The biology of brown trout, Salmo trutta L., in the River Scorff, Brittany: a synthesis of studies from 1973 to 1984

Abstract. The following results were obtained from brown trout, Salmo trutta L., from the Scorff Basin between 1973 and 1984. Individual growth varied depending on the environment (brook or river, upstream or downstream) and the year. The differences were seen in young fish and the effect of different years was temperature related. In spawners there was always a greater ratio of males (the majority aged 1+ years). The majority of females matured for the first time at 2+, although 1+ females were frequent in brooks. Maturation at 1+ depended on growth at 0+. Spawning took place in tributaries downstream of the basin, in the principal river and in upstream tributaries. Survival rates were estimated in one stream, where it was confirmed that the majority of recorded mortalities were 0+ fish. In spawners, annual survival of females was better (50%) than in males (30%). A biological cycle is proposed, taking into account the existence of an intermediate generation in the brooks, which places itself between migrating spawners from the main river and juveniles (1+ and 2+) produced from the streams and ensures essential trout recruitment in the Scorff.