Fish: Diseases and Pests

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					                       Fish: Diseases and Pests
                                    Booklet No. 535
                          Fisheries and Aquaculture: FACS - 3
I.     Introduction
II.    Diseases of Fish
       A. Viral diseases
       B. Bacterial diseases
       C. Fungal .diseases
       D. Protozoan diseases
       E. Helminth diseases
       F. Crustacean diseases
       G. Nematode diseases
       H. Environmental diseases
       I. Nutritional diseases
III.   Pests of Fish
IV.    Conclusion


       Like all other organisms, fish also get sick and is affected by various kinds of diseases
and pests, especially in the intensive and commercial fish culture, resulting in huge loss to the
farmers. Some of the diseases are epidemic and contagious and therefore, special precautions
are needed to prevent them. Therefore, some basic theoretical knowledge about the common
diseases and their management practices is essential for the growers to take precautions as
well as to treat the diseases. This knowledge is especially needed for the common people and
the social workers who are promoting fish farming as an income generating programme among
the people.

Dr. K.T. Chandy, Agricultural & Environmental Education

I. Introduction

       Fishes are infected by many disease causing organisms such as virus, bacteria, fungi,
protozoa, nematodes, and internal parasites like tape worms, crustaceans etc. Besides these,
there are problems caused by harmful insects, voracious fishes, amphibians, reptiles, birds and
mammals. I have tried to enumerate the different organisms causing diseases to the fishes, just
to give an idea of the various sources and causes of diseases. Each disease is very
complicated and different from the other. Therefore, dealing with fish diseases is not as simple
as people usually think.

        The fish farmers are usually poor and illiterate, so they are unable to understand that
there are millions of micro- organisms which cause various diseases to the fishes, because they
cannot see these micro-organisms. However, there is a need to educate them in these
complicated matters. It is not only the fish farmers but also all the social workers who are
working among the poor farming people, need to be educated in the basics of fish farming and
fish diseases. Quite often they prepare a project on fish farming for the people without knowing
anything about the fish farming and finally end up in utter failure with bitter experience.
       It is not possible to describe all the diseases that occur to the fishes as there are too
many in number. Only the most common diseases, that are affecting the Indian and Chinese
carps, a few diseases affecting the eels and catfishes are de- scribed here. It must be clear to
the reader that an ordinary illiterate person normally cannot identify all the diseases of human
beings and animals however much he tries to educate himself on those diseases so also a fish
farmer will be seldom able to identify correctly each disease. This booklet never intends to make
anyone a specialist in fish diseases, but it is meant to introduce a common man into the whole
world of fish diseases:

II. Diseases of Fish

       As already mentioned, diseases of fishes are caused by various types of micro-
organisms like virus, bacteria, fungi, protozoa, etc. Hence, the best way to study them is to
study under the classification of causal organisms.

A. Viral diseases
         Virus is considered to be the lowest form of life and the link between the living and non-
living. Some of the important viral diseases of fish are briefly described here.

1. Spring viremia of carp (SVC)
        Spring viremia of carp is also known as infectious dropsy of carp (IDC). This is caused
by a virus belonging to the Rhabdovirus group with ribonucleic acid (RNA) structure and is
called Rhabdovirus caprio. It occurs in the spring when the water temperature is raising (13 to
200 C) but it occurs most commonly when the temperature of the water is 170 centi- grade. It is
noticed that the outbreak ceases when the temperature exceeds 200 centigrade. This disease
usually occur in fish which are to one year old. It spreads fast and the mortality ranges between
80 to 90 % in, 2 to 3 weeks time.

