THE GROWTH OF THE BROWN TROUT (SALMO-TRUTTA L) IN THE BASIN OF SCORFF RIVER

The growth of brown trout (Salmo trutta L.) has been studied with 0+ to 2+ year old fish belonging to five cohorts, living in four different environmental conditions (lower and upper part of River Scorff, a tributary and a subtributary of this river). Size of trout decreases in all age classes from the lower to the upper part of the main river, from the main river to the tributary and from the tributary to the subtributary. The monthly growth rate, expressed as G = 2.85 (LnL2 - LnL1)/(T2 - T1), decreases in each environmental condition from the first to the third year of life. Differences in the monthly growth rate according to different environmental conditions could only be observed in the 0+ and 1+ age classes. For 0+ trout, this effect depends on population density. In the younger trout population, growth rate changes also from one year to another, depending on the temperature of the water. The relation between length (Lf) and the sum of degree-days from the mean date of emergence (Tmj) is described for the first year of life. A good correlation is found for an exponential model: Lf = 106.7 (1 - exp [ - 0.0005 (Tmj + 468)]). Results are discussed and allow to draw some conclusions on the main factors influencing the growth of brown Trout up to the sexual maturation.