When freshwater prawn farms are in tropical areas where adult prawns are available year-round, the word broodstock usually refers only to the females that are kept in hatcheries until their eggs hatch, after which they are discarded or sold. The individual value of egg-carrying females is low, especially because they are usually sent to the market after the eggs have hatched, so there is no need to economize in the number used. An indication of the number of berried females required is given in Box 4.
Different considerations apply when freshwater prawns are being grown in temperate regions, as discussed later in this section. Some hatcheries also hold a supply of adult males. Few tropical farms maintain freshwater prawn broodstock in dedicated ponds (a practice which is commonplace in many fish farms), despite the potential advantages (e.g. the ability for selection).
Freshwater prawn eggs are carried under the tail of the adult female prawn (known as ‘berried’ or ovigerous females) and are easily visible (Figure 4). In the tropics, berried females can be obtained year round from farm ponds containing adult animals but the quantity of berried females available may vary according to the time of year. They can be obtained by cast netting but are frequently selected at times of partial or total harvest. Berried females can also be obtained from rivers, canals and lakes in areas where they are indigenous (native). Some hatcheries prefer to use berried females from natural waters based on the belief that wild females produce better quality larvae than pond-reared ones. However, collecting ovigerous females from the wild often results in considerable egg loss during transport, so many hatcheries prefer to use adjacent rearing ponds for their supplies. The dangers of doing this are discussed later in this section of the manual.
In the wild, berried females are most abundant around the beginning of the rainy season. When M. rosenbergii is reared in areas where the climate is sub-tropical or temperate (usually originating from stock introduced from another area), broodstock are typically obtained from ponds during the harvest at the end of the growing season and maintained indoors in environmentally-controlled conditions during winter. When freshwater prawns are introduced into an area where they are not found in the wild, great care must be taken to follow national and international guidelines for introductions, including quarantine. A basic code of practice for introductions is given in Annex 10. The topic of quarantine is fully discussed by Bartley, Subasinghe and Coates (1996). From a hygienic point of view it is better to import PL from sources where no diseases have been reported, rather than berried females. The permission and assistance of the local Department of Fisheries should be sought on this topic.
The topic of broodstock selection and the advantages of maintaining specific broodstock facilities have been discussed by Daniels, Cavalli and Smullen (2000). Genetic selection has been reviewed in Karplus, Malecha and Sagi (2000). Until recently, very little progress had been made in the genetic improvement of Macrobrachium although this topic has long been recognized as an area of research that could be expected to yield significant improvements.
Freshwater prawns that originate from eggs that hatch early appear to have an advantage in grow-out because they are the first ones to establish themselves as dominant blue claw males (BC). However, there is no evidence that these ‘early hatchers’ have any genetic advantage over the ‘late hatchers’. Therefore it would be pointless to select larvae from only one part of the spawning period to stock larval tanks. Moreover, selecting eggs from only one part of the spawning period could lead to a reduction in genetic variation and an increase in inbreeding. Proper genetic resource management combines selection and conservation of genetic diversity (Tave 1996, 1999).
FIGURE 11
The eggs of Macrobrachium rosenbergii are carried by the (‘berried’) females until they are ready to hatch; as they ripen, they change from orange to grey/black (Hawaii)
SOURCE: TAKUJI FUJIMURA, REPRODUCED FROM NEW AND VALENTI (2000), WITH PERMISSION FROM BLACKWELL SCIENCE
Most farmers select larger females, which usually carry more eggs, but this may not be good practice. Selecting fast-growing, berried females from ponds three months after they were stocked, rather than choosing large females six months after stocking, has a positive genetic effect on weight at harvest. Collecting the faster growing females and rearing them in dedicated broodstock ponds would enable you to use selection to improve grow-out performance and also give you the ability to hold the animals until their clutch size becomes larger (after later mating moults).
Experiments have shown that cutting off one of the eyestalks (ablation) of female broodstock increases the number of mature females in a captive broodstock and diminishes the time between each spawn. Young females (about 4 months old after stocking at PL size) spawn about 20 days after eyestalk ablation and spawn again after about 30 days.
There is tendency for the performance (growth rate, survival, FCR) of farmed Macrobrachium rosenbergii during grow-out to decline after several production cycles where the berried females used in the hatcheries have been drawn from grow-out ponds. This phenomenon, caused by inbreeding and sometimes known as genetic degradation, has been noticed in a number of countries including Martinique, Taiwan Province of China, and Thailand. In countries where M. rosenbergii is indigenous the problem has occurred because of the ‘recycling’ of animals (broodstock for hatcheries being obtained from grow-out ponds and the process being repeated for many generations). In countries where this species is not indigenous the problem may be worse because the farmed stock has normally originated from a very small number of females (or PL), which were introduced to the country many years ago. When the problem of declining yields (and therefore incomes) occurs, it naturally results in the initial enthusiasm of farmers fading. The solution to the problem must be two-fold: using more wild broodstock, and genetic improvement.
Work on genetic improvement began in Thailand in 1998 and one company has recently introduced a new strain of M. rosenbergii that it claims has markedly improved performance (Anonymous, 2001b). This manual does not endorse any specific commercial product or source of PL but welcomes this potential solution to the problem of genetic degradation, in principle.
In the tropics, where berried females are readily available, special broodstock holding facilities within hatcheries are not necessary, although the advantages of maintaining special broodstock ponds have already been mentioned. However, in temperate zones where freshwater prawns are reared in the summer, indoor broodstock facilities are essential.
In temperate zones it is necessary to provide holding facilities for over-wintering. Broodstock need to be maintained for up to six months and the temperature needs to be above 25°C to prevent loss of eggs. To conserve water and maintain good water quality, a recirculation system is suggested, similar to that used in recirculation hatcheries, as described later in this manual. Nylon mesh netting should be hung vertically or horizontally in the water column (buoyed with PVC piping and floats) and placed on the bottom of the tanks. This minimizes the total tank volume needed, reduces cannibalism, and increases fecundity. The use of large mesh sizes reduces the amount of fouling.