        This disease is common among the carp in Europe and fortunately, it is not yet reported
in our country. Because the virus cannot survive in temperatures above 20oC and our country
mostly falls within the tropical and subtropical climates. The affected fish show darkening of their
body, and then tend to assemble at the water inlet. They exhibit sluggish breathing and finally tilt
to one side. In the advanced stage of disease, symptoms like abdominal distension, protrusion
of the eyes, reddening and swelling of the anus, anaemia and haemorrhagic spots in the gills
are seen. Various internal organs like intestine, spleen, kidney, etc, show swelling and

        Scientists are of opinion that IDC is often mixed with bacterial disorders. However use of
antibiotics like chloramphenicol, streptomycin and oxytetracycline in the fish breeding ponds is
found to be effective. The antibiotics are better, if given through the artificial feed or through a
bath or through injection. The dosage depends on the strength of the medicine and as per
manufacturers instruction. The farmer should seek the help of specialists on fish diseases.

2. Swim bladder inflammation (SBI)
       It is caused by SBI virus belonging to the Rhabdovirus group with Deoxy ribo Nucleic
Acid (DNA) chromosome structure. This is also a common disease in the Europe. Usually,
young fishes up to two months are infected more and the mortality could reach 100 per cent. In
the grown up fishes the incidence of this sickness is rare. Unlike the SVC/ IDC this disease
occur at medium temperatures. The suitable temperature range for the replication SBI virus is
found to be between 15 –28o C and no growth takes-place above 33°C and below 5°
        In the initial stages the inflammation occurs in the wall tissue of the swim bladder,
especially at the bottom of the swim bladder walls. Gradually, bleeding spots appear and brown
or black spots appear leading to necrosis and the swim ' bladder is destroyed. By this time the
infection would have extended to the other internal organs also.

         Administration of antibiotics either by injection or orally through feed is found to be
effective to control this disease. A dose of 40 mg of chloramphenicol per kilogram weight of the
fish either as abdominal injection or orally through feed is found to be effective. In the case of
yearlings 3 -4 administrations lasting for 10 days, and at 5-8 day intervals 20 -30 mg of
methylene blue per individual is reported to be useful. For two-year old fish 2-4 administrations
of 35-40 mg and for adult fish administration of food for 3 days in which 3 gm per kg of dry feed
followed by feed without drug and the cycle being repeated three times.

3. Viral renal disease of eels
        The virus is identified as EV-E virus but to which group it belongs is not yet confirmed.
This disease is particularly affecting the eels. It causes inflammation mainly on the gills and
kidneys and hence is called "gill kidney in11ammation". Eventually the inflammation leads to
necrosis and degeneration of these organs and death of the eels.

        Fortunately, so far this disease is not yet reported in our country. Providing antibiotics
through the feed as mentioned in the previous diseases described is probably the best way to
control this disease.

4. Channel catfish viral disease
        This disease is mainly affects the catfish and hence it is called catfish virus belonging to
Herpes virus which has the DNA structure of chromosomes. It usually affects the finger- lings of
less than four months old during the period between June to September when the temperature
exceeds 230 centigrade.

       The affected fish exhibit abnormal swimming patterns, circling on their sides, exhibiting
convulsions, sinking to the bottom of the pond and finally floating on the surface of the water
and die. They exhibit abnormal distension and protrusion of the anus in the terminal stages. The
disease is also characterized by the protrusion of eyes and extreme fading of gills.

       Treatment with antibiotics as mentioned in the previous diseases is found to be effective
besides the prophylactic measures like cleanliness and proper maintenance of the pond.

5. Pox disease of carps
        The virus causing the pox disease is identified as Carp pox virus belonging to a group of
virus called DNA Herpes- virus. This is also called papilloma disease since it is characterized by
the dense formation of papilloma in various sites on the trunk, head, and fins of carp fish. The
papilloma are white or pink in colour.

         The disease does not seem to be fatal however, it affects the growth and development
of the fish. The virus is not yet isolated and hence its preventive and curative measures are not
finalized. However, administration of antibiotics already mentioned through feed is found to be

6. Lymphocystis disease
        It is fairly certain that Lymphocystis is caused by virus. Though it is commonly occurring
in the wild fish the cultivated fishes are also affected by it. However it is not reported to be really

       Small blister-like swellings appear scatteringly or in clusters on the surface of the trunk,
head, fins and other parts of the body. The disease occur in high temperatures (summer
months) and automatically disappear when the temperature becomes less.

