What Causes Fish Kills?

July 1998

More than half of the fish kills worldwide are the result of natural causes, including infectious diseases, bacteria, protozoans, strandings of school fish, or life cycle events such as spawning or migration. Fish kills also can be caused by oxygen depletion in the water resulting from stormwater runoff, algal blooms, high water temperatures and low water flow. Because there are many reasons for fish kills, it is often difficult to determine exactly what may be the reason for the death of large numbers of fish in our rivers or sounds.

North Carolina often experiences fish kills during summer. One of the main reasons for that is something scientists call anoxia and fishermen call “dead water.” Anoxia is a layer of water that forms along the bottom of rivers and sounds that is so depleted of oxygen it cannot support life. Anoxia has always been a problem in the deepest parts of our sounds during the summer. During hot, dry summers, when there is little wind and rainfall, the water begins to stratify, or form horizontal layers. The warmer, fresher water coming downstream settles atop the cooler, saltier water, creating what is often called a “salt wedge.” Flounder have been known to back halfway up a bank to escape the water’s clutches. Crabs will scurry onto dry land, and eels will lay their heads on the bank gasping for air.

The upper layer of water always contains plenty of dissolved oxygen because of the exchange that takes place at the surface and because plants and algae produce oxygen through photosynthesis. However, in the saltier, lower level very little photosynthesis goes on because sunlight cannot filter through. When the two layers do not mix, what little oxygen exists in the lower layer is used up and “dead water” is the result.

Excess nutrients flowing into our coastal waters can compound problems caused by anoxia. Nutrients such as nitrogen and phosphorus are food for algae and other microscopic organisms. As algae multiply, or bloom, they remove the oxygen from the water. As they die, the algae sink and decompose. This processes uses up even more oxygen and results in the death of fish, crabs, oysters, clams and other creatures. These nutrients can enter estuaries via any number of sources upstream, including livestock operations, agricultural runoff, sewage treatment plants and industrial discharges.

Another cause of fish kills in some of North Carolina’s waterways may be linked to Pfiesteria, a toxic dinoflagellate, sometimes called “killer algae,” which paralyzes fish and then eats their flesh. Scientists suspect Pfiesteria is stimulated by nutrient pollution.

While a wide variety of fish have been shown to be vulnerable to Pfiesteria when exposed to it in a laboratory, little is definitively known about the relationship between fish species and Pfiesteria in the wild. Scientists do know these organisms graze on other algae and their growth can be stimulated by nutrient-induced algal blooms. It is quite possible that stress caused by low oxygen conditions makes fish more susceptible to attacks from organisms such as Pfiesteria.

What should you do if you see a fish kill? The North Carolina Department of Health and Human Services recommends the following precautions:

1. Do not eat any part of a fish with sores or other indication of disease.

2. Do not collect for consumption dead or dying fish (floaters).

3. Do not use fish harvested in a dead, dying or diseased condition as bait or to feed to domestic animals.

4. When you are uncertain about the cause and side effects of a fish kill, protect yourself by avoiding consumption of any fish, shellfish or crabs harvested in the immediate vicinity of the fish kill.

5. Do not swim in waters near a fish kill. This advice also applies to other recreational activities that would involve skin contact with the waters of a fish kill site.

6. If your work requires contact with water, you should postpone any planned activities in the vicinity of an ongoing fish kill. If water contact cannot be postponed, protective gear should be used to reduce water contact.

7. Items that have been immersed in the waters of a fish kill site should be handled with suitable protective gear (e.g., gloves).

8. A person who falls into the water at a fish kill site or who has another unprotected water contact should change any wet clothing and wash the exposed area(s) with soap and clean water or a solution of 1 part household bleach to 10 parts water. (Do NOT use undiluted bleach.)

9. Pets should not be allowed to swim in the vicinity of a fish kill.

10. Persons who experience illness that they think may be related to exposures at a fish kill are advised to promptly consult a physician.