boun d behaviour of the latter. These differences increased in sympatry at the expense of Atlantic salmon, which was handicapped by its smaller s ize. Under limiting trophic conditions, the growth and sedentary behav iour of Atlantic salmon were significantly altered by the presence of coho salmon,:thus justifying explicit doubt on the interest of introdu cing coho salmon into French rivers.