Fish farming is the main form of aquaculture, while other methods may fall under mariculture. Commercial fish farms have raised nearly forty percent10 of seafood eaten by consumers around the world. Fish farming involves raising fishes commercially in tanks or enclosures, usually for consumption and business purposes. The fish species are divided into marine aquaculture and freshwater aquaculture. The most common marine aquaculture fish species raised by fish farms for consumption are Salmo salar (Atlantic salmon), Thunnus thynnus (Atlantic bluefin tuna) and Gadus morhua (Atlantic cod) while the common freshwater aquaculture species are tilapia (tilapia), Cyprinus carpio (common carp), Oncorhynchus mykiss (rainbow trout) and Clariidae (catfish).
Pelagic fish like salmon and tuna mature in holding pens rather than the open ocean. Catfish and tilapia on the consumer market usually comes from farms rather than wild fish populations. Increasing demands on wild fisheries by commercial fishing has caused an impact to the eco-system. Fish farming offers a substitute to the increasing market demand for fish and their protein. As fisheries dwindle, farms attempt to domesticate more and more species of fish to meet a global growing demand.
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There are two kinds of aquaculture: extensive aquaculture, which is based on local photosynthetical production and intensive aquaculture, in which the fish are fed with external food supply. The management of these two kinds of aquaculture systems are completely different.
Microbial diseases which impact fish farming
As the common species of farmed fish come from marine or freshwater aquaculture, they are exposed to different pathogens due to the environment they are in. The fishes can be exposed to different kinds of diseases such as bacterial, fungal and virus. Regardless of the type of disease that the fish is suffering from, there will be telltale signs that a fish is feeling unwell. Some of the common indications include difficulty in swimming, breathing difficulty, swimming near the surface of water, loss of appetite, not interacting with other fishes. As both marine and freshwater fish species experience different diseases, treatment will differs.
The characteristics of bacteria that cause disease in fishes can be group into Gram-positive and Gram-negative bacteria. The common bacterial disease experienced by marine and freshwater fishes is shown in table 1 on the following page.
Motile Aeromonad Septicemia causes red spots on the body or fins of the fish, scale protrusion (Fig. 1) or haemorrhage (Fig. 2). This disease easily infects freshwater fishes like carps or catfishes and is usually stress related. Transmission is usually by contaminated water or ingestion of infected fish. Common treatment is by antibiotics (Oxytetracycline, Chloramphenicol) and controlling the further spread of disease is to have a better sanitation, quarantine of infected fish and stress prevention.
Streptococcosis affects mostly marine fish species and fishes suffering from it have a darkened skin, are disorientated in swimming and have exophthalmia (Fig. 3). Transmission is usually by infected fish, contaminated water and feeding of trash fish. Treatment can be by antibiotics (Erythromycin), vaccines (autogenous) or immunostimulants in feed. The disease can be controlled by quarantined, better sanitation, diet, medication and disinfection.
Edwardsiellosis affects both freshwater and marine fishes. Fishes will show signs of ulcer (Fig. 4), pale gills (Fig. 5), lesions, abscess formation and scale erosion. Water from infected source, carrier animal faeces, water and mud are the common source of transmission. Treatment is usually by antibiotics, improvement of water quality, stress (over crowding, malnutrition) reduction. The disease can be controlled by strict quarantine of infected fish, stress prevention and better hygiene.
Fungi are a group of organisms called heterotrophs that require organic compound for growth and reproduction. Unlike plants, they are incapable of manufacturing their own nutrients by photosynthesis. Fungi are ubiquitous and there is no known place where they do not exist. In most cases, fungi serve a valuable ecological function by processing dead organic debris. However, fungi can become a problem if fish are stressed by disease, poor environmental conditions, receive poor nutrition, or are injured. If these factors weaken the fish or damage its tissue, fungus can infest the fish. Fungi can also prevent successful hatching when it invades fish eggs7. The common fungal diseases in fishes are Saprolegniasis, Branchiomycosis and Ichthyophonus.
