12. SAMPLING AND ANALYSIS

12.1 Estimating the number of shrimp in a pond

For management pruposes, knowing the number of shrimp in a pond is very important. For example, if the number of shrimp is drastically reduced due to a catastrophic mortality caused by disease or low dissolved oxygen levels, it might be advisable to drain the pond and start over again with a new stock. In ponds where supplemental feed is given, accurate estimation of numbers is critical, as the amount of feed provided is usually based on the estimated weight of shrimp in the pond. If the estimated number of shrimp is high, too much food will be given and the excess will decay and pollute the pond. If the estimate is low, not enough food will be given and the shrimp will not grow well.

Arriving at a reliable estimation of the number of shrimp in a pond is difficult. This is because the distribution of shrimp within a pond is not even, they tend to group together. The concentrations of shrimp do not remain in one area of a pond, instead they move around within the pond. Also it has been observed that shrimp are usually more abundant in the corners of a pond.

A general idea of the number of shrimp in a pond can sometimes be obtained by walking around a pond at night with a bright light and observing the number of shrimp swimming near the edge. The swimming activity of shrimp is affected by many things such as moon phase, food supply and water movement. As a result the number of shrimp observed can vary widely on different days. A more reliable estimation can be obtained by sampling. The following methods have been tried with varying degrees of success.

12.1.1 Use of screened frame boxes. A wooden or iron frame covered with mosquito netting which has a known area (usually 1 m²) is placed on the pond bottom. In one method the sides of the frame are high enough to extend out of the water. The shrimp trapped in the frame are caught with a scoop net. Another type of frame has sides only 30 cm high, but a net covers the top. The shrimp trapped in this frame are counted by a diver. To obtain a reliable sample it is necessary to sample at least 10 locations in a 0.5 ha pond. One sample should be taken in each corner and six in the middle. An average number of shrimp per sample (m²) is arrived at and this is multiplied by the number of square metres in the pond to arrive at the number of shrimp in the pond.

12.1.2 Beam trawl. A beam trawl with a two-metre opening is dragged across the pond. The trawl is set on one side, then a worker carries a long rope around the pond to the opposite side. Then the trawl is pulled directly across the pond. The width of the pond is multiplied by the width of the beam trawl to obtain the number of square metres of pond bottom sampled. The number of square metres sampled is then divided by the number of shrimp caught in the beam trawl to get the average number of shrimp per square metre.

The method works best in ponds with a level bottom and with no structures placed in the pond for shelter or windbreaks, etc. It can not be used with “lumut” and disturbs the bottom in a “lab-lab” pond. It is difficult to sample corners.

12.1.3 Cast net. Sampling with a cast net should be done at night when the shrimp are active. Sampling during the day is not effective. It is difficult to estimate the area covered by the cast net as each throw is different. Also some shrimp frequently escape from the net.

12.1.4 Marking. Mark a specified number of shrimp by cutting off one uropod. Replace the shrimp in the pond. After one or two nights, sample the shrimp in the pond again. The total number of shrimp in the pond can be estimated by multiplying the number of shrimp marked by the number of shrimp in the second sample and then dividing by the number of marked shrimp recovered in the second sample.

12.2 Sampling for growth

Sampling should be done once a week. Measurements of 50 to 100 shrimp should be adequate. It is better to take several samples instead of one large sample. If too large a sample is taken, there is a danger that the shrimp might die before they could be returned to the pond. It is better to sample in the cool of the morning or evening. Measurements should be made as soon as possible after the shrimp are caught. Shrimp should not be stored for any length of time due to possible death by cannibalism or loss of weight due to starvation. The shrimp can be prevented from jumping out of the holding container by placing a few branches with leaves on them in the container.

A variety of methods can be used to obtain a sample, but an important point to consider is that methods of sampling which disturb the pond bottom also destroy food. In such cases, the number of samples taken should be made as few as possible to obtain the information required. Samples of postlarvae can be obtained by placing twigs or branches around the pond and then lifting up the branches catching the postlarvae in a scoop net as the branch is lifted out of the water.

There are several different ways of measuring the length of shrimp. Total length is measured from the tip of the rostrum to the tip of the telson. There is some criticism of this measure because the rostrum is frequently damaged or broken and the shrimp cannot be measured. However, since it is difficult to measure standard length or carapace length of postlarvae and juveniles, it is perhaps better to use total length, so that only one type of measurement can be used throughout the shrimps growing period. Standard length is a measurement from the post-orbital notch to the tip of the telson. Carapace length is a measurement from the post-orbital notch to the posterior margin of the carapace.

