Changes in recruitment of Atlantic salmon (Salmo salar) population in the River Oir (Lower Normandy, France): relationships with the characteristics of spawners and with the stream modification

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The River Oir is a small tributary of the Sélune (Armorican massif) and is used as a spawning area by the Atlantic salmon (Salmo salar L). Since 1985, the population dynamics of its salmon population have been studied from a data set regarding parr density, number and age structure of migrating fish (smolts and adults). Parr densities (1.5 to 17.4 fish/100 m² of suitable habitat) and smolt production (0.25 à 9.2 fish/100 m²) fall within the range of values of other nearby rivers but vary strongly according to the year and to the cohort. These fluctuations of abundance are linked to the number and to the size of spawners (with a strong decrease of multi-sea winter fish) and to the possibility for the adults to reach the best spawning grounds, depending on discharge conditions