COVER
THA/75/012/WP 10

A MANAGEMENT PERSPECTIVE ON STRESS AND INFECTIOUS DISEASES IN CLARIAS FARMING




by

Alex N. Fedoruk

Programme for the Development of Pond Management Techniques
and Disease Control (DoF - UNDP/FAO THA/75/012)
Thailand

National Inland Fisheries Institute
Bangkok, Thailand
1981

PREFACE

The “Programme for the Development of Pond Management Techniques and Disease Control (THA/75/012)” was implemented in Thailand during 1979–82 as a joint project by the Department of Fisheries (DOF) and UNDP/ FAO. The purpose of the project was to improve DOP support services for Clarias farming through strengthening:

  1. the skills of Fisheries staff in aquaculture disciplines such as disease diagnosis and treatment, pond management and extension,

  2. the research on solutions for problematical aspects of Clarias culture,

  3. the system of relaying problems from the farms to DOF and of transferring improved technologies, and

  4. the equipment and facility base of DOF for working on aquaculture problems.

Although the UNDP/FAO participation was structured to terminate in August 1981, DOF committed continuation of the project to at least August 1982.

This report is one of several Working Papers prepared on various aspects of the project. A list of titles of reports completed in the series is annexed.

Inquiries concerning the subject matter of any particular report should be directed to the author.

c/o The Director
National Inland Fisheries Institute
Kasetsart University Campus
Bangkhen, Bangkok 9
Thailand

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A MANAGEMENT PERSPECTIVE ON
STRESS AND INFECTIOUS DISEASES IN CLARIAS FARMING1

Alex N. Fedoruk2

Outbreaks of infectious diseases in fish usually occur when three things happen at the same time:

Factor 1. disease causing organisms are present

Factor 2. a fish host for the disease is available

Factor 3. the resistance of the host fish to the disease is reduced.

One of the main objectives of fish culture management is to prevent the simultaneous occurrence of these factors.

Organisms which cause diseases in fish are called “pathogens” and they are usually always present in ponds and natural waters (Factor 1). Preventing the occurrence of pathogens in fish farms in Thailand is often very difficult and costly so little management is applied to control this factor.

1 A reference paper to be used for developing a contribution to the “Manual of Farming Pla Duk (the walking catfish) in Thailand”.

2 Senior Fisheries Biologist, UNDP/FAO Project THA/75/012, Bangkok, Thailand.

Fish, or the host for an infectious disease, occur in nearly all waters in Thailand whether naturally or through stocking (Factor 2). Two factors necessary for a disease outbreak, the pathogen and the host, are thus automatically brought together. For the disease to happen, however, the pathogens must come in contact with the fish, particularly a vulnerable fish. The chances for contact are relatively small in natural systems where the pathogen and the fish are normally spread widely apart. In aquaculture, however, the fish are crowded together thereby creating an increased opportunity for a pathogen to infect a host. Although the host is provided in fish farming, management does allow some control on the numbers of fish in a given area or volume of water hence some control on the chances of contact with a pathogen.

Fish become vulnerable to disease when their natural defence mechanisms for resisting the infection are weakened or lost (Factor 3). Pathogens thus take hold and rapidly multiply in or on the host producing more pathogens to further infect the same fish or to infect other fish nearby. Fish afflicted by a disease either recover or die. Recovered fish, however, are generally smaller than normal since growth was arrested while they were diseased; some become permanently damaged and are unable to resume normal growth.

Every species of fish is equiped to thrive within a range of living conditions called “normal” for a given species. While all fish can tolerate changes of conditions within the normal range, they become weakened and lose resistance to disease when the changes are sudden and approach or exceed the limits of the normal range. The changes which weaken a fish are called “stressors. When fish are unable to adjust to a change they become stressed. How a stressor actually reduces disease resistance is incompletely known. Researches on fish, however, repeatedly show that stressed fish readily become diseased fish. Fisheries workers in some parts of the world counteract the problem of reduced resistance related to stressors by applying antibiotics which provide the fish with temporary protection against pathogens such as bacteria. Studies in Thailand showed that although some antibiotics were helpful in controlling Aeromonas (bacteria) diseases of Clarias, the cost of the drugs was in excess of the returns that could be fained from saved fish.

Factor 3 occasionally arises in natural systems but it can often occur in fish farming operations where fish are being raised under conditions that are not “normal” for the species. Examples of stressors which are believed to have adverse influences in Clarias culture operations in Thailand include:

Temperature

Clarias spp., that are farmed in Thailand appear to thrive best at about 27 to 32°C. This range derives from records of general temperatures which prevail in the comparatively successful Clarias operations in the country. Temperature in excess of 32 but rarely as high as 37 or 38°C have been recorded; although no particular set backs in fish performance were noted or related to them, upper threshold limits undoubtedly occur and remain to be determined by research. Experiences involving the rearing of Clarias at lower temperatures show that fry are unable to undure exposure below 19°C and growth of larger fish is extremely slow in the range 19 to 24°C.

