Mackerel gill mycosis
Branchial mycosis is an infection of the gills caused by filamentous fungi (often of the Branchiomyces genus or opportunistic aquatic molds). It affects marine fish such as mackerel, particularly when the water quality is poor or the fish i...
What is it?
Branchial mycosis is an infection of the gills caused by filamentous fungi (often of the Branchiomyces genus or opportunistic aquatic molds). It affects marine fish such as mackerel, particularly when the water quality is poor or the fish is weakened. The fungal filaments invade the gill tissues, reducing the fish's ability to breathe properly. This disease is mainly documented in breeding or captivity, more rarely in the wild.
Key point
This disease can progress rapidly and cause high mortality in a group of fish if water quality is not corrected quickly.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Affected mackerel shows accelerated or irregular breathing, because the damaged gills struggle to capture oxygen. \nIt can swim near the surface or near water outlets, seeking a more oxygenated place. \nA general depression sets in: the fish becomes less active, stays away from the group. \nThe appetite decreases markedly. \nWe sometimes observe friction against the walls or decor, a sign of irritation. \nIn advanced stages, the gills may appear discolored, mottled or necrotic upon close inspection.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by the proliferation of filamentous fungi that colonize the gill tissue. These fungi take advantage of water loaded with organic matter, poor oxygenation or an imbalance in the environment to develop. Mackerel, a pelagic fish sensitive to stress and water quality, is particularly exposed to it in captivity.
Poor quality water (excess organic matter, high ammonia) strongly favors the appearance of the disease. \nToo high a density of fish in a pond or fishpond increases the risk. \nSignificant stress, linked to transport, handling or sudden temperature variations, weakens the fish's defenses. \nPre-existing injuries to the gills make it easier for the fungus to take hold.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is based on observation of respiratory symptoms associated with a visual or microscopic examination of the gills, carried out by a veterinarian specializing in aquatic pathology. A gill tissue sample allows the fungal filaments to be identified under a microscope and the cause to be confirmed.
Treatment
The treatment is based above all on the urgent improvement of water quality. Antifungal treatments adapted to the aquatic environment can be used under the supervision of a specialist. General support for the fish (stress reduction, isolation) is part of the treatment.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Maintaining excellent water quality is the most effective measure: appropriate filtration, regular renewal, monitoring of parameters. \nAvoid overpopulation in ponds or fishponds. \nLimit sources of stress during handling and transport. \nPromptly isolate any fish showing suspicious signs to prevent spread in a group.
Possible complications
Without intervention, progressive destruction of gill tissue can lead to severe respiratory failure. \nWeakened fish also become more vulnerable to secondary bacterial infections. \nMortality can be significant in the event of massive damage to a group of fish in captivity.
When to see a veterinarian
The fish breathes abnormally quickly or irregularly.\nIt remains motionless or stands near the surface or oxygenation areas.\nSeveral fish in the same tank show similar signs.\nThe gills appear discolored or damaged upon observation.
Prognosis
The prognosis depends on the speed of treatment and improvement of water conditions. If detected early, the disease can be controlled. At an advanced stage, with significant destruction of the gills, the prognosis becomes more guarded.
Frequently asked questions
Is mackerel gill mycosis contagious between fish?
The responsible fungus grows mainly because of water quality, but it can spread more easily in a pond where several fish share the same weakened environment.
Can this disease be prevented without veterinary intervention?
Good water management and regular monitoring greatly reduce the risk, but veterinary advice remains essential as soon as symptoms appear.
Does this disease also affect other marine fish?
Yes, gill fungi can affect several species of fish living in similar conditions, especially in captivity or breeding.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 26 September 2026
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