Premières observations sur une population de juvéniles de saumon atlantique (Salmo salar L.) reconstituée dans l 'Ourthe en 1989.

In 1988, our laboratory started the « Salmon 2000 Project », aiming at the reconstitution of populations of Atlantic salmon (Salmo salar L.) in the River Meuse basin, where the species had disappeared since the 1930's. This paper presents the first results on the biology of a reconstructed salmon population at Esneux, in the River Ourthe, one of the main tributaries of the River Meuse. On June 12th, 1989, 1,000 salmon parrs were restocked in this station, averaging 46 mm (fork length) and 1,0 g of body weight. 80 days later, 60 individuals were captured by electrofishing at their site of release. Their lengths ranged from 70 to 135 mm (mean = 101 mm) and the average weight was 12,3 g, after 80 days at a mean water temperature of 18,55 "C. We used three methodologies based on microhabitat and mesohabitat concepts to determine the preferences of salmon parrs towards river habitat characteristics, and to define a strategy aiming at a comparative study of habitat segregation among rheophilous species. The habitat study shows that salmon parrs exploit preferentially riffle areas (mean slope = I ,55 cm/m), on a substrate principally composed of cobbles ( 1 0-20 cm). The densities ranged from 0 to 16 parrs/100 m2 and were statistically (p < 0.05) and positively correlated with the proportions of depth class 20-40 cm and bottom current speed class 50-70 cm/s. These results are similar to those obtained on natural populations of Atlantic salmon living in comparable waterbodies and tend to prove the good adaptation of the reconstructed population in the River Ourthe. From this first field experiment, methodological aspects and new research perspectives and axes are discussed.