The egg-carrying capacity of the females is reduced at higher broodstock densities. A maximum stocking rate of one adult prawn per 40 L of water is recommended. For every twenty females, you should hold one or two BC males and two or three OC males (each >35 g), if eggs are required 3-4 months after the adults are stocked. If newly hatched larvae are not required until six months after the adults are stocked into broodstock facilities, the number of OC males should be adjusted to three or four per 20 females (to allow for male mortalities).
The total quantity of broodstock to be maintained in temperate facilities obviously depends on the final demand for PL. Only about 5% of the females will spawn together and an adult mortality of 50% should be anticipated during the holding period. Assuming an average of 45 000 larvae/45 g female, obtaining a single batch of 100 000 larvae at the end of the holding season would therefore require you to over-winter about 90 females, each about 45 g in weight (plus, using the proportions and timing indicated in the previous paragraph, 5-9 BC males and 9-18 OC males). This would provide a batch of 100 000 larvae at least once a week, thus allowing your hatchery to supply enough PL to stock 1 ha of ponds (assuming a stocking rate of 5 PL/L and a 50% hatchery survival rate to the PL stage) per week. These numbers can be adjusted according to your needs. It would be foolish to base the whole cycle of operations on a single tank, however; many accidents and other unforeseen circumstances can arise. It is therefore suggested that you split whatever broodstock animals you hold into a minimum of three holding systems.
Managing broodstock in outdoor facilities in the tropics is similar to managing grow-out facilities. However, in temperate climates where broodstock are over-wintered, special care is necessary to ensure good health and maintain maximum survival. Broodstock should be disinfected upon arrival at the hatchery by placing them into freshwater containing 0.2 to 0.5 ppm of copper sulphate or 15 to 20 ppm of formalin for 30 minutes. However, it should be remembered that the use of these chemicals in aquaculture is prohibited or controlled in some countries. Aeration should be provided during these treatments. Similar precautions should be taken in handling berried females which are brought into tropical zone hatcheries from ponds or the wild. Adult prawns can then be transferred to holding tanks which contain freshwater at an optimum temperature of 27-31°C.
The water quality for indoor broodstock holding facilities should be similar to that for hatcheries. The selection and sex ratio of males to females has been discussed earlier. A nutritionally complete diet is essential to promote superior egg production and quality. Commercially pelleted grow-out feeds can be used but need supplementation. Broodstock should be fed at a daily rate of 1-3% of total biomass, adjusted to match consumption. Half of the pelleted ration should be substituted with the equivalent amount of pieces of beef liver or squid (or similar fresh feeds, such as mussel flesh), cut to the appropriate size, at least twice per week. 1 kg of a wet feed is roughly equivalent to 200 g of pelleted diet. Thus, (for example) if the normal daily ration you are providing to your broodstock is 30 g of the pelleted diet, on two days per week you would need to replace half of it with 75 g of the fresh feed. The daily food ration should be given in two equal portions, normally in the early morning and late afternoon. Two broodstock diets designed for Macrobrachium rosenbergii are described in Annex 3.
Specific separate facilities for hatching freshwater prawn eggs are rarely used in commercial hatcheries. The most common system for hatching utilized in tropical hatcheries is described in the hatchery management section of this manual. However, especially in temperate hatchery facilities, a separate hatching facility is easier to control. In this system, berried females can be collected from the holding system and placed into a tank where the eggs are allowed to hatch, and stage I larvae are obtained either with a collecting device, as mentioned below, or simply netted from the system. Figure 12 shows a hatching system that consists of a 300 L rectangular hatching tank and two 120 L circular tanks, one for collecting larvae and one to house a biofilter. Up to sixty females with brown to grey eggs can be placed into the hatching tank, which contains adequate habitat structures (e.g. a piece of pipe for each individual). The hatching tanks need to be covered to exclude light and the interior should be painted with black epoxy-resin paint, except around the area where the overflow pipe is located, which should be painted with a lighter colour, such as beige (or, if the tank is translucent, left unpainted). Black painted grating (e.g. egg crating or louvre material) is used to divide the tank into two chambers.
The largest chamber, occupying about 80% of the total tank volume, is used to hold the females and to keep them separate from the larvae as they hatch. Water overflows into the collection tank and then passes through a 180 µm mesh screen, located around a central standpipe, into a biofilter. The larvae will flow with the water leaving the hatching tank because they (being positively attracted to light) move towards the lighter area of its wall, which is illuminated. Water is returned to the hatching tank from the filter tank by airlifts (Figures 13a and 13b). Hatching usually occurs at night but as the hatching tanks are covered, larvae can be collected during the daytime. The water in this system should be preferably maintained around 28°C. If you use slightly saline water (~5 ppt) it will result in greater hatchability. Recently, some evidence has been published (Law, Wong and Abol-Munafi, 2001) indicating that the hatching process is extremely pH sensitive. If this is corroborated, the pH may need to be adjusted to 7.0-7.2 for hatching. pH outside this range appears to result in substantially reduced hatching rates. The light regime for the broodstock is not important but direct sunlight should be avoided. To enhance water quality for the hatching larvae, it is recommended that berried females should not be fed at all during the 2-3 day period prior to egg hatching. Larvae are then removed from the collection tank and transferred to the hatchery phase. Further details of this and alternative hatching systems are provided in Daniels, Cavalli and Smullen (2000).
FIGURE 13b
Airlift pumps keep the water moving and oxygenated (Peru)
SOURCE: OSCAR ORBEGOSO MONTALVA
