Water Quality And Fish Health

Water Quality and Fish Health Dr. Subhendu Datta Sr. Scientist Kolkata, India

Introduction Health condition of any animal deteriorates when the environmental environmental condition is not congenial. A simple example may be when the temperature started dropping during during the onset of winter season, we face all the symptoms associated with cold and cough (e.g. sneezing, running nose occurs). Good water quality is the key to successful fish/prawn production. production. Fish and prawn under normal condition is in a state of equilibrium with its its environment and diseasedisease-producing organisms. Any alteration in the environment disturbs this equilibrium resulting resulting in stress to the fish/prawn and they become more vulnerable to disease producing producing organisms. organisms. The presence of harmful impurities in water is mainly due to natural natural contamination or as a result of human activities polluting water may cause health problems to fish/prawn. A number of abiotic factors beyond optimum levels are responsible for poor health of fish/prawn.

1

Temperature Fish/prawn respond to an increase in environmental temperature particularly in summer season by increasing the respiration rate as well as cardiac rate and cardiac output. Fish/prawn eventually dies from abnormally high temperatures because of lack of oxygen and malfunction of enzyme systems. Most of the tropical species may die when temperatures drop below below 6°C. Exotic carps and Indian major carps cannot survive at temperature above 500C. Optimum water temperature range for major carps is 20° 20°-38° 38°C. Fish/prawn can tolerate upward changes in temperature better than than downward changes. It is always advisable to gradually acclimatize fish/prawn to water temperature change of 3° 3°C in either direction.

Temperature (contd…) During extreme summer due to high temperature fishes/prawns refuse feeding or become easily prone to intestinal inflammation. inflammation. When temperature exceeds tolerance capacity, excitability and convulsive movement of fishes are observed with dark colouration throughout the gills followed by collapse and death. Low temperature affected by cold shocks (3(3-5°C) with the symptom of disturbances in fish equilibrium causing damage of skin, oedema, oedema, intestinal damages, haemolysis and death due to paralysis of respiratory centres. Sudden drop of water temperature causes pathological changes in gills resulting impaired respiration and abnormally copious mucus mucus production, gills turn white indicating circulatory disorders and it also causes disarray in the embryonic development in fishes.

2

Oxygen It is the most important environmental factor influencing the health health condition of fish/prawn in a water body and thus being the limiting limiting factor in fish production. Dissolved oxygen value of 5-10 ppm is optimum for normal growth and reproduction in tropical waters. At levels below 3 ppm fish/prawn may live but will not feed or grow but the concentration < 1 ppm is lethal to many species if sustained for a long period. Low oxygen levels are frequently a problem during the summer due to high temperature with heavy algal and or phytoplankton bloom. As a result of photosynthesis during bloom period the dissolved oxygen values fluctuate during the day with maximum values during late afternoon (2(2-3 PM) and minimum values in early morning because during night dissolved oxygen in water is utilized for respiration respiration but no production of oxygen due to lack of photosynthesis.

Oxygen (contd…) On the other hand, during day time both production and utilization of oxygen takes place by photosynthesis and respiration respectively. Typically, the fish/prawn will be found dead or in severe stress at dawn when oxygen levels may approach zero. zero. Typically larger fish die first and water often changes in smell and colour. colour. Inadequate supply of oxygen leads to embryonic mortality. Sudden death of vegetation and blooms further complicate the problem by increasing the biological oxygen demand (BOD) in the decay/decomposition/oxidation process. Algae growth is restricted in the winter that is why low oxygen is not a problem during colder months. months.

3

Oxygen

(contd… contd…)

The basic problem during summer months is generally overcrowding with an abundance of water nutrients (nitrates, phosphates) resulting from from fish/prawn wastes, decaying of artificial fertilizers, sewage, feeds, and organic organic manures. This decaying process also reduces the available dissolved oxygen oxygen from the water due to oxidation process. High salinity, low atmospheric pressure, shortened day length, and and cloudy weather also decline the dissolved oxygen levels from the water body. The basic requirement of dissolved oxygen for fish/prawn varies between species. Besides, younger fish/prawn requires more oxygen than adult, active active fish/prawn requires more oxygen than resting fish, fish normally requires more more oxygen as temperature rises, oxygen requirement increases after feeding because because oxygen is required to digest the food, stressed fish/prawn requires more more oxygen and when fishes are transferred suddenly from cold to warm water and vice versa. In deficiency of oxygen the defensive mechanism of fishes/prawns is no longer maintained at optimum level, hence the possibility of infestation infestation with various pathogens is maximum. maximum.

