6.4 Health, predation and disease

WATCHING FOR SIGNS OF PROBLEMS

Continuous exchange of a small proportion of the water is the normal way of maintaining good water quality. However, some farmers change water more suddenly every two weeks, and in much larger proportions, because this tends to make the prawns moult.

The more that moult (and are therefore soft-shelled) at the same time, the less potential losses there may be due to cannibalism. A scum of phytoplankton may cover the surface of the pond. This will cause low DO2 problems at night and should be controlled by a reduction in feeding and by exchanging water.
Low DO2 should be suspected if prawns begin to crawl out of the ponds or congregate at the edges of the pond in daylight. If this problem occurs,

Figure 78
Measuring transparency can be very simple, even when the design of the Secchi disk is unconventional (Peru)

SOURCE: OSCAR ORBEGOSO MONTALVA

 

flush the pond. The need to do this in emergency situations illustrates the importance of having sufficient water available. Very high pH levels in freshwater prawn ponds can cause prawn mortalities, both because of the direct effect of the pH itself and because of the greater solubility of waste ammonia at high pH. High pH is often caused by dense phytoplankton blooms.
If you see sudden heavy mortalities, or observe small numbers of mortalities over a period of time, you should carefully investigate the cause. Prawns covered with algae or showing signs of not having moulted recently may indicate either that culture conditions are poor or that the animals are not healthy. Poor farm management, resulting in poor water quality and/or disease may be to blame.
However, external factors may also be responsible. The most likely source of external water pollution is from pesticides and herbicides. For example, pesticides used on neighbouring banana farms and herbicides used for the elimination of water hyacinths in irrigation canals have been blamed for prawn mortalities in the Caribbean and Thailand respectively.
Thus the importance of site selection and water source is obvious. Further reading on this topic is contained in Boyd and Zimmermann (2000), Correia, Suwannatous and New (2000) and Daniels, Cavalli and Smullen (2000).

DEALING WITH PROBLEMS OF PREDATION

Predation is one of the greatest problems for any aquaculture enterprise, including freshwater prawn farming. Predation is caused mainly by other aquatic species, birds, snakes and humans. Two of the greatest sources of loss in freshwater prawn farming are human predation and operator error.
Freshwater prawn farms are more prone to human predation than many fish farms because of the high value of the product and because prawns are relatively easy to catch.
The temptation to catch a few kilograms of prawns by cast-net at night (a kilogram of which may be as valuable as a tenth of a month’s individual income to some) is sometimes too great to resist. You cannot eliminate any form of predation, including human poaching.
However, you must minimize it by good management. Perimeter fences, dogs, lighting, and reliable watchmen help. If your farm is big enough to financially support it, you may be able to achieve some protection from human predation if you stock some PL into local public waters, thus generating a positive attitude towards your farm. If you own a small farm you may find it useful to form a cooperative with other farmers within the community. The activities of such groups are normally protected by the local community. You may also lose prawns through operator error and poor management. For example, water levels may be allowed to become too low and therefore temperatures too high, or DO2 levels may be allowed to fall too low. Both errors will cause animals to die. Not maintaining outlet structures properly allows prawns to escape.
Normally, insects (mainly dragonfly nymphs), carnivorous fish and birds are the most serious predators in freshwater prawn farming. In the past, chemicals have been used to kill dragonflies and other insects but this is not recommended because it may negatively affect the pond ecosystem. Mosquito fish (Gambusia affinis) and related species were also once stocked in freshwater prawn ponds to control insects. M. rosenbergii postlarvae themselves, if they are stocked before the insects hatch, can control the dragonfly population. You can effectively control unwanted fish by using rotenone or teaseed cake between cycles, as discussed earlier in this manual. You can prevent the entry of fish and some insects by passing the intake water through suitable screens or gravel filters (Figure 79). Most commercial prawn farms rely on simple net filters. If fish eggs and larvae do get into your ponds (which they will!), it is not a complete disaster because, by the time they get to a dangerous size, many will be seined out during the cull-harvesting of prawns. The ideal would be to exclude all predators but this is not possible. The most important thing is to stock the prawns very soon after each pond is filled, so that predators and competitors have less chance to become established. The presence of many frogs and toads in a pond usually indicates that predatory fish have been fairly efficiently excluded.
It is suggested that you use small 60 cm high netting fences around ponds for the prevention of invading catfish and snakehead fish (Figure 80). These fences may also prevent the entrance of amphibians, reptiles and some mammals. You will also find that the seining that you do during the cull-harvesting of prawns can remove large predators, such as fish, turtles and snakes. Birds are very difficult to repel or control. Netting or string can be stretched across the top of ponds as a deterrent. You can use various bird-scaring devices. In general, you should not shoot invading birds because you may be breaking local bird conservation regulations [a list of endangered birds is available in the IUCN Red List (IUCN 1996)].
The use of dogs as bird scarers may be more efficient and cheaper than shooting them.
Competent management of prawn competitors and predators includes stocking prawns as soon as the pond is filled, seining periodically, and totally draining and treating the ponds at least once per year.

