HYGIENE, HEALTH AND MANAGEMENT PROBLEMS

Good hygiene

Good hygiene is essential for hatchery success. Ideally, you should not use the same equipment for more than one tank. Thus each tank would have its own dedicated nets, siphon tubes, spare filters, etc. This is time and money consuming and rarely practised. However, some much more important guidelines must be followed.

Water should never be transferred from one larval tank to another. Submersible pumps, which are often used for water transfer in hatcheries, should never be placed in the larval tanks because they are a potential source of disease transfer. Always drain your larval tanks by gravity or siphon and only use submersible pumps in water storage or mixing tanks. Disinfect all mobile equipment (buckets, siphoning tubes, nets, beakers, pipettes, etc.) on a daily basis. Dip them in a 500 ppm active chlorine solution9, rinsing them very thoroughly with water and storing them dry. Between larval rearing cycles, routinely disinfect the larval tanks. Failure to do this usually results in massive blooms of organisms, such as Zoothamnium, Epistylis, hydroids, etc., which are harmful to the larvae.
Disinfection does not eradicate these organisms but does effectively control their growth.
Scrape your tanks between larval cycles, fill them with a 500 ppm active chlorine solution for one day, rinse them very thoroughly with water, dry them in sunlight for one day, and rinse them thoroughly again before use.

Good hygiene and management standards are even more essential in recirculation than in flow-through hatcheries. Mistakes lead to larval mortalities. Before use, immerse all new tanks, filter containers and filter media, and every other piece of hatchery equipment in running freshwater for one or two weeks to eliminate potentially toxic substances.
Then clean and flush the system with filtered freshwater prior to filling it with brackishwater. After filling the system with brackishwater, disinfect it by adding 5 ppm of available chlorine (see Box 6) a few days prior to stocking. Run the whole system normally, including the filter, providing aeration; this should remove all the residual chlorine in a few days. It is possible to do this more quickly by using sodium thiosulphate but this is not recommended, because of the problems noted earlier in this manual. Ozonisation or UV light can be employed to avoid the use of either chlorine or thiosulphate but, of course, these add to the capital costs and have not yet been noted in commercial freshwater prawn hatcheries. Some additional recommendations for hygiene in recirculation systems are given in Box 12.

General management problems

Most hatchery problems are caused by poor management. The commonest cause of larval loss is not mortalities due to poor water quality or disease, but physical losses due to simple operator error during tank cleaning and siphoning, water exchange, etc. Poor internal (i.e. within the hatchery) water quality is usually caused by poor management, for example: inadequate water exchange, poor daily observation of the larvae, overfeeding, total failure of the aeration equipment due to poor maintenance of the blower or emergency

9 Both dry and liquid sources of chlorine vary considerably in their chlorine content from product to product and from batch to batch. It is best to determine the level of chlorine in each batch before use, to ensure that you are actually using the quantity of chlorine that you think you are adding. The methods for determining the level of chlorine in commercial bleaching powder or liquid bleach are contained in Annex 4, Table 6. The method for making a 200 ppm active chlorine solution is given in Annex 4, Table 3; to make a 500 ppm active chlorine disinfecting solution, use 2.5 times the quantity of liquid bleach or bleaching powder calculated from that table.

BOX 12

Additional recommendations for recirculation system hygiene

PREVENT the introduction of disease organisms or chemical contaminants by workers and visitors entering the hatchery by using chemical footbaths, and washing hands prior to and when returning to work.

KEEP all equipment, supplies, and food clean at all times. Special care should be taken before a new larval cycle is begun.

