Hydroelectric power stations in rivers produce potential injury to downstream migrating fish as they pass through the turbines. Highest mortality rates are found for eel and ranges between 15 and 38%. The impact of fish passage through a series of hydroelectric power stations in one river can be very high. This might be the case for river systems like the river Rhine or Meuse where many hydroelectric power stations have been installed. Countermeasures to protect downstream migrating fish at hydroelectric power stations are under research. One optional countermeasure is turbine management i.e. to close down the turbines during migrating periods of silver eel and to offer them a save passage over the weirs.