Institute of Aquaculture, University of Stirling, U.K.
This paper discusses the effects and control of disease in enhanced fisheries. Infectious and non-infectious causes of disease are given and the effects of disease, including direct mortalities as well as losses due to reduced growth, fecundity, product quality and social factors, as well as the costs of control measures are discussed. The importance of husbandry and environmental factors in the occurrence of disease outbreaks is highlighted and the impacts of the introduction of pathogens and the introduction of exotic fish species are described. Methods for the prevention of the introduction of pathogens are described, including the use of quarantine procedures. The occurrence of disease in caged fish is assessed and methods of health management for both caged fish and those stocked in capture fisheries are described and discussed.
Although the effects of disease in intensive or even semi-intensive aquaculture are well documented and of enormous economic importance, in capture fisheries, even those which are enhanced by stocking, outbreaks of disease are poorly documented. This is generally true in all parts of the world, including developing and developed countries.
The most notable exception to this rule is epizootic ulcerative syndrome (EUS) of freshwater fish in south-eastern and southern Asia which caused such devastation as it spread westwards through Asia since 1975. Losses due to this disease are difficult to quantity because of the sheer scale of the problem but have been put at US $3.38 million during the initial outbreak in Bangladesh in 1988 (Barua, 1995).
2. CAUSES OF DISEASE
Diseases are usually classified as infectious or non-infectious. The former include those caused by pathogens, i.e. viruses, bacteria, fungi and parasites. The latter include nutritional diseases, genetic conditions and environmentally-based pathogens. Invariably the outbreak of a disease is caused by a combination of factors, such as the presence of a pathogen and unfavourable environmental conditions which will reduce the immune capacity of the fish (Braaten and Hetkoen, 1991).
3. EFFECTS OF DISEASE
The most obvious effect of the occurrence of disease is mortalities amongst the fish population. In epizootic situations, such as outbreaks of EUS, disease can be acute, and mortalities can be large, sudden and easily visible. However, this is unusual and perhaps more likely is a chronic situation where mortalities occur at a low level over a long period of time and may not be noticed. In this situation diseased fish may be mostly predated.
It is probable that other effects of disease may be more insidious, but nonetheless can cause serious economic loss. These effects include: loss of growth, reduction in fecundity, loss of product quality, loss due to control measures, and loss due to social factors.
Loss of growth
This may occur in any disease situation, especially where the condition is relatively longstanding, e.g. chronic bacterial or viral infections. It may manifest itself as a relatively poor growth rate or as a reduced condition factor. The condition may be relatively transient or it may be permanent. In aquaculture there may be a reduced feed conversion, which in intensive systems can be very significant economically.
Reduction in fecundity
Diseased fish may show an absolute reduction in the quantity of sexual products, especially eggs, or perhaps more importantly, in the quality of eggs. This will then be translated through into poorer egg and fry survival. A reduction in fecundity can be due to a general loss of condition in diseased fish, or to the destruction of gonads by infectious agents.
Loss of product quality
The presence of infection may lead to the occurrence of lesions in the musculature of fish which renders them unacceptable for human consumption. They may take the form of haemorrhages or necrotic tissue, or loss can be caused by the presence of parasites. In some instances lesions or parasites may be easily visible externally, e.g. in EUS, or they may not be noticeable until the fish is prepared for consumption, e.g. parasite cysts. In some instances there can be the potential for zoonotic infections where human parasites use fish as intermediate hosts.
Loss due to control measures
In some cases it may be possible to apply control measures against diseases. For example, it might be appropriate to spray insecticide to control Argulus or to apply lime to combat EUS. In any such case there will be costs involved. The most immediate of these will be the cost of the chemicals involved and the labour involved in their application. These can be considerable and in developing countries may be prohibitive. In addition there is always the possibility of residues remaining in fish when they reach the consumer. In some countries fish health legislation may prevent the movement of fish into or out of waters where specific diseases occur.