       Antibiotics may be used at the beginning of the infection. However, proper sanitation and
other prophylactic measures are necessary to control the incidence of the disease.

       Proper feeding through the natural and artificial feeds is also a necessary element in the
prevention and control of this disease.

B. Bacterial diseases
      There are a number of bacterial diseases occurring among the cultivated fish. The
common ones occurring in India are briefly described here.

1. Tail and fin rot
        As the name indicates this disease is characterized by the white margins on the fins and
putrefaction of the fins and tails. All the Indian major carps such as catla, rohu and mrigal are
susceptible to this disease. It is caused by the bacteria called Myxobacters and Aeromonas sp.

        This disease occurs mostly among the young ones during summer months. Besides the
high temperature high organic material settled at the bottom of the pond is also a predisposing
condition for the outbreak of this disease.

       As curative measure feed the fishes with a feed mixed with either the antibiotic
terramycin or sulphadiazine at the rate of 100 mg per kg of feed. The fishes are also given
common salt bath or formalin bath in 3% salt solution. Dipping in a solution of 250 ppm
potassium permanganate for , two minutes is also found to be effective.

2. Eye disease
        This disease is more common among the catla fish. The eyes look reddish the lens
become cloudy affecting the movement and feeding. Eventually the fish become weak and die
resulting in poor production.

       The treatment for this disease is the same as those pre-scribed for the previous bacterial
disease namely the salt water bath, antibiotics and dip in a solution of potassium permanganate.

3. Dropsy disease
       Dropsy disease is characterized by the accumulation of water in the body cavity or scale
pockets. It is mainly because of the dysfunction of the kidney. Excessive accumulation of the
organic matter is considered to be one of the factors causing the occurrence of this disease. It is
caused by the bactcria called Aeromonas species. This disease is common during the summer

        The treatments are the same as those prescribed for the above bacterial diseases.
General sanitation and removal of excessively accumulated organic matter are additional help in
controlling this disease.
4. Ulcers
         The ulcers commonly occur among the Indian carps catla, rohu and rnrigal during the
summer months and it is caused by some unidentified bacteria, Ulcers are formed on the body
of the fishes. These ulcers vary in sizes from pimple like structures. to very conspicuous sores.

         Besides contamination from other affected fishes excessive accumulation of putrefied
organic matter at the bottom of the ponds and canals is a favourable Condition for the incidence
of this disease.

       Besides general sanitation and removal of excessive organic matter treatments
prescribed for the other bacterial diseases are administered for this disease also. However,
treatments should be done at the beginning of the incidence of the disease.

5. Columnaris disease or reddish blotches
       Columnaris disease is characterized by the reddish bruises, oozing of blood and
formation of clots at the posterior part of the fish. Though all the members of the carp family are
affected by this disease silver carp, rohu and catla are found to be more susceptible. The causal
organism is still to be identified and named.

       Unlike other bacterial diseases described so far this disease occur mostly in the post
winter months. Besides excessive accumulation of the decomposed organic matter higher
stocking density is also a reason for the incidence of this disease.

       The treatments are same as those prescribed for the other bacterial diseases besides
the general sanitation and prevention of excessive accumulation of the organic matter.

C. Fungal diseases
       These diseases are caused by various organisms belonging to fungi family. There are
many fungal diseases affecting the fishes in the commercial culture. Only the important and
more relevant ones are discussed here.

1. Saprolegniasis
        This is also known as "water mold disease" or "aquatic fungus disease". This is caused
by a group of fungi belonging to Saprolegnia mold. The hyphae of the mold grow extensively
giving the appearance of cotton like growth on the outer tissue of the body of the fish and can
be clearly seen by any body. Several species of this fungus are known to cause the disease
showing the main symptom of cotton like structures.

