THE USE OF ROCK SALT, SEA SALT, ROCK SALT BRINE AND SALT STOCK SOLUTION FOR LARVAL CULTURE OF MACROBRACHIUM ROSENBERGII
chacheongsao Fisheries Station
Inland Fisheries Division, Department of Fisheries
Ministry of Agriculture and Cooperatives
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THE USE OF ROCK SALT, SEA SALT, ROCK SALT BRINE
AND SALT STOCK SOLUTION FOR LARVAL CULTURE
OF MACROBRACHIUM ROSENBERGIIa
The demand for giant freshwater prawns has increased considerably in domestic and foreign markets. This has resulted in higher prices and thus categorized prawns as an aquatic animal of great economic value. Many people have taken up prawn farming as their profession. Normally all larval stages require brackish water. Specimens reared in pure freshwater all die within a few days. After metamorphosis freshwater should be provided for them until they are fully grown - ready for mating and egg-laying, then they have to be introduced once more into brackish water.
Therefore, prawn farms which have to cater for breeding, hatching and rearing should preferably be situated near the sea since, transportation of seawater to a farm located far away from the sea costs a lot of expenses.
In order to eliminate factors that limit production in the field of breeding and rearing of juvenile prawns, a research, training and development project on giant freshwater prawns has been carried out by the Chacheongsao Fisheries Station, Freshwater Fisheries Division, Fisheries Department. The work described in this paper reports the result of an experiment where rock salt, sea salt and north-eastern brine were successfully mixed with freshwater and salt stock solutionc to rear larvae. The mixture ratio has to be appropriate to the quantity of mineral salts required for the breeding and rearing of juvenile prawns so that it simulates the mixture of seawater and freshwater used normally. Success in this work would reduce considerably the cost of transportation of seawater to the farm. It is hoped that in the future the successful result obtained from this experiment would be utilized in breeding and culturing of giant freshwater prawn all over Thailand, in particular in the north-eastern part of Thailand, where plenty of rock salt and brine can be obtained for this purpose.
a This Working Paper is a translation of part of a RTG Department of Fisheries Report, 1980.
b Fisheries Biologist, Chacheongsao Fisheries Station, c/o Department of Fisheries, Ministry of Agriculture and Cooperatives, Bangkok, Thailand.
c The term ‘salt stock solution’, which is used throughout this report, means the liquid residue after the production of sea salt in evaporation ponds. It is the liquid effluent which will not crystallize and is discharged as a very strong solution of mineral salts, other than Sodium chloride (NaCl). This solution is believed to be the key to the successful use of salt for making re-constituted seawater or brackishwater.
Moreover, the mixture can also be used to culture other aquatic animals requiring brackish or sea water. This will render great advantages to the research and culture of aquatic animals in areas remote from sources of brackish or sea water.
to make full use of rock salt, sea salt, the brine obtained from the north-eastern region of Thailand and salt stock solution in the culturing of aquatic animals;
to conduct a feasibility study of larval freshwater prawn culture in order to eliminate the factors that limit the production of prawn farming (in other words to expand the zone of larval prawn farming as wide as possible);
to promote the development of a practical mass rearing technique for aquaculture and for the accumulation of breeding stock.
Since 1978, the Chacheongsao Fisheries Station has experimented to find ways and means of utilizing artificial seawater and saltwater obtained from the rock salt of the northeast of Thailand. The study was undertaken by studying the basic data necessary for the experiment, for instance the characteristics of estuarine-water, the physiology of crustacea, the chemical composition of freshwater and seawater and the management system of salt farms both of the sea salt and rock salt type. Results obtained from preliminary experiments in hatching and rearing juvenile prawns in artificial seawater and salt water of rock salt had shown that the juvenile prawns could be hatched and reared in the mixture of water for a certain period of time but the survival rate was very low. It was therefore impossible to carry out mass production, since the expense was too high due to the high price of the chemicals utilized in, the experiment. However it has also been discovered that the effluenta of salt farms contains a solution of minerals of high salinity, which is an important factor in the composition of seawater, only some of the salts having been crystallized out during salt production. Only a small quantity of the salt stock solution would be required to mix with a large quantity of freshwater in order to obtain various mineral salts in the required proportions. The addition of crystallized salt to this mixture may produce water good enough for rearing larval prawns. After initially satisfactory results were obtained, this experiment was undertaken during the months of July-August 1980, utilizing mixtures of rock salt, sea salt, brine from the NE of Thailand, salt stock solution and freshwater. In this experiment, 2,833 postlarvae were produced in the trial tank containing about 140 liters of freshwater. It was concluded therefore that mixtures of water used are suitable for breeding juvenile prawns at a satisfactory survival rate and consequently the technique can be used for mass production.