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Saprolegniasis is a fungal disease of fish and fish eggs. Commonly caused by the Saprolegnia species. They are common in fresh or brackish water. This disease will attack an existing injury on the fish and can spread to healthy tissue. Poor water quality (water with low circulation, low dissolved oxygen or high ammonia) and high organic loads, including the presence of dead eggs are associated with Saprolegnia infection. Fishes suffering from the infection will have fluffy cotton-like material (white to shades of grey and brown) found on their skins, fins (Fig. 6), gills, and eyes or on the eggs. Saprolegniasis can be prevented by good water quality (for example good sanitation) and circulation, avoidance of crowding and having nutritious diet. Common treatment includes potassium permanganate, formalin and povidone iodine solutions.
Branchiomycosis or commonly known as "gill rot" is caused by Branchiomyces sanguinis and Branchiomyces demigrans. Both species are found in fishes suffering from environment issues such as low pH (5.8 to 6.5), low dissolved oxygen or high algal boom. Fishes suffering from "gill rot" may appear lethargic and often seen gulping air at the water surface. A closer examination of their gills will show that it appears to be striated (Fig. 7) or marbled with the pale areas representing infected and dying tissue. A trained diagnostician should be engaged for the verification of this disease.
High mortalities rate in fishes suffering from fungal disease are often associated with this infection. Avoidance is the best control for "gill rot". A good management practise will create conditions that are undesirable for fungal growth. If an infected fish is found, it should not be transported and great care must be taken to prevent the movement of the disease to a noninfected area. Formalin and copper sulfate are commonly used to stop mortalities. Tanks and any holding areas should be disinfected and dried to eradicate any possible traces of the fungal. If fishes are reared in ponds, it should be dried and treated with quicklime (calcium oxide).
Fig. 8: Liver of fish with white sores
Ichthyophonus disease is caused by Ichthyophonus hoferi. It grows in fresh and saltwater but is restricted to a cooler environment (2ï‚°C to 20ï‚°C). This is commonly spread by fungal cysts, which are released in the faeces or cannibalism of infected fish. The primary route of transmission is through ingestion of infective spores. Fishes suffering from mild infection will not show any external sign of the disease. If the skin of fish appears to have a "sandpaper texture", it is a sign that the fish is badly affected by this disease. This "sandpaper texture" is caused by infection under the skin and muscle tissue. Some fishes will experience curvature of the spine and it can be observed with the naked eye. Post-mortem examination of infected fish will show the internal organs swollen with white to grey-white sores (Fig. 8). There is no known cure; as such fishes will carry this infection for life. Prevention is the only control to this infection.
To avoid introducing infective spores, it is always a good practise to never feed raw fish or raw fish products to cultured fishes. Using cooked feed is a good practise to prevent exposure to any infective spores. If unfortunately, Ichthyophonus disease is identified by a trained diagnostician, the infected fish has to be removed and culled. Complete disinfection of tanks, raceways or aquaria is encouraged. Ponds with dirt or gravel bottoms would require months of drying to totally eliminate the fungus.
Viruses are very small infectious agents that multiply only within the living cells of an animal or plant host. Other microorganisms, such as bacteria or fungi, have organelles for their own metabolism, but viruses do not. They must utilize the machinery of the infected host cell for growth and reproduction8.
