Growth of Young Migratory Brown Trout Salmo trutta Correlated with Water Temperature in Norwegian Rivers

Abstract

(1) The objective is to test the generality of Elliott's (1975a) growth model developed on brown trout, Salmo trutta, fed on maximum rations, and the data are observed growth rates of parr of anadromous brown trout from twelve Norwegian rivers. (2) The mean annual instantaneous growth rates of the twelve Norwegian brown trout populations in their second year of life (1 +) varied between 1-14 and 1 94. In five of the rivers, the annual growth rates were not significantly different from those computed from the model, whereas values in four rivers were significantly higher than those computed. Values in three rivers were significantly lower than those computed. The mean growth rates on a 1-year basis varied between 76 and 136% of the computed maximum for that temperature regime. In thirty-eight of the fifty-four comparisons, recorded growth rates were not significantly different from those computed from the model, but ten were significantly higher and six significantly lower than model predictions. (3) In three rivers brown trout were sampled twice annually, once just after the ice break in late April or early May, and again in early August. The daily growth rates in early summer in all three 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. (4) The growth rates of brown trout in some Norwegian rivers exceeded the established rates for trout fed on maximum rations. This indicates genotypic differences in the growth rates of Norwegian populations and the experimental fish in England, or higher food intake in wild trout exposed to diel fluctuating temperatures than among experimental fish held at constant temperatures.