FIELD ASSESSMENT OF THE INFLUENCE OF TEMPERATURE ON GROWTH-RATE IN A BROWN TROUT POPULATION

Growth rates of age-0 brown trout Salmo trutta during the growing peri od (May-September) were estimated in nine consecutive year-classes in three sites of Arroyo Chabatchos (Esva River basin, northern Spain). O bserved growth rates were highly correlated with average water tempera ture, which explained 48, 68 (90% excluding the severe drought of 1989 ), and 89% of the interannual variation in respective growth rates at the three sites studied. Observed growth rates varied from those predi cted by a model. by Elliott and others, which predicted higher-than-ob served growth rates in winter and negligible growth in summer. Observe d growth rates were consistently higher than those predicted in two si tes, and the only four instances (out of 27 possible) of lower-than-pr edicted growth rates occurred in fish at the site at which observed gr owth was slowest. As expected from the model, the predicted growth rat es were also correlated with water temperature, but the slopes of the observed and predicted regressions differed significantly. The faster growth at higher temperatures exhibited by Chabatchos brown trout may be a local adaptation of the thermal regulation of growth. This adapta tion does not seem to have induced a shift to a higher optimal growth temperature but only a slower reduction of growth rates with increasin g water temperature. This allows Chabatchos brown trout to maintain gr owth that is greater than other brown trout at comparable temperatures , enabling these fish to achieve larger sizes in most growing seasons.