Oxygen

(contd… contd…)

When water is having lower concentration of oxygen, fish begin to to rise to water surface or crowd near inlets particularly in the early morning hours gulping air with the mouth wide open and the gills of stressed fish become pale. pale. If the low dissolved oxygen condition persists for a long period in a pond it may produce significant sub lethal and lethal effect in fish. In extreme depletion of dissolved oxygen level, fish may die due to asphyxia. asphyxia. The oxygen can be elevated manually by splashing the water with bamboo sticks, sticks, which helps in dissolving atmospheric oxygen in water or by use use of commercial aerators or by pumping the water and spraying over the surface simultaneously. Low levels of potassium permanganate (2(2-4 ppm) ppm) have been used as algaecides as well as to quickly raise the oxygen; however, the increased BOD resulting from decaying algae will further complicate the problem. problem. Surface agitation, increased inflow of aerated water, and thinning thinning the population of fish are effective methods used singly or in combination to remedy the problem.

4

Following precautionary measures may be taken to minimize the oxygen depletion in the pond: Feed should be given in the afternoon or evening not in early morning as oxygen requirement in fish after feeding increases and dissolved oxygen is minimum in pond during early morning. During application of organic manure in the pond, by determining the BOD value of the manure from dry matter content of it the maximum amount of oxygen that would be consumed for stabilization of a given quantity of manure at a particular temperature can be predicted. Thus, the quantity of manure to be applied daily without the hazard of oxygen depletion can be calculated taking into consideration the availability of dissolved oxygen during 24 hours in the pond water.

Oxygen (contd…) Proper care has to be taken during death of phytoplankton and/or algal blooms to compensate the oxygen used for decomposition of dead blooms by microorganisms. During summer season as the oxygen declines due to higher temperature and increased respiration of bacteria, proper oxygenation methods should be applied. applied.

5

Ammonia The total ammonia concentration in water comprises two forms, viz., NH3 (unionized ammonia) and NH4 (ionized ammonia). They maintain equilibrium as per the equationequationNH3 + H2O

NH4 + OH-

The unionized ammonia fraction is more toxic to fish and the amount of the total ammonia in this form depends on the pH and temperature of the water. As a general rule, higher the pH and temperature, the higher the percentage of the total ammonia is present in the toxic unionized form. form. In aquatic systems ammonia accumulates as a result of the normal metabolism of the fish where it is excreted by kidneys as well as as by the gill tissue.

Ammonia (contd… contd…) Ammonia is also formed by the normal decomposition processes of protein (uneaten/excess artificial fish or prawn feed), organic manure, inorganic ammonia based fertilizers and dead phyto or zooplankton. Industrial and domestic wastes released in the water areas produce produce ammonia. In ponds, high ammonia levels are result of insufficient insufficient water flow for the amount of fish/prawn stocked. Fate of ammonia in the water body depends on oxygenation of the water. Such as in oxygenated water; ammonia produced is oxidized to nitrite and harmless nitrate (nitrification process) whereas in deoxygenated waters nitrate is converted to harmful nitrite and ammonia (denitrification (denitrification process). process). Two types of bacteria facilitate the oxidation of ammonia: Nitrosomonas sp. converts ammonia to nitrite and Nitrobacter sp. converts nitrite to nitrate. nitrate. Nitrates are utilized by plant and bacteria or denitrified to gaseous gaseous nitrogen and eventually fixed into plants by specific bacteria.

