ANNEX 8
Size management

AS MENTIONED IN SEVERAL SECTIONS of this manual, freshwater prawns do not grow at an even rate (Annex 8, Figure 1). This makes the management of size an essential component of the efficient husbandry that is needed to ensure their successful farming. A considerable volume of knowledge about the different (male) morphotypes which play a part in the phenomenon of uneven growth has been gained since the previous FAO manual on freshwater prawn farming was written. The topic has been reviewed in depth by Karplus, Malecha and Sagi (2000), from which the material in this annex has been derived. The main purpose of Sections 1-3 in this annex is to provide an introduction to the scientific background; it is necessary to study this to understand the phenomenon of size distribution and management. Practical advice on management contained in the main text of the manual has taken these factors into account. Section 4 of this annex provides a check-list of the various techniques which it is essential that farmers apply to get the maximum output of marketable prawns.

1. Major male morphological characteristics

Firstly, it is necessary to understand what is meant by the various morphotypes. Three major morphotypes have been described for sexually mature male M. rosenbergii (Annex 8, Figure 2). The most immediately distinctive feature is the size and colour of the claws, and the robustness of their spines:

• Blue claw males (BC) have extremely long blue claws (second pereiopods) with longer and stronger spines than OC males;
• Orange claw males (OC) have golden coloured claws that are generally shorter and have shorter and less strong spines than those of BC males; and
• Small males (SM) have small, slim, almost translucent claws.

There are also a number of intermediary forms between these major morphotypes. The transition from the small male (SM) to the orange claw (OC) morphotype is gradual. The OC is therefore sometimes referred to as the strong orange claw (SOC), and an intermediate stage between these two forms, the weak orange claw male (WOC), has been recognized in research work. Another intermediate form, this time between the orange claw (OC) and the blue claw (BC) is known as the transforming orange claw (TOC); this is the last stage of the SOC male before it transforms into the BC male, as described later in this annex.

Karplus, Malecha and Sagi (2000) also describe some other external features which can be used to delineate the various morphotypes, such as the length and orientation of the spines on the claws, but these are less immediately obvious than claw colour and size. There are also a number of internal morphological and physiological differences, as well as differences in moult frequency. SM have relatively large testes that both produce and store sperm. The testes of BC serve mainly as a sperm reservoirs. The three orange claw male forms (WOC, SOC and TOC) represent a series of gradual changes between SM and BC. Firstly, the abundance of mature sperm found in the testes of SM declines and almost disappears in the early OC stages. At the same time, the rate of production of spermatocytes (cells from which spermatozoa arise) increases as the SM moults into the OC phase. The OC phase is also characterized by frequent moulting. Another differential feature is the size (weight) of the midgut glands, especially the hepatopancreas. The hepatopancreas weight of the rapidly growing SOC is much greater than in all other morphotypes. The slow-growing SM and BC have the lowest relative midgut gland weight, while the WOC and the TOC males have intermediate values.

ANNEX 8, FIGURE 2
The major male morphotypes of Macrobrachium rosenbergii are called blue claw (BC), orange claw (OC), and small male (SM) (Israel)

SOURCE: ASSAF BARKI, REPRODUCED FROM NEW AND VALENTI (2000) WITH PERMISSION FROM BLACKWELL SCIENCE

2. Behaviour

The behavioural characteristics of the morphotypes described above are of essential importance in the management of freshwater prawn grow-out facilities. BC males are aggressive, dominant and ‘territorial’, OC males are aggressive, subdominant and ‘non-territorial’, and SM males are submissive and ‘non-territorial’.

FIGHTING (AGONISTIC) BEHAVIOUR

The fighting behaviour of the three male morphotypes differs as males follow the developmental pathway from SM through OC to BC. There is less physical contact and fewer displays of claw position and movement occur in SM than in OC and BC. As claw size increases there is an increased risk of severe injuries caused by claws during interactions amongst OC and BC prawns. Male prawns have a hierarchical relationship. BC males are dominant over OC males which, in turn, are dominant over SM. Interactions between BC males are often only for show, with little physical contact. Those between BC and OC males involve more physical contact but BC males generally use threat displays and mere approaches in their relationships with SM. BC and OC males with equal claw size are evenly matched but a BC with larger claws than an OC, even if the OC is much larger, has an advantage. The dominance of BC over OC seems to confer priority of access to preferred areas (e.g. shaded protected crevices) but true territoriality (defending a fixed exclusive area to keep intruders out) has not yet been adequately demonstrated. However, laboratory studies have shown that competitors are evicted from the vicinity of a limiting resource, such as shelter, food, and receptive females.

