Fish Disease – Freshwater Aquaculture

Andy Goodwin, Fish Pathologist/Inspector(AFS-FHS), Professor/Associate Director UAPB Aquaculture/Fisheries Center

Aquatic Animal Health and Disease

Wet mount of Trichodina from catfish skin (photo by Andy Goodwin).

When fish are living under good conditions with favorable water quality, nutritious food, and optimal temperatures, disease generally occurs only when new bacteria, viruses, or parasites are introduced into the fish’s habitat. When fish are living under less than optimal conditions, their resistance to disease is reduced, and they may be infected by common diseases that are often present in the environment at low levels. For this reason, it is very important to make sure that we do all that we can to ensure a good environment for our aquatic animals and protect them from the introduction of exotic diseases.

Maintaining a Good Environment

For private ponds that are managed for recreational fishing or ornamental fish, the most important environmental considerations are these:

Water quality
1. Oxygen: A problem when there are excessive nutrients, dense growths of plants or algae, when herbicides have been used, when fish are stocked and fed at high levels, and when the weather is hot and cloudy.
2. pH: Some kinds of soil chemistry can make your water very acidic or very alkaline.
3. Alkalinity: This is a measure of minerals in the pond that help keep the pH at the right level.
Food availability
1. Proper stocking, liming, and fertilization ensure that there is adequate natural food.
2. Supplemental feeding of quality fish feeds helps too.
Pond balance
1. Proper stocking not only ensures good growth but also may include fish that control aquatic plants and snails that carry fish parasites.

For commercial aquaculture ponds, tanks, and raceways, the most important environmental considerations are these:

Water quality
1. Dissolved gasses including oxygen and carbon dioxide
2. Waste products like ammonia and nitrite
3. Alkalinity for pH control
4. Calcium and magnesium (hardness) and salt levels
5. Temperature appropriate for the species
Food quality
1. Appropriate protein and fat levels
2. Quality ingredients
3. Correct pellet size and form
Stocking
1. The right species
2. The right stocking density

Preventing the Introduction of Exotic Diseases

For private fishing ponds, the primary consideration is the health of the fish that you put in the pond. For sportfish, this means that the fish should come from a reputable farm and that the fish should look healthy and free of disease when you purchase and stock them. It may even be worthwhile to ask the supplier about their fish health inspection program. Many sportfish farms have very rigorous disease prevention programs, and they will be happy to share that information with you. You should never stock fish that you have caught in the wild. The wild fish may carry diseases that you do not want in your pond.

For ornamental fish ponds, the most important consideration is to buy healthy fish that come with some assurances about their disease status. Of special concern are koi. There are some very serious exotic koi diseases that you must avoid. If you buy healthy-looking fish produced in the United States by a reputable breeder, the chance of having problems with koi ulcer disease or koi herpes virus (KHV) are fairly low; however, new ornamental fish should always be quarantined for several weeks (preferably at water temperatures in the 70s) before they are put in your pond. It would also be a good idea to ask the breeder about their koi herpes virus prevention program. Good koi breeders will be happy to tell you about their disease prevention measures.

For commercial aquaculture, preventing the introduction of exotic diseases is of critical importance. All farms that produce aquatic animals should have plans to prevent the introduction of exotic diseases. The main routes of introduction are (in order of importance) infected animals, contaminated water, contaminated equipment, and wild animals that visit the farm facility. To avoid disease problems, farms must produce their own stock or buy stock only from sources with good fish disease testing programs and good disease prevention plans. If possible, the farm should use water supplies that do not harbor exotic diseases (wells and springs are very safe; run-off water is good; surface water from lakes and rivers may carry significant risks). Equipment returning to the farm from other places should be cleaned and disinfected before entering. Visits to the farm by wild animals that may carry diseases must be discouraged.

Recognizing Fish Health Problems

It can be difficult to monitor aquatic animals for health problems, especially when the animals live in large ponds or in water that is not crystal clear. The list below describes some of the clues that may suggest that your aquatic animals are sick.

A reduction in feed consumption not clearly related to low oxygen or temperature changes
Any change in behavior including:
1. Fish congregating at the surface or at the water’s edge
2. Fish congregating around fresh water inflows or aerators
3. Fish swimming slowly or in circles
4. Fish “flashing” (rubbing against the bottom or other hard surfaces)
Damage to skin and fins including:
1. Fin erosion
2. Skin ulcers
3. Skin discoloration (darker, lighter, reddening)
4. White or tan fuzzy patches
5. White spots
6. Bumps on the skin
7. Open sores
8. Cloudy or swollen eyes
Dead and dying fish (the obvious one!)

What to Do When You Suspect That Your Aquatic Animals Are Sick

The first step whenever any aquatic animal health problem is suspected is to look at the water quality. Commercial fish producers usually have some water quality testing equipment. All producers should be able to test for oxygen levels, ammonia, pH, and nitrite and should recognize optimal and acceptable levels of these chemicals. Sportfish farm pond owners and ornamental fish owners will often need to seek out professional assistance with their water quality testing.

