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Diagnosis of Fish Diseases in the Upper Midwest

Background Information

Fish diseases are a common problem in fish culture. A basic understanding of the nature of fish diseases is important for fish culturists. With this information, the culturists will be better equipped to prevent and handle disease outbreaks.

What Is An Infectious Fish Disease?

Fish, like all animals, are subject to a variety of diseases. They can suffer from infectious diseases caused by pathogens, which are organisms capable of causing disease only when the host’s resistance is lowered. These include parasites, bacteria, or viruses.

Fish can also suffer from environmental and nutritional diseases. A fish disease is a state of imbalance between the fish and its environment, which may lead to death.

Under natural conditions, fish seldom suffer from severe disease outbreaks. However, when fish are crowded and reared under unnatural conditions, the potential exists for a serious disease outbreak.

What Causes Fish Disease?

Fish diseases are the result of interaction between a pathogen, a fish (host), and a stressful environment (Figure 1). Even if the pathogen is present, a disease outbreak will not occur unless the environment becomes too stressful for the fish (Figure 2).

Venn diagrams describing how diseases occur when a pathogen is present with a host fish in a stressful environment.

Intensively raised fish are stressed by fluctuating water temperatures, changes in water quality, overcrowding, handling, and transport. Fish can handle these stresses up to a point but when they can no longer adjust, they may succumb to disease and eventually die.

How Can Disease Be Prevented?

Prevention of disease is a primary goal in aquaculture. Ideally, the fish facility should be maintained pathogen-free but this cannot always be done. If the fish are bought, the buyer should only obtain fish that are certified disease-free. The fish should be stocked in an environment with sufficient oxygen, low levels of ammonia, and minimal organic loading (overfeeding fish will result in an accumulation of food, causing poor water quality). Finally, the fish must not be handled too often or too roughly. Keep the amount of stress encountered by the fish to a minimum. For example, transportation and handling stress can be reduced by adding 0.1% to 0.3% salt and enough calcium chloride to raise the total water hardness to 50 parts per million for soft waters. Additional calcium chloride is not needed in harder waters.

What Are the Warning Signs of Diseases

In spite of the best preventive efforts, diseases may occur. It is vital to detect the disease as soon as possible. The best way to detect a disease at its onset is by observing the fish when they are feeding. If any of the following are observed, a disease outbreak may be starting and immediate treatment is necessary.

Behavioral Signs

  • Failure to feed properly
  • Flashing (turning on their sides)
  • Rubbing on the bottom
  • Gathering around the water inflow
  • Reduced vitality
  • Gasping at the surface

Physical Signs

  • Blistered areas
  • Swollen bellies
  • Popped-out eyes
  • Bloody (hemorrhaged) areas on fins
  • Discoloration or erosion of body parts
  • Excessive mucus
  • Growths on the body

Fish mortality rates should be carefully monitored and recorded every day. This can indicate the trend of the disease and provide clues as to the cause. Figure 3 (Meyer et al. 1983) illustrates the relationship between cumulative mortality and time (days), which helps trace the cause of the problem. Line (A) represents a die-off due to a severe environemental failure (e.g., low oxygen or a toxic chemical). Line (B) is die-offs due to a severe disease outbreak; line (C) is die-offs due to a persistent disease. A diagnostic laboratory can use this information to help distinguish between an infectious disease and other types of disease.

Diagnostic Information

In most cases, fish culturists will need the assistance of a trained pathologist to diagnose and treat a disease.

How Can An Infectious Disease be Diagnosed?

If an infectious disease outbreak is suspected because of a warning sign(s), then samples of fish should be sent to a disease specialist for analysis. Contact a disease specialist or laboratory for specific instructions on how many fish to send and how to ship them.

The following list consists of the kinds of fish to sample (in order for preference from excellent to fair), that can best be used by a disease specialist to determine the cause of a disease:

  1. Fish, delivered live, exhibiting behavioral symptoms such as lethergy, listlessness, or erratic swimming. Probability of finding the cause is high.
  2. Fish, delivered live, exhibiting overt physical symptoms such as open sores, eroded areas on the body surface or gills, bloody fins, and swollen or clubbed gills. The probability of finding the cause is high.
  3. Dead fish that still have red gills and somewhat normal amounts of color and mucus. This is a fair sample; the probability of identifying the cause of death depends on how long the fish have been dead.

The following list consists of the kinds of fish samples that cannot be used by a disease specialist to determine the cause of the disease:

  1. Several fish taken at random from a seine sample. This is a poor sample; the probability of identifying the cause of death is low since a majority of the fish may be healthy. The fish that are sampled must have the symptoms.
  2. Dead fish that have lost body color and mucus layer and have white, mushy gills or fish that have been frozen. These are unusable samples.

Who Can Be Contacted For Advice and Diagnosis Services?

The first contact for Minnesota residents is the Minnesota Department of Natural Resources. They will provide advice, and if their schedule permits, diagnositic services. Residence of other states should contact the appropriate state fisheries agency.

Fish Pathology Laboratory
Minnesota Dept. of Natural Resources
500 Lafayette Rd., Box 25
St. Paul, Minnesota 55155-4025
Phone: (651) 259-5100

A second contact is United States Fish and Wildlife Service Fish Health Center. They will provide advice and diagnostic services. The center for Minnesota and nearby states is:

United States Fish and Wildlife Service
Fish Health Center
555 Lester Ave.
Onalaslka, WI 54650
Phone: (608) 783-8441

Selected Further Readings

  • Procedures for the Detection and Identification of Certain Fish Pathogens, by David McDaniel, American Fisheries Society, 1979.
  • The Third Report to the Fish Farmers, by Harry K. Dupree and Jay V. Huner, United States Dept. of the Interior Fish and Wildlife Service, 1984.
  • A Guide to Integrated Fish Health Management in the Great Lakes Basin, by Fred P. Meyer, James W. Warren and Timothy G. Carey, Great Lakes Fishery Commission, 1983.
  • Fish Hatchery Management, by Piper et al., United States Dept. of the Interior Fish and Wildlife Commission, 1982.


Meyer, F.P., J. W. Warren, and T.G. Carey (ed.). 1983. A Guide to Integrated Fish Health Management in the Great Lakes Basin. Great Lakes Fishery Commission, Ann Arbor, Michigan. Spec. Pub. 83-2:272 p.

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This page last modified on May 04, 2016     © 1996 – 2020 Regents of the University of Minnesota     The University of Minnesota is an equal opportunity educator and employer.
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