6.5 Monitoring performance and record keeping
The growth rate and survival of each population of prawns depends on many factors, including density, predation, feed and temperature. Since these factors are so site- and operator-specific it is not wise to predict what they will be in this manual, for fear of causing expectations which may not be realized. However, Box 24 gives examples of growth and production rates that have been reported in the scientific literature. Survival rates during the grow-out period should not fall below 50%. Under the semi-intensive management system described in this manual, productivity ranges of 1 000-3 000 mt/ha/yr are typical but can be exceeded.
As your farm operates, you will develop your own experience of growth rate and productivity during grow-out. You can only achieve this by careful monitoring and record keeping.
It cannot be over-stressed that you should keep adequate written records of such things as water quality, stocking rate and date, daily feeding quantities, dates on which water changes are made (and how much), harvesting dates and quantities, etc. Only in this way can you build up a picture of how each pond behaves under a certain management regime (and every pond is different) and accurately apply your experience to future pond
BOX 24
Examples of freshwater prawn (M. rosenbergii) growth and production rates
AVERAGE YIELD: The following yields are on an annual basis, except where indicated.
TROPICAL MONOCULTURE: 800-1 200 kg/ha (Brazil); 1 500 kg/ha (Dominican Republic); 1 200-2 500 kg/ha (Guadeloupe); 2 000-2 250 kg/ha/6-7 month crop (India); 900-3 150 kg/ha/crop (Malaysia), 520-1 926 kg/ha (Martinique); 1 286 kg/ha (Polynesia); 909-1 909 kg/ha (Puerto Rico); 2 000 kg/ha (Taiwan Province of China); 1 500 kg/ha (Thailand); 3 100 kg/ha/crop (Thailand, with paddlewheel aeration); 600-750 kg/ha (Viet Nam).
TROPICAL POLYCULTURE AND INTEGRATION: 200-300 kg/ha (Bangladesh); 200-500 kg/ha (India); 200-300 kg/ha (Viet Nam).
TROPICAL INTEGRATED CULTURE: 100-300 kg/ha (Viet Nam, in ricefields).
TEMPERATE CULTURE: [The growing season is typically 4.0-5.5 months per year] 2 250-3 000 kg/ha/crop (China, in monoculture); 1 200-1 800 kg/ha/crop (China, in polyculture with carps when the emphasis is on prawn production); 300-900 kg/ha/crop (China, in polyculture with carps when the emphasis is on fish production); 1 200 kg/ha/crop (USA, in commercial monoculture); ~3 000 kg/ha/crop (USA, in experimental monoculture with substrates).
MONOCULTURE IN GEOTHERMALLY HEATED WATER: 2 500-3 000 kg/ha (New Zealand).
GROWTH RATES: The following are very approximate data from experimental work - exact rates depend on environmental conditions and, in the case of grow-out, on the way in which size variation is managed; intermediate cull-harvesting will pull out marketable animals which grow at a much faster rate than the average.
PL AND JUVENILES with starting weights of between 0.01 and 0.3 g can grow between 5 and 30 mg/day over periods of between 60-75 days in indoor and outdoor nursery facilities.
0.25 g JUVENILES reached an average of nearly 34 g in ~132 days when stocked in temperate zone ponds at ~4/m2 or about 26 g when stocked at ~8/m2.
0.33 G JUVENILES, stocked at about 6/m2 in another temperate zone experiment, reached an average of 30 g in 106 days in ponds without substrates and nearly 37 g in ponds provided with substrates.
Recently, the production of prawns averaging 52 g in temperate zone ponds with substrates has been reported but details of stocking rates, size of juveniles stocked or time of rearing have not yet been published.
SOURCE: ALSTON AND SAMPAIO (2000); DANIELS, D’ABRAMO, FONDREN AND DURANT (1995); NEW (1995; 2000b); REDDY & RAO (2001); J. TIDWELL (PERS. COMM. 2001); TIDWELL, COYLE AND SCHULMEISTER (1998).
management in order to operate your farm profitably. This applies equally to hatchery management.
Methods of monitoring feeding rate and phytoplankton density and for the control of the latter have been dealt with earlier in this manual. It is good practice, if possible, to monitor other water quality parameters such as pH, temperature and dissolved oxygen routinely, so that you can link production rates with the environment of each pond and the way in which you manage it. This will give you the information you need to take actions to prevent a recurrence of problems (such as low dissolved oxygen levels, for example).
Figure 81
A large BC Macrobrachium rosenbergii broodstock male from the CAUNESP (Aquaculture Center, São Paulo State University, Brazil) being measured in the ‘scientific’ way (from behind the eye orbit to the tip of the telson)
141-1
SOURCE: DEBORAH ISMAEL
Ideally, you would like to determine the average size and the number of prawns in your pond at any time. In this way you could tell whether growth and survival rates are satisfactory, or not, and determine a daily feeding rate based on a percent age of the pond bio mass. Unfortunately, there is no accurate way known of determining the standing crop of freshwater prawns in a pond unless the pond is regularly seined. Even when a reasonable estimate of pond biomass can be obtained, daily feeding rates based on a percentage of biomass should not be applied blind ly but should be tempered by observations on consumption and phytoplankton density.
If you are growing freshwater prawns for the first time, you must realize that individual prawns within a population grow at different rates. Some will grow very fast, others hardly at all. This normal characteristic of the animal has been described in Annex 8. The disparity in growth rate is more pronounced among males than females and in mature populations of freshwater prawns.
You should regularly measure growth rate, either by weight or total length. Measurement of length from the eye orbit to the tip of the telson (Figure 81) is the most reliable technique (because the rostrum of some animals becomes shortened by damage) but, in farming practice, total length from the tip of the rostrum to the tip of the telson is usually measured, often by ruler. Figure 82 gives the relationship between total length and live weight for a mixed-sex population of freshwa ter prawns. Males weigh slightly more than females of the same length, but not markedly so. A
FIGURE82
There is a relationship between the total length and the weight of your prawns; this shows a typical length/weight relationship for Macrobrachium rosenbergii
SOURCE: WICKINS (1972)
method for sexing small (juvenile) prawns is shown in Figure 3. The differences between larger females and the various male morphotypes have also been described earlier in this manual.
If you have never grown freshwater prawns before you may notice that you do not see the prawns after they are stocked; they are difficult to see and to catch at this stage. Do not be discouraged! At this time, you will be giving quite large quantities of food but, after a while, you may begin to think that the feed (and your money!) is being wasted. You cannot see many prawns, so you wonder: have they all died, escaped, or been eaten by pred ators ? Do not decrease the amount of feed or stop feeding altogether. About two months after stocking, you will begin to see (by now quite large) prawns again. If you wait until the stage when you can see the prawns in the pond to start feeding again, the productivity of your crop will have been permanently reduced. This is a common experience of new farm ers of this species. Have patience, and examine the perimeter of your ponds by night, with the aid of a flashlight.