Fish Culture Research Institute
2. FISH DISEASES
3. TECHNICAL ADVICE FOR PLANNING FISH BREEDING SYSTEMS
4. PLANNING FISH PONDS
5. INTENSIVE FISH BREEDING SYSTEMS
In fish culture, a number of fish diseases may cause large losses from time to time. In this short paper the different fish diseases are not detailed. There is a short introduction on development of fish diseases and some practical information which can be useful for aquacultural engineers in planning fish breeding systems. The advice perhaps is not always very exact but it is geared to the present state of fish disease research. This is characterized by rapid progress in recent years, together with intensification of fish culture. Unknown diseases are appearing, and new views and theories are being made or taken over from other fields of science. Since fish disease research is not sufficiently advanced, we can give general advice only, but at the present level of our knowledge even this can be useful in practical planning work.
2.1 Important Types of Fish Diseases
A fish is a cold blooded animal that lives in water, so there are some characteristic differences from other warm blooded animals.
- symptoms generally are simple and similar in various diseases, e.g., signs of O2 deficiency are very frequent. Fish go to the inlet water, air breathing 'piping' and so on. Mucus production on the gills and skin is very frequent, too.
- in the case of fish no individual prevention and treatment is possible. Only in special cases are individual treatments applied. Normally the population of a pond are controlled by medicated foods/bathings.
- Intensification results in new diseases.
The modern concern for fish diseases was introduced by Snieszko, 1974, who applied the stress theory to fish and water animals. The theory was established by the fact that the effect of water environment was much higher on animals living in water than the air effect on air living animals. Snieszko put the environment-caused stress in fish disease in the foreground of sciences. A triple circle figure (Figure 1) well demonstrates this theory.
The essence of the theory is that environmental stress effects impair fish condition, and the weak fish are invaded by parasites.
The environment in a fish culture system depends on:
- water quality
- food quality/artificial foods
- breeding circumstances.
Parasites of fish are always present in water and mud. Healthy fish, generally adults, are always hosts of pathogenic agents. For this reason every fish is a potential host organism, and all waters where any marine life exists should be considered infected'. The important and well-known fish parasites are facultative pathogens.
The task for aquacultural engineers is double:
- make the best conditions for the fish in the planned systems
- keep out parasites from the water of the systems.
2.1.1 Not infectious diseases
2.1.2 Infectious diseases
The cause of diseases: lethal effect on environmental factors (Figure 2).
Figure 1. Sublethal environmental effect
Figure 2. Lethal effect on environmental factors
Important kinds of environmental diseases:
- O2 deficiency
- temperature (hot-cold) caused diseases
- food-caused diseases (deficient artificial foods).
The first is a sub-lethal environmental effect (see Figure 1).
Important agents of diseases:
- viruses (obligate pathogens)
3.1 Location of Fish Farms
3.2 Organizing Water Supply System
3.4 Nursery Ponds
3.5 Ponds for Larger and Marketable Fish
The price of investment may be frequently higher than losses caused by diseases. For this reason the economical points of view are primary in the case of planning fish breeding systems, but if it is possible, economic consideration of fish disease aspects is advisable (Molnár and Szakolczai, 1980).
It is desirable to place new fish farms and other systems at the source area of rivers, far from well-known, constantly polluted waters. At the source area the river is generally clean, and wild fish (the hosts of fish parasites) are not present. However, the area of a new farm generally depends, from an economic point of view, on whether the water supply system of a given area is readily available.
3.2.1 Insufficient water supply (figure 3)
3.2.2 Good water supply (figure 4)
The water quality deteriorates progressively in the four ponds, with the concentration of toxic materials from fish metabolism and the number of parasites always increasing. This system should never be planned. However, in the case of a given situation, we suggest the first pond for breeding small fish, and ponds 2, 3 and 4 for larger fish and adults; older fish are not so sensitive.
In this case the quality of water is similar in all the ponds. Keep wild fish out from water supply canal and stock pond, because of parasites.
Hatcheries always need the best quality water. Plan to have well or spring water for water supply. If well or spring water is not available the hazard is higher. In this case plan stock pond (without fish) and use filtered water. Filtering (e.g., sand) keeps out or reduces the number of parasites.
