Symptoms and Treatment Methods of Aquatic Fungus Disease in Koi

The term "fungal disease" is heard frequently, and there are many examples showing that even in mammals, including humans, this disease is difficult to cure. So-called "fungi" mainly refer to algae-like fungi, or aquatic fungi and filamentous fungi belonging to the Fungi Imperfecti. These fungi attach to the remains of animals and plants, using the remains as a nutrient source, and grow filamentous hyphae. These hyphae belong to lower plants that do not contain chlorophyll, and according to reports from academic societies, there are more than ten types of hyphae, including aquatic fungus disease. What will be discussed here is aquatic fungus disease, which breaks out from late autumn through the overwintering period, also commonly known as "cotton-cover disease" or "mud-cover disease."

Feeding and Water Quality Management

The season when water temperature begins to drop has arrived, and this is also the time when you can enjoy the results of raising koi with beautiful colors and sturdy bodies. When feeding high-protein baits such as pupae, wearing a short-sleeved shirt will feel slightly meaningful! If the water temperature drops below 22°C, you should switch to regular food as early as possible, and to make the koi more valuable for appreciation, it is best to reduce the amount of food. During this period, if you force-feed too much food, various organ disorders will be triggered during overwintering, and there is a risk of failure in keeping.

Generally speaking, after about three months you should stop the color-enhancing food and switch to regular food, and the feeding amount should also be gradually reduced. In this way, the koi can be raised very beautifully. The necessary condition at this time is that the keeping water should be maintained in good condition. Up to now, you should have fed a lot of food, and the amount of excrement must also be considerable! You must check whether the filter tank is functioning properly. In addition, when the temperature difference between day and night is large, you should try to control the temperature difference to about 2°C. Adjusting the amount of fresh water is also one method.

Onset

This is the zoospore of aquatic fungi (a type of asexual spore that moves by flagella), which commonly exists in a wide range of waters, attaching to wounded parts of koi, and using their necrotic tissue as an energy source to reproduce, causing disease. In addition, this aquatic fungus is also called a "saprophytic fungus."

The causes of injury leading to aquatic fungus disease are rough handling of koi, bites from weasels or martens during overwintering, or abrasions caused when parent koi spawn naturally. Aquatic fungi multiply on such affected parts. Because the damage caused by aquatic fungi in the aquaculture industry is very serious, the scope of damage is extremely wide, from farmed salmon and shrimp trout to soft-shelled turtles. For prevention, the antifungal agent malachite green can be used.

Symptoms

Aquatic fungus disease mainly occurs from late autumn to overwintering at low water temperatures (below 15°C). However, in early summer during the spawning period, when parent koi spawn naturally, abrasions may sometimes form because of unexpected obstacles such as tree roots or branches, and if not treated, the symptoms will quickly become severe, the parent koi will die, and major losses will be suffered. Great care must be taken.

There will definitely be necrotic tissue at the site of external injury, and using the necrotic tissue as a nutrient source, the zoospores attached to the wound gradually invade the surrounding tissue, causing the affected area to expand. At the wound site, the white, long hyphae unique to aquatic fungi grow in clumps, looking as if white cotton wool is attached. Therefore, it is commonly known as "cotton-cover disease" or "mud-cover disease."

In cases of chronic symptoms, when touching the affected area (palpation), the koi's muscles feel slightly stiff; this is because the hyphae are reproducing and elongating within the muscles. As the condition drags on, the hyphae elongate even further and invade important organs. By this time, the koi will rapidly weaken and die.

Treatment

The antifungal agent malachite green is very effective at suppressing the development of aquatic fungi, so since long ago, in the farmed fish industry, beginning with artificial insemination of salmonid fish, malachite green has been favored for prevention. Coloring agents decompose rapidly under sunlight, so it has become common knowledge to apply malachite green at dusk on clear days and during the day on cloudy days.

[Long-duration medicated bath]: In a pond with 0.2-0.3 of malachite green per ton of pond water, carry out a medicated bath for 24-48 hours.

