. Introduction

Fishes are cold blooded vertebrate and have aquatic adaptation by means of gills, paired fins and lateral line system etc. Rapid growth of human population and shortage of protein rich food, resulted in increasing use of aquatic animals like fish as food, which is easily digested, tasty and nutrition rich. To meet this increasing demand farmers are attracted towards the field of aquaculture which can provide crucial economic support also. However, like human and other animals, fishes are also affected by several diseases. Various stress factors like inadequate physiochemical and microbial content of culture water and poor nutrition status can lead infection by parasite, opportunistic bacterial and fungal pathogen and cause threat of disease in fishes [1-4]. Beside this acute level of polluted water can directly bring abnormalities in fish seeds and adults.  Because of these; in fish morbidity and mortality rate increases. It is important to keep them healthy at early stage by better caring so as to increase production and quality.

2. Types of diseases in aquaculture:

Occurrence of diseases in aquaculture is a result of interaction between host, pathogen and environment. There are three types of diseases, which include bacterial, viral, fungal and parasitic. Parasitic infection and diseases are some of the factors that delay the productivity in aquaculture and affects to both, food fishes and ornamental fishes [5]. These parasites cause mortality in cultured fishes resulting in economic loss to the farmers [6]. Seasonal changes like, oxygen depletion, high temperature, change in pH, water depth and stocking density affect the incidence of parasites in fishes [2]. Inadequate water aeration would distress feed-remnants and fecal deposits at the bottom of pond and become a suitable medium for parasite growth [7]. Fish parasites can inflict a different variety of damages such as irritation, wound, injury or atrophy of tissues and occlusion of the alimentary canal and blood vessels on many fishes [8].

Figure 1 : Various types of fish diseases

2.1 Sign of Sickness in Fishes:

Sickness in fishes may lead to various ailments and diseases which can be identified by some common external symptoms listed below [9]:

(1) Fish becomes inactive.

(2) There is a loss of balance and the fish is incapable of maintaining its position in the body of water.

(3) Fish tends to lie on its side either resting at the bottom or floating at the surface.

 (4) Tails and fins do not seem to function normally with vigor.

(5) Continuing discolorations.

 (6) Gills appear pale.

(7) Sick fish is nervous and less responsive in its reaction to external disturbances.

(8) Slimy, grey excretion on skin.

(9) Fish scratches to bleeding.

(10) Directionless movement.

Sick fishes eventually become more prone to secondary infections due to their lowered body resistance. It is, therefore, becomes more relevant to identify and categories their sickness whether it is nutritional, environmental or parasitic in origin.

Table 1. Bacterial diseases:

Bacterial diseases:

Bacterial fish diseases are widespread and pose a significant challenge in terms of health management. These bacteria are generally saprophytic in nature and turning pathogenic only under conditions where the fish's physiological equilibrium is disrupted, nutritional deficiencies occur, or when various stressors like poor

water quality and overstocking create opportunities for opportunistic bacterial infections [10-14].

Bacterial infections have commonly been detected in fish eggs and fingerlings, leading to significant mortality rates. Several significant bacterial diseases, such as Motile aeromonad septicaemia, Edwardsiellosis, Pseudomonas septicaemia, Flexibacteriosis, Vibriosis, Bacterial gill disease, Mycobacteriosis, and Enteric septicaemia, have been frequently reported in carp culture in India [9,11,15]. The detail of common bacterial diseases in table 1.

Table 2: Fungal disease

Fungal diseases:

Most fungal infections recorded in fish culture are caused by species belonging to the class oomycete fungi, Saprolegnia, Achlya and Aphanomyces. Diseases caused by these fungi are collectively called “saprolegniasis” [11,19]. The fungi attack and grow on the outer surface of the body such as, skin, gills, fins, mouth and eyes. The fungal mycelium first grows between epidermal cells and deeper, toward the bones. During infection, fugal hyphae invade surrounding tissues and may infect eye ball and brain. The detail of common fungal diseases and their treatment reported in India are present in table 2.

3. Prophylactic measures:  

It is correctly said that" an ounce of prevention is worth a pound of cure." A proper management of the fish farm may lead to the prevention of a large number of fish diseases. Seriously implementing the following prophylactic measures can prevent most fish diseases [9,11,15].

1. Disease should be detected early and proper treatment initiated instantly because it prevents contaminate the entire pond full of fishes.

2. When the pond was infected before harvesting, the disinfection should be done with KMnO4 solution.

3. The bottom of the pond should be thoroughly dig and drying is done till the cracking of soil, and it should be treated by lime.

4. While refilling the ponds the quality of water should be thoroughly checked. The pond should be regularly checked for leaks to prevent entry of any unfiltered water into the fish pond.

4. Disease management practices:

After the pond is prepared and feeding regimes are established, the fishes are stocked into the ponds, keeping in view the following points [9,11,15].

1. A good quality of water with proper oxygen and food conditions are required to be maintained throughout.

2. Before putting the fishes into the pond, they should be examined carefully and only healthy fishes should be allowed to enter.

3. Handling of fishes should be as brief and as gentle as possible to reduce stress.

4. New fishes should be kept in acclimatized for over 15 days, before being added to pond.

5. Transportation should be conducted at low fish densities to prevent overcrowding.

6. A regular watch for trouble and disease for the healthy growth of fishes.

7. The predators should be carefully removed and the behaviour of fish should be constantly observed.

8. Any change in normal behaviour may be a sign of disease e.g. rubbing the body, gasping for breath, refusal to eat etc.

The principal physio-chemical and biological conditions (Table 3) also determine the healthy growth of fishes. Some fishes are more sensitive and more tolerant to any variation. It is therefore essential to design the species composition and restore the vital environmental conditions during the period of their growth.

Table 3: Ideal Water Quality Parameters for Fish Farming. 

5. Conclusion

Disease prevention and mitigation in aquaculture is crucial for increasing productivity, diminishing economic losses by maintaining the health of aquatic organisms. Several strategies are employed for prevention and control of infectious diseases in aquaculture such as biosecurity, stress management, good management practices (GMP) and the efficient usage of vaccine, antibiotics, chemical therapeutants. The present review emphasis on common fresh and marine water bacterial and fungal fish diseases, symptoms, features and their treatment. Hence, there is urgent need to promote higher and sustainable fish production in aquaculture industry.