58. Environmental Implications
of Disease Treatments in Aquaculture
Central Marine Fisheries Research Institute, Madras Research Centre, 68/1, Greams Road, Chennai-600 006
Coastal aquaculture, which is practiced
traditionally in Kerala, West Bengal and Goa in India from time
immemorial, has developed into a big industry in India. The aquaculture
production has increased from less than 5000 metric tonnes per
annum to 74,850 tonnes in 1994-95. The mushrooming, unplanned
development of shrimp farms in some parts of India has generated
social and environmental conflicts. A major disease outbreak
in 1994 and the white spot disease syndrome, which struck subsequently,
devastated shrimp culture throughout east and west coasts of India
and resulted in increased use of chemotherapeutics for disinfection
and treatment. Recently, probiotics - most of which are exotic
strains - and immunostimulants are also being used in shrimp culture
in India. The paper describes environmental and biological implications
of various forms of disease treatments practiced in coastal shrimp
farming in India.
The world aquaculture production has reached 25.5 million metric tonnes with fish and shellfish (including crustacea) contributing 18.5 million mt (Pillay, 1997) and the FAO estimate for aquaculture production by the year 2025 is 51.8 million mt. Asia dominates the aquaculture scene by contributing more than 88% of total production (Phillips, 1995).
Based on export needs and optimum per capita requirement of 11kg of fish per year, the marine fish production in India has to be 5 million mt by 2020, of which 1.62 million mt should come from aquaculture (Devaraj, 1997). Similarly, the projected figure for freshwater fish production in India by 2020 is 3.1 million mt, which means that the freshwater aquaculture production should nearly double the present level of 1.60 million mt. (Devaraj, 1997).
The demand for increase in aquaculture production has brought to light the conflicts between multiple users of natural resources like land and water. The rapid stride in aquaculture development, all over the world, coincides with the general awareness of environmental issues especially after the Earth Summit in Rio de Janeiro in 1992.
Major coastal activities that threaten the biodiversity of coastal environment are discharge of domestic and industrial sewage, oil exploration, refining and transport, mechanized fishing activities, recreation and tourism, use of chemicals, fertilizers and pesticides in agriculture, in land activities like construction of dams and canals along major rivers and its tributaries, reclamation of coastal lands and lastly, coastal aquaculture. Most important marine pollutants in India are untreated sewage which increase total suspended solids, biochemical oxygen demand, chemical oxygen demand, phenols and sulfites; heavy metals from industrial discharges; petroleum hydrocarbons and agricultural pesticides like DDT, BHC, indosulphan and heptachlor.
However, aquaculture is being implicated
for any environmental degradation like deterioration in water
quality, habitat destruction, loss of biodiversity in addition
to the social conflict in the thickly populated coastal zones
of Asian countries. The aim of this paper is to discuss the environmental
implications of coastal aquaculture in India, with special emphasis
on the effect of chemotherapeutics widely used in India.
Coastal aquaculture in India
Coastal aquaculture is centuries old in
India and is practiced in the traditional tide fed impoundments
in the states of Kerala, West Bengal and Goa. Commercial shrimp
farming in India was initiated in early 1980's and rose to industrial
proposition by late 80's. 74,850 mt of shrimps were produced through
culture in about 1,10,000 hectares of brackish water area during
1994-95 and currently 1,20,000 ha are under shrimp farming (Paulraj
et al., 1997). The tiger prawn, Penaeus monodon
and the Indian white prawn, Penaeus indicus are the main
species cultured. The production averages 700 kg per hectare per
year with a maximum of over 10 tonnes per hectare reported in
few semi-intensive shrimp farms.
Disease outbreaks in shrimp farms in India
The first major disease outbreak in commercial shrimp farming in India was reported in July, 1994 in Kandleru Creek in Andhra Pradesh, along the south east coast. About 2500 ha of land on either side of the narrow creek have been identified for shrimp farming and 600 ha are now under cultivation with another 600 ha being under development. The Kandleru river is 100 km long, and at Krishnapatnam bay it enters the sea. The river has tidal flow up to 50 km towards upper reaches. Most of the farms are located in the 40km stretch from the bar month. The mass kill in July 1994, happened after incessant rains and flooding of the area due to south west Monsoon. The disease was initially reported as Yellow Head Virus (YHV) disease. But a recent report suggests that it could have been the first major outbreak of the White Spot Disease Syndrome (WSS) in India (Shankar and Mohan, 1996). In November, 1994, the WSS, which is caused by a Systemic Ectodermal and Mesodermal Baculovirus, made extensive damage to shrimp culture in India and continued its devastation in 1995 and early 1996 throughout the west and east coasts of India. Both P. monodon and P. indicus in extensive, improved extensive and semi-intensive farms, irrespective of the source of water for culture, were affected by the disease and the total loss was estimated to be about 6000 million Indian rupees (US$_35.75 rupees) (Karunasagar and Karunasagar, 1997). Such major disasters are not uncommon in shrimp farming and have been reported in 1987, 1989 and 1993 in Taiwan, in 1989 in Sri Lanka, in 1991 in Thailand and in 1993 in China (Phillips, 1995). The severity of the white spot disease syndrome has since decreased in India and shrimp farming was limping back to normalcy till a court order issued by the Supreme Court of India in January 1997 suspending coastal aquaculture within 500 meters from the high tide level in Coastal Regulation Zone. Deliberations are going on at various levels in the Government and industry to find a way out of this impasse and to pave way for regulated, sustainable coastal aquaculture.
