Although this is sometimes a natural condition, the increased area of water affected, extended length of each episode, and higher frequency in recent decades are due to human activities. The effects of hypoxia include fish kills and shellfish bed losses. These losses can have significant detrimental effects on the ecological and economic health and stability of coastal regions health and stability of coastal regions. Hypoxia is primarily a problem in estuaries and coastal waters, although it can also be a problem in freshwater lakes. The direct effects of hypoxia include fish kills, which not only deplete valuable fish stocks and damage the ecosystem, but are unpleasant for local residents and can harm local tourism.
Oxygenated water is necessary for aquatic animals to breathe.
Mobile animals, such as adult fish, can often survive hypoxia by moving into oxygenated waters. When they cannot, such as when young fish need to spend time in the habitat that has become hypoxic area, the result is a fish kill. This causes a severe reduction of the amount, or in extreme cases the complete loss, of animal life in hypoxic zones. Because hypoxia often occurs in estuaries or near shore areas where the water is poorly mixed, nursery habitat for fish and shellfish is often affected. Without nursery grounds the young animals cannot find the food or habitat they need to reach adulthood.