AVOID disease transfer by using equipment for only one recirculation system. Keep another set for each system.

power supplies, or individual tank aerators being blocked or left turned off. Losses also occur because late-stage larvae (close to metamorphosis) jump quite a lot and some get stranded above the water line. Some hatcheries cut out a strip of mosquito screen and stick it to the inside of the tank with epoxy-resin in the region of the water surface level. Since the water level varies it is suggested that the strip of mosquito netting should be about 12 cm wide. This helps to prevent larvae becoming stranded.
It cannot be over-stressed that while this manual attempts to lay down guidelines for a particular method of freshwater prawn culture, successful hatchery operation is a blend between factory discipline and husbandry. The most important thing to remember is that if you do not pay close attention to your animals, your hatchery will fail. You and your staff must always closely observe the behaviour of your larvae and the condition of your tanks.

Problems of disease and predation

Several diseases affect freshwater prawn larvae. These are summarized within Table 10.
Some preventative measures are listed in Table 11. More information is provided in Johnson and Bueno (2000). If you see problems which may be caused by disease you should seek the advice of local aquatic animal health specialists (where available) and microbiologists for identification and treatment purposes. Some brief notes are provided in Box 13.
Some hatcheries use formalin (200-250 ppm daily dip for 30 min) as an effective remedy for protozoal and hydrozoan infections and fungal diseases. Where the treatment period is short it is best to apply it when the tank water level is very low (10-15 cm) so that it can be rapidly flushed with ‘new’ 12 ppt water after treatment. The flushing process should continue for one hour. Aeration must continue during treatment as normal. Formalin can also be used at a lower level of 25-30 ppm for a longer period, followed by a water change after 24 hours.
Mortalities from hydroid infestation can also be reduced by transferring healthy larvae to newly disinfected tanks every 5-10 days. Malachite green (0.2 ppm daily dip for 30 min) has also been used for treatment. However, you are recommended not to use this substance because it may be toxic to hatchery workers; its use is banned in some countries. Similarly, the use of copper sulphate (previously suggested as a 0.4 ppm dip for 6 hours) is not now recommended.
Antibiotics are sometimes used to control filamentous bacteria (Leucothrix spp.).
Some hatcheries use lime (CaO) as a prophylactic between larval cycles. Others apply antibiotics and/or sulpha drugs as prophylactics but this practice is definitely not recommended in this manual, partly because their use in aquaculture may become banned in future and partly because of the danger of inducing the development of resistant disease strains. The use of these and other treatments for disease problems is described in Johnson and Bueno (2000).

TABLE 10
The major diseases known to affect freshwater prawns, and their exterior symptoms

NOTE: THIS TABLE DESCRIBES THE EXTERIOR SYMPTOMS AND THOSE THAT CAN BE SEEN BY SIMPLE MICROSCOPIC EXAMINATION. DISEASES DETECTABLE BY OTHER TYPES OF INVESTIGATIONS ARE MENTIONED IN THE ORIGINAL REVIEW (JOHNSON AND BUENO, 2000) TABLE10

As noted earlier in this section, severe disease problems very frequently begin because of basic management failure. Such failures will be minimized if you follow the recommendations in this manual. However, diseases will still sometimes occur. You are recommended to totally discard any seriously infected batch, to drain the tanks, and to disinfect all the hatchery equipment. Do not waste time on treating a serious infection. Cut your losses, try to find out the basic cause of the problem, eliminate it, and start a new larval batch.

TABLE 11
Prevention and treatment* of freshwater prawn diseases

ALTHOUGH THESE TREATMENTS HAVE BEEN REPORTED IN THE SCIENTIFIC LITERATURE, THEY ARE NOT RECOMMENDED IN THIS MANUAL. IF SEVERE INFECTIONS OCCUR, POPULATIONS SHOULD BE DESTROYED AND SYSTEMS DISINFECTED AND DRIED OUT BEFORE RE-USE.

