Disease management

Seabass is prone to diseases caused by parasites, bacteria, fungi and virus. But diseases and abnormalities due to environmental stresses and nutritional deficiencies have also been recognized.

Viral disease

Lymphocystis Disease

Lymphocystis disease is commonly found in seabass raised in cages especially among juveniles 4–7 cm in total length. It has been observed at all temperatures in rather high salinity. A gross sign of the disease is massive enlargement of the cell within the dermis layer of the fish skin, which resembles the cauliflower disease. Transmission is from fish to fish.

Bacterial diseases

Fish reared in intensive culture conditions are exposed to extreme environmental fluctuations, and they may be more sensitive to stress than wild populations.

a. Aeromonas bacteria

Whenever fishes are exposed to environmental stress or injury, aeromonas causes serious outbreaks of homorrhagic disease with high mortality. Temperature, pH, high CO2 and DO depletion, decomposition products, and free ammonia in the water, all of these can be considered as possible factors for Aeromonas infection. When seabass are overcrowded and water salinity is low for long periods, the diseased fish caused by A. punctata could be observed. Gross signs and behaviours are usually shown by hemorrhage on the fin and tail. In a heavy case, erosion of tail and fin can be seen.

b. Vibrio bacteria

Diseases caused by Vibrio sp. typically appear as ulcerative hemorrhagic septecaemia. The typical symptoms of vibrio disease include congeston of the fins, eccymoses and petechiae on the body surface and frequently, haemorrhages and ulceration of the skin and muscle tissue. The tissues surrounding the infected anus (the vent) are usually reddened and inflamed. Internally, there is congestion and haemorrhagia of the liver, spleen and kidney, frequently accompanied by the presence of necrotic lesions. The gut and particularly the rectum may be distended and filled with a clear viscuos fluid.The body is completely covered by a thick layer of mucous. Occasionally, small-unbroken lesions are present. There may be a reddening of the caudal funs and vent. Internal organs appear normal. Young fish die more rapidly than adults.The pathogenic vibrio which have been isolated from seabass include Vibrio parahaemolyticus, V. anguillarum and V. alginolyticus.

c. Columnaris disease

Columnaris disease caused by Flexibacter columnaris is one of the diseases commonly found in juvenile seabass which are raised in water of low salinity during rainy and winter seasons. Gross signs are observed by saddle-shaped lesions in the mid-body position about the dorsal fin of the fish. The bilaterally symmetrical lesion appears as a fuzzy, pale yellow white plague, with dark margins, often eroding in the epidermis. Clinically, the condition may be chronic, acute or peracute. The gram negative, aerobic bacillus (about 12 um) can be isolated from the lesion of the diseased fish.

Parasitic Protozoa

Protozoans are probably the most important group of animal parasites affecting fish. Many reports from all over the world indicate great losses in fish culture caused by protozoans. Obligate parasites such as the ciliate ichthyophthirius and certain species of the cnidosporidians are responsible for many of these losses. Many species, which are considered as commensal protozoans, may become pathogenic under certain conditions. Environmental factors affect the susceptibility of fish to certain protozoans. Oxygen concentration and temperature are the factors affecting both hosts and parasites. Since many protozoans transfer from fish to fish through the water, fish population density is an important factor. Tremendous infestation of protozoans can occur in a relatively short time where fish populations are dense. Other factors, such as host size, age, host specificity, immunity, and the aforementioned influences of host condition also play an important part in the host reaction to invasion by protozoans. Most host reactions to invasion by protozoans are directed towards expelling or isolating the parasite,

Protozoans cause harm to fish mainly by mechanical damage, secretion of toxic substance, occlusion of the blood vessels, depriving the host of nutrition and rendering the host more susceptible to secondary infections. Some of the most common clinical signs are changes in swimming habits, such as loss of equilibrium, flushing or scraping, loss of appetite, abnormal colouration, tissue erosion, excess mucous production, haemorrhage, and swollen body or distended eyes.

a. Cryptocaryon sp.

Cryptocaryon is a marine counterpart of the freshwater Ichthyophthirius species and similarly causes the white spot diseases in marine fish. Its morphology and life cycle is quite similar to that of the “Ich”. The surface of invaded fish reveals white pustules or numerous minute, greyish vesicles which are nests of cilliates burrowing under the epidermis. They feed on the host's cells underneath the epithelium and cause heavy irritation resulting at first in excessive production of mucous and finally completely destroying the fine respiratory platelets of the gill filaments. On the skin, this parasitic protozoan causes considerable lesions resulting in destruction of large areas of the epidermis. Secondary infection may complicate the situation and the host dies. The incidence of Cryptocaryon sp. in seabass showed a distinct peak during low water temperature period, with a marked prevalence during February. This ciliated protozoan probably causes more damage to fish populations over the entire world than any other single parasite.

b. Trichodina sp.