MATING BEHAVIOUR

Females approach males 2 to 3 days before their pre-mating moult. At first the female is chased away but later, after several hours of persistence, is allowed to remain near the male. About a day before the pre-mating moult the female is already totally accepted by the male, positioned below it or between its long second pair of claws. As a result of this early pair bonding, fertilisation can occur from several minutes to half an hour after moulting. It has been reported to occur up to nearly 22 hours later than the moult but this has been because researchers were pairing the males and females themselves, rather than allowing the natural bonding described above to occur. All three male morphotypes have similarly high rates of fertilising receptive females. Despite the fact that the spermatophores of SM are about half the size of those of BC males, the number of viable embryos following mating is dependent only on the female size, not on the type of male morphotype.

Males do not attack or injure the females that they have just fertilized. BC tend to guard the female for two or three days following mating, by which time the female’s exoskeleton is hard enough to withstand attacks by other prawns. However, OC do not appear to groom or protect the females. There are reports of injuries inflicted by OC on females during this period (especially when more than one OC is present) but the information is, at present, conflicting. Small males have been observed to mate with females by sneaking between a receptive female and her guarding BC male. A single SM has little or no chance to gain access to a receptive female guarded by a BC male. However, SM mating can be achieved when there are three or more SM present; while the BC is chasing away some of the runts, the female remains unprotected. Occasionally, following their pre-mating moult, females have been observed with several spermatophores attached to their sperm receptacle. There is some evidence that females are more attracted (through chemoreception) to BC and are more aggressive towards other male morphotypes. However, unfertilized females quickly lose all their eggs, which may be the reason why females that have undergone the pre-mate moult cooperate during mating with any of the male morphotypes.

3. Importance of population structure in freshwater prawn farming

The characteristics of size distribution in freshwater prawns (Annex 8, Figure 1) has been mentioned many times in this manual. This section of the annex describes how various factors affect the size distribution of the prawns in your ponds.

THE EFFECT OF THE SEX RATIO

The proportion of females under grow-out conditions tends to be greater than males, possibly for the following reasons:

• females may already outnumber males at stocking; and
• selective male mortality may occur in crowded pond populations.

Since the highest prices are generally obtainable for the largest animals, a preponderance of females in a population may seem to be a disadvantage at first glance. It would appear to indicate that there would be a strong incentive to rear all-male populations of prawns. However, the effect of density on average weight is more extremely pronounced in all-male, compared to all-female populations. The use of all-male populations would therefore not remove the need to manage size variation and harvesting procedures very carefully. If maximizing the total weight of prawns produced per hectare is the main goal, the rearing of all-female populations at very high densities would be sensible. However, if maximizing the income from the pond is the main goal, proper management of mixed-sex or all-male populations would be best, since the larger-sized prawns normally have the greatest unit value. Manual sexing has been done on an experimental scale but this requires extremely skilful workers and is very labour-intensive. It is likely that commercial preparations of the sex-controlling androgenic hormone to sex-reverse the broodstock used to generate monosex populations will become available in future.

THE EFFECT OF DENSITY

The proportions of the various male morphotypes change significantly with density (Annex 8, Figure 3). High density results in a larger proportion of SM. The frequency of the large BC males is highest at low densities. At high densities many prawns are in close contact with BC males, which inhibit their growth.

THE EFFECT OF UNEVEN MALE GROWTH RATE

Newly metamorphosed postlarvae are relatively even in size but size variation soon becomes noticeable. Individual prawns grow at different rates. This is known as heterogeneous individual growth (HIG). Some exceptionally fast-growing individuals (sometimes called ‘jumpers’) may become up to 15 times larger than the population mode within 60 days after metamorphosis, forming the leading tail of the population distribution curve. Jumpers became obvious within two weeks after metamorphosis. Slow-growing prawns (laggards) only become apparent later, about 5 weeks following metamorphosis. It has been suggested that growth suppression in laggards depends upon the presence of the larger jumpers. Male jumpers develop mainly into BC and OC males, while laggards develop mainly into small males.

Once this specific growth pattern has been established, juvenile prawns continue to show different growth patterns, even when they are isolated. Many studies have been conducted on the effects (on harvest production and average animal weight) of grading prawns into different fractions depending on size but are outside the scope this manual to record. For further reading on this topic see the review by Karplus, Malecha and Sagi (2000). This research has provided important clues towards improved management of grow-out populations in freshwater prawn farming and form some of the background for the comments on grading in this manual.

