Fish kills can be caused by algal blooms, harmful bacteria and viruses, or pesticides and herbicides in the water. When large numbers of fish die in water, they decompose, which further decreases the amount of dissolved oxygen available in the water for fish to breathe. This downward spiral is called a fish kill. Once a fish kill starts, little can be done to stop it.

Algal blooms are caused when excessive amounts of nutrients enter slow-moving bodies of water. Nutrients, such as nitrogen and phosphorous, act as fertilizers that help algae grow in water. Algae use dissolved oxygen to fuel night-time growth, when sunlight is unavailable for photosynthesis. When dissolved oxygen levels fall beneath certain levels, fish and other aquatic creatures can no longer survive.

Additional Resources

Information on harmful algal blooms (HABs) and health effects of toxic algae

Health effects of the toxic algae Pfiesteria, with information hotlines you can call for more info

Everything you ever wanted to know about toxic algae

NC Division of Water Quality’s Algal & Aquatic Plant Assessment Program

Summary of past fish kills in NC

Detailed information on the causes and prevention of fish kills