Members of the genus Trichodina with about 60 species described from marine fish and related peritrichous ciliates are the most common parasitic protozoans that are especially harmful to young fish. The species attach themselves to the gills of marine fish. More than half of juvenile seabass heavily infected with this parasite died. Trichodina also causes problems to crowded seabass in cages.

Clinical signs of trichodinosis include excess mucous production, flushing, debility and hyperplasia and necrosis of the epidermis. The fin may become badly frayed in heavily infected fish and this may be accompanied by sluggishness and loss of appetite. Excessive number on the gills of infected fish interferes with respiration.

c. Henneguya sp.

Henneguya is a flaggelate found to attach mainly to the gills of seabass in cages. In heavy infections, it may be found in the skin. Gross signs are hyperplasia, bronchitis plus necrosis. Life cycle involves a free-swimming dinospore, which moves by means of 2 flagellae. It attaches to the host and transforms to a sac-like trophont, which has elaborate attachment mechanism. It feeds and grows and detaches from host, sinks to substrate where it encysts and produces dinospores.

d. Epistylia sp.

Epistylis is another protozoan found in seabass especially in freshwater environment. Epistylis belongs to the sub-class Peritrichia, and is common ectocommensals; however, it occasionally turns pathogenic. It attaches to the fish with its stalks. This protozoan may be present at a variety of temperatures and its number may be large enough to cause a grey mat on the epithelial surface.

Parasitic Helminthes

Worm diseases with the possible exception of those produced by monogenetic trematodes have not yet appeared to be a serious problem in seabass culture. This is probably due in large part to their complex life cycle and the difficulty in completing such cycles in the culture system. Helminthis parasites, which have been found in seabass, include monogenetic trematode, digenetic trematide, nematide and acanthocephala.

a. Monogenetic trematodes

Monogenetic trematodes can be observed throughout the year. Abundance of these parasites and their seasonal distribution have not been studied. It has been reported that temperature apparently plays an important role in determining outbreaks of certain parasitic Monogenera. Peak infections of monogenetic trematodes usually occur among young susceptible fish. Such behaviour is advatanegous for the spread of the organism in a fish population.

A monogenetic trematode which has been found to be a dominant species in the gills of seabass is Diplectanum latersi. Dactylogyrus sp. may be present but this has not been fully identified.

b. Digenetic trematodes

Lecithochirium sp. was found in the intestine of seabass especially in wild fish. Incidence of infection was 86.0 percent and average parasite burden was 5.5 Another digenetic trematode which was commonly found in the intestine of wild seabass is Pseudometadena celebesensis. Its incidence of infection and parasite burden were 100 percent and 9.3, respectively.

c. Nematodes

Although many species of nematodes are found either as adults or larvae in fish, few have been implicated as serious pathogens of their hosts. In seabass, nematode of the genus Cucullanus was found more common in the gut of larger fish than in that of young fish.

d. Acanthocephala

Acanthocephalid worms, despite their fearsome-looking proboscis with its rows of hooks, have not been observed as serious pathogens of fish. The great majority of acanthocephalus in seabass are found as adults in the gut.

Parasitic Copepods

The parasitic copepods are among the most devastating of fish parasites. The mature female usually attaches to the fish and feeds on the host. After copulation the female matures and produces egg sacs while the male dies.

a. Caligus sp.

Caligus sp. has caused big problems in cultured seabass. They attach to the gills, buccal and opercular cavities, occasionally on the skin and fins of the seabass. Heavy infections can cause mass mortalities especially in young fish.

b. Lernanthropus sp.

Lernanthropus are found attached to the gill of seabass especially in cage cultured fish. Large numbers of this parasite can cause anaemia to the fish host.

Treatment

Treatment is usually in the form of chemotherapy, possible combined with some of the preventive measures listed above. Chemical control should be a “last resort” in disease control.

Chemical Prophylaxis

To treat the pond accurately, the volume of water in the pond must be known. To determine the volume of a pond, multiply the number of surface unit area of water by the average depth of the pond.

The following chemicals are often used in the treatment of various fish diseases:

• 15 ppm of formalin (which contains 37–40% formaldehyde). This should be new stock.
• 1.0 ppm of malachite green (should be zinc-free) for Ich.
• 2.0 ppm of potassium permanganate.
• 0.25 ppm of dylox (dipterex).

For small ponds, dilute the chemical in a bucket of water and distribute evenly over the pond using a dipper. In large ponds, the chemical should be mixed in a large drum and distributed evenly over the pond.

General Treatments

When treating fish, it is advisable to know the quality of the water because such things as pH and temperature greatly affect treatment results. Treat a few fish first and see how they react before treating the entire group. Use only the drugs and chemicals that have been cleared by an authorized agency for use on food fish. Here are some general treatments for specific groups of pathogens:

Remember the following precautions during treatment:

• Know the accurate volume of the water
• Know the percent active ingredient of the chemical
• Evenly distribute the chemical
• Use only the chemicals approved for use on aquatic animals for human consumption.

Read more on causative agents, symptoms and treatments...