6

Ammonia

(contd… contd…)

Generally closed systems do not have enough plant life to remove nitrates, leading to the necessity of clearing nitrates by water replacement. replacement. Nitrates are not toxic but they act as growth promoting substances substances for many bacteria, which are undesirable in closed system of fish/prawn culture. culture. 0.020.02-0.04 ppm ammonia is considered as safe concentration for many tropical fish and prawn species whereas 0.050.05-0.39 ppm and 0.400.40-2.5 ppm produce subsublethal and lethal effects on the many fish/prawn species respectively depending on the oxygen, pH, and temperature of the water. Ammonia stress in fishes/prawns causes gill hyperplasia, reduced activity, and growth. Liver, kidney and brain damage also occur. In prawn surfacing occurs. occurs. Ammonia in water is a predisposing factor to bacterial gill disease. High level of ammonia probably interferes with respiration resulting resulting in physiological oxygen depletion. depletion. High levels of nitrite produced from denitrification process also cause mortality to fishes/prawns.

Measures to reduce the effects of ammonia Appropriate measures should be taken for maintaining safe ammonia concentration in water for successful aquaculture to produce healthy fish/prawn. Normally at high salinity, low dissolved oxygen, and high carbon dioxide concentration, the toxicity of ammonia to fish/prawn is increased. increased. Aeration will increase the dissolved oxygen concentration and decrease the pH thereby reducing toxicity. toxicity. Aeration will bubble out some of the gaseous unionized ammonia from water. Ammonia will get oxidized to water and gaseous nitrogen (4 NH3 + 3O2 = 2 N2 + 6H2O).

7

Measures to reduce the effects of ammonia (contd…) Healthy phytoplankton population removes ammonia from high ammonia content. The manure should be dried to allow ammonia gas to escape and then applied in the pond. Sodium chloride is used to reduce the toxicity of ammonia in water. For example in Clarias farming, sodium chloride is used at the rate of 200200-300/ kg/1600 m2 to reduce toxicity of ammonia. Biological filters may be used to treat water for converting ammonia to nitrite and then to harmless nitrate through nitrification process.

Nitrite Nitrite is an intermediate product in the biological oxidation of of ammonia to nitrate called the nitrification process. In most natural water bodies and in wellwell-maintained ponds nitrite concentration is low. In water bodies with high organic pollution pollution and/or low oxygen concentration nitrite concentration may increase. Less than 0.02 ppm nitrite concentration in water is considered to be safe to the fish/prawn life, whereas, 0.020.02-0.90 ppm and 1.01.0-10 ppm are sub lethal and lethal levels for many warm water fish species respectively. respectively. Nitrite is highly toxic to fish. When fish absorbs nitrite it reacts reacts with haemoglobin to form methaemoglobin. methaemoglobin. This methaemoglobin gives brick red colour to fish gills and it also unable to carry oxygen leads leads to death of fish. To maintain safe nitrite level in water, correct stocking, feeding, feeding, and fertilization practices should be maintained. The pond should be kept well oxygenated. Biofiltration is done through special filters by which biological conversion of nitrite to harmless nitrate occur.

8

pH (Hydrogen Ion Concentration) Fish can tolerate wide range of pH and the optimum pH range for most fish and prawn species is from 7 to 8.5. Too acidic and alkaline pH is detrimental to fish. pH 9.0 produces sub lethal effects on many fish species, pH 1010-10.9 is lethal to many fish/prawn species if exposed over a prolonged period otherwise in short duration sub lethal effects occur, pH 11 is lethal to all fish/prawn species. At pH 55-6, poor pond productivity and reduced fish/prawn growth occurs, pH 4.14.1-4.9 produces sub lethal effects, and at pH 4, direct mortality occurs occurs in many fishes. There are several factors that influence the acidity of waters. As mentioned earlier that high level of free carbon dioxide increases the toxicity of acids. acids. The primary effect of acidity is to disrupt the ionic balance of fish/prawn. Thus, an increase in the concentration of calcium, magnesium, sodium and chloride cations will help to protect fish from the harmful effects of acids. The fry stage or hatchlings of fish are normally most vulnerable to acids. acids. Some acid ponds can be successfully used for fish farming if fingerlings rather rather than fry are stocked.

Several measures can be taken for rectifying alkaline and acidic water bodies For alkaline waters Ensuring good water management may rectify rapid fluctuations in pH caused by excessive phytoplankton populations. Water body should have an alkalinity alkalinity of more than 50 ppm as calcium carbonate. By application of acid forming fertilizers. By application of gypsum (5(5-6 tons/ha) or raw cow dung 20 tons/ha. Apply Dolomite in saline water For acidic waters By application of lime: Limestone CaCO3, slaked lime Ca(OH)2, quick lime CaO or dolomite are used to rectify the acidic water bodies depending upon upon the pH. Salt water like seawater may be flushed through water bodies of coastal farms to neutralize acidity.