Changes in air temperature through a 24 hour period in areas of Thailand where Clarias farming occurs can often be 12 to 16 °C. The water mass in ponds, however, has a capacity for buffering or resisting change and the greater the water mass the less will be the change. Although water temperature patterns follow air temperatures the magnitude of change in the water is not as great, and the change in deeper ponds is less than in shallower ponds. Where diurnal air temperatures changes by 12 to 16°C the temperature in ponds with 1 to 1.5 m of water may only change by 3 or 4°C whereas in ponds with 60 cm of water the change can be 7 or 8°C.

Management to offset temperature stressors can entail (i) growing Clarias only at times of the year when warmer temperatures prevail (May to November) and (ii) use ponds that are at least 1m deep.

Nutrition

Clarias are capable of eating a variety of substances but to ensure good health they must receive a balanced diet of carbohydrates, proteins and lipids along with an assortment of vitamins and minerals. Growth in particular is dependent upon protein and a Clarias diet should have at least 35% protein (by weight) if production targets in an aquaculture operation are to be met. Furthermore, the capability for growth changes with the age of the fish. Young fish can grow much faster than older fish hence they have a requirement for proportionally more food.

Stresses related to nutrition in Clarias culture in Thailand have been noted to occur. Diets are sometimes lacking in necessary ingredients or they are not balanced. Vitamin C deficiency, resulting in poor growth, incomplete bone formation, spinal curvature and fractured backbones has been confirmed in a number of cases. The problem can be counteracted by applying commercial vitamin C to feedstuffs, by using whole broken rice or rice bran instead of polished rice as a food component or even by mincing fresh plant materials (eg. prineapple skins, leafy plants) into the food mixture. Excesses of rice in a mixed trash fish diet are also suspected of being a problem. With too much carbohydrate from rice, fatty tissues accumulate around some of the internal organs of the fish and interfere with normal functions. A general tendency of under feeding young fish and over feeding occurs in many Clarias farms in Thailand. Young fish are thus starved, and stressed and are vulnerable to disease. Exact feeding rates have not yet been determined for all diets and circumstances but some evidence indicates young Clarias (0.6 to 5 g) fed on a trash fish diet should receive 12 to 16% of their body weight/day whereas older fish (100 to 150 g) may only need 2 to 3%/day. Over feeding results in degraded water quality which can also stress fish.

Physical Injuries

Physical injuries to fish can be due to attacks by predators (other fish, birds, snakes, etc.) and parasites, to spinal wounds received from other Clarias when agitated in a crowded circumstance, and abusive handling by man. Injured fish are stressed fish. Open wounds become ready sites for infection by a number of pathogens. Some instances have been noted where young Clarias have been permanently crippled following rough handling.

Handling

Handling of fish by man is comparable to an attack by a predator from a fish's point of view and it is a stressing experience. Repeated and abusive handling intensifies the stress. Young fish appear to be particularly susceptible to handling as may be indicated by mortalities which frequently follow initial stocking in new ponds.

Parasites

Stresses caused by infestations of external parasites are not always direct. Infections of Clarias fry and fingerlings by the protozoan parasite, Trichodina, for example, may cause the fish to stop feeding. The fish thus exhibit signs of malnutrition and starvation. The combined effects of parasitism and starvation compound the stress condition and increase the vulnerability of the fish to bacterial infections, handling effects and other stressors. When large fish are infested by parasites they sometime become agitated and stressed, stop feeding or even try to remove the parasites by rubbing their bodies against a rough surface. Wounds result which become easy sites for infection by other pathogens.

Practices for controlling the occurrence of parasites include treating the ponds with chemicals before they are stocked and sometimes after they are stocked or bathing young fish in special solutions prior to stocking. Applying these chemicals, however, must be done in carefully regulated doses since excesses can themselves be a stressor. Lime spread on the bottom of a dry pond can kill some parasites while also helping to establish a suitable chemical environment when the pond is infilled. Formalin and “dipterex” have been frequently used to control parasites of Clarias. Some evidence has now emerged, however, to indicate that some deformities in young Clarias can be related to excessive concentrations of formalin which interfere with normal bone development.

Crowding

All animals are affected by crowding and the limits of tolerance vary from species to species. Some fish can be reared closely together while other are so territorial that they are unable to accept the nearby presence of others of their species (eg. as shown by males of “pla kat”, the Siamese fighting fish). Failure to provide adequate space for the number or density of fish in a culture system can lead to the development of stress.

