| Summary: | This study reports on the first stage of a project for radio-tracking of Atlantic salmon (Salmo salar L.) as they migrated back up the Aulne river. The objective was to make a survey of anadromous migration on the water course of which 70 kilometres downstream are channelled. The behaviour of 40 salmon was thus studied at 26 obstacles from the beginning of July to the middle of December 1999. The mean percentage of fish passing observed at the obstacles fitted with old fish-pass facilities proved to be less than that obtained for more recent fish-passes installed since 1994 (63% as against 94%). The study of the cumulative effect of the obstacles indicates that only 2% of the population are likely to get through the channelled part and reach areas which are suitable for reproduction. The most evident blocking was recorded on those obstacles which are the furthest downstream on the migration itinerary. The environmental conditions, such as sluggish flows and deteriorated water quality might make it difficult for salmon entering the river to progress. Moreover the salmon appear to behave in a particular way in this strongly anthropic environment. They withdraw into faster flowing areas during the summer and migrate massively downstream just before the reproduction period. 35% of the radio-tagged salmon leave the Aulne before the spawning period, enter the Brest harbour and go up other neighbouring water courses. La présente étude constitue le premier volet d'un suivi par radiopistage de saumons atlantiques (Salmo salar L.) au cours de leur remontée sur l'Aulne. L'objectif est de réaliser un état des lieux de la migration anadrome sur ce cours d'eau dont les 70 km aval sont canalisés. Le comportement de 40 saumons a ainsi été étudié de début juillet à mi décembre 1999, au niveau de 26 obstacles. Le pourcentage moyen de franchissement observé au niveau des seuils équipés d'anciens dispositifs de franchissement s'avère inférieur à celui obtenu au niveau des passes à poissons récentes installées depuis 1994 (63 % ...
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