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Fish Disease and Fish Kills

Report Fish Kills

Please report sick, dead or dying aquatic species with this form or by calling the local Fisheries Management Unit Office or the Main Fisheries Division Office 517-284-5830. Please report fish kills over 25 fish as soon as possible.


Report a fish kill

More information about fish kills

Possible Diseases

  • A disease that may affect any fish cultured intensively. BGD is caused by a number of different bacteria that infect the gills of fish. The disease is characterized by the presence of large numbers of filamentous bacteria on the gills accompanied by clubbing and fusing of the gill filaments. This reduces the ability of the gills to supply oxygen to the blood and results in mortality if left unchecked. The onset of BGD is usually linked to a decline in environmental conditions in combination with overcrowding.

    The best treatment is prevention through maintenance of optimum rearing densities and keeping rearing units as clean as possible to prevent the growth of bacteria. A flow through treatment of Chloramine-T for 60 minutes at 8.0 parts per million is usually effective in eradicating the disease. The U.S. Food and Drug Administration (USFDA) have not yet approved Chloramine-T for general use. State hatcheries use this drug under a special permit (INAD) issued by the USFDA.

  • A disease of trout and salmon caused by the bacterium Renibacterium salmoninarum. The disease is systemic but derives its name from off-white bacterial lesions in the kidney. In later stages many organs may become affected and the body cavity may become filled with fluid.

    This disease has been thought to be responsible for mortalities of chinook and coho salmon in Lake Michigan over the past decade that led to dramatic declines in those fisheries. In 1993, Fisheries Division began screening all salmonid adults and eggs during spawning operations to detect the presence of BKD. All fish and eggs that test positive are destroyed.

    The bacterium that causes BKD is endemic in Michigan lakes so total eradication is not probable. The screening process during spawning is the best available control measure to reduce the incidence of the disease by using fish that are less susceptible to it. This process uses the natural selection process to reduce the impact of the disease on our fisheries.

  • A common disease in earthen bottom ponds and lakes, this disease is caused by a parasite (larval trematode) that burrows into the skin of a fish causing the formation of a cyst approximately one millimeter in diameter. This parasite has a complex life cycle that requires fish eating birds or mammals, snails, and fish at different stages in order to survive.

    In general, even heavy infestations of these parasites do relatively little damage to the fish. There is some evidence that heavily infested juvenile fish may experience excessive blood loss, physiological stress, and even death. Also, fish with heavy infestations on the eyes may be blinded.

    While not recommended by Fisheries Division, it is possible to reduce the incidence of the parasite by removing or reducing the intermediate hosts such as snails. This will disrupt the life cycle of this parasite. It is recommended that individuals contact their local fisheries biologist for more information.

    Control of this parasite is really not necessary as these parasites are incapable of infesting humans and the fish are safe to eat. It may be more aesthetically pleasing to skin a heavily infested fish prior to eating. In any case, cooking kills the parasites.

  • A disease of young salmonid fishes caused by a myxobacterium Cytophaga psychrophila. Coldwater disease occurs when water temperatures fall between 45o to 50o F. This disease is characterized by lesions in the caudal peduncle or on the isthmus or other areas of the body. The caudal fin is often completely destroyed and the fish may become dark colored.

    An effective treatment for coldwater disease is medicated feed containing oxytetracycline (Terramycin) at a rate of 50 mg/kg/day for 10 to 14 days. Where possible, raising the water temperature above 50o F will cause the infestation to subside.

  • A bacterial disease caused by the bacteria Flexibacter columnaris. This disease is usually associated with some kind of stress condition such as high water temperature, low dissolved oxygen concentration, crowding, or handling. Symptoms of this disease include grayish-white spots on some part of the head, fins, gills, or body usually surrounded by an area with a reddish tinge. The columnaris lesions on different species of fish vary in size, location, and appearance. On fingerling rainbow trout, a lesion usually originates on the back of the fish and progresses down each side resembling a saddle. On crappies, the lesions are generally confined to the fins and gills and rarely extend to the body. The lesions on bullheads generally appear as small circular areas with sharp distinct outlines. Although columnaris most commonly involves external infections it can occur as an internal systemic infection with no visible external signs. Scrapings from a columnaris lesion placed under a microscope will reveal long, thin, rod shaped motile bacteria. The bacterial clumps form microscopic columns or dome shaped masses, hence the name columnaris.