a “salt stock solution”
Three sexually mature female prawns (with a length of approximately 12 cm. each) were culled with ripe eggs of a grey colour;
One fibre glass tank of 50 × 100 × 100 cm. was used to stock all female prawns culled for hatching;
Four cement ponds of 1.5 × 15 × 1 m. were used to prepare rearing water;
Analytical equipment for measuring the quantity of mineral substances and water;
Air pump. The air pump system normally in use at the Station was used;
Sixteen 160 liter salt jars from Rajburi Province to be used as tanks in the experiment;
Salt and brine obtained from rock salt farms in the NE of Thailand, sea salt, salt stock solution, freshwater and other necessary mineral salts;
Equipment for filtering water;
Food used for feeding juvenile prawns for instance, Artemia steamed ground eggs;
Facilities for siphoning out the sediments and unused food particles and facilities for cleaning.
The jars were used for rearing juvenile prawns in different trials. They were filled with 1.30 liters of water and numbered T1 R1 - T4 R4.
Four types of water were used in the trials. Each mixing tank was 1.5 × 15 × 1 m. The trial compared the rearing of larval prawns in water composed of mixtures of rock salt, sea salt, brine, salt stock solution and freshwater with a normal sea water/freshwater mixture.
Seawater was pumped into the tank and mixed with freshwater to get a salinity of 9 ppt. This was the control and is the type of water normally being used at present in the hatching and rearing of juvenile prawns.
Freshwater was pumped into the tank and rock salt added to it. Salt stock solution of about 350 ppt was added to the tank in the ratio 1 to 200 of liquid. Air pumps were used at all times to thoroughly mix the different mineral salt solutions with water and to obtain an even salinity. The quantity of mineral salt was tested once again and the mixture adjusted to simulate the composition of natural brackish water. The mixture of water finally had a salinity of 9 ppt.
As for Type 2 except sea salt was used instead of rock salt.
A mixture of rock salt brine and salt stock solution was added to freshwater in a 1:300 solid:liquid ratio to produce 9 ppt water.
The food used to feed juvenile prawns in these trials consisted of two categories as follows:-
Artemia. Throughout the trial, Artemia produced in Thailand by the Chacheongsao Fisheries Station was fed to the juvenile prawns. Artemia cysts were hatched in brine with a salinity of 20–30 ppt, with full aeration provided, for approximately 12–36 hours.
Steamed ground eggs. Steamed ground eggs were prepared by beating the yolk and white of hen's eggs thoroughly and adding fish flesh or oyster meat or animal's blood, vitamin and a variety of mineral salts and other substances. Water in a 1:1 ratio to mix eggs was added. The mixture was thoroughly blended by passing it through a meat grinder. The ground mixture was filtered through a strainer made of fine cloth in order to retain only the solid part and remove dissolved substances. The solid part was cooked by steaming. It was then passed through strainers with different mesh sizes to produce particle sizes suitable for use for feeding of larval prawns.
On the 4th of September 1980, three female prawns were culled. All of them carried their brood of grey eggs. They were introduced into a 0.5 × 1 ×1 m. tank. Freshwater was added to the level of 30 cm. Aeration was provided during the whole incubation period. On the 5th of September the eggs hatched.
Four replicates of each type of water was used and the rearing tanks were marked T1 R1, T1 R2 etc., T2 R1, T2 R2 etc., etc.
On the 5th of September 1980, 1 day old juvenile prawns were transferred from the hatching tank to the trial tanks and stocked at approximately 4,000 prawns per 130 liter jar.
When the juvenile prawns were 3 days post hatching, Artemia nauplii were fed day and night for a period of 5 days. After this, the daytime feeding consisted of steamed ground eggs which were at a frequency of approximately 5–6 times a day. Artemia was fed at night as before.