Loss due to social factors
The occurrence of severe disease may cause a loss in morale and even deter participants from continuing with fishery enterprises. It may also become more difficult to obtain financial support for affected enterprises.
4. DISEASE AND ENVIRONMENT
Pathogens are present in all fish in all environments, but in fact outbreaks of disease occur only rarely. This reflects the fact that usually some adverse environmental or husbandry factors are necessary for disease outbreaks. Such conditions cause stress in affected fish populations, probably reducing their immune competence and rendering them more susceptible to pathogens.
It follows that to effectively combat disease and prevent further outbreaks it is essential to identify and eliminate these underlying factors wherever possible. The actual nature of these factors will vary considerably from case to case, but often adverse changes in water quality, such as eutrophication or pollution, for example due to pesticides or herbicides, may be involved.
Fish in natural or semi-natural situations, which may be taken to include most stocked fisheries, are usually at a relatively low population density and provided there is adequate feeding and reasonable water quality they will not be stressed and will usually be unlikely to succumb to disease.
5. INTRODUCTION OF DISEASE AGENTS
The most serious outbreaks of infectious disease have occurred when a pathogen is transferred to a new geographical area where it comes into contact with native fish or shellfish species which have no resistance to it. If the pathogen is sufficiently virulent then epizootics of disease may result.
The best documented example of such a transfer in the developing world is the transfer of Epizootic Ulcerative Syndrome into South-east Asia. The aetiology of this disease has been the source of some dispute but it now seems certain that it is caused by a particularly virulent fungal agent, Aphanomyces invaderis, which is able to rapidly invade the muscle of susceptible fish causing extensive and unsightly lesions. A great variety of Asian fish species are susceptible to the fungus. For a comprehensive review of EUS see the review by Roberts et al. (1994) and the proceedings of the regional seminar by Roberts et al. (1995).
Epizootic ulcerative syndrome would appear to be identical to “red-spot disease” reported from brackish waters of Australia. The disease apparently first occurred in Asia in Papua New Guinea in 1975, and has now spread as far west as Pakistan. How the fungal agent reached Asia is uncertain, nor is it known how it has been transmitted between countries in Asia. Movements of fish may in the past have been responsible, although “natural” movements between and in watersheds have certainly taken place.
When it first appears in susceptible fish populations EUS causes very severe mortalities in both farmed and wild fish, but subsequent outbreaks are much less severe, probably because of the development of resistance among the fish populations, and/or some attenuation of the virulence of the fungal agent.
A further excellent example of the potentially disastrous effects of pathogen transfer is the translocation of the ectoparasitic monogenean Gyrodactylus salaris on Atlantic salmon smolts from Sweden into Norway. Such smolt movements were made in order to supplement the number of smolts available to the rapidly expanding Norwegian salmon farming industry in the 1970s (Johnsen and Jensen, 1988).
Gyrodactylus salaris is a relatively non-pathogenic parasite of salmon in countries bordering the Baltic Sea, but Norwegian salmon were extremely susceptible to the parasite and when G. salaris entered wild populations it reached epizootic proportions in many rivers, decimating local populations. Experimental studies with British salmon have shown that they too would be highly susceptible to G. salaris should the parasite ever reach the UK.
The case of G. salaris is salutary, since it was previously unheard of for a parasite on a particular fish species from one locality to be so completely different in its pathogenicity to the same fish species in an adjacent country in the same land mass.
There are a number of other examples of pathogen transfers with serious consequences but the transfer of pathogens with fish species need not always have dramatic adverse effects. For example, the movement of Chinese carps over much of the globe has also led to the translocation of many of their parasitic species. However, these have either been too specific to their original hosts to successfully transfer to native species, or, if they have done so, they have not been especially pathogenic.