    Columnaris is most commonly seen in warmwater species when water temperatures are above 68 degrees F and in salmonids species when temperatures are above 59 degrees F. Under appropriate conditions columnaris can spread rapidly and cause catastrophic losses in a matter of several days.

    External infections of columnaris can be treated with copper sulfate at 0.5 parts per million (in ponds) or a Terramycin bath at 15 parts per million active ingredient for 24 hours. Internal infections can be treated with feed containing Terramycin.

  • Fungus, often called 'water mold' is a common parasitic disease affecting all species of fish. The most common fungi species affecting fish are of the genera Saprolegnia, Achlya, and Dictyuchus. Fungi infestations are generally considered to be secondary infections that occur following an injury or in association with poor environmental conditions.

    Signs of fungal infections include cottony or fuzzy appearing growths usually associated with discolored areas or lesions. The fungal growths contain flagellated spores that are capable of moving out and infecting other fish. If left untreated, fungal infestations will spread to all fish in an infested rearing unit.

    Eggs should be treated daily with a flow through treatment of formalin at 1,600 2,000 parts per million for 15 minutes to prevent fungal infestation. Hydrogen peroxide can also be used. Fisheries Division is also currently evaluating non-chemical approaches to treating eggs. An effective treatment for fish is a flow through treatment of a 0.5 1.0 % salt (sodium chloride) solution for 15 minutes repeated daily as needed.

  • A bacterial disease of salmonids that is usually characterized by boils or furuncles on the skin of affected fish although this is not always the case. The causative agent is the bacterium Aeromonas salmonicida. Furunculosis is systemic in nature as it travels through the bloodstream and affects all parts of the body, especially the vital organs. The rapid multiplication of bacteria in the bloodstream causes smaller blood vessels to rupture allowing the bacteria to spread to surrounding tissue. When allowed to advance to this stage, the disease is always fatal.

    Furunculosis is endemic in a number of Michigan waters but is not generally a problem in nature. In crowded hatchery settings, the disease can cause high mortalities if not recognized and treated at its onset.

    Medicated feed containing the antibiotic Terramycin or Romet are often effective in treating furunculosis.

  • Piscirickettsia - A New Pathogen in Lake St. Clair Muskies - A Fact Sheet of our Current Knowledge

    Resource Background: The muskellunge fishery of Lake St. Clair has a reputation among muskie anglers for unusually high catch rates. The fishery draws anglers from the local area as well as from across the continent. Numerous charter fishing businesses specialize in muskie trips. Countless individual anglers fish exclusively for muskellunge while the season is open. A large Detroit area angling club is dedicated to the sport of muskie fishing. Many Lake St. Clair muskellunge anglers espouse a strict catch-and-release ethic. The Lake St. Clair muskellunge fishery is completely self-sustaining, thus any threat to the continued successful recruitment of muskellunge in the lake is a serious concern for fisheries managers.

    Pathogen Detection in Lake St. Clair: : In May of 2002, the Michigan Department of Natural Resources (MDNR) collected several muskellunge exhibiting unusual red skin rashes from the Anchor Bay area of Lake St. Clair. The rounded rashes reached up to one inch in diameter (Figure 1). Laboratory analysis at Michigan State University by Dr. Mohamed Faisal, revealed the association of these lesions with a Gram-negative, curved, intracellular bacterium that occurred as rings or curved rods (Figure 2). Based on their shape and culture characteristics, the organism was determined to be a Piscirickettsia-like organism.

