The biology of brown trout, Salmo trutta L., in the Scorff River, Brittany : a synthesis of studies from 1972 to 1997

Abstract

This paper synthetizes the studies carried out on the biology and ecology of the brown trout (Salmo trutta L.) in the Scorff basin between 1972 and 1997. After a brief introduction on taxonomy and the general ecological characteristics of the species, the main results are shown in terms of a description and the dynamics components of the population.

The repartition and abundance of brown trout varied according to age, environment (brook, downstream and upstream main river), habitat type and year. The highest densities were observed in brooks and headwater streams where the young of the year were mainly localized, while the oldest fishes (one to six years old) colonized the main river. Depth and shelter were the two habitat parameters adjusting brown trout distribution within the basin. Yearly variations of density were high (average ratio of 1 to 10).

Individual growth varied depending on the environment (trophic capacity) and year (water temperature). The differences were seen early in the first year of life.

In spawners there was always a greater ratio of males (sexual maturation at two years old). Most females matured for the first time at three years old, although two years old females might be frequent in brooks. Maturation at two years old depended on the growth during the first year of life.

Spawning took place in the tributaries in the downstream of the basin, and both in the main river and the tributaries in the upstream part of the basin.

Survival rates were estimated in a brook, where most mortalities were recorded during the first months of life and more particularly during the under gravel phase (a strong influence of fine sediments).

These results allowed to propose an explanatory scheme regarding the different recruitment strategies in the brown trout within the Scorff basin. They notably showed the existence of an intermediate generation in some brooks, which took place itself between migrating spawners from the main river and juveniles (mainly 1+) produced from these brooks and ensured the essential trout recruitment in the middle and downstream part of the main river.