An Evaluation of the Accumulated Damages to Downstream Migrating Salmon Smolts and Silver Eels While Passing a Chain of Hydro Power Stations

The European Water Framework Directive stipulates the aims for achieving a better ecological status for running waters. In parallel, the energy production from hydropower shall increase to promote climate protection and secure energy supply. It is, therefore, important to reduce the negative effects from the utilization of hydropower as much as possible.  Damage to fish caused by turbines accumulate in a chain of dams. In particular, the survival rate of diadromous species from a multiple hydropower stations can fall to levels that might not guarantee the survival of fish populations. The survival of a fish stock depends on site-specific damage rates, the number of hydropower stations encountered during migration and the location of spawning grounds and juvenile fish habitats. The German Federal Environmental Agency has commissioned a research project to examine the correlation between these factors. The test area of the project was the River Weser. The first stage was to assess the damaging rate of fish downstream from turbines. Fish were caught with nets and damage levels were compared with results of known mathematical models. At the same time, a parallel research project assessed the damage levels caused by turbines at a project using an Archimedes screw turbine. A model for estimating survival rates of fish along migration routes in river systems was formulated. The model accounted for losses during up and downstream migration and which occurred during the maritime part of the life-cycle, reproduction rate and survival rate of Salmon smolt. The outcome of this research found that fish populations can only sustain themselves if there are a small number of hydropower stations and few obstructions to upstream migrating fish. The quality of fish-passes and fish protection systems must correspond to the number of hydropower stations along the migration route.  Strategies for management of river systems were formulated from these data. The key results are as follows: fish-friendly turbines and turbine management can be used in large rivers, while in small and middle-seized rivers, mechanical fish protection barriers have to be employed. The maximum distance between bars of protection barriers for silver eel shall not be larger than 15 mm and 10 mm for Atlantic salmon smolt. The approach velocity must be 0.5 m/s of less. Such systems have been already constructed for European rivers. They are applicable for a maximum discharge volume of 30 cms per screening unit. Such facilities have been already installed at some projects.