The shrimp can be weighed individually or all together. For production purposes, the length of shrimp is not needed and an average weight obtained by weighing the whole sample at one time is all that is required. This procedure saves a lot of time and a less delicate balance is required. The total sample is weighed at one time by placing the shrimp in a bucket containing water. The bucket containing water is weighed before the shrimp are added. The weight of the shrimp is the difference between the two weights. The average weight of an individual shrimp is calculated by dividing the total weight of the sample by the number of shrimp in the sample.

When taking individual weights, each shrimp should be dried before weighing. This is done by gently patting the shrimp with an absorbent towel or cloth.

12.3 Salinity

There are many ways to measure salinity. Elaborate salinometers, electric instruments, refractometers and chemical methods are available. These are useful for scientific research, but are usually too expensive for fish farmers.

A hydrometer is a less expensive instrument. It is a kind of calibrated-floating tube. It measures the weight, or specific gravity of liquids according to how high the tube floats in the liquid. As salt is added to water, the water becomes slightly heavier. As the water becomes heavier, objects floating in the water are pushed higher out of the water. The hydrometer uses this principle to indicate the amount of salt in the water. Commercially-made hydrometers calibrated at certain temperatures are available from scientific equipment dealers, with instructions for their use. They do not cost too much, but as they are made of glass, they break easily.

It is possible for a shrimp farmer to make a simple hydrometer for measuring salinity. A strong, rigid, narrow mouth plastic bottle of about 100 cc capacity would be a good choice, but a glass bottle could also be used (Fig. 12). The mouth of the bottle is plugged with a stopper that is fitted with a light stem such as a piece of bamboo. Enough rocks should be put in the bottle so that the tip of the stem just sticks out of the water a short distance when the bottle is put in freshwater. The point where the stem comes out of the water is then marked on the stem. Next the bottle is floated in seawater which has been collected from a distance offshore so that it is not diluted by freshwater runoff from rivers. The stem will stick farther out of the water now because the salt in the seawater makes the bottle float higher. A mark should be made on the stem at the new point where the stem comes out of the seawater. Next a mixture is made of half seawater and half freshwater. The bottle is floated in this water and a third mark is made on the stem to indicate half strength seawater. Now the bottle can be used as a reference to judge the approximate salinity as compared to seawater and freshwater. Most accurate results are obtained when the temperature of the water being tested is the same as the water used when the stem was marked (Anonymous, 1974).

12.4 Temperature

Hand-held thermometers are adequate for measuring water temperature. One should not hold the thermometer in the pond water and then lift it out to read it. This procedure can cause the thermometer reading to be several degrees off. The thermometer should be read while in the pond water.

A record of the extreme temperatures in the pond over a period of time can be obtained by laying a maximum-minimum thermometer on the pond bottom.

12.5 Turbidity

Turbiditv is caused by particles suspended in the water. It can be caused by phytoplankton, mud or other substances. One way of measuring turbidity is with a Secchi disc (Fig. 17). A Secchi disc is about 30 cm in diameter, painted white and black or just white, and has weights or heavy objects hanging on it to make it sink straight down in the water. The disc is suspended on a rope or a long piece of wire that is marked off in centimeters. The disc can be made from metal or wood as long as it will sink. A tin can pounded flat can be used (Druben, 1976).

When the Secchi disc goes into the water it will sink and disappear from sight at some depth. The marks on the rope are read at the point where the disc just disappears from sight. In a pond being managed for phytoplankton the disc should disappear at a depth of about 25 to 35 cm.

12.6 Determining lime requirements for pond soils

A soil sample should be taken as described in Section 3.2.

The composite soil sample is mixed thoroughly and spread in a thin layer to air dry. After drying, the sample is crushed gently into a powder and sieved through a screen with 0.85 mm openings. A p-nitrophenol buffer of pH 8.0 ± 0.1 is prepared by diluting 20 g p-nitrophenol, 15 g boric acid, 74 g potassium chloride, and 10.5 g potassium hydroxide to 1 liter with distilled water.

Place 20 g of dry soil in a 100 ml beaker, add 20 ml of distilled water and stir periodically for one hour. Then measure the pH of the muddistilled water mixture with a glass electrode pH meter. Add 20 ml of the p-nitrophenol buffer and stir periodically for 20 minutes. Set the pH meter at 8.0 by using a mixture of 1 part p-nitrophenol buffer and 1 part distilled water. Next read the pH of the sample (soil, distilled water, buffer mixture) while stirring vigorously. The pH value of the soil in water and the soil in buffered solution is used to obtain the lime requirement from the following table. If the pH of the soil in the buffered solution is below 7, repeat the analysis with 10 g of dry soil and double the amount of lime required given in the table (Boyd, 1976).

Lime requirement in kg/ha of calcium carbonate (neutralizing value of 100) to increase total hardness and total alkalinity of pond water above 20 mg/l (from Boyd, 1976)