Gulf Restoration
Conserving the Nature of America

Effects of Water Quality on Fish in the Gulf


The conservation actions that we take to improve fish and wildlife habitat will provide improved water quality and improved habitat for fish and wildlife in the Gulf. These activities include reforestation, restoring riparian areas, reducing nutrient input to streams, implementing best management practices for agriculture and forestry, stormwater protection, removing old and defunct dams, and restoring hydrology and fish passage, among others.

Many of these actions are designed to reduce nutrient input into streams. According to the Environmental Protection Agency, excess nutrients from the Mississippi River and seasonal stratification (layering) of waters in the Gulf results in what's known as the hypoxic zone every summer. Nutrient-laden freshwater from the Mississippi River flows into the Gulf of Mexico. This freshwater is less dense and remains above the more dense saline Gulf water. In addition to the saline gradient caused where the freshwater and saline water meet, the freshwater is warmer than the deeper ocean water, further contributing to the stratification. This stratification prevents the mixing of oxygen-rich surface water with oxygen-poor water on the bottom of the Gulf. Without mixing, oxygen in the bottom water is limited and the hypoxic condition remains.

Direct effects of hypoxia include fish kills, which deplete valuable fisheries and disrupt ecosystems. Mobile animals (e.g., adult fish) can typically survive a hypoxic event by moving to waters with more oxygen. Less mobile or immobile animals, such as mussels or crabs, cannot move to waters with more oxygen and are often killed during hypoxic events. Ultimately, hypoxia causes a severe decrease in the amount of life in hypoxia zones. Hypoxia also affects the ability of young fish or shellfish to find the food and habitat necessary to become adults. As a result, fish and shellfish stocks may be reduced or become less stable because less young reach adulthood. Hypoxia can also affect species that rely on fish for food. Such species might have to leave an area to find the necessary food to survive.

To learn more about hypoxia, view this instructional video on

Many of the migratory fish, birds, and other wildlife use both the Gulf and inland habitats during various parts of their lifecycle, thus healthy habitats in both places are necessary for healthy fish and wildlife populations. Examples of these migratory fish include Alabama shad, Gulf-strain striped bass, and American eel.

Oysters and other shellfish and estuarine dependent species must have good quality and quantity of water coming from the watershed to thrive. In Apalachicola Bay, a famous oyster fishery, the oyster populations have decreased in recent years due to a decline in the quality and quantity of water coming from the Apalachicola River and the ACF basin.




Two oystercatchers walk in an oyster bar

Oystercatchers, a migratory bird species, love to eat oysters, an immobile species that is vulnerable to die-off during hypoxic events. Photo: Dave Addison, American Oystercatcher Tracking Project

An American eel on top of a leaf

An American eel. Photo © Leo Miranda

Last updated: September 23, 2013