Compared with electricity produced using fossil fuels, hydropower is a competitive and renewable source of energy, producing no air pollution or greenhouse gases. However, the regulation of river systems for hydropower purposes often has a negative impact on the production of fish and other aquatic organisms. The hydropower licence-holders have therefore been ordered to compensate for the loss of fish production, usually by paying financial compensation or by restocking.

Up until now the management measures implemented have not been particularly flexible and have focused on restocking in an attempt to compensate for the lack of natural reproduction. However, fish production in alpine reservoirs seems to be less dependent on such compensatory measures. Recent studies show that it is possible to achieve substantial populations of high quality fish, provided that there is sufficient prey to feed the numbers of fish concerned. The governing factors seem to be biological conditions, such as natural reproduction (lake spawning), available food species and existing fish species, as well as abiotic conditions such as climate fluctuations, river management regimes and water quality.

The project will be divided into the following three sub-projects:

The structure of the food chain
Bottom-dwelling (benthic) crustaceans, such as Gammarus lacustris, Lepidurus arcticusand Eurycercus lamellatus [GMF1] are often the most important for the production of trout in mountain lakes. Successful management of the reservoir requires that the river management regime is flexible enough not to restrict the development of benthic food chains. The main task of this sub-project is to identify factors in the environment which are favourable for the benthic food chain in which crustaceans are important food species. We will use new methods (analysis of stable N and C isotopes in fish scales) to determine the significance of the benthic food chains in reservoirs with various production conditions. In addition, we will examine developments over time, based on archived scale material. 

Reproduction
The objective here will be to determine the extent to which lake spawning is of significance for the maintenance of trout populations in alpine reservoirs. Recent surveys indicate that the scale of lake spawning may have been underestimated. In lakes which are involved in restocking programmes, we will use stable isotopes in an attempt to differentiate between naturally occurring fish and restocked fish, and archived scale material to highlight the importance of natural reproduction.

Population dynamic model 
A model of the population dynamic for trout will be developed, in which reproduction and mortality is expressed as a function of the river management regime, fish density and climatic conditions. By simulating the population dynamic and the distribution of fish size under various filling regimes, we will be – providing that cost data associated with various filling regimes are available – in a unique position to calculate the optimal filling regime for trout production.