The mass kill of shrimps in July and November 1994 ushered in large scale use of chemotherapeutics in Indian shrimp farming. Till then, lime was the main disinfectant used at the time of pond preparation.
Major inputs in shrimp farming in India
Many small, medium and large scale hatcheries, with capacity to produce few thousands to 300 million seeds of P. mondon and P. indices, have been established in India by mid 1990's. Main inputs in hatcheries are live feed in the form of phytoplankton, rolifers and artemia nauplii, chemical disinfectants, antibiotics, and fungicides. Use of beneficial bacteria (probiotics) and immunostimulants have also been initiated recently.
(ii) Pond preparation
Mahua oil cake, tea seed cake and liquid ammonia are used to eradicate predators in the traditional, tide fed impoundments, where ponds cannot be completely drained. The pump-fed ponds along the east coast of India are drainable and lime and bleaching powder are extensively used as disinfectants during pond preparation. Organic and inorganic fertilizers like cow dung, urea, superphosphate and materials like zeolite to remove NH3, H2S and other toxic pollutants are also being used for pond preparation. Recent introduction is the application of probiotics like sludge digesting bacteria.
After the wide spread disease outbreak in 1994-95, most of the farms have converted few ponds as reservoir for disinfection of incoming water. Commercial bleaching powder or calcium hypochlorite Ca(OCl)2 with active ingredient ranging from 24-30% is used for disinfection at 30-35 ppm level in these reservoirs. Water from the reservoir is dechlorinated by aeration and used in the growout ponds.
Major inputs during on-growing include
seed, feed and feed supplements, fertilizers and prophylactic
and therapeutic chemicals like disinfectants, antibiotics, fungicides
etc. Zeolite and tea seed cake are occasionally used to clear
water of unwanted microorganisms, protozoans and metabolites.
Probiotics (bacillus, nitrosomonas and nitrobactor, pseudomonas
etc), immune enhancers and immunostimulatns are also being introduced
to ongrowing ponds.
Environmental implications of chemotherapeutics
Different methods of treatment in aquaculture and its advantages and disadvantages are given in Table 1 and the disinfectants and drugs used for treatment in India are given in Table 2.
Coastal aquaculture is spread all along the long coast line of India with intense activities in certain areas, especially around some creeks and canals where a number of farms are located. Except for few corporate farms which draw sea water directly for farming, most of the small, medium and big farms draw water from estuaries, creeks or canals. In most cases the same waterbody is used for discharge of effluents. Such systems are ecologically fragile and the aquaculture effluents in the form of sediments, suspended particles, organic and inorganic nutrients and chemotherapeutics can cause imbalances in the environment, if proper flushing is not possible. Aquaculture is the only activity where there is a danger of the effluents reentering the system.
Compared to countries like Thailand where nearly fifty percent of mangrove areas have been converted for shrimp farming, very limited farms have developed in mangrove areas in India. Mangrove is the natural breeding and nursery ground of many aquatic animals and its large scale destruction can cause reduction in recruitment to fishery. Mangrove soil is acidic and large volume of lime has to be used to make it suitable for shrimp farming. Continuous use of lime will increase the pH of soil in the mangrove area, which might effect the flora and fauna of the system.
2-3 ppm chlorine is usually used to disinfect
drinking water, while 30-35 ppm level is used in coastal aquaculture
since higher concentrations are required in saline waters. In
addition, chlorination is also intended to kill small aquatic
invertebrates which might act as reservoirs of disease. Water
in the reservoir is usually dechlorinated before being utilized
for on-growing. Improper dechlorination might cause health problems
to the farmed animal itself and when discharged, might kill the
microflora and smaller invertebrates in the environment. The effect
of chlorination in preventing outbreak of viral disease in shrimp
farms is yet to be demonstrated scientifically and the widespread
use of chlorination has to be discouraged, till it is evaluated.
Table 1: Different types of treatments
used in Aquaculture
|Prophylactic treatment||Routine, before diagnosis is done||Decreased chance of going off feed. Predictable amount of drugs.
Easier to use with large population
Usually over used. Costly if no disease.
To be avoided when using antibiotics
|Mataphylactic treatment||Middle of disease treatment.
Treat after a level of disease is noticed and before full scale outbreak.
|Treatment not given when not needed. Decreased mortality. Decreased resistance. This is the method of choice.||Need to establish initiation parameters. Medicated feed may not be kept on site.
Require prompt initiation.
Requires regular monitoring
|Therapeutic treatment||Treat at disease outbreak.
Positive diagnosis required before treatment
|No unnecessary treatment.
|Requires diagnostic facility.
Requires regular monito-ring. Limited usefulness. Response time usually slow. Medicated feed may not be available at times of need.