During the nursery period the small fish need good water quality and parasite-free water.
Figure 3. Common inlet water supply system
Figure 4. Separate inlet and outlet water supply system
Water supply is best from well or spring. If this is not possible, supply nursery ponds with the best quality water available in the given area. Keep out fish/parasites from the inlet water.
The larger and marketable fish are not so sensitive. Being a host of parasites, they can live in a worse environment. Be careful! In spite of lower sensitivity we must supply them with good quality water, if possible! If this is not done, their condition deteriorates and benefit is lower.
4.1 Water Change
4.2 Aeration of Ponds
4.3 Bottom Treatments
4.4 Removal of Mud
4.5 Bird Problem
Sometimes very quick water change is necessary in the fish ponds. It is advisable to prepare new ponds in these cases.
Quick water change is important:
- in case of O2 deficiency of water
- absolute O2 deficiency: really low O2 content of water
- relative O2 deficiency: is caused by gill damages in case of sufficient O2 content of water
- some treatments of fish need quick water change, for example malachite green treatment of white spot disease (Ichtyophthiriosis).
Advisable to plan aeration systems. O2 deficiency frequently needs this kind of treatment. O2 content of the water may be increased by sprayed water, like irrigation systems.
The bottom of the ponds should always be planned flat, without holes and puddles which hinder bottom treatment, like dry bottom, freezing and disinfection (the pond being empty, of course). Treatment is important in killing parasites in the bottom mud.
A thick mud layer on the bottom is dangerous for fish, as parasites can well survive the bottom treatments and dangerous chemical processes may produce toxic chemicals in the mud, for example H2S.
Removal of mud is important from time to time. Ponds should be suitably designed for this work.
The presence of birds is not good on a fish farm.
- birds eat fish and cause direct losses
- the presence of birds is necessary for the life cycle of some fish parasites. These parasites cannot exist without birds. The hunting of water birds (herons, egrets) is generally forbidden as they are normally protected. They can be driven off by the sound of carbide guns.
In intensive breeding systems (e.g., recirculation systems) fish are crowded or overcrowded. This is a stress factor and increases the hazard of diseases. In the case of intensive systems fish health measures are more rigorous for both small and adult fish. It is advisable to supply water from a well or spring.
In recirculation systems generally three or four pieces of equipment provide constant good water quality.
Aeration supplies suitable O2 content of the water. Sedimentation collects on the bottom of the tank (faeces, food wastes). Mechanical filters remove small waste materials, and by nitrification/denitrification the toxic ammonia from the fish metabolism is micro-biologically subtracted from the water.
A UV lamp kills pathogenic fish parasites, especially bacteria. A UV lamp or osonation system are the disinfecting units of recirculation systems. These are well-known and useful in cold water (salmonid rearing) systems. The suggested wave-length is 2 537 Å, and the capacity of the lamp is 25-30 000 MWS/W . s/cm2.
In warm water systems (22-25°C), UV light is not so effective (Farkas et al., 1983). The UV light generally kills bacteria and the inlet water is practically sterile, but the number of pathogen germs are significantly increasing in the fish basins and may cause disease in spite of UV treatment. This phenomenon originates from the warm water which is optimal for the multiplication of bacteria.
The number of germs remains very high in outlet waters and in the sedimentation tank, but is radically decreased by filters (zeolit, gravel). In warm water systems the high number of fish pathogens seems to be a self-preservation process in the fish basins; for this reason the UV treatment is not sufficient to stop fish diseases. Other measures are important together with the UV lamp (bathing, medicated foods, disinfection of the whole system).
Farkas, J., J. Oláh and K. Magyar, 1983, Effect of UV lamp on microflora of recirculation fish breeding system. Paper presented to the Eighth Scientific Conference on Fisheries, Szarvas, 1983 (in Hungarian)
Molnár, K. and J. Szakolczai, 1980, Fish diseases. Budapest, Mezögazdasági Kiadó, 254 p. (in Hungarian)
Snieszko, S.F., 1974, The effects of environmental stress on outbreaks of infectious diseases of fishes. J. Fish. Biol., 6:197-208