SOURCE: JOHNSON AND BUENO (2000)

BOX 13

Notes on potential disease problems

MANY DISEASE problems are often secondary to, or aided by, a primary failure in tank hygiene, insufficient water exchange, inadequate feed quality, an inappropriate feeding regime, and low dissolved oxygen levels. All these result in poor larval condition.
Protozoa are a common cause of larval ‘disease’. The most common belong to the genera Epistylis, Zoothamnium and Vorticella. These protozoa move about and attach themselves to the body surface and the gills of the larvae. They are normally cast during the moulting process but can seriously affect larval movement, feeding, and gill operation. They are also often evident on tank surfaces. Ciliates feed on bacteria and the link with poor tank maintenance is obvious.
The medusan stage of small hydrozoans has been reported to actively prey both on brine shrimp nauplii and freshwater prawn larvae.
Problems with hydrozoans are particularly acute when surface water sources are used. The importance of using ground water and, if this is unavailable, proper water treatment is therefore emphasized.
Bacterial infections take several forms. The first are chitinolytic bacteria, which erode the surface of the exoskeleton, often following physical damage, and appear as black or brown spots or lesions (sometimes referred to as shell disease) or can cause loss of appendages. These moderate to mild occurrences are infrequent and rarely fatal and often disappear when larvae moult. More advanced bacterial involvement of the exoskeleton and underlying tissues may cause high larval mortality from bacterial necrosis. Bacteria, especially of the filamentous type, may also settle on the surfaces of the gills and interfere with respiration. Internal infections arising after pathogenic bacteria enter via the exterior surfaces or digestive tube may also cause serious disease. Damage is done throughout the body or principally in organs such as the digestive gland.
The muscles of larvae subjected to stress or low oxygen levels sometimes become opaque or whitened and often recover if the exterior problem is cured. This syndrome has been associated with excess sunlight but may indicate advanced disease by viral or other pathogens.
Several viruses have been reported to affect freshwater prawns but not all have yet been associated with morbidity or mortality. Most virus problems have occurred during grow-out. However, in the late 1990s heavy mortalities in some Caribbean Macrobrachium hatcheries were attributed to a nodavirus. The visible symptom was a whitish tail.
Fungal infections of larvae may be eliminated by better food hygiene and a reduction in larval density but have caused serious problems in Macrobrachium hatcheries, notably in Taiwan Province of China. The fungal infection experienced in the latter hatcheries could be easily identified by the presence of a mycelial network, which could be observed through the exoskeleton (shell) of sick or dead larvae.

An important disease in M. rosenbergii hatcheries, whose exact cause is unknown, is referred to as the ‘mid-cycle disease’ (MCD). As its names indicates, it is most noticeable in the middle of the larval rearing period (days 15-22 when the larvae are at stage VI-VII), when heavy daily mortalities may occur. Mortalities may even start to become obvious as early as day 10. The disease is recognisable by the larvae becoming bluish-grey and swimming slowly in a spiralling pattern, as well as by a reduced rate of consumption of Artemia and poor growth rate. You can reduce the incidence of this disease by cleaning, disinfecting and drying out hatchery equipment between cycles and taking special care in general hygiene throughout the larval cycle. If there is a severe outbreak of this disease, a full clean-up of the hatchery should be considered. This would mean killing all the larvae and thoroughly disinfecting the whole hatchery installation, so it is not a decision to be taken lightly.
Another disease which has mainly been noted to affect larvae is known sometimes as the ‘exuvia entrapment disease’ (EED) or as the ‘moult-death syndrome’ (MDS) or as the ‘metamorphosis moult mortality syndrome’. The characteristic of this disease is that the larvae get trapped in the old exoskeleton (exuvia) during moulting. It is mostly noticed towards the end of the larval rearing cycle, especially at the moult which occurs when stage XI metamorphoses into the PL stage. The mortality rate at this point can be very high. The cause of EED is not known; it may have multiple causes. It may imply that the diet is nutritionally inadequate and requires enrichment. Difficulties in shedding the old exoskeleton during moulting have also been observed in juvenile and adult prawns. The moulting process is stressful and may be difficult for weakened animals, and it is at this time that hidden problems become noticeable.