Untreated sewage effluent and agricultural run-off carrying fertilizers are examples of human-caused eutrophication. Eutrophication generally promotes excessive plant growth and decay, favouring simple algae and plankton over other more complicated plants, and causes a lack of oxygen needed for fish and shellfish to survive.
Fish kills in bayous, streams, and other freshwater habitats are often caused by oxygen depletion associated with excessive levels of fertility and algae. Like most plants, many algae produce oxygen during the daylight as a by-product of photosynthesis. At night these algae consume oxygen, but usually much less than was produced during the daylight. Many common situations, however, can reduce the amount of oxygen a bloom produces without reducing its nighttime oxygen demand. Extremely calm or cloudy days may reduce photosynthesis and oxygen production. This type of oxygen depletion may kill fish directly or weaken their immune systems through prolonged stress. Hydroelectric dams affect particularly migratory fish such as salmon and steelhead trout. Impacts of Hydropower include:
Migration blockage—Hydropower dams block the continuous flow of rivers and streams. As fish attempt to move up and down rivers, hydropower dams physically block their movement. In some cases it is possible to build passages that allow fish to migrate upstream around a dam.
Habitat changes—Hydropower facilities can change water volume, temperature, depth, and velocity, and alter dissolved oxygen sediment loads.
Turbine strike—Even if fish can migrate upstream past a dam, they can suffer injury or death during their downstream migration if they come in contact with a dam’s turbines.
Fish become prey—Hydropower dams might attract predatory species because of increased concentration of fish due to migratory delays, disoriented or injured fish from turbine strikes, or predator-friendly habitat.