GROWTH OF YOUNG MIGRATORY BROWN TROUT SALMO-TRUTTA CORRELATED WITH WATER TEMPERATURE IN NORWEGIAN RIVERS

Mean annual instantaneous growth rates of 12 Norwegian brown trout populations in their 2nd year of life (1+) varied between 1.14-1.94. In 5 of the rivers, annual growth rates were not significantly different from those computed with Elliott's (1975) growth model, whereas values in 4 rivers were significantly higher, and in 3 rivers were significantly lower. Mean growth rates on a 1yr basis varied between 76-136% of the computed maximum for that temperature regime. In 3 rivers brown trout were sampled twice annually, once just after the ice break and again in early August. Daily growth rates in early summer in all 3 rivers were higher than the corresponding growth rates computed from Elliott's model. In late summer the observed compared to the computed growth rates were usually lower than in early summer, and in some cases no growth was observed after early August. The growth rates of brown trout in some Norwegian rivers exceeded the established rates for trout fed on maximum ratios, indicating genotypic differences in the growth rates of Norwegian and English populations, or higher food intake in wild trout exposed to diel fluctuating temperatures than among experimental fish held at constant temperatures. -from Author