9

Alkalinity Alkalinity refers to the concentration of bases in water and the capacity of water to accept acidity i.e. the buffering capacity. If the water contains various dissolved salts or bases such as carbonates, bicarbonates etc., not only will the pH be naturally higher than neutral but these negatively charged ions will combine with hydrogen ions which essentially essentially avoids a pH drop. Proper alkalinity in ponds is important for successful fish production. production. Waters with a low alkalinity i.e. total alkalinity less than 20 ppm as CaCO3, have a very low buffering capacity and consequently are very vulnerable to fluctuations in pH, for example, during rainfall and phytoplankton blooms. Such fluctuations may be directly harmful to fish populations. Ponds with low alkalinity also tend to be much less productive than high alkalinity alkalinity ponds, although ponds with alkalinity greater than 500 ppm may also be unproductive because of limitations carbon dioxide availability at such high concentrations. Alkalinity value ranging between 100100-250 ppm is ideal for fish/prawn, fish/prawn, whereas the value less than 20 ppm creates stress in fish/prawn, Low alkalinity ponds can be treated with lime to rectify it.

Total Hardness Cations of alkali earth metals; mainly calcium and magnesium constitute constitute the total hardness of a water body. body. The total hardness concentration in majority of the water areas should be similar to the total alkalinity. This is because the calcium and magnesium ions are commonly bound to the main alkalinity bases, carbonate and bicarbonate. bicarbonate. There are several factors that influence hardness in water areas. areas. In water bodies where the total hardness concentration is more than total alkalinity the calcium and magnesium cations are bound to anions other than carbonate carbonate and bicarbonate viz., sulphate and chloride. Total hardness value of more than 50 ppm is satisfactory for pond productivity and should help to protect fish/prawn against harmful effects of pH fluctuations and metal ions but total hardness value of less than 20 ppm creates stress in fish/prawn. Ponds with low hardness can be treated with lime for rectification. rectification.

10

Carbon dioxide Carbon dioxide is present in the atmosphere in very small quantity. For this reason, in spite of its high solubility in water, its concentration in most water bodies is low « 6 ppm. ppm. It occurs in waters in three closely related forms viz., i) Free carbon dioxide ii) bicarbonate ion (HCO3-) iii) carbonate ion (CO3-2). The amount of each forms present; depend on the pH of water. For example, in neutral or acidic waters high concentration of free carbon dioxide i.e. the toxic form is frequently found.

The concentration of free carbon dioxide may rise under following circumstances: If it is an acidic ground water. In water areas with large phytoplankton bloom, carbon dioxide may reach high levels due to a) respiration of phytoplankton at nighttime b) during cloudy weather c) decay of dead phytoplankton. In water area is heavily loaded with organic manure and feed. In natural waters high concentration may occur after herbicide treatment.

11

High external concentration of carbon dioxide interfere the uptake of dissolved oxygen from water causing respiratory problems and stress. Thus the effects of high carbon dioxide are acute at low dissolved oxygen concentration. Up to 11.9 ppm of carbon dioxide is tolerable to fish at low oxygen concentration. 1212-49.9 ppm produces sub lethal effects may include respiratory stress and the development of kidney stones (nephrocalcinosis) nephrocalcinosis) 5050-60 ppm is lethal to many fish species with prolonged exposure.

Several measures can be taken for controlling high carbon dioxide dioxide concentration. Such as, Repeated aeration of water Increasing the pH of water by adding hydrated lime (calcium hydroxide). It acts according to the reaction: Ca(OH)2 + 2 CO2 Ca(HCO3)2 Experiments have shown that approximately 1 ppm of hydrated lime can remove1.68 ppm of free carbon dioxide. Therefore in water bodies with low alkalinity care must be taken not to apply excessive excessive lime because it may cause the pH to rise, creating stress to fish. fish. The phytoplankton population and the organic loading in a water body should be regulated by correct stocking, feeding, and fertilization.