A virus has two parts. The internal part is the virion, or virus particle, which is composed of nucleic acid, the same material that makes up genes. The virion is enclosed in an external protein coat called a capsid. The type of nucleic acid they contain broadly categorizes viruses; the two basic types of nucleic acid are ribonucleic acid (RNA) and deoxyribo-nucleic acid (DNA). Because they are so small, viruses are often difficult to detect. Viruses are often both species-specific and tissue-specific. This means that they may only grow in certain types of cells from certain animals. This can make it difficult to isolate viral agents from many fish because there may not be a commercially available cell-line for an individual fish species. Many cell-lines, which are commercially available, originate from coldwater fish such as salmonids, and may be less suitable for warm water species. Reflected in table 2 on the following page are the common viral disease in the region of Asia and Australia
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Epithelioma Papillosum (Fish Pox) is caused by Cyprinid Herpesvirus 1. It is usually observed in carps or other cyprinids. Infected carp shows white skin lesions (Fig.9), which may spread to other parts of the body. Subsequently, they loose balance and swim laterally - with a bent body. There are four progressive stages of the disease. The disease may have an acute course with high losses or it may be chronic with sporadic losses. In acute cases, the fourth stage of the disease is reached rapidly. Mode of transmission is still unknown but whenever it occurs, it appears to spread throughout the population very quickly. Currently, there is no vaccine available to immunise cyprinids against this disease.
Lymphocystis is caused by lymphocystivirus and it infects a variety of fishes (marine and freshwater species). This disease causes cell enlargement on the skin or fins (Fig. 10). The disease usually runs its course in four or more weeks (depending on species involved, water temperature, and other variables) and then the enlarged cells rupture or slough off and release the viral particles into the water. While infected, the fish may become slowed or weakened, or more visible, and thus be more prone to predation or attack. If there are mouth lesions, the fish may have difficulty in feeding or may not be able to feed. The low mortality rate of lymphocystis is mostly due to secondary bacterial or fungal infections.
After lymphocystis lesions are lost, the host tissue heals up. Adhesions and scarring can occur during healing. If the gills are affected, the fish can have difficulty breathing, especially if gill surface areas are destroyed (no longer present), or adhesions or scarring occur and gill surfaces are thus reduced in surface area or functional quality for oxygen uptake. The disease is usually spread by epidermal abrasion or viral particles present in water. There is no vaccine for lymphocystis yet.
Infectious Pancreatic Necrosis Virus (IPNV) is caused by Birnavirus and it infects a variety of fishes (marine and freshwater species). It is a highly contagious systemic birnavirus disease of young fish. The disease can be transmitted either through water route and ingestion of infected material or from spawning carrier brood fish. Susceptibility to the disease decreases with increasing age. The first sign of an outbreak is usually a sudden and progressive increase in daily mortalities in the hatchery, particularly in the faster-growing individuals. Fishes will have a swimming motion that resembles spiral swimming motion.
The disease will cause darkening pigmentation, pronounced distended abdomen (Fig. 11), long thin whitish faecal casts, mild to moderate exophthalmia and the gills are typically pale. After a post-mortem examination, absence of food but presence of clear or milky mucus in stomach and anterior intestine is pathognomonic. Three commercial injectable vaccines (two inactivated and one recombinant) are currently available.
The innate and adaptive immune systems in fish of commercial interest
Fishes are divided into three classes: Osteichthyes (bony fish), Agnatha (jawless fish) and Chrondrichthyes (cartilaginous fish). The common fish farm species belongs to the Osteichthyes group. Their cells of the immune system and types of immune organs are different from the other two classes. As the different group of fishes have different immune organs, they are susceptible to different diseases and treatment will differs. Besides the risk of being exposed to pathogens, seasonal cycle and the environment they are reared in also affect the health of juvenile as well as brood fish.
Due to the susceptibility to diseases and viral infections, these have become a significant problem in fish aquaculture and it is the major cause of death for brood fish. The innate and adaptive immunity of farmed fishes has become the topic of interest for many researchers as by having a better understanding of these two immunities, the chances of immunodeficiency in farmed fishes can be reduced and aquaculture industry can progress further.
Innate immunity is the first line of defence against any infecting microbes and there is no memory response. Fishes rely heavily on the innate immune system due to their evolution history. Innate immunity is broadly classified into three groups: physical, humoral and cellular. Physical factors plays a significant role in the defence system of fishes as it act as a barrier for infecting microbes. A systematic infection is prevented when microbes are unable to enter the body of the fishes. Skin and scales is the physical barrier that a fish has. In certain species for example trout, mucus is produced as a form of physical barrier.