Disease problems can also arise when a fish species is transferred to a new geographical area and comes into contact with native pathogens to which it is highly susceptible. Thus, the rainbow trout was transferred to Europe from North America in the 19th century and has proved very susceptible to certain parasite species. However, although locally serious this has not resulted in large-scale epizootics.
It appears that the risks involved from the transfer of susceptible fish are rather less than if pathogens are transferred into a new area.
6. PREVENTION OF INTRODUCTIONS
The movement of live fish always carries the risk of introduction of pathogens to a new environment. This may happen at all levels, from intercontinental translocations, to those within regions, between countries, or even between watersheds. There is no way in which a live fish can be guaranteed to be free of pathogens. A number of conventions have been established for the translocation of fish (e.g. EIFAC, ICES) and these address the question of pathogen transfer. The recommendations within these conventions are soundly based and should be rigorously followed, especially for movements of species between continents or within large regions. The question of fish health control and legislation is discussed by Wootten (1991).
Before a movement of fish is made, at whatever level, it is necessary to assess the risk involved. This entails a search for information on known pathogens within the region, country or watershed of origin which might be dangerous to native species, and pathogens within the receiving country/watershed which might be dangerous to the introduced species. It is probably pointless in most cases to proscribe all pathogens and only specific pathogens which are potentially sufficiently dangerous need be considered.
Ideally the fish to be introduced should be from a source which has been tested by a competent laboratory for pathogens. Unfortunately in some regions such facilities do not exist, or the cost may be prohibitive.
Since no diagnostic test is 100% accurate it is preferable to import fish only from sites with a long history (at least several years) of freedom from specific pathogens. This gives greater assurance that pathogens are indeed absent. Sources of fish should be tested according to internationally recognised diagnostic methods and in sufficient numbers to minimise the risk of failure of detection. A number of protocols exist of which the most recent is that produced by the Office International des Epizooties (OIE, 1995). Although it is obviously desirable that the fish population from which the importation is drawn be properly tested, this in itself is a poor indicator of freedom from a pathogen given the difficulties of diagnosis. Shipments of fish should, wherever possible, be accompanied by reliable certification, preferably from central government authorities in the country of origin, although the importer should specify in detail what testing is required. Guidelines on certification of fish and shellfish are provided by the Office International des Epizooties (OIE, 1995a).
At a more local level such rigorous testing is often out of the question, although it is worth pointing out that in Europe and North America testing for specific pathogens is often routinely carried out on many fish farms since this is now expected by buyers whether they require fish for ongrowing or for stocking.
Quarantine is often cited as a means of appraising the health of translocated fish, and is even used at local level. In theory, the holding of fish in quarantine for a specified period to determine whether they are carriers of dangerous pathogens is good practice. However, in many countries adequate facilities do not exist to make quarantine an effective proposition.
For introductions of new species, or what are considered high-risk translocations, the quarantine unit should be secure in the sense that there is no possibility of escape of fish or of untreated effluent. In addition adequate laboratory facilities should be available in order to test fish for specific pathogens. A whole generation of fish should remain in quarantine before release into unprotected waters is allowed.
Any fish holding system that allows the escape of fish or untreated effluent cannot be considered a true quarantine system for high risk movements of fish. Once a pathogen enters open waters it will probably be impossible to eradicate it.
At a local level there is often some virtue in holding new groups of fish separately for a short time before their release, in order to determine whether they are likely to rapidly become diseased. This gives the option of treating them or destroying the shipment. However, it must be appreciated that the act of holding fish for any length of time in crowded conditions may itself cause an outbreak of disease.
8. TREATMENT OF STOCKED FISH
The treatment of stocked fish once released is extremely difficult because of the volume of water involved. It is obvious that the larger the water body the harder any form of control will be, and similarly it will be more difficult to make any effective intervention in open rather than closed water systems.