As soon as the egg mixture began to be used, when the larvae were 7 days old, tank cleaning was done everyday in the morning with flexible tubing, which was used to siphon out wastes and unused food from the bottom of the shaded part of the tank. After siphoning, the water level was restored to normal. During the day time, a plastic pail, converted into a filter and aerator (Figure 1) was used. The filters were removed at night to prevent losses of Artemia.
Larvae reared in water of all 4 types produced no great differences in growth rate. In all cases, some post-larvae were observed at day 18 and metamorphosis was completed by day 31.
The results of the trial are summarized in Table 1. Survival rates to metamorphosis ranged from 25–54% in the control, 31–50% in the rock salt/ salt stock solution; 33–49% in the sea salt/salt stock solution; and 32–53% in the rock salt brine/salt stock solution. An average of 13 postlarvae per liter of rearing water were produced in all treatments (Table 2)
Our experiments in breeding and rearing prawn larvae in mixtures of rock salt, sea salt, and brine from the NE of Thailand with freshwater and salt stock solution were successful. No significant differences were obtained in the various treatments.
Utilization of mixtures of salt stock solution and freshwater and various kinds of salt, as mentioned above, would reduce the expenses incurred in direct transportation of seawater or the cost of chemicals needed for making artificial seawater.
This experiment should serve as a guideline for expansion of the area where larval prawns can be reared, both in Governmental Units and in the private sector, in areas remote from the sea (which include the Northeastern and other regions of Thailand).
The experiment also serves as a guideline for mixing water for brackish water and seawater aquaculture in conditions and locations where good quality or quantity of seawater cannot be obtained.
This experiment contributes to efficiency in the development of practical mass rearing techniques for Giant freshwater prawn farming and in the provision of animals for stocking purposes. It also aids the general expansion of freshwater prawn farming.
The experimental results could contribute to finding ways and means to improve the quality and mineral composition of larval rearing water.
This experiment was only a beginning; there is much more detail needed to complete the feasibility study. The next step will be to conduct more experiments to improve our technical knowledge in order to prevent, and at the same time to solve operational problem which are confronted. This work will promote higher yields and higher larval survival.
The writer is grateful to Mr. Somsuk Singholka, Cheif of Chacheongsao Fisheries Station, for his enthusiastic support and his review of this report.
Thanks also are due to Mr. Chalieng Chaitiemvong, Chief of the Operations Unit, Freshwater Fisheries Division and to all colleagues who have contributed valuable advice and assistance in publishing the report.
I also thank Mr. Vidtaya Charoenphol for his support of the rearing trial by providing all the tanks used and to Mr. Lieng Sodsai and Mr. Um Ruamsap for their assistance in the experiment.
The writer is also grateful to his teachers who gave him his knowledge in this field of work. Thanks are due also to all of his classmates (Class 30, Fisheries 21), Kasetsart University and all of those who contributed assistance and moral support to the writer both directly and indirectly, in enabling the smooth and rapid progress of this work.
Table 1: Summary of yield of postlarvae from each treatment (Initial stocking 4,000 per tank)
|Trial No.||Number of postlarvae|
|1.||Seawater plus freshwater (T1)||1,689 (42%)||1,001 (25%)||2,175 (54%)||1,900 (48%)||6,774||1,694 (42%)|
|2.||Rock salt plus salt stock solution (T2)||1,225 (31%)||1,709 (43%)||1,701 (43%)||2,006 (50%)||6,641||1,660 (42%)|
|3.||Sea salt plus salt stock solution (T3)||1,325 (33%)||1,500 (38%)||1,829 (46%)||1,947 (49%)||6,601||1,650 (42%)|
|4.||Brine from the NE of Thailand plus salt stock solution (T4)||1,460 (37%)||1,292 (32%)||2,147 (53%)||1,844 (46%)||6,743||1,686 (42%)|
Table 2: Postlarval yield per liter
|Trial No.||Result of yield of giant freshwater prawn (prawn/liter)|
|1.||Seawater plus freshwater (T1)||13.06||7.70||16.73||14.62||13.03|
|2.||Rock salt plus salt stock solution (T2)||9.42||13.45||13.08||15.43||12.77|
|3.||Sea salt plus salt stock solution (T3)||10.19||11.54||14.07||14.98||12.69|
|4.||Brine from the NE of Thailand plus salt stock solution (T4)||11.23||9.94||16.52||14.18||12.97|
Figure 1: Detail of the filter used in larval rearing.