Freshwater fi sh communities have always been valuable resources

for society, particularly the species that migrate between

freshwater and the sea. Historically, eel, salmon and trout were

so abundant that good fi shing luck could turn a capable fi sherman

into a wealthy person. Unfortunately, migrating fi sh became

threatened by over-fi shing quite early in human history. The

more recent construction of dams and other habitat destruction

associated with development of towns, industries and intensive

agriculture have reduced populations further. The largest Danish

river, the River Gudenå, illustrates this historical decline and

also the diffi culty of re-establishing healthy fi sh communities

once the original populations have deteriorated or, in the case of

salmon, become extinct. Efforts are currently needed to be made

to effectively restore and manage fi sh communities in streams.