12

Hydrogen sulfide Hydrogen sulfide above optimum level in water can cause stress to to fish and prawn. Hydrogen sulfide is the product of the anaerobic action of bacteria bacteria during organic matter degradation, which accumulates and forms a thick layer of organic deposit at the bottom. The bottom soil turns black and a rotten smell is discharged when when it is disturbed. Unionized hydrogen sulfide is toxic to fish, but the ions resulting from its dissociation are not very toxic (H2S = HS + H+, HS = S+2 + H+). Hydrogen sulfide concentration of 0.010.01-0.5 ppm is lethal to fish and any detectable concentration in water creates stress to fish. At a concentration of 0.10.1-0.2 ppm, ppm, prawn loses their equilibrium, creates sub lethal stress, and at a concentration of 3.0 ppm prawns die instantly.

Some measure can be taken to rectify increase in hydrogen sulfide. Such as frequent exchange of water to prevent building up of hydrogen sulfide in the water body if pH of water is increased by liming the toxicity of hydrogen sulfide decreases.

13

Suspended solids Lots of solid materials are present in the water body, which are retained when the water is filtered through a 0.45 µm mesh size filter paper. Natural weathering of the rocks, land erosion, or pollution is the the factors responsible for origin of the suspended solids in the large water water body. But in the small culture area suspended solids in water is constituted constituted by phytoplankton bloom, uneaten feed particles, and faeces. faeces. The effects of suspended solids depend on the nature of the solid. solid. Abrasive particles such wastes from coal washing or long spined diatoms or sometimes, copepodic zooplankton is harmful to various stages of fishes than soft materials. Gill tissue is the most susceptible. Gill damage through excessive mucus production or clogging. Bacterial gill disease is common to high levels of suspended solid load in water. water. Suspended solids and turbidity are important in reducing the penetration penetration of light in the ponds, reducing the productivity, and increasing the risk of deoxygenation. deoxygenation.

Adequate water depth is needed not only for optimum growth of fishes fishes and prawns but also to provide enough space and oxygen to them. The ponds where water source is monsoon rain, after the end of the the season water levels starts decreasing gradually and shortage of water is is quite common during summer season, which is the most crucial time for fish culture since the fish growth rate is faster in this period. period. In fact, during the time of lowest water level the ponds contain the maximum biomass. In shallow and seasonal ponds sufficient phytoplankton population population fails to appear and soft sediment layer is vigorously stirred up by fish, fish, making the water more turbid, thereby reducing the photosynthetic process process by limiting light penetration. Eventually total amount of dissolved oxygen may not be at time sufficient sufficient to meet the demand for respiration of total community and the chemical chemical oxygen demand of the sediment, resulting sometimes in mass mortality mortality of fish and planktonic collapse.

14

On the other hand in the dipper perennial ponds where the water column is more than 3 m, fish life is again adversely affected. In such ponds, the photosynthetic or oxygen producing zone is less in comparison with oxygen consuming zone, which leads to negative oxygen balance and create to stress to fish health. Suspended solid concentration of up to 10,000 ppm (up to 4% by volume) is tolerable to freshwater carps, carps, Tilapia sp. and catfishes, although effect will depend upon the nature of the suspended particles. Persistent turbidity problems in water if caused by fine clay mineral particles can be treated with alum @ 2525-45 kg/ha or lime.

Metals Originally the natural weathering process determines the background background level of metals in a particular water body. Though it is a natural process, the rate of weathering is influenced influenced by man made changes in land use pattern and by acid rain. Effluents from mining, industry, and domestic use are also the source source of metals (Zn, Cu, Hg, Ni, Pb, Pb, Cd, Cd, Cr, AI, Fe etc.) in the water body. If the water is hard (300 ppm as CaCO3) with a pH of 8, heavy metals will precipitate as carbonates or sulphates. In softer, low pH water (less than 100 ppm as CaCO3) the metals are in their ionic form and are more toxic to fish. In acid waters high levels of metals with low pH are recorded during during period of high rainfall. Most metals dissolve more easily in acid water of pH < 7 and therefore acid water usually contain higher concentrations of metals metals than neutral or alkaline waters.