Humoral factors are circulating in the fish through bodily fluids. These fluids will contain peptides and proteins that react against microorganism and inhibit their growth. This process involves antibodies with the aid of cytokines and T helper cells. The common humoral factors are lysozyme and immunoglobulin (Ig). Lysozyme functions by hydrolysing the cell of gram-positive bacteria and with the assistance of complement system, gram-negative bacteria growth can be inhibited. Lysozyme can be found in the head kidney of fish. Ig is a form of antibody and bony fish contains four classes of Ig. They are IgM, IgD, IgZ and IgT. IgM and IgD are known to be similar to the mammalian form although it differs in the number of monomers it may contain. Even though IgM and Ig D are similar to the mammalian form, there is no class switching in fish Ig. Each Ig has different function in the immune system.
Cellular factors include the natural killer (NK) cells and phagocytic cells. NK cells are a type of cytotoxic lymphocyte that constitutes a major component of the innate immune system. NK cells play a major role in the rejection of tumours and viral infected cells. Phagocytic cells protects by ingesting any harmful bacterial and dead or dying cells. Phagocytic cells in fish include monocytes, macrophages and granulocytes.
Adaptive immunity is the next line of defence for bony fish. It is this immunity that allows the fish to response to specific pathogens due to the memory response that it has. Maternal immunity being a form of adaptive immunity has become the paramount importance for protection of young fishes at early stage of life. Maternally derived immunity is indispensable in young vertebrates since they are most vulnerable from birth to the development of indigenous antibodies. Fish offspring, like other vertebrates, are dependant on the maternally derived nutritional and immune factors for their sustainability15. Although resistance can be conferred by inoculation of individual fish but this is not practical on a large scale14, as it will involve high cost and intensive manpower.
Transferring of immune factor by an immunocompetent female to an immuno-logically naÃ¯ve neonate occurs transplacentally or through colostrums, milk or yolk. Mouth bearers are able to transfer some immunity to larvae through the mucus secreted from mouth cavity. This secretion is able to produce enhanced antimicrobial factors and have specific immunoglobulin. The larvae metabolize antibodies raised in the maternal circulation gradually but retain the ability to bind antigens and confer relatively. In most fish, antibodies were found from day zero to larval stage day fourteen (Fig. 12); amount gradually decreases as they grow.
If immunity persists in the offspring, then large numbers of fish could be protected by direct inoculation of relatively few mother fish. This in turn becomes a cost saving solution for fishes to obtain immunity. Although immunity is transferred to the offspring directly, factors like environment, nutrition, affects how well immunity is transferred maternally.
Vaccines and their importance in fish farming
Vaccination plays an important role in large-scale commercial fish farming and has been a key reason for the success of cultivation of different fish species. In general, empirically developed vaccines based on inactivated bacterial pathogens have proven to be very effective in fish. Fewer commercially available viral vaccines and no parasite vaccines exist.
Vaccine development is extremely expensive and there are only a few viral diseases of fish, which have sufficient economic impact to warrant investment in more viral vaccine. Fishes are ectothermic and this makes their immune response not as predictable as that of warm-blooded animals. Therefore this makes booster shots required for certain vaccines. At present, most commercially available vaccine offers protection from common bacterial agents. Vaccines are usually administered by injection or by immersion bath.
In general, fish farming may be a fast growing industry but the development in improving fish immunity is still slow. This is mainly due to the unpredictable immune response that fish has. Inoculation of individual fishes results in high cost and industrial players usually rely on maternally derived immunity to ensure the fries are adequately protected before proceeding on to immunise the fishes. Environmental factors, diet and psychological welfare (stress related) affect the health of fishes (larvae to adult) as such these cannot be overlooked. Not all diseases can be treated; as such proper sanitation is required to prevent any outbreak or containment of the disease.