Twenty years of hydropower regulation in the River Alta: long-term changes in abundance of juvenile and adult Atlantic salmon

The River Alta, northern Norway (70°N), was regulated for hydropower in 1987. Densities of juveniles and catches of adult Atlantic salmon have been studied since 1980-1981 to examine the effects of regulation. The need to control environmental variables during electrofishing was emphasized, as flow variables explained up to 42% of the variation in estimated juvenile densities. The number of spawning redds was counted along the river from 1996 to 2005. The annual number of spawning redds was correlated with the catch of multi-sea-winter salmon (predominantly females). In the upper 7 km section, just downstream of the power station outlet, juvenile densities were reduced by 80% from pre-regulation levels to minimum levels in 1992-1996. This was followed by partial recovery during 1997-2005, although not entirely back to pre-regulation levels. In contrast, the general trend in the middle part of the river was a linear increase in juvenile densities during 1981-2005. Decreased juvenile densities in the upper section was subsequently followed by reduced catches of adult salmon in this part of the river. The relative catches of smolt year classes migrating to sea in the upper section was reduced by up to 75% from 1991 onwards. Spawning and recruitment in the upper section have increased in recent years, probably back to the introduction of catch-and-release angling and an increase in salmon runs. However, present day smolt production in the upper section is still reduced compared to the middle part of the river, 18 years after regulation. The decreased densities of juvenile salmon in the upper section were probably caused by several factors, of which stranding mortality due to sudden drops in the water level and increased winter mortality due to changed environmental conditions, especially reduced ice-cover, may be the most important. In conclusion, the regulation caused a considerable reduction of the salmon production in the upper 16% of salmon reaches, but did not affect the salmon population negatively further downstream. This study illustrates that apparently small environmental disturbances can cause large changes in Atlantic salmon abundance in high latitude populations.