15

Following factors influence toxicity of metals to fish/prawn The presence of organic substances like sewage effluents, organic manure, humic acids have all been shown to reduce the toxicity of metals, because organic organic substances complex the free ions, i. e. the toxic component of metal and render them them harmless. Calcium is one of the most important inorganic substances affecting metal metal toxicity. High levels of calcium in water protect the gills from metal damage by by slowing down the diffusion in of metal ions. Acidity and low alkalinity make many metals more toxic because they change the metal into the more harmful soluble form. Increased temperature and low oxygen level usually increase toxicity of metals because at low oxygen concentration fish pumps more water hence more toxic toxic metals over their gills. Moreover at higher temperature, water contains less oxygen oxygen making the problem worse. More than one metal can act together to produce more pronounced toxicity than a single one. Fish acclimated relatively high concentration of metal in water are better able to withstand a potentially toxic concentration than nonnon-acclimated fish. In general small fry are more vulnerable than older fish.

The toxic effects of most metals are nonnon-specific. Acute responses exhibit gill damage in the form of swelling of epithelial cells, separation of gill epithelium, severe osmoregulation problems, and mucus production. production. This results in loss of body salts, poor oxygen exchange, and eventual death. Chronic effects range from proliferation of epithelial cells, fusion of secondary lamellae, clubbing at the end of gill filaments and changes in the blood chemistry. chemistry. The response of fish to some metals is more specific e.g. the neurotoxic effects of lead. However, since most effects are nonnon-specific it is essential in many suspected case of heavy metal poisoning to carry out analysis of heavy metal content of water and fish tissues together with measurements of oxygen, alkalinity, hardness and pH to establish the cause of death.

16

Recommended safe level of metals suggested by EPA AI

0.1 ppm at pH 5.25.4

Cd

i) 0.004 ppm in hard water i.e.400 ppm of CaCO3 ii) 0.0004 ppm in soft waters i.e. 40 ppm of CaCO3

Cu

i) 0.005 ppm in water having 10 ppm of CaCO3 ii) 0.02 ppm in water having 50 ppm of CaCO3 iii) 0.04 ppm in water having 100 ppm of CaCO3

Fe

Pb

1.0 ppm is safe but 1.210.5 ppm is lethal to common carp, death attributed to the precipitation of ferric ydroxide on gills forming brown deposit on it

i) 0.005 ppm in water having 10 ppm of CaCO3 ii) 0.03 ppm in other types of water iii) 0.05 ppm produces sublethal effects, toxic to nervous systems in fishes, black tails are diagnostic of lead poisoning

Hg

Ni

Zn

i) 0.01 ppm in water i) 0.01 i) 0.0002 having ppm 10 ppm ppm in at 20 ppm water of hardness ii) 0.0005 CaCO3 of water Ii) 0.05 ppm in ii) 0.04 tissues ppm in ppm at water (wet 320 ppm having weight hardness more basis) of water than 50 ppm of CaCO3

Pesticides The widespread use of pesticides in pest control of crops and forestry forestry has threatened the fishery waters. Although many pesticides are useful in fishery management there are many others, which are dangerous for fish life and can produce several several types of deleterious effects. Three types of pesticides such as organochlorines, organochlorines, organophosphate, and carbamates are of importance causing pollution in waters. These pesticides may come into the water bodies accidentally or by deliberate application. Accidental entry may happen due to i) run off from sprayed agricultural agricultural field ii) washing of sprayed equipments, plants crops in water etc. deliberate deliberate application occurs during i) control of aquatic weeds ii) control control of mosquito larvae iii) elimination of unwanted fish from the pond iv) control control of insects v) control of parasite.

17

Formulation and chemical stability of pesticides are two important factors, which can influence the toxicity of pesticide in water. The active ingredients of any pesticide are the main toxic components and its formulation in oil emulsion, wettable powders, or granules very often determines the toxicity to fish. For example, it is reported that DDT is more toxic to fish in an oil base emulsion than in water. Water based pesticides are more easily washed away than oil based emulsion, which tend to be more persistent. Granule formulation releases the toxic component of the pesticide over a longer period of time thereby reducing the toxicity to fish due to dilution. As per as stability is concerned organochlorines are very stable and are recorded to persist in the environment for a longer duration. duration. Other pesticides e.g. organophosphates are known to be less stable stable and easily degradable becoming less toxic to fish.

