3 Broodstock

3.1 Obtaining and selecting egg-carrying females

OBTAINING BERRIED FEMALES

When freshwater prawn farms are in tropical areas where adult prawns are available yearround, 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 eggcarrying 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.
If your hatchery is close to the ponds containing berried females, you can transport them in buckets of water. If you need to transport them longer distances they can be held in tanks or double plastic bags, using techniques similar to those for moving PL, as described later in this manual, except that the rostrum of each animal should be blunted with scissors or inserted into a plastic tube to prevent the bags being punctured. In addition, it is recommended that you shade the animals from light during transport; UV light may harm the eggs. Tying the chelipeds with rubber bands or covering them with plastic tubing also reduces the danger of the plastic bags being punctured. Some people wrap the animals in cloth or plastic or nylon screens or enclose them inside perforated PVC pipes, which are then placed into double polyethylene bags. This is not recommended, because immobilisation results in increased mortality rates during transport. The use of small bags containing only one animal and transported in darkness reduces egg losses. You need to take great care in catching, handling and transporting berried females to minimize egg loss and damage.

BOX 4

Numbers of berried females required

N TROPICAL conditions, assuming that each berried female available is capable of producing enough eggs to provide 20 000 viable stage I larvae, you would need about 50 berried females for each larval cycle of a hatchery using a total larval tank volume of 50 m3 (e.g. ten 5 m3 tanks) producing a total of 500 000 PL per cycle (this also assumes a larval survival rate of 50% to metamorphosis).

Berried females should be carefully selected. Choose animals that are obviously healthy and active, well pigmented, with no missing appendages or other damage, and carrying large egg masses. The ripeness of the eggs is also important. As the eggs ripen, their colour changes from bright orange to brown and finally to greyish-brown a few days before hatching (Figure 11). Those carrying brown to grey eggs are the best ones to bring into the hatchery, as their eggs will hatch within 2 or 3 days. It is best to ensure that you do this so that the whole larval batch is of a similar age. This will increase the efficiency of your feeding operations and reduce cannibalism. The number of females required depends on the volume of the hatchery tank(s) to be stocked with larvae, and on the number of eggs carried by each female.

GENETIC IMPROVEMENT

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.

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

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).
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.