Young Clarias seem tolerant of comparatively high densities. Under highly controlled conditions they have been reared successfully at densities of 1,000/m2. Most fingerling farmers, however, stock at the rate of 300/m2 partly as a means of reducing the chances of infection by pathogens. Clarias fry producers in Thailand, however, found through trial and error that the best stocking density for mature “pla duk” is about 1/100 m2 thus indicating a crowding limit when fish are breeding. Fish produced in grow-out phases of Clarias culture are harvested while they are still immature (150 to 200 g) and instincts for breeding space may not yet have emerged. Consequently, it is unknown whether or not a crowding stressor operates in farming table-size Clarias.

Studies by the Thai Department of Fisheries generally indicate that Clarias survival and performance improves as the density of fish reduces. Recommendations have therefore been advanced that stocking rates in grow-out ponds should be between 40 to 80 fish/m2. The reason for the evidently better results may be due to densities (i) that limit the chances of disease infection (ii) that limit the generation of adverse chemical conditions of the water which are linked to the numbers of fish present, and (iii) that limit the incidences of Clarias wounding one another with their spines which otherwise create sites for easy disease infection. Some farms, however, persist with higher rates and in cases where management controls (eg. adequate water exchanges, good feed, etc.) are applied the operations are equally successful when stocking at 120–200/m2.

Pollutants

Pollutants are foreign substances, generally chemical compounds, that enter the fish systems from an outside source. Likely and suspected pollutants that can create problems in Clarias culture are agricultural pesticides, and wastes and industrial wastes. Those contaminants can either kill fish directly or cause physiological stresses in the fish. Some pesticides can remain in the tissues of apparently healthy fish and under conditions of stress, due to another factor, may be released into the blood stream of the fish and compounding the susceptibility to disease or even resulting in death. Some of the contaminants alone may be tolerated by Clarias but it is known in the cases of other fish that multiple contaminants strengthen the adverse effect of one another thereby increasing the stressing or lethal effect.

Many of the Clarias farms in Thailand use khlongs (canals) as a source of water. These canals receive discharge from a variety of land uses including industrial and agriculture. Hence, the pathway exists for the introduction of pollutants to Clarias farms. These kinds of toxicants have been suspected of being the basis for diseases and deaths in some instances where problems have arisen in Clarias farms. The specific contaminants, however, have not yet been identified. Furthermore, some farms have experienced problems when rice fields are drained for harvesting. Since the use of pesticides in rice fields is widespread in some locales, the discharge water going into the khlongs could contain the contaminant.

Preventing the introduction of pollutants from khlongs to Clarias ponds is problematical. If pollution is suspected the farm must confront choices of not using the water at all or using it with the gamble that the effects of the contaminants will not be serious. In circumstances where alternate water supplies do not exist then farms may consider developing reservoirs to hold “good” water for use in cases of special needs.

CO2, NH4, H2S

Waste products are generated within Clarias culture ponds. Some derive from the fish themselves and some from uneaten food as in the case of excessive feeding. These products can be toxic at certain concentrations or at lesser concentrations be sources of considerable stress. Three products believed to be of particular concern are carbon dioxide (CO2), ammonia (NH4) and hydrogen sulphide (H2S).

Solutions for coping with accumulations of these waste products usually rest with exchanging water at a rate high enough to dilute or flush-out the wastes, or to remove the wastes in a filtration system which allows re-entry of the cleansed water back into a pond. The latter application has not yet been effectively made in Thailand but experiments are underway to prove a practical method. An additional practice which can help prevent the problem is the use feeding rates that are suited to fish needs thus avoiding the accumulation of wastes from uneaten food.

Research to determine the exact concentrations of CO2, NH4 and H2S that cause problems in Clarias culture is presently underway. Methods to control undesirable concentrations are also being studied.

Some Clarias farms in Thailand have access to ample water and can regularly flush ponds. The availability of flushing water is, however, frequently limited in the majority of farms where the problem of wastes becomes more acute. Water in the ponds of these farms generally exhibit two basic states throughout a crop cycle of grow-out fish.

In the first state, over the initial 40 to 50 days, some oxygen is always present to assist in the decomposition of ammonia compounds. High CO2 concentrations and nitrite levels, a stressing and toxic compound related to ammonia, periodically occur during this period when H2S problems are usually absent. CO2 and NH4 related stressors may be significant contributors to a major disease outbreaks that have been noted to occur repeatedly from day 30 to day 50 in Clarias grow-out operations. No definite methods are yet known for controlling excesses of CO2 and NH4 short of flushing the ponds. Adverse effects can be reduced by also ensuring the fish are free of other stresses wherever possible.