Following factors can influence the toxicity of pesticides to fish fish

The joint action of mixture of pesticides can often be lethal to fish than effect of a single pesticide. The toxicity of pesticides to fish varies with fish species and the size of fish. For example, fish fry are most susceptible than adult fish. fish. Fish is more susceptible to pesticide poisoning if the water quality quality is bad. For example, at a higher temperature and low dissolved oxygen condition in water most fishes pump in more water and hence more pesticides over their gills. gills. In water bodies with high turbidity the effect of pesticide pollution is diminished. Most pesticides are easily bound to the large amount of organic matter present in such water bodies and are consequently rendered rendered less toxic.

18

Recommended safe levels of different pesticides in water and tissue suggested by EPA Common Name DDT BHC Dieldrin Endosulfan Endrin Dichlorvos

Dimethoate Edifenphos Malathion Parathion

Carbaryl Carbofuran Quinalphos

Trade Name

Safe Level (ppm) Water

Organochlorine Gesarol Neocid 0.002 < 0.18 Octalox 0.005 Thiodan 0.003 Tafdrin 0.002 Organophosphates Nuvan < 0.003 Captan < 0.005 Zineb < 0.010 Rogar 0.11 Hinosan < 0.003 Cvthion 0.008 Folidol < 0.003 Metacid-50 <0.003 Carbamates Sevin < 0.006 Furadan 0.001 Ekalus < 0.0015

Tissue 0.1 0.1 0.1 -

Other environmental mediated diseases Gas bubble disease: disease: Gas bubble disease can occur in fish under condition in which there is an excess of gases (CO2, N2, O2 and H2S) in the water and normally happens due to high organic load at pond bottom. bottom. During decomposition of fertilizers and manures gases are released in the form of bubbles. bubbles. The fingerlings of fish try to ingest them mistaking it for planktonic food and accumulate in the intestine. intestine. Sometimes gas bubbles may enter into the blood circulation and transported to brain or heart, fish can die suddenly with no other other signs. Bubbles just under the surface of the skin may be seen.

19

The young fishes show erratic movement and gradually die exhibiting the whirling movement. The abdomen is swollen and the balance of fish is lost due to accumulation of large gas bubbles in the intestine. Haemorrhaging of the fins is common due to the occlusion of small blood vessels by gas bubbles. With stopping application of unfermented fertilizers and addition of fresh water in the ponds, the condition can be checked.

Algal toxicosis The pea soup coloured bloom of algae (Microcystis sp. and Anabaena sp.) may occur in the ponds due to excessive use of fertilizers and feeds. feeds. Overcrowding of algae causes its mass mortality because of lack of nutrients or lack of carbon dioxide. The dead and decomposing cells release enough breakdown products or toxins called algal toxins, which are harmful to fish/prawn and produce serious loses in pond culture. There is clogging of gills by the algae causing respiratory distress. distress. Small fishes die first after erratic swimming movement and convulsions occur with acute kidney inflammation in severe cases followed by death.

20

For controlling the condition following measures can be taken Copper sulphate is applied in the pond @0.5 ppm. ppm. (Suspended particles may be settled by application of lime) Algal bloom can be restricted by application of Takazine – 50 (Cymazine) Cymazine) @ 2-4 kg / acre. acre. If the pond water is covered by floating weeds, Wolfia. Wolfia. sp (microweeds (microweeds)) or Lemna minor, Lemna major, Spirodella for one week then also the algal growth is checked due to lack of penetration of sunlight. Dry Cow dung cakes @ 200 kg/ha is sprinkled over the surface of water. These process blocks the sunlight penetration in water. Some surface area of pond is covered with water hyacinth thereby blocking sunlight penetration in water.

Conclusion In aquatic environment fishes continually adjust themselves to the change of physicophysico-chemical parameters which impose a great stress on their limited homeostatic mechanism. Such stress causes totally upsetting the defensive mechanism or immune system resulting in susceptibility of various diseases. Besides abnormal environmental quality also causes direct adverse effect on fish. Proper monitoring and management of culture system can only help in healthy fish/prawn production and make the culture operation profitable.

21