Fish Diseases and Management

1.1.3 Role of water quality in fish diseases

Maintenance of good water quality by making inlet water free from toxic substances and pathogens is one of the integral parts of the health management in aquaculture. It has been advised that the shrimp farmers maintain a reservoir pond for pre-treating the inlet water. Water quality deterioration if noticed in a farm has to be attended immediately. Increasing aeration, reduction of feeding rates, control of phytoplankton bloom and water flow management are the important steps that are to be undertaken without any delay for abating low dissolved oxygen content. Water exchange during periods of poor water quality also helps to regain the health status of the cultured fish.

Uneaten food, faecal waste, dead plant matter will increase the organic load. Addition of calcium hydroxide or calcium carbonate enhances respiration of acidic soils. For optimum oxidation and respiration of organic matter, soil moisture has to be between 12% to 20% and pH at 7.5 to 8.0. Low or fluctuating pH, low alkaline or low hardness condition can be corrected by adding agricultural lime in recirculation or flow through systems by dipping lime into the water or passing water through a bed of oyster shells. Introduction of biofilter has been found to be very much essential in closed circulatory system to keep up the water quality. It is also advisable to sterilise the incoming water to hatcheries for fear of entry of potent viral pathogens, which could often be accomplished by the ozone treatment.

Water quality criteria for optimum fish health management of warm water and cold water species of fish (mg/l, except pH)

Characteristic	Cold water	Warm water
O2	5– Saturation	5–Saturation
pH	6.5 – 8	6.5 – 9
Ammonia (un-ionised)	0 – 0.125	0 – 0.02
Calcium	4 – 160	10 – 160
CO2	0 –10	0 –15
H2S	0 – 0.002	0 – 0.002
Iron (total)	0 – 0.15	0 – 0.5
Manganese	0 – 0.01	0 – 0.01
Nitrate	0 – 3.0	0 – 3.0
Phosphorus	0.01 – 3.0	0.01 – 3.0
Zinc	0 – 0.05	0 – 0.05
Total Hardness (CaCO3 )	10 – 400	10 – 200
Total alkalinity (CaCO3)	10 – 400	10 – 400
Nitrogen (gas saturation)	<100%	<100%
Total solids	0 –80	1. -500

A disease is the sum of the abnormal phenomena affecting the body displayed by a group of living organisms in association with a specified common characteristic or set of characteristics by which they differ from the norm of their species in such a way as to place them at a biological disadvantage. The reason for the occurrence of disease outbreaks is varied, representing complex interactions between the host and the disease-causing situation. Some bacteria are primary pathogens while others are opportunistic, (saprophytic) water borne bacteria that would colonize the exposed tissue. The opportunistic pathogen can cause a disease when the disease resistance of the fish reduces (in an adverse physiological state) as in an environmental stress and an increase in number and/or virulence of the pathogens. A disease may even result from the synergistic interaction of two or more organisms. Within fish farms, outbreaks of disease may begin suddenly, progress rapidly usually with high mortalities and disappear with equal speed (acute diseases) or develop quite slowly with less severity but persist for greater periods (chronic outbreaks).

According to Kinne (1980) four major groups of diseases may be identified in terms of epizootiology. These include ‘sporadic disease’ which occur sporadically in comparatively few members of a population; ‘epizootics’ which are large scale out breaks of communicable animal disease occurring temporarily within limited geographical areas; 'panzootics', which occur in large areas; and enzootics which persist or re-occur as low level out breaks in certain areas.

The importance of diseases in aquaculture is dependent on the type of culture system employed. There are three basic types of culture systems in use in modern day fish or shrimp farming: 1) Intensive culture system, in which animals are raised in high density, intensively managed tanks and ponds 2) semi intensive systems in which fish or shrimps are raised in moderate densities in ponds , cages or tanks with some management of the system practised and 3) extensive systems where they are grown in low intensity ponds or pens in natural bodies of water and where little or no management is practised. Hatcheries are a part of the intensive or semi intensive culture systems. As the stocking density in a culture system increases the possibility of disease occurs. Many bacteria causing diseases in aquaculture are a part of their normal microflora. They establish lethal influence as a result of other primary conditions such as nutritional disorders, extreme environmental stress and wounds. In the ponds when the waste materials like uneaten feed, and metabolites of animal increases, bacteria multiply in large numbers. When the load of bacteria in the medium is high and the animals are stressed due to any of the conditions, opportunistic bacteria can cause diseases.