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On occasion, one of the Wildlife Resources Commission's fisheries biologists will use this "Ask A Biologist" column as a forum to answer a question that may require a more detailed response (see question below). If you have a specific fish or fish-related question, please submit it to wrccomments@ncwildlife.org. It may be used in this column to help others learn more. |
[Editor's Note: This column was posted July 10, 2006] By: Carl Kittel and David Deaton “If I catch fish that look sick (have red sores or cotton-like patches on the body and/or fins or other signs of disease) what should I do? What causes these sores? Should I be concerned about the fish population? Is there any risk to me?”There are many factors that can cause a fish to get sick or develop sores. Fish are constantly exposed to various bacteria, fungi, viruses and parasites that occur naturally in the water. Generally the fish can deal with these with little or no problem. However, if fish are stressed or weakened by poor environmental conditions, they may not be able to fight off disease and may become sick. Some common diseases encountered in wild fish in North Carolina include white fuzzy patches on the skin caused by fungi or protozoans, red sores caused by bacteria, and black or yellow spots in the flesh of certain fish species caused by parasites. Some causes of stress include traumatic injury, spawning activity, rough handling and changes in water quality. When water temperature increases and/or fish are spawning, they are more likely to develop signs of disease. In most cases, the fish recover when conditions improve and stressors are reduced. Disease outbreaks typically don’t have a serious population level effect on natural fish populations. Usually only a small portion of a population will be seriously affected by a disease, and the population will rebound quickly. The risk of an infected fish transmitting disease to a human is extremely small. There are rare diseases that can be passed from aquatic organisms to people but scientists will try to make people aware of the situation if there is an outbreak of such a disease in our area. What You Should Do
Pictures of Common Fish DiseasesPhotos Courtesy of Southeastern Cooperative Fish Disease Project Picture 1:
Largemouth bass with “yellow grub.” This parasite is a digenetic trematode from the Clinostomum genus. It uses the fish as an intermediate host during its complex life cycle. The yellow cysts that are found in the flesh and under the skin are larval worms that remain there until the fish is eaten by a bird to start the cycle over again. Humans are not affected by this parasite. Picture 2:
The cottony growth on this catfish is caused by the fungus Saprolegnia,which is found everywhere in freshwater ecosystems. Fungi attack fish that have an injury or have a weakened immune system. Since Saprolegnia is considered a “water mold,” there is little to no risk of humans becoming infected from contact with these fish. Picture 3:
The small black spots present on this fish are caused by the immature larval trematode parasite Apopphallus brevis. It is often called “black spot” or ‘black grub” disease. The fish acts as an intermediate host during its life cycle. The parasite does not infect humans and can be killed by cooking the fish. Picture 4:
The red sore on this bass is caused by the protozoan parasite Epistylis.
There are many causes of red lesions on the skin of fish. A common cause
is the bacteria Aeromonas. Fish that are stressed from
spawning activity, injury or poor water quality can be susceptible to
these types of infections due to weakened immune system. Carl Kittel is a fish production coordinator. David Deaton is a biologist at the Marion Fish Culture Technology Center.
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