COPING WITH DISEASES AND OTHER PROBLEMS

Diseases in freshwater prawn ponds are relatively unusual, compared to other forms of aquaculture. However, this may be a function of the relatively low stocking densities used

Figure 79
A simple gravel filter on a farm supply system helps to exclude predators (Brazil)
Figure 80
Netting can be used to protect freshwater prawns from predators that arrive overland (Brazil)

SOURCE: WAGNER VALENTI SOURCE: JULIO VICENTE LOMBARDI

so far. If stocking rates are increased, more problems may occur in future. Furthermore, diseases have been known to occur in freshwater prawn grow-out when the quality of the water (either of the intake or within the pond itself) is poor. Proper attention to the possibility of disease and other problems is therefore essential. The potential problems are discussed in this section of the manual.
The general issues of health, defence mechanisms against disease, and diagnosis have been dealt with by Johnson and Bueno (2000); their review also contains general information on sanitation, quarantine and therapeutic treatment. There are number of other problems in freshwater prawn grow-out, which include the result of nutritional deficiencies, fouling or parasites.

Diseases of known cause

A summary of the infectious diseases currently known to affect freshwater prawns during the grow-out phase is given in Table 10, while Table 11 lists some actions that can be taken to reduce the incidence of these problems (prevention). Some treatments that have been used, often experimentally, after the appearance of diseases are also listed in Table 11.
However, treatment is not normally practical. The continuous use of antibiotics and other chemicals is also not recommended, either in hatcheries or grow-out systems. It is not thought practical to treat prawns in commercial grow-out facilities at this moment (practical and environmentally acceptable treatments may evolve in the future).
Prevention (through good management) is always better than attempted cure. There are potential human health hazards and food safety issues concerning the use of antibiotics.
Some, such as chloramphenicol, are banned substances in aquaculture. If you do use antibiotics, you must consult your local aquatic animal health specialist and only use approved substances in the correct dosages. You must also follow the specialist’s advice on how long before harvest to stop using the product, to make sure that there are no residues in your harvested prawns.
Disease problems may originate during the transfer of animals from one site to another, including the introduction of animals into a location where they are not indigenous.
Comments on the care which should be taken with introductions have already been mentioned in this manual.

Diseases of non-specific cause

All life stages of freshwater prawns are also subject to a disease known as muscle necrosis.
Affected prawns show a whitish colour in the striated muscle of the tail and appendages. The necrotic areas may increase in size and become reddish, a colour similar to cooked prawns, due to the decomposition of the muscle tissue. Secondary pathogens (bacteria and the fungus Fusarium) have been found to be associated with muscle necrosis (see Table 10). Prawns suffering from chronic muscle necrosis do not survive. Population mortality rates vary from insignificant to 100%. This disease is associated with poor management and occurs particularly when stocking rates and handling stress are high and when environmental conditions are poor (low dissolved oxygen level; temperature fluctuations; and, in the hatchery, salinity fluctuations). Follow the good management practices suggested in this manual and you will minimize the occurrence of such problems.
Other diseases of uncertain origin affect freshwater prawn larvae; these have been described earlier in this manual.

Parasites

Parasites seem to be quite rare in cultured M. rosenbergii. Freshwater prawns have been found to be hosts for the bopyrid isopod Probopyrus. These attach themselves to the interior of the gill chamber, usually resulting in a visible swelling. This would normally only be a problem if it became common in a captive broodstock, because it is reported to interfere with egg production. The only other problem that might occur if this parasite became common in grow-out would be its affect on the appearance of prawns sold head-on; so far this has not been described.
Wild-caught freshwater prawns of various species have been observed to be intermediate hosts for trematode worms. Prawns have also hosted the Asian lung fluke but are thought to have an unimportant role in its transmission to mammals.

Fouling

The general body surface of the prawn can serve as a substrate for filamentous bacteria and algae, and single or colonial protozoa. More information about the specific organisms that cause fouling (which include Zoothamnium, Epistylis, Vorticella, Leucothrix and many others) and references to further reading on this topic are given in Johnson and Bueno (2000). Moulting temporarily frees prawns from these fouling micro-organisms. The problem is particularly noticeable in large animals, especially blue-claw (BC) males, which moult less often.
Although these organisms do not invade the tissues they make it difficult for the prawns to move and to feed, particularly in the larval and postlarval phases. Extreme infestation on the gills can impair their function, and may cause mortalities in juvenile or adult prawns. Heavy infestation over the exterior surface can also reduce the market value of prawns. Infestation by filamentous algae has been observed to occur in grow-out ponds with high transparency (above 40 cm). This problem can be therefore be reduced by encouraging lower water transparency through feed management.
You can cut down the incidence of problems in hatcheries caused by fouling organisms by good management, especially the correct treatment of incoming water, the proper cleaning of tank bottoms, and the treatment of Artemia cysts. In both hatcheries and ponds the avoidance of over-feeding and increased water exchange help to minimize the fouling of animals. A number of chemical treatments against fouling organisms have been suggested (Johnson and Bueno 2000) but are not recommended in this manual.