       Almost all the species of fishes are affected by this disease. High amount of decaying
organic matter settled at the bottom of the pond is an addition cause for the incidence of this
disease. A bath of 5% solution of common salt is found to be eftectivein controlling this disease.
By treating the pond with Malachite green at the rate of 0.1 gm per litre of water is another
method of controlling the disease. A bath of 0.5 gm of copper sulphate per litre of water or one
gm of potassium permaganate per litre of water is another alternative to the bath at common

2. Branchiomyces
        Branchiomyces is also called gill rot from the symptom seen in the fishes. Almost all the
species are affected by this disease. The blood vessels are blocked due to the disease, leading
to the death of the fishes. It is caused by the fungus called Branchimyces. Accumulation of
highly decayed organic matter at the bottom of the pond is also said to be the,.
reason for the incidence of this disease.

        Addition of lime at the rate of 150 kg per ha and bath to the infected fishes in 3-5 %
solution of sodium chloride for 3-4 minutes are also found to be effective in controlling this

D. protozoan diseases
        Protozoa are microscopic and unicellular organisms found every where. Some of these
are pathogenic and some are nonpathogemc.. Numerous protozoan parasites live on the bodies
of the fish causing various types of diseases. They attack the skin both on the surface and
underneath, the gills and later on spread to the other organs. Some of the common protozoan
diseases are described here.

1. Gill spot disease
        Gill spot disease usually occur during the post monsoon and winter months. The young
ones 'of catla fish are more susceptible to this disease. The exact causal organism is called
Thelohane/lus catlae.

       The affected fishes are seen with white cysts slowly spreading over to the gills and other
parts of the body. The growth is retarded. Excessive mucus secretion from gills and irregular
growth of gills epithilium are also observed.

        Higher rate of stocking, general weakness due to loss of appetite or lack of feed
availability in winter and .he presence of the spores of the causal organisms in the pond are the
factors encouraging the incidence of this disease.

         Dipping in 2 -3 % salt solution for 3 -4 minutes and pond treatment with a mixture of
malachite green at the rate of 0.1 mg per litre of water and formaldehyde at the rate of 25mg per
litre of water are the recommended control measures. Liming of pond for sanitation helps in
controlling the disease. Proper feeding of the fishes is necessary to prevent as well as cure this

2. Scale and body spot disease
        The scale and body spot disease occur during pre/post and winter months mostly to the
young ones of mrigal and rohu. It is caused by the protozoa called Myxobolus mrigalae and
rohitae. The affected fishes show white cysts embedded in the scales and body surface leading
to emaciation, degeneration of the scales and ulcer formation,

        The presence of the spores of the protozoa, the excessive deposition of the decaying
organic matter and the lack of proper feed are the aggravating conditions for the incidence of
this disease.

       The treatment for this disease is the same as those pre- scribed for the first protozoan

3. Trichodinosis
        Trichodinosis is a ciliate protozoan disease affecting all the Indian and Chinese carps
during the post monsoon and winter months. Excessive secretion of mucus from gills and body,
pale gills clubbed or worn out are some of the symptoms observed in the affected fishes.
       The treatments for this disease are the same as those prescribed for the other protozoan
diseases already described.

4. Chilomastosis
        Chilomastosis is a disease caused by a group of organisms belong to phylum protozoa
and class mastigophora. There are many types of mastigophora. They parasitize on different
parts of the body and cause diseases. They can be prevented or eradicated by giving the same
treatment explained for other types of protozoan diseases.

5. Ciliates
        Ciliates are protozoans having hair like structures on their body, called cilia. The cilia are
used for transportation, feeding, sensing, protecting etc. There are many types of ciliates which
cause various .types of protozoan diseases to the fishes.

6. Coccidiosis
        Coccidia are parasitic protozoans usually residing in the alimentary canal of the fishes
and disturb the proper functioning of the digestive system thus digestion and absorption of the
food materials. There are many species of coccidia causing various types diseases to the

7. Myxosporidiosis
         Myxosporidiosis is the name used for a number of sick-nesses caused by a number of
organisms belonging to the protozoa and its subdivisions. They cause various types of diseases
to the fishes.