When water quality problems are ruled out, the next step is to consider parasites and other diseases. Many aquatic animal diseases produce symptoms that are very similar. A correct diagnosis usually requires an aquatic animal health laboratory with the expertise to recognize fish parasites by microscopy and the ability to culture and identify bacteria, fungi, and viruses. When a disease problem is suspected, the first thing to do is to contact a fish health professional or fish disease diagnostic laboratory for assistance.

Working with a Fish Health Professional or Fish Disease Diagnostic Laboratory

Finding help with a fish health problem can often be quite challenging. Assistance might come from state or university fish health laboratories, veterinary schools, extension programs, wildlife agencies, or veterinarians who include aquatic animals in their practice. For commercial fish farmers, the best way to find fish health assistance is to consult your state aquaculture coordinator, your state aquaculture extension staff, your state veterinarian, other local fish and shellfish producers, or the American Veterinary Medical Association at www.aquavetmed.info. For private sportfish ponds, you may find help at the same places as the fish farmers, but it is also likely that your state wildlife agency will be able to guide you. Ornamental fish owners may find assistance at the same sources as fish farmers, but they may want access to services like X rays, ultrasound, blood chemistry, and surgery that are only available through veterinarians.

Once you have identified a fish health professional, the next step is to call that professional and ask what kinds of information and samples will be needed. The information needed will often include answers to the following questions:

How big is the pond/tank?
What species of animals are dying?
How many fish are dying?
How long have they been dying?
Are there sick fish swimming around right now?
How are those sick fish behaving?
What do they look like (sores, spots, and the like)?
Are they eating?
Have you used any treatments?
Have you treated weeds or algae recently?
Have there been any recent changes in the color or clarity of the water?
What has the weather been like (temperatures, wind, and rain)?
When did you last stock new fish?
When did you last harvest fish?

In some cases, help may come to your farm or pond, but often you will be asked to send samples to a laboratory. These samples may include water, live fish, fish on ice, or other materials needed for testing. Only the laboratory can provide you with detailed directions. It is very important to always call the lab prior to sending any samples for testing.

Treating Fish Diseases

Drugs and chemicals to treat diseases in aquatic animals are highly regulated by the FDA and EPA. There are very few legal drugs, especially for food animals, and very strict limitations on their use. All drugs and chemicals must be used exactly according to their labels or under direct veterinary supervision. Any violation is punishable by state and federal law, and any product treated with an illegal drug or chemical is “adulterated” and would be ordered destroyed. When treating an aquatic animal disease:

Be sure that the disease has been correctly diagnosed (often this requires assistance from a fish health professional).
Be sure that the chemical or drug to be used is appropriate for the disease, the fish species, and the water quality (including the temperature).
Be sure that the chemical or drug is legal for the use.
Be sure that the chemical or drug is used exactly according to the label or according to guidance from your veterinarian.
For food animals, be sure that the required withdrawal period (time elapsed between the last treatment and sale of the animals as food) is followed.

For commercial fish production, there are also several legal vaccines that can be used to prevent fish diseases. As with drugs, these vaccines must be used according to their labels.

Fish Health Inspection and Certification

Commercial fish farms that ship fish alive, especially interstate or internationally, are often required to conduct inspections for fish diseases. Each state, federal, and international jurisdiction may write and enforce its own unique fish health regulations. Before shipping any live product anywhere, it is imperative that you make sure that you understand the regulations of all the legal jurisdictions involved, that you have complied with all of the requirements and obtained all of the needed permits, and that your shipment (number, species, dates, etc.) is exactly according to those permits and regulations. Any interstate violation of fish health regulations can put you and your farm at risk for severe penalties (including felony convictions) and fines under state laws and the Federal Lacey Act.

Importation of fish into the United States is under the jurisdiction of the U.S. Fish and Wildlife Service (USFWS) (salmon, trout, and other salmonid fish species), the U.S. Department of Agriculture-Animal and Plant Health Inspection Service (USDA-APHIS), and the destination state. Federal and state permits may be required prior to shipment into the United States. Fish are often restricted to entry through certain ports where federal inspectors are available. Before importing fish, consult with the USDA-APHIS-National Center for Import and Export (NCIE) for advice and with the agriculture and wildlife agencies of the destination state. The NCIE can also assist you with exports to other countries.

For other shipments of live animals, sometimes even within your state, be sure that you have complied with all state aquatic animal health regulations. The regulatory agency with jurisdiction over aquatic animals varies from state to state, and you may need to consult both the state agriculture agency and the state wildlife agency. Extension fish health experts, other farmers, regional aquaculture centers, state aquaculture coordinators, and fish health testing laboratories may be valuable sources of information and assistance; however, penalties for errors can be severe, so careful shippers always confirm their regulatory knowledge with an official representative of the regulatory agency.

edited by
Robert Durborow, State Extension Specialist for Aquaculture, Kentucky State University