It follows then, that it will be especially advantageous to ensure that the health status of fish is as high as possible before they are stocked. The concept of stocking fish of specific pathogen-free status has already been discussed. It is in any case important to use fish of as high a quality as possible, preferably from known, reputable sources. Fish must be transported in a correct manner in oxygen-filled plastic bags, or aerated tanks. They must not be overcrowded or exposed to excessively high temperatures. Badly-transported and thus highly stressed fish will be very vulnerable to outbreaks of disease.
Prophylactic treatments of introduced fish may be worthwhile to reduce pathogen load. For example, bath treatments in formalin can be useful in controlling ectoparasites. Similarly antibiotic baths can on occasion be of some value. Vaccination would represent the ideal prophylactic measure to protect introduced fish, but very few effective vaccines are commercially available, and these are mostly against salmonid diseases. They also tend to be relatively costly.
At the time of stocking water quality should be as appropriate as possible, extreme conditions should be avoided. It may be possible to improve water quality parameters. For example the addition of lime is often recommended as a prophylactic measure against EUS. Outbreaks of EUS are usually associated with drops in water temperature post-monsoon. The addition of lime to raise pH just before this may have some beneficial effect.
As the example of EUS indicates, many diseases are seasonal in their occurrence. This is often associated with changes in water temperature. It is obviously better not to stock near to the time at which an outbreak of a particular disease is known to be likely to occur. Thus, with EUS it would obviously be inappropriate to stock infected waters close to the annual post-monsoon fall in water temperature.
Conversely it may be appropriate to attempt to maximise harvesting before the likely outbreak of a disease to recover as many fish as possible before losses occur.
Treatment of stocked fish is, in most cases, likely to be impossible. In relatively small water bodies it may be possible to add treatment chemicals by spraying them on the water surface. Ectoparasitic crustaceans such as Argulus or Lernaea may sometimes be controlled by spraying with organophosphate insecticides such as dipterex to a concentration of 1 ppm active ingredient. However, it is difficult to obtain the correct concentration throughout the water body and the treatment is unlikely to be completely effective. The cost of such a treatment can also be high in large water bodies. In theory, other chemicals could be similarly applied, but in practice this is rarely attempted.
Fish species will vary in their susceptibility to any given disease. If a disease is endemic in a water body, then it may be possible to avoid stocking with particularly susceptible species. Alternatively some form of selective fishing may be used to remove susceptible species. This might practically be achieved by altering mesh size or fishing gear.
Some pathogens, particularly helminth parasites, tend to accumulate in older and therefore larger fish. Selective fishing might also be used in this case in order to remove larger, more heavily infected fish and thus to lower overall parasite numbers within the water body.
If the impact of disease was particularly severe and long lasting then a drastic, but potentially effective method might be a complete harvest or kill of all fish stocks, followed by restocking after a fallow period. The length of this period would depend on the nature of the pathogen and the time it might be expected to survive in the environment without the presence of a fish host. A kill of all fish stocks could only be attempted in a relatively small water body and would be most effective in closed water bodies where there was less chance of pathogens and recentering the system.
The impact of selective fishing or a complete harvest or kill-off would be very severe in economic terms and cause severe hardship to fishing communities. The potential benefits and disadvantages of such a course of action must be carefully assessed, as well as the likelihood of its success.
9. CAGE CULTURE
Cage culture, like any other form of intensive aquaculture, will be affected by disease, but because the fish are within a confined space and probably fed artificially, treatment is a much easier proposition than in open waters.
Concern is often expressed that cage cultured fish will act as a reservoir of infection for free-living fish in the same water body. If an epizootic occurs within the caged fish the danger is that sufficient infective stages of pathogens will escape to cause an outbreak of disease in the free-swimming fish. In practice this has not been found to occur, presumably because the population density of the latter fish is too low to allow sufficient pathogen transmission to cause overt disease. Experience suggests that the flow of pathogens is usually from free-swimming fish to the caged fish.
The prevention of disease in cage culture must depend on stopping the entry of pathogens into the system, maintenance of good environmental and husbandry conditions, and, if necessary, some form of treatment.
