Densité et microrépartition des différentes espèces de poissons dans la Basse-Nivelle, petit fleuve côtier des Pyrénées Atlantiques

Abstract

The density of fishes in the lower part of the river Nivelle is 227,6 kg/ha, the eel is the most important item with 108,3 kg/ha. Three species of salmonid are present : the brown trout (42 kg/ha), the rainbow trout (36,2 kg/ha) and the atlantic salmon (6,7 kg/ha). Wit h 23,8 kg/ha gudgeon is the most abundant of small species. Eel population is trimodal, the first mode is constituted by underyearlings, but it is difficult to say what the two others are. There are no juveniles in trout populations. Salmon population shows a majority of underyearlings (91 % ) with a good growth but a poor density (2,8/100 m²) . Microdistribution of different species is characteristic. The highest densities are found in deepest waters and the lowest one in shallow pebbly riffles or muddy stretches . There are good relations between water velocity , deepness, salmonid stock and eels densities. After the first summer in river eels change their habitat, they come from shallow riffles to deeper waters. Shelters are important for eels, brown trout and salmon. Catchment injuries are the principal cause of pisces regression during the last decade.