Atlantic salmon (Salmo salar) in the regulated River Alta:: Effects of altered water temperature on parr growth

The chief objective of this study was to analyse the effects of altered water temperature, due to the hydropower regulation of the River Alta, on growth of Atlantic salmon parr. The river was developed for hydroelectric purposes in 1987. A 110 m high concrete dam was built in the main river 49 km upstream from the outlet to the sea. The outlet of the power station is located 2.5 km downstream from the dam. The annual regime of water temperature has been altered downstream from the power station because of the regulation. It has decreased 1-2degreesC during June, July and the first half of August, while it has increased up to 3degreesC during late summer. During winter, water temperature has increased from 0degreesC to about 0.3-0.4degreesC. Atlantic salmon is the predominant fish species in the river. They can penetrate 46 km from the sea, up to the outlet of the power station. In this paper I have studied the relationship between growth of juvenile Atlantic salmon and water temperature in the upper part of the river. At similar temperatures, the growth rate of salmon parr in the River Alta is higher in early summer than later in the growing season. In early summer the salmon grew faster than the maximum rate predicted by a recently published model. Therefore, I adjusted the model to describe growth rates of salmon in early summer (ice break to mid-August), using data derived prior to the hydropower development (1981-1986). The new model proved effective at describing growth rates of fish in early summer following the hydropower development (1987-1996). After development, growth rates decreased during early summer, but increased correspondingly later in the season. There was close agreement between these growth changes and the altered annual regime of river temperature. Overall, only minor changes in annual growth rates have been observed after the hydropower development. Copyright (C) 2003 John Wiley Sons, Ltd.