THE SOCIAL CONTROL OF GROWTH

Social interactions between freshwater prawns are extremely important in regulating growth. In freshwater prawns the most important social interactions are the growth enhancement of OC males (what is known as the ‘leapfrog’ growth pattern) and the growth suppression of SM by BC males.

Growth enhancement of orange claw males

The change of OC males into BC males is sometimes called a metamorphosis because the differences between these morphotypes are so dramatic. An OC metamorphoses into a BC after it becomes larger than the largest BC in its vicinity (Annex 8, Figure 4). As a new BC male it then delays the transition of the next OC to the BC morphotype, causing it in turn to attain a larger size following its metamorphosis. The newly transformed BC is larger (sometimes much larger) than the largest BC previously present. This is known as the ‘leapfrog’ growth pattern, because the weight of one type of animal leaps over another.

BC males dominate OC males, regardless of their size, probably because of their larger claws. A prawn that has metamorphosed into a BC male and is larger than any other BC in its vicinity (following the ‘leapfrog’ growth pattern) becomes the most dominant prawn in the vicinity until it is overtaken by another prawn metamorphosing from OC to BC. The ‘leapfrog’ growth pattern results in the gradual descent in the social rank of existing BC males. When a new and larger BC appears on the scene, the ‘social ranking’ of all BC males present before that event fall.

Growth suppression of small males

The growth of runts (SM) is stunted by the presence of BC males. Food conversion efficiency seems to be the major mechanism controlling this growth suppression in runts. Runts have poorer (higher FCR) feed efficiency when BC males are present. This seems to be governed by physical proximity; the phenomenon has not been demonstrated when these two types of prawns are separated, even when they are in the same water system and can see each other (i.e. chemoreception and sight are not factors).

As noted earlier in this annex, SM are sexually active. While they stay small they attract less aggression from dominant BC males (which are busier interacting with OC males) and are probably less vulnerable to cannibalism since they can shelter in small crevices. Being small and highly mobile, runts can find food on the bottom before being chased away by larger prawns, whether they be males or females. If BC males are removed from the population, some runts will increase their growth rate, and transform into OC males and, finally into BC males, following the normal ‘leapfrog’ growth pattern. This highlights the importance of regular cull-harvesting.

4. Managing grow-out in the light of heterogeneous individual growth (HIG)

Several important characteristics of freshwater prawns that affect the potential harvest from grow-out have been described above. These create various management options; a check-list of those procedures is provided below. The first three techniques have been described in the main text of this manual. The final technique (monosex culture) is a possible future development and is not part of the current manual. In grow-out management:

• there is an opportunity to improve the final harvest, both in terms of average market size and total production, by grading, because the prawns that are going to grow fastest become identifiable within 2-5 weeks after the PL metamorphose from larvae.
• there is an opportunity to increase the final harvest, both in terms of average market size and total production, by cull-harvesting. This removes the BC males, many of the OC males, and the larger females, thus encouraging small males to grow faster in the absence of dominant males (sometimes known as compensatory growth).
• there are opportunities for increasing productivity through the use of substrates. These provide refuges for newly moulted prawns (which increases survival) and decrease the frequency of fighting (which reduces growth suppression). This results in fewer SM, more OC and BC males, and higher average harvest weights.
• the potential advantages of monosex culture, not only because of the differential growth rates of males and females but also because there is less HIG in females than in males, may eventually become exploitable.
  Further reading on the experimental results of following these management options is provided in the review by Karplus, Malecha and Sagi (2000).

ANNEX 9
Farm-made pond feeds

THIS ANNEX PROVIDES a very brief introduction to farm-made grow-out feeds and their use for freshwater prawns. Further reading on this topic is provided in New (1987) and New, Tacon and Csavas (1995).

1. Feed preparation

The following general instructions are for the preparation of diets, which may be fed moist or dried.