Please adjust according to the size or symptoms of the koi. If this agent is used for a long period, it may sometimes accumulate in the koi's body, and many documents point out that malachite green has carcinogenic properties. Therefore, the concentration and bath time during use carry danger, and everyone should pay close attention and handle it carefully and cautiously. When giving emergency treatment to many koi, a medicated bath using this agent can be used. As for externally visible wounded areas, Isodine, which has iodine as an ingredient, must be applied promptly. Treating external injuries as early as possible is an essential requirement for koi lovers.

After koi suffer from hole disease and bell-shaped ciliate disease, part of the muscle becomes exposed. Moreover, at low water temperatures, zoospores will likewise attach. Therefore, you must use absorbent cotton to remove moisture from the affected area, and then apply Isodine solution. It not only has bactericidal and insecticidal effects, but can also promote the regeneration of new granulation tissue.

Regarding other treatment methods, for koi with relatively small affected areas and whose strength has not yet declined, apply a 3%-5% potassium permanganate solution and Isodine solution several times to the sore area. In this way, the hyphae will gradually die, and dirt can also be removed. Therefore, the unclean feeling of the affected area will gradually disappear. When injectable antibacterial agents, liver-strengthening agents, and vitamin agents are used at the same time, recovery can be hastened. However, for koi whose affected areas are stiff and which are treated too late, the prognosis will be poor.

Precautions When Having Parent Koi Hatch Fry

It seems that quite a few people are extremely interested in having parent koi hatch fry or in raising young koi. To raise young koi, you must first spread lime in the spawning pond to adjust the water quality containing aquatic fungi. In addition, algicides must be used to suppress the reproduction of various algae. Without experience, you may sometimes suffer miserable failure. As for such problems, you might as well ask your seniors for advice.

If tap water is used, the number of bacteria will be relatively small. In river water or spring water, however, zoospores of aquatic fungi are often present. But even if good-quality water is poured into the spawning pond, after five days, many organisms harmful to raising young koi will increase sharply.

One week before the scheduled spawning day, after spreading an appropriate amount of lime in the spawning pond, which both adjusts the pH and disinfects, let it dry in the sun that same day, and do not spread lime again until the spawning day. If an algicide is used, this work should be stopped more than three weeks before the scheduled spawning day. In addition, around three days before the scheduled spawning day, stop the water-filling work as well. When placing the parent koi into the pond, the ideal water temperature is 15-17°C.

Natural spawning should target the period from late May to early June, when a large belt-shaped high-pressure system arrives overhead. For the several days from spawning to hatching, no changes such as a sharp drop in water temperature are an absolutely necessary condition for increasing the hatching rate. Therefore, you must always pay attention to weather information.

In addition, during the period before the parent koi spawn, keep them in a pond where the water temperature will not change abruptly, roughly 12-17°C, and feed them some fresh river snails every day to maintain and enhance the parent koi's strength and vitality. Once the weather conditions approach the ideal state, the scheduled spawning day will be three days later, so water should be poured into the pond as soon as possible. Then, on the morning of the day before the scheduled spawning day (before about 8:00 a.m.), gently place the parent koi into the spawning pond. Since it is a clear day, the water temperature will rise by 5-6°C from morning until dusk. Therefore, the water temperature is usually between 18-23°C, which can be said to have reached a temperature suitable for spawning.

Once this excellent water temperature state is reached, around 5:00 p.m., the parent koi will begin to feel restless and swim around the pond searching for a spawning place. This behavior is called "nest-seeking behavior," and female koi are especially active. At this time, if a spawning bed (chemical fiber brocade) is quickly placed into the pond, the parent koi will spawn at dawn and finish after 2-3 hours. Although they spawn, there are many unfertilized eggs or immature eggs that stop developing, and these will become a breeding ground for aquatic fungi and have an adverse effect on fertilized eggs. Therefore, at dusk on the day before the scheduled spawning day, 0.3 ppm of the antifungal agent malachite green must be spread into the pond.

If female koi exhibit "nest-seeking behavior," they will definitely spawn the next morning. However, if the female koi's physical condition is poor, they must be carefully observed throughout the following day. In addition, if they still have not spawned on the fourth day after filling with water, you must give up, replace the water in the spawning pond, and start over from the beginning! If spawning occurs long after filling with water and no antifungal agent is used, either the hatching rate will drop markedly or harmful organisms will appear frequently. Therefore, sometimes not even a single fry can survive.