    Pathogen Facts:

    • Piscirickettsia salmonis (P. salmonis ) was the first "rickettsia-like" bacteria to be recognized as a pathogenic agent of fish.
    • P. salmonis infects a wide range of salmonid fish species causing a systemic infection associated with high mortalities in Chile, Norway, Ireland, and Canada. This organism has never been found in any non-salmonid fish species.
    • Similar agents, known as Piscirickettsia-like organisms, have been found to be associated with increased mortality rates in diverse fish species, including white seabass, black seabass, tilapia, blue-eyed plecostomus, and dragonet. Three of these Piscirickettsia-like organisms have already been reported in non-salmonid fish species in North America.
    • Piscirickettsia are not known to infect human or other terrestrial animals.

    Pathogen Impacts in Lake St. Clair: While the Piscirickettsia-like organism has been found in Lake St. Clair, DNR surveys and reports by fishermen have not indicated negative impacts in the muskellunge fishery.

    Fisheries Division Action Plan: The discovery of this infection in Lake St. Clair muskellunge has raised the following questions:


    • How widespread is the disease in the lake's muskellunge population?
    • Does the organism affect the longevity of infected fish?
    • What could be the source of the infection?
    • What measures should be undertaken to prevent the spread of this infection?
    • Are other fish species affected?
    • Will the infection affect the abundance of muskellunge in the lake?

    In response to public concerns, the DNR has assembled a team of fishery biologists, managers, and specialized veterinarians that continue to monitor this case. Muskie anglers wishing to assist in this effort and anglers that catch a muskie in Lake St. Clair bearing red sores similar to those shown below are asked to report their observation to the Lake St. Clair Fisheries Research Station using the following contact information.


    This roundworm (nematode), commonly called the red worm, is abundant in yellow perch of Lake Huron and western Lake Erie, but is also found in other localities. As high as 86% of fish examined from areas of Lake Huron have been found infested with as many as 78 worms per fish, although it is usually found in lesser abundance.

    This parasite is pink to red in color, very slender, and may reach 2 inches (5 cm) in length. It is usually found encysted within the body cavity of the fish, but in very heavy infestations, or in fish not dressed quickly upon capture, the worms may move out of the cysts and be found free in the body cavity or even in the flesh.

    The life cycle of the red worm of perch is not known but it is believed to be similar to the closely related species (P. nodulosa) found in the common sucker which use various species of a crustacean (Cyclops) as an intermediate host

    Red worm encysted in the body cavity of a perch. Note the reddish worm at the tip of the pointer.

    In this case, larvae developing from eggs of the adult female worms in fish escape into the water and are eaten by species of Cyclops. Within the copepod the larvae attain a certain growth stage, and remain in this state until eaten. When a fish feeds on the colpepod, the larval worm leaves the crustacean, grows, and migrates through the tissues of the fish, eventually encysting in the body cavity. 

    Although this parasite has not been implicated in mortalities of fish, heavy infestation must certainly affect the normal growth and vitality of such fish. These nematodes are incapable of infesting man, regardless of how objectionable a parasitized fish might be aesthetically. As with all parasites, thorough cooking kills this worm.

  • This is a common disease often seen in intensive fish culture. It is caused by a bacterium whose exact identity is unknown. The bacterium first attacks the adipose fin, which will display a noticeable white line along the outer margin. As the disease progresses the adipose fin is destroyed and the bacteria will enter the caudal peduncle (tail) of the fish. Eventually the bacteria will cause the vertebral column to become exposed and become infected. When the disease is allowed to progress past the adipose fin it is always fatal.

    Tail rot is associated with high rearing density and poor sanitary conditions in the rearing unit. The best treatment is prevention by keeping rearing units as clean as possible to prevent the growth of bacteria. A flow through treatment of Chloramine-T for 60 minutes at 8.0 mg/L is effective in eradicating the disease. Chloramine-T has not been approved for general use by the USFDA. Michigan hatcheries use this drug under a special permit (INAD) issued by the USFDA.

  • Viral hemorrhagic septicemia (VHS) is an extremely serious viral disease of fresh and saltwater fish. It has recently spread into the Great Lakes region of the United States and Canada. VHS virus has been found in Lake Huron, Lake St. Clair, Lake Erie, Lake Ontario, Lake Michigan, and the St. Lawrence River in New York. The virus also has infected several inland lakes in New York, Michigan, and Wisconsin. The disease can cause large-scale fish kills and have severe economic consequences.