1. Mix all your dry ingredients (except vitamin mix, if used) thoroughly, preferably in a mechanical mixer.
2. Add your vitamin mix, if your formula contains one, and remix as in No. 1 above.
3. Add any liquid ingredient (such as fish oil) or any wet materials (such as chopped trash fish). [Note of caution: commercially processed shrimp by-products, such as shrimp meal and shrimp head meal, are very valuable from a nutritional point of view in feeds for freshwater prawns. There is no danger that such ingredients will cause disease problems (Flegel, 2001). However, the use of raw (unprocessed) shrimp or prawn wastes, such as prawn heads, may introduce viruses (e.g. WSSV) into your farmed animals. Although this may not cause any visible symptoms it may make them carriers of the disease for other crustaceans.]
4. Remix all your ingredients thoroughly.
5. You will now need to add up to 30-35% of water. The exact quantity depends on the moisture of the ingredients. You will need to add enough water to produce a very thick paste. Add water a little at a time and test the mixture. It is easy to add more water (but not possible to remove it if you add too much at first!). You can test the consistency (stickiness) of the diet by squeezing it within your clenched hand. If the ‘sausage’ of mixed diet (consistency of bread dough) which emerges from your fist between your first finger and your thumb is too crumbly, you need to add more water. If it runs out like a liquid, you have already added too much water!
6. Continue mixing. You can help to mix your ingredients thoroughly by passing the mixture through the coarse die of a meat mincer (see No. 7 below). This works well and also helps to bind the diet together.
7. You can now shape the mixed diet into small balls or discs by hand. However, it is best if you extrude the mixed diet through a meat mincer, this time using small die holes (1/8-in diameter) to produce a well-bound spaghetti-like material, which breaks easily into pellets when dried. Annex 9, Figures 1 and 2 show this process.
8. You can feed your mixed diet as it is (in a moist form) if you can use it on the same day as you make it. Alternatively, you can stir the extruded ‘spaghetti’ to form 1-2 cm long pellets and sun-dry them for later use. Drying on a concrete surface in direct sunlight for six hours (Annex 9, Figure 3) may be sufficient to reduce the moisture content of the pellets to a level (about 10-12%) at which they can be stored without excessive deterioration. Dried feed takes less space and is much easier to transport to the pond (Annex 9, Figure 4) and to feed. A simple solar dryer (Annex 9, Figure 5) can be constructed for drying pellets during the monsoon season; however, this is difficult to use if you are producing a lot of feed. Many farms find it more feasible to choose days on which the weather is forecast to be dry to manufacture and dry their feed. Others find that the feed can be collected up from the concrete drying surface if a rain cloud approaches and then put out again when the sun returns. This sounds difficult to do but the author of this manual has observed it being done! Whatever, method you choose, it is important that the feed is dried in as short a time as possible to prevent fungal growth.
9. Dried pellets can be stored up to 2-3 months. You must store them in the coolest conditions possible. It is essential to keep them dry and to protect them from rats and other animals during storage.

ANNEX 9, Figure 1
Some farmers make their own equipment for extruding farm-made feeds (Thailand); this photo shows the tray where the mixed feed is pushed into the grinder, whose die plate is at the far side (not visible)

SOURCE: HASSANAI KONGKEO

2. Feed formulae

Some examples of feed formulae for the pond rearing of freshwater are given in Annex 9, Tables 1-7. Please note that these formulae are only examples; many other feeds have been used, or are possible, depending on the availability of raw materials. These examples have been extracted from D’Abramo and New (2000), where the original references are cited. Diets 1-4 are practical feeds that have actually been used in freshwater prawn grow-out. Diets Nos. 5-12 have been used in experimental work.

ANNEX 9, Figure 2
In this photo, freshwater prawn feed is being extruded from the die plate of a meat chopper (Thailand)

SOURCE: MICHAEL NEW

ANNEX 9, Figure 3
Farm-made feeds need to be dried if not fed directly after manufacture; this can be done by spreading the feed out on concrete or on trays for sun-drying (Brazil)

SOURCE: DENIS LACROIX

ANNEX 9, figure 4
Feed that has been extruded through a mincer and sun-dried is easy to transport to the ponds (Thailand)

SOURCE: HASSANAI KONGKEO

ANNEX 9, TABLE 1
Farm-made grow-out feeds No. 1 and No. 2

* PLEASE NOTE THE CAUTIONS ON THE REGULAR USE OF ANTIBIOTICS IN THE TEXT OF THIS MANUAL.
** NO DETAILS AVAILABLE.
*** WEIGHT BEFORE BOILING.

ANNEX 9, TABLE 2
Farm-made grow-out feed No. 3

* COMPOSITION UNKNOWN.
** A LOCALLY AVAILABLE VITAMIN MIXTURE BEING SOLD FOR FRESHWATER PRAWNS. IT WAS STATED TO CONTAIN VITAMINS A, D, C, AND E, AND AN UNSPECIFIED ANTIBIOTIC (OXYTETRACYCLINE ?) [SEE NOTE BELOW PREVIOUS TABLE]. THE MANUFACTURER SUGGESTED A VITAMIN MIX INCLUSION RATE OF 0.5-1.0%.

