Temporal Changes in the Spatial Distribution of Juvenile Sea-Trout Salmo trutta in a Small Stream

Autumn population density of juvenile sea-trout Salmo trutta L. in 50 m sections of a small stream was estimated by electrofishing for three consecutive years. The mean density over the years varied with the biotope for all age-classes (0+, I+, II+ and older). 0+ and I+ maintained low densities in slow-flowing, soft-bottom sections and larger in sections including riffles. II+ and older showed largest density in sections with both riffles and pools. The spatial distribution of 0+ and I+ among the sections was shown to vary from year to year, and the observed patterns is proposed to be explained by a simple model based on (1) biotope quality, expressed as common needs for the two age classes, (2) intercohort competition, and (3) long-term stationarity.