    What are the symptoms?
    At a low level of infection, fish might not display any noticeable symptoms. As the infection intensifies, fish will display widespread hemorrhages (bleeding) throughout body surface (eye, skin and fins) and within the internal organs (swim bladder, intestine, kidney, etc.). Because of the bleeding, gills and liver might appear pale. Sick fish often will be listless, swim in circles, and frequently are observed at the surface of the water.
    NOTE: Confirming VHS infection requires laboratory testing. A diagnosis cannot be made based solely on observation because many different diseases of fish have very similar symptoms.

    Will the virus affect humans?
    No. The virus will not affect humans regardless of whether you touch or eat it because it dies at human body temperatures.

    Where has VHS been found in Michigan waters?
    To date, we have found VHS in the following waters: Lake Huron including Saginaw Bay, the St. Clair River, Lake St. Clair, the Detroit River, Lake Erie and all tributaries up to the first dam or barrier. VHS also has been documented in Budd Lake in Clare County and in Baseline Lake in Washtenaw County. As other areas are identified positive for VHS, they will be listed online at

    What about other waters in Michigan?
    While VHS has not yet been confirmed in all waters of the state, other areas are at risk for VHS infection. To prevent or slow the spread of VHS to other waters, anglers are reminded to keep the following tips in mind when using baitfish:

    • Learn to identify the species of baitfish you are using. Species known to be susceptible to VHS and typically used as live bait include emerald shiners, spottail shiners, and white suckers. Other species occasionally used as bait that are susceptible to VHS include bluntnose minnows, trout perch, gizzard shad, shorthead redhorse, and silver redhorse.
    • Request that your local bait store sell baitfish that are certified disease-free.
    • Purchase and use only baitfish that are certified disease-free.
    • Never move live fish between bodies of water.
    • Check the Michigan Fishing Guide for current regulations and follow the regulations restricting baitfish use.
    • Disinfect bait bucket between uses.

    How does the disease spread between waters?

    1. Moving VHS-infected fish from one body of water to another. This includes live gamefish caught in an infected body of water and released in another water, or live baitfish caught or used in an infected body of water and transported, used or released into another body of water.
    2. Moving infected water from one body of water and discharging into another. Examples would be the discharge of infected water and fish from ships, discharge of infected water from live wells on fishing boats, and discharge of infected bilge water from recreational and fishing boats.
    3. Stocking or releasing VHS-infected fish or water from infected fish hatcheries.
    4. The natural movement of infected fish from one body of water to another.

    Protect the waters that supply our hatcheries
    To protect hatchery stocks of fish from possible VHS infection, the use of baitfish and roe (fish eggs) is prohibited in certain waters of the state, including portions of the following waters in Benzie, Chippewa and Marquette counties. 

    • Benzie County: Brundage, Kinney and Stanley Creeks.
    • Chippewa County: Pendills Lake (T47N, R4W, S25, 26) and Pendills, Sullivan and Viddian Creeks.
    • Marquette County: Cherry Creek.

    VHS Clinical Signs (photos)


    This is the common "grub" found in our freshwater fish as a yellow worm up to 1/4 inch (6.4 mm) long just under the skin, or in the flesh. Yellow grub has been reported from so many kinds of freshwater fish in North America that apparently no fish is immune to it. The grub is the larval stage which must be eaten by fish eating birds, such as herons and bitterns, to develop. The grub matures in the throat of the bird, and eggs wash into the water from the bird's mouth when feeding. The eggs hatch and the first larval stage (miracidia) swim by means of fine hair like cilia until they find a snail of the genus Helisoma. Unless they find this snail they die within several hours. In the snail they go through several developmental stages during which they multiply a thousand fold, finally leaving the snail as free swimming cercariae. Unless the cercariae find a fish within a few hours, they die. When they find a fish, they burrow through the skin and encyst, where they develop into metacercariae, which are the yellow grubs. There they remain until eaten by the bird host, thus completing the life cycle.