ANNEX 9, TABLE 3
Farm-made grow-out feed No. 4

* AT LEAST 50% PROTEIN.
** NO DETAILS AVAILABLE. NOTE: NO OTHER SUPPLEMENTARY VITAMINS ADDED.

ANNEX 9, TABLE 4
Farm-made grow-out feed No. 5 and No. 6

* NO DETAILS AVAILABLE.

ANNEX 9, TABLE 5
Farm-made grow-out feed No. 7 and No. 8

* NO DETAILS AVAILABLE.

ANNEX 9, TABLE 6
Farm-made grow-out feed No. 9 and No. 10

* CRYSTALLINE CaCO3
** NO DETAILS AVAILABLE.

ANNEX 9,TABLE 7
Farm-made grow-out feed No. 11 and No. 12

NOTE: NO VITAMIN OR MINERAL MIX

ANNEX 10
Basic code for introductions

ANNEX IS DERIVED, with acknowledgements, from part II of a draft framework for the responsible use of introduced species that was prepared for EIFAC by Bartley, Subasinghe and Coates (1999).

The basic code for introductions applies to the intentional movement of aquatic species in fisheries, biological control, aquaculture, and for research. Therefore, someone, some organization, some private business, or some government agency (referred to below as ‘the entity’) must knowingly engage in the act of transporting the species. Guidelines and policy concerning species introduced inadvertently through ballast water or on ship’s hulls are addressed elsewhere, for example by the International Maritime Organization. Development projects that involve geographic changes, such as river diversion, dredging of canals to connect distinct water bodies, etc. also may involve the subsequent introduction of exotic species and therefore this framework could also be used in the review and evaluation of those projects.

The basic code contains the requirements that:

0. the entity moving an exotic species develop a proposal, that would include location of facility, planned use, passport information, and source of the exotic species;
0. an independent review that evaluates the proposal and the impacts and risk/benefits of the proposed introduction, e.g. pathogens, ecological requirements/interactions, genetic concerns, socio-economic concerns, and local species most affected, would be evaluated;
0. advice and comment are communicated among the proposers, evaluators and decision makers and the independent review advises to either accept, refine, or reject the proposal so that all parties understand the basis for any decision or action, thus proposals can be refined and review panel can request additional information on which to make their recommendation;
0. if approval to introduce a species is granted, quarantine, containment, monitoring, and reporting programmes are implemented; and
0. the ongoing practice of importing the (formerly) exotic species becomes subject to review and inspection that check the general condition of the shipments, e.g. checking that no pathogens are present, that the correct species is being shipped, etc.

The Code is general and can be adapted to specific circumstances and resource availability, but it should not lose any of the above requirements nor should it lose the rigour at which the requirements are applied. For example, a regulatory agency may require a proposal to contain a first evaluation of the risk/benefits and this evaluation would then be forwarded to an independent review or advisory panel; or the advisory panel could make the first evaluation of a proposal. Similarly, States may require quarantine procedures to be explicitly described in the proposal before approval is granted.

ANNEX 11
Glossary of terms, abbreviations and conversions

Terms

THE FIRST SECTION OF THIS GLOSSARY defines unfamiliar terms used in this manual. The definitions are intended to make the terms understandable to the novice rather than to the biologist.

Abbreviations

NOT ALL OF THE FOLLOWING abbreviations have been used in this manual. However, they are provided to help you when you read other documents.

Conversions

THIS SECTION OF THE ANNEX should be used in conjunction with the abbreviations section. Please note that the words gallon and ton have different values depending on whether the source of the text you are reading is ‘British’ or ‘American’ in origin.

LENGTH:

WEIGHT:

VOLUME:

CONCENTRATION - DISSOLVING SOLIDS IN LIQUIDS:

CONCENTRATION - DILUTION OF LIQUIDS IN LIQUIDS:

AREA:

TEMPERATURE

PRESSURE:

Scientific units

Scientists have a different way of writing some of the units described in this glossary. They use what is called the Système International (SI). The units are referred to as SI units. For example: 1 ppt, which can be written as 1 g/L (see concentration above) is written as 1 g L-1 in scientific journals. 1 g/kg is written as 1 g kg-1. 12 mg/kg would be written as 12 mg kg-1. 95 µg/kg would be written as 95 µg kg-1. A stocking density of 11 kg/m³ would be written as 11 kg m-3. This system of standardisation is not normally used in commercial aquaculture hatcheries and grow-out units and has therefore not been used in this manual. More information about this topic can be found on the internet by searching for SI Units (e.g. www.ashree.org/book/siguide.htm)