The second state of the water in Clarias ponds occurs from about day 60 to the termination of the grow-out cycle. It is characterised by the little or no oxygen, regularly high levels of CO2, a regularly high level of NH4, but with a low incidence of nitrites, and a frequently increasing level of H2S. These waste products may relate to another commonly experienced disease outbreak which occurs around days 90 to 120 after initial stocking.

Again, no definite ways are yet proven practical for controlling the CO2, NH4 and H2S levels apart from flushing. The CO2 levels in this second phase are due to excretion by the fish themselves and the decay of other wastes in the absence of oxygen. Some experiments are underway to see if spraying and agitating water will reduce the CO2 levels. The NH4 levels arise for the same reasons as CO2 but the particularly bad component; nitrite, is prevented from being released because of other chemical conditions. Whether or not NH4 itself is a problem in this phase is uncertain. However, additional tests are being conducted to see if ammonia can be reduced in filters and through spraying and agitation. The H2S accumulations arise from the decay of uneaten food. The compound does not form as long as iron is available in the water In some cases the available iron is used up over the first 100 or so days so that H2S only begins to accumulate after this time. Additional research is being conducted to test the effectiveness of adding iron over the latter stages of a grow-out cycle in Clarias pond culture a means of controlling excessive H2S.

SUMMARY

Disease organisms (Factor 1) and fish hosts for disease (Factor 2) occur in both natural and aquaculture systems in Thailand. When stressed fish with a reduced resistance to disease (Factor 3) also occur, outbreaks of disease, particularly in aquaculture systems, commonly follow.

Control of factors 1 and 2 through management in Thai Clarias operations is generally limited and impractical. The key for disease control in these operations lies in the prevention of of factor 3 conditions through pond management practices. Likely stressors and ways to control them in Clarias culture in Thailand are summarized in Table 1.

Table 1. Stressors and practices for controlling them in Clarias culture.

StressorManagement Practice
temperature shock
  1. avoid raising Clarias, particularly fry, during low temperature periods
  2. use deep ponds (lm plus); increased volume buffers sudden and excessive temperature changes
inadequate diet (quality and quantity)
  1. use balance diet
  2. add vitamin C
  3. use feeding rates that correspond with the age of the fish.
physical injuries
  1. control predators
  2. avoid disturbances when the fish are crowded
  3. handle fish carefully
handling
  1. limit the number of times fish are handled
  2. handle fish carefully
parasites
  1. control parasites in ponds by carefully regulated treatment with chemicals
  2. bathe young in mild concentrations of chemicals before stocking.
crowding
  1. limit stocking rates that relate to age or size of fish as follows:
    1. fingerling farms - 300 +/m2
    2. breeding farms - 1/100m2
    3. grow-out farms - 40 to 80/m2 or up to 150/m2 if adequate exchange water is available.
pollutants
  1. avoid using source water with known or suspected pollutants wherever possible
  2. create stand-by reservoirs of uncontaminated water for special needs
CO2, NH4, H2S
  1. flush ponds with exchange water
  2. avoid excessive feeding of large fish during the last 60 to 80 days of a grow-out cycle
  3. keep in touch with DoF research which may be able to show practical ways of controlling waste accumulation by:
    1. water agitation
    2. filtration
    3. chemical treatment

The programme for the Development of Pond Management
Techniques and Disease Control
(DoF-UNDP/FAO THA/75/012)

REPORTS

THA/75/012/WP 1 Report on Aquaculture Training Undertaken at the International Center for Aquaculture, Auburn University, U.S.A. Chanchai Sansrimahachai

THA/75/012/WP 2 Third Semi-Annual Report (Sept. 1/80-Feb. 28/81) of Progess on the Programme for the Development of Pond Management Techniques and Disease Control (DoF-UNDP/FAO/THA/75/012)". Alex N. Fedoruk

THA/75/012/WP 3 Management in Clarias Culture, Thailand. James Muir

THA/75/012/WP 4 Collecting Clarias Fry from Natural Waters. Montree Muangboon

THA/75/012/WP 5 Preliminary List of Diseases of Cultured Clarias in Thailand. National Inland Fisheries Institute, Thailand and Institute of Aquaculture, Stirling, Scotland.

THA/75/012/WP 6 Electrophoretic Analysis of Tilapia from the Dusit Palace Stock, Thailand. Brendan McAndrew

THA/75/012/WP 7 Water Quality Conditions as Disease Related Stressors in Clarias Pond Culture. Vijai Srisuwantach, Rangsarn Soungchomphan and Pathipath Sae-Eng

THA/75/012/WP 8 Analysis of NIFI Clarias Diet No. 12. Albert J. Tacon and M. Beveridge

THA/75/012/WP 9 Summary of the Report “Raising Clarias Fry on an Artificial Diet” Prasert Sitasit and Alex Fedoruk

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