8. Microsporidia diseases
        These are diseases caused by microsporidians which parasitize on the fishes. From the
name itself we can understand that they are spore producing organisms to multiply themselves.
Their infestation results in various types of diseases.

E. Helminth disease
        Helminths are worm like organisms causing diseases to the fishes. Helminths can be
round and long or flat and long. Trematoda diseases are caused by flat helminths or
platyhelminths. The important helminth diseases are dactylogyrosis- gyrodactylosis, black spot
and ligulosis.

1. Dactylogyrosis and Gyrodactylosis
       The name of the disease and the name of the organism are the same. Both the Indian
and Chinese carps are affected by these diseases. They infest the fishes during the post
monsoon and winter months. Fading of the normal colour, dropping and folding of fins,
feebleness and frequent surfacing are the symptoms observed. Excessive secretion of the
mucus and damage of the epithelium of the gills are also observed.

       Higher stocking rate and loss of appetite during winter are aggravating factors for the
incidence of this disease.

        Dipping treatment of the infected fishes for 3-4 minutes in formalin dissolved at the rate
of 200 -250 mg in a litre of water or in 2-3% salt solution can control the disease.

2. Black spot
       Black spot usually affects the Indian carps and is caused by the organism called
Diplostomum spp and Dilostomulum spp. They usually infest the catla fishes during the post
winter months. Black cysts are found allover the body of the fish. Excessive secretion of the
mucus from the gills is noticed. The infected fish show signs of irritability and tendency to
frequent surfacing.

         This disease can be controlled by giving a bath to the infected fishes in 3-5 % solution of
salt or in 200 ppm tormalin solution. Eradicatition of molluscan population tram the pond vicinity
reduces the incidence of the disease.

3. Ligulosis
        Ligulosis is a helminth disease affecting the catla fishes. The infected fishes show
abnormally enlarged abdomen and dark coloured body, Birds around the ponds is an
aggravating factor for the spread of this disease, The causal organisms spread through these
birds especially through their droppings. Therefore control of these birds itself is a means of
controlling this disease.

4. Tape worms
        The nature and functioning of tape worms is familiar to all. It is a deadly parasite in the
sense that once it is infested it cannot get rid oft' easily as it can hide its microscopic head in the
tissue of the host even if the body segments are detached.

F: Crostacean diseases
        Crustaceans are the organisms with segmented body and jointed legs. Some of these
are parasites on the fish causing diseases to the fishes. Argulosis, Lernaeosis and Ergasilosis
are the three important crustaean diseases that infest the fishes.

1. Argulosis
         Argulus sp is a crustacean organism affecting the Indian and Chinese carps during the
summer and pre-monsoon months. The organism attaches itself to the body of the fish causing
irritation and as a result we can notice rubbing behaviour of the fish, trying to get rid of the
irritating organism. Continued infection leads to emaciation and pigmentation on the body
surface. Excessive deposition of decayed organic matter at the bottom of the pond is an
additional cause for the infestation of the disease.

        Hence prevention of deposition of decayed organic matter is a prerequisite to control the
disease. Pond treatment with gamrnaxine at the rate of 0.2 mg/litre of water or bath in 3% salt
solution of infected fishes for 3-5 minutes are the measures that can control this disease.

2. Lemaeosis
        Lemaeosis disease is caused by Lernae sp which affect the Indian carps. The incidence
of this disease is found to be common in the summer months. The body of the infested fishes is
covered with pin like white bodies allover. There will be excessive secretion of the mucus. Due
to severe infection the gills may be damaged.

       As in the case of other diseases described excessive deposition of the decayed organic
material is one of the causes of infestation by this organisms.