As discussed earlier, as far as possible pathogens should not be introduced with fish stocked into the cages and good quality, healthy stock should be used. Prophylactic treatments might be advisable. Transport of fish should be carried out as described earlier in order to prevent undue stress.
As with fish stocked into open waters, if an endemic disease has a particular seasonal occurrence, then introduction of fish into cages should take place after the period of highest risk. For fish already in cages then an early harvest may be advisable. If particular fish species are especially vulnerable to endemic disease then it may be worthwhile to stock non-susceptible species.
Once fish are stocked into cages then good husbandry is the most effective way of minimising the risk of disease outbreaks. As far as possible environmental conditions should be appropriate for the fish species under culture, although this is obviously more difficult in cages in a large water body, than for example in ponds. Poor husbandry, e.g. excessive handling, or bad water quality may by itself cause significant pathology.
Of great importance is the maintenance of correct stocking density. Overstocking will cause deterioration of water quality, fish will become stressed and the rate of pathogen transmission will increase. This combination of factors will often lead to outbreaks of disease. Reduction of stocking density is often an excellent management technique to control an outbreak of disease, with or without the intervention of therapy. Correct feeding, using good quality material, is also of major importance.
Monitoring of fish health is vital to the early identification and successful control of disease outbreaks. Once a disease is well established in stocks of fish it is very difficult to control quickly and some losses are almost inevitable. A reduction in feeding behaviour is often the first sign of an impending disease outbreak, well before any lesions or mortalities occur. On the other hand, in acute disease outbreaks mortalities may occur without any obvious clinical signs of disease. If possible it is always worthwhile to seek expert advice for the diagnosis and control of disease outbreaks.
In many cases chemotherapy may be the best way to control an outbreak of disease. A number of therapeutants are available against pathogens, particularly parasitic, bacterial, and external fungal infections (see Schnick, 1991 for review). Therapeutants are not available against viral infections.
In cages chemotherapy is usually administered as a bath, i.e. the chemical is added directly to the water, or, if the fish are fed on an appropriate artificial diet, it may be incorporated into the feed. The former method is normally used for external infections and the latter for systemic infections, especially those caused by bacteria.
There are a number of problems associated with therapy, especially in developing countries. Treatment chemicals are generally expensive, often beyond the reach of poor farmers. Treatments tend to be harmful to fish, even at normal doses, thus organophosphate used to control ectoparasitic crustacean will significantly depress cholinesterase activity in fish. If chemicals are used at the wrong dose, or too often, they may cause significant pathology and mortalities in fish. Treatments may cause significant environmental damage, adversely affecting natural flora and fauna, although this is rarely long lasting as most chemicals are diluted or broken down fairly rapidly. The possible impact of chemical treatments on other water users must also be borne in mind, especially if water is used for domestic purposes. Treatments, especially antibiotics, may also leave residues in the flesh of fish. Treated fish should therefore be left for an appropriate withdrawal period before consumption.
Treatment, if it is a practical proposition, should be used with care, and perhaps be considered as a last resort if other measures such as reducing density fail. It is always imperative that any underlying unfavourable environmental or husbandry conditions which may have precipitated the outbreak be corrected, since otherwise it is very likely the disease will re-occur.
A number of comprehensive accounts of disease in fish and their treatments are available (Roberts, 1979; Stoskopf, 1992; Noga, 1996) and should be consulted for details of specific problems.
Disease, with the exception of EUS, is relatively rare in stocked capture fisheries.
The most serious outbreaks of infectious disease occur when pathogens are transferred to new geographical areas.
The prevention of the introduction of pathogens is best achieved by control of movements of live fish, including effective health certification and quarantine.
The control of fish disease in fish stocked into capture fisheries is extremely difficult, and any control measures possible may cause severe economic hardship.
Cage culture will be affected by disease because of the intensive nature of production, although control measures are relatively easily applied.
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