Table 2: Chemotherapeutants used in
shrimp farming in India
|Disinfection and Prophylactic treatment||Systemic treatment|
Chlorine as bleaching powder.
Quarternary ammonium compounds like Benzalkoniam Chloride.
Tea seed cake.
Mahua oil cake.
|Antibiotics like penicillins and tetracyclins (Amoxycillin, Ampicillin, oxytetracycline etc).
Antibacterials - Potentiaed Sulphonamides like sulphamethaoxazole with Trimethoprim.
Nitrofurans like Nitrofurazone, Furazolidone etc.
Other chemical disinfectants like copper compounds, Iodophores, formaline, malachite green etc can create health hazards for the persons handling them as well as can kill non-targeted organisms if discharged to the main waterbody through effluents.
Use of antibiotics, antibacterials and antifungal drugs like penicillin, tetracycline, potentiated sulphonamides, quinolenes and nitrofurans are also being used in shrimp culture in India, though in limited quantities. Unlike many other countries, where strict regulations are enforced in use of antibiotics, no such enforcement is made in India and even new generation drugs developed to treat specific human diseases are easily accessible. This can lead to indiscriminate use of broad spectrum antibiotics which might create more problems than it can eliminate.
It is reported that only 20-30% antibiotics are ingested by fish and the remaining 80-70% reach the environment (Dehadrai, 1977). Even the antibiotics ingested by aquatic animals may be excreted as such or as metabolites which might sometimes he harmful to the animal as well as human consumers. Pathogens such as Vibrios and Aeromonas can develop resistance to antibiotics very quickly. The presence of residual antibiotics in the sediments will promote resistance and may have human health implications if this resistance is passed on to potential human pathogens (Kong, 1995). The residual antibiotics also might destroy the beneficial microflora in the aquaculture system and at the discharge point and might affect other non-target organisms. Antibiotics tend to accumulate in muscle and other organs of the fish/shrimp and might be hazardous to humans. It is extremely important to allow effective withdrawal time whenever antibiotics are used to treat shrimp diseases. Biomagnification of antibiotics through the food chain in the ecosystem is another point to be studied before it is discharged into the surrounding environment. High dosage of antibiotics are often used in shrimp hatchery which might increase its survival and might produce apparently healthy seeds. Such seeds usually perform very badly when released to on-growing ponds due to its reduced natural resistance.
Probiotics, which are beneficial microorganisms that will prevent multiplication of pathogenic ones and will enhance the health of the host, are widely used in human and veterinary medicines. Many probiotics have recently been introduced to shrimp farming and is reported to yield better survival and growth. Adequate caution is required in use of many brands of probiotics available in India since most of the beneficial strains are exotic and its behaviour and efficacy in Indian situations have not been tested. Indigenous probiotics must be developed and used for shrimp farming in India.
Immune enhancers and immunostimulants
are now being utilized for shrimp farming in India. In the absence
of specific immune mechanisms in shrimp, vaccines are impractical.
However, shrimps possess very efficient non-specific immune mechanisms.
Three factors involved in immune system of shrimps have been identified
and are being used effectively as immune enhancers in countries
like Malaysia, Indonesia and Philippines (Kong, 1995). ß
glucans and some lipopolysaccharides simulate the cell wall configurations
of vibrios and are effectively used as immunostimulants
in shrimp culture. Both immune enhancers and immunostimulants
have to be repeatedly used since shrimps do not possess immune
memory. Immune enhancers and immunostimulants are now being used
in many shrimp farms in India.
Towards Sustainable Aquaculture
To establish sustainable, ecofriendly aquaculture, a comprehensive planning with long-term perspectives is required. Selection of appropriate site and adoption of models that will suit the carrying capacity of the system are few of the prerequisites for sustainable aquaculture. Low to medium production will sustain for longer period and will have low input of undesirable materials. Maintaining high water quality by proper monitoring and management is an insurance against any disease in aquaculture. Production of high health shrimp, which is specific pathogen free and specific disease resistant is one of the strategies towards sustainable aquaculture. Use of highly digestible, low protein, highly efficient quality feeds and adoption of appropriate feeding strategy would reduce wastage and would greatly enhance the scope of ecofriendly aquaculture.
While it may not possible to eradicate diseases in aquaculture, treatment has to be resorted to only at times of necessity and extra caution has to be taken while using antibiotics and other chemotherapeutics that will possibly have an impact on the ecosystem. Use of indigenous, effective probiotics and immunostimulants are some of the exciting propositions for sustainable aquaculture in future. Closed system of culture or partial reuse of water in aquaculture systems will reduce the possible impact of effluents in the ecosystem.
With world production of fish from capture
fishery almost reaching a plateau, it is only through aquaculture,
especially coastal aquaculture and mariculture, that future protein
needs of the world can be met. The need of the hour is to progress
towards regulated, sustainable, ecofriendly aquaculture and not
to cry foul and stop aquaculture activities by over emphasizing
its possible negative impacts on the environment.
The author is thankful to Dr. M. Devaraj,
Director, Central Marine Fisheries Research Institute, Kochi,
India for encaragement in preparing this paper.
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