       The treatments for this disease is same as those pre- scribed for the other crustacean
3. Ergasilosis
        Ergasilosis is a crustacean disease caused by Ergasilus sp and occurs mostly in the
summer and winter months. The infected fishes appear restless, frequently coming to the
surface and fading of the normal colour is seen. Excessive secretion of mucus and the gills
becoming pale are other symptoms observed in the infested fishes. This disease also is encour-
aged, as we have seen in other diseases, by the excessive loading of the pond bottom with
highly decayed organic material.
        This disease can be controlled by the same treatments as those prescribed for the other
crustacean diseases.

G. Nematode diseases
       Nematodes are microscopic type of worms that are found in the soil, plants and animals.
Some nematodes live on fishes as parasites. Nematodes which parasitises on fish are divided
into two: those which use fish as the final host and those which use fish as the intermediate
host. Majority of the first ones resides in the alimentary canal while some in the gills, eye
sockets, body cavity, muscles etc. So far the problems due to infestation of nematodes have not
come up to any significant level. However, the farmers should be aware of the existence of such
organisms that are harmful to the fishes.

H. Environmental diseases
       The physical and chemical characteristics and general condition of water, temperature,
amount of dissolved oxygen, amount of decayed organic matter accumulated etc. are the
environmental factors that contribute to the growth and well being of the fishes positively or

        If the amount of dissolved oxygen is less then the fishes show stress by gasping for air
at the surface of the water.

       If there is excessive accumulation of highly decaying organic matter then as we have
already seen a number of diseases occur.

       If the water is muddy or turbid the sunrays will not reach the bottom of the pond and the
growth of plankton will be affected.

       Similarly, if the water is too acidic or alkaline it will affect the fishes. For example, if the
pH of the water is below 5.5 it becomes toxic to most of the fishes. And it the pH is at or below
5.00 mortality may start. There will be excessive secretion of the mucus, the body will be
covered by a thin whitish film and the gills turn brownish in colour. Ponds with such low pH
should be treated with 500 kg calcium carbonate (lime) per hectare. As far as possible run off
water or water from the melted snow should not be allowed to enter the pond.

       If the water is too alkaline (PH above 9.0) then also it is dangerous for the fish. The pH
should be brought down to normal or slightly alkaline by the application of gypsum at the rate of
about 250 to 500 kg per ha depending on the pH value of the pond.

       Excessive shading, periodic flooding by the run off water, incoming of sea water into the
pond, excessive drought and water scarcity are some of the other environmental factors that
adversely affecting the health of the fish.
I. Nutritional diseases
         Artificial feeding is the main source of feed for the fishes cultured in ponds. Over feeding
as well as under feeding will cause diseases and mortality among the fish. In certain fishes over
feeding results in Lipoid hepatic degeneration characterized by a yellow-brown colour of the
liver. Treatment includes avoiding over feeding.

        Entritis is a feeding disease. If we press the abdomen of the affected fish a yellow-red
liquid will come out through the anus.

        The feed should be balanced to supply all the essential nutrients to the fish in their
proper quality and quantity. The feed should be distributed regularly at different locations close
to the bank of the pond and ensure that all the fishes get sufficient feed and at the same time no
excess feed is supplied at a time to avoid chances of overfeeding. For more details about
feeding of the fish consult Booklet No. 525 on "Fish Feeding".

III. Pests of Fish

        As for all the living beings so also for the fishes there are several natural enemies which
are called in general pests.

        Some of them are insects, amphibians, reptiles, bird, mammals and even fishes
themselves. Among them some are permanent enemies (of eggs, fry, and adults) while others
are occasional enemies and those competing for feed. However, they are described here under
insects, amphibians, voracious fish, reptiles, birds and mammals.

1. Insects
       The most note worthy among the handful insects are the water beetles, water bugs and
dragon flies. They attack the eggs and fry. They also compete for feed. In some insects, only
larvae are harmful while in other both the larvae and adults are handful.

a. Water beetles
         Water beetles are 'found abundant in pond with lot of aquatic plants. The great diving
water beetles is the most important among them. The adults of these beetles eat on the fry and
are very voracious in nature. The larvae suck in the eggs and small try. They can do a lot of
harm in the nursery ponds. The adults of black beetles, another harmful beetle, being
vegetarians are not dangerous to the fishes however
their larvae are voracious eaters of eggs and fry.

