A lone salmon jumps a fish ladder in its journey upstream to spawn.

In severely altering their surrounding environment, dams alter a river's ecological support capabilities, particularly harming fisheries. Trying to remedy this issue has been analyzed by many dam engineers who have come up with fish ladders and waterway bypasses, but the continual decline in fish population numbers shows that dams have detrimental effects on fish life cycles. In this section those effects on natural fish migration dams cause from wither blocking or destroying spawning habitat is looked at, in addition to the economic implications fish losses have on regional industries and their people.

The Ardnacrusha Dam on the River Shannon in Ireland has had extensive studies taken following the effects of the dam on the river's silver eel population, producing results in line with many of the above mentioned issues.