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

In 1988, the Laboratory of Fish Demography of the University of Liege and several partners (University of Namur, Ministry of Walloon Region) started the "Salmon 2000 Project" aiming at reconstructing 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 juvenile population of salmon reconstructed from hatchery reared fish at Esneux in the River Ourthe, one main salmonid tributary of the River Meuse. On June 12, 1989, 1000 hatchery-reared salmon parrs were stocked in this station, averaging 46 mm fork length and 1,0 g body weight. Eigty 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 of growth at a mean water temperature of 18,55 °C. We used three methodologies based on microhabitats and mesohabitats 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 in the Ourthe preferentially exploit riffle areas (mean slope 1,55 cm/m), on a substrate principally composed of cobbles (10-20 cm). The population 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. The results are similar to those obtained in native 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 field experiment, methodological aspects and new research perspectives and axes are discussed.