       As a preventive measure do not place nursing ponds under water more than 15 days
before stocking in order so that harmful larvae have no time to develope. Secondly clean up
swamps and grassy ditches in the neighbour hood of the fish farm.

b. Water bugs
        Water bugs are of several types. They are harmful to the fishes at varying degrees:
some eating up the eggs, some fry etc. But they are in real competitors for feed. Cleaning up of
all aquatic weeds regularly will control the water bugs.

c. Dragonflies
       The nymphs of dragon flies are aquatic, though the adults fly in the air, and they can eat
up eggs and small fry. Yearly drying of the pond is the best way to control the nymphs of the
dragon flies.

2. Amphibians
         The larvae and adult of the amphibians are harmful as predetors and competitors of
feed. Frogs are the most harmful among the amphibians to fish. Some of the species of the
frogs live on the fry while the tadpoles mostly eat on the eggs.

       The adults and the tadpols can be scooped out using a scooping net. They also can be
destroyed by quicklime.

3. Reptiles
       Reptiles especially the crocodiles can destroy the eggs, fry and even the adult fishes of
bigger size.

4. Birds
        Kingfisher, grey heron, duck, swans, water hens and other water bird are predators of
fish. Some of them prey on fry, some on eggs, some on the small and medium size fish and
they do the harm at various degrees. These birds could be chased away using appropriate birds
scarers or could be caught on suitable bird traps.

5. Mammals
        Otters (Lutra lutra), musk rats, brown rats, water rats and water shrew are some of the
mammals that prey on the fishes. They live in the pond area in burrows on the sides of the
pond. These burrows themselves are dangerous to the ponds. Several of them destroy or feed
particularly on eggs and fry while others feed on adult fish. Among them the others live only on
fish and they can do great harm. Use locally available methods to trap and eradicate the

6. Voracious fishes
        By nature, some fishes are more voracious which can be noticed from the way they eat:
eating fast and greedily. When such fishes are grown along with the less voracious fishes the
latter ones will be at a disadvantage of getting less feed for themselves.

7. Carnivorous fishes
        Carnivorous fishes are those which prey on other fishes eggs, fries or growing or adult
ones. In each locality there are several types of carnivorous fishes and the fish farmer should
identify them using the local peoples knowledge. Catfish is a ubiquitous carnivorous fish which
can do a lot of havoc to the cultivated fishes.

IV. Conclusion

       From the description of the diseases and pests of the fish certain things are very clear.

       Excessive deposition of decayed organic matter at the bottom of the pond leads to the
incidence of a number of diseases though organic matter is essential for the maintenance of the
flora and fauna in the pond. Annual or biennial emptying of the pond and removing the
excessively deposited organic matter and allowing the bottom of the pond to dry for at least 15
days is probably the most important preventive measures to control most of the diseases.
Secondly, sanitation in the surrounding area of the pond should be maintained. Thirdly the
fishes should be fed properly with feed balanced in all the nutrients. Both over feeding and
under feeding are harmful. Fourth: ensure that no animal burrow the sides destabilizing the side
walls of the pond. Fifth: maintain the optimum range of pH of the water in the pond. Sixth: use
only eggs and fries from uncontaminated breeding stock or from reliable nurseries. Seventh:
ensure that no wild fish enter the pond. Eighth periodic bath of the fish with 3-4% solution of salt
for 3 -5 minutes is very effective. Therefore at the time of the construction of the pond itself
facilities for such baths and treatments should be foreseen. Ninth, provide antibiotics along-with
feed as a preventive measure to most of the bacterial diseases. Tenth, regular liming of the
pond should be carried out. If these ten points are put into practice the fish farmer can make
sure of a successful crop of fish.


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