Many fish species undertake more or less extended migrations as part of their basic behaviour. Amongst the best known examples in Europe are salmon (Salmo salar), sturgeon (Acipenser sturio) and eel (Anguilla anguilla), which are diadromous species with their life cycle taking place partly in fresh water and partly in sea water, often swimming several thousands of kilometers to reach their spawning grounds.
A serious decline of migratory fish populations has been reported over the last decades, due to a combination of negative factors related to human impact, i.e. overfishing, water quality deterioration, habitat loss or alteration, invasive species competition etc. A particularly negative impact is associated to the construction of dams that fragment the waterways and imped fish migration.
A fish pass (or fishway, fish ladder, fish bypass) is a structure built for facilitating fish migration past obstacles like dams, dykes and locks, enabling fish to pass around the barriers by swimming and leaping up a series of relatively low steps into the waters on the other side. However, opening up these corridors also presents the risk of facilitating the spread of invasive alien species. To counter this effect, fish passes can be monitored with special cameras and capture cages for the isolation of unwanted invasive fish can be installed. An example of such a system has been implemented in Italy for the eel corridor between Ceresio lake (Lugano lake) and Po river funded by a LIFE project.