T. Tattanon and S. Maneewongsa
A. SPAWNING OF SEABASS BY STRIPPING OF SEXUALLY MATURED SPAWNERS
Induced spawning of seabass by hand stripping of eggs and milt from wild spawners collected from the natural spawning grounds is one of the best methods for collecting adequate amount of eggs for mass production of seabass fry. This method is suitable for the hatcheries that are near the spawning grounds and have no spawner in grow—out culture for broodstock.
2. Spawning Techniques
The process of stripping the spawner from natural spawning grounds needs a fair amount of skill to avoid injuring the eggs as well as the fish.
Stripping ripe female spawners
This is usually done by two men working as a team. One man grasps the ripe female over a container while the other gently extrudes the eggs by applying gentle pressure on the abdomen from the anterior towards the posterior with the thumb and forefinger; the eggs will flow into the container. The characteristics or stage of eggs that are ready to be fertilized are enumerated below:
Eggs round and with distended surface (no wrinkles).
Yolk distributed in whole area of inner egg shell, no pervitelline space.
Eggs are separated and do not stick together in groups.
Eggs are transparent but when in groups, they appear to be light yellow or golden in colour.
Eggs float in water at 28–32 ppt salinity.
Diameter of the egg averages 0.8 mm.
No yolk vesicle.
Stripping the Male Spawner for Milt
For stripping the male spawner for milt, the same technique as with the female spawner is used. Good sperms must not be thick and stocky with plasma. When it is poured from a container, it should flow easily, and on examination under the microscope, the sperms can be seen to be moving rapidly.
Sometimes the mature male spawner is encountered before you can find suitable running ripe female. In this case, the milk can be placed in glass vials or test tubes, stoppered and kept in the ice box. By this method, the sperms remain viable up to five days.
Method of Fertilization
As soon as the female has been stripped, ripe male has to be stripped to the container with the eggs. The eggs and sperms are then gently mixed with chicken feather until well mixed. This is the dry method of egg fertilization.
After that, add some water and mix again. Then the egg have to be washed again repeatedly with clean saline water.
3. Care of Fertilized Egg, Embryo and Early Larvae
The fertilized eggs are put in vinyl plastic bags with some seawater of proper salinity (28–32 ppt) and transferred to the hatchery. Before putting the eggs into the hatching nursery tanks, the unfertilized eggs have to be taken out and the fertilized ones washed again. If necessary, the eggs are disinfected with acriflavine before being placed in the tanks.
If the hatchery is far from the spawning ground, aeration should be supplied to the bags of eggs and placed in low-temperature conditions. This will prevent development of egg which will appear to be damaged. At lower temperatures (about 27 degrees Celsius) hatching will occur in about 17 hours. At higher temperatures (30 degrees Celsius) the eggs hatch earlier or in about 12 hours. After hatching, care of the young larvae starts.
B. NATURAL SPAWNING OF SEABASS UNDER CONTROLLED ENVIRONMENT
Artificial spawning of seabass was first discovered in 1971. Broodfish were caught from natural open water during the spawning season and in 1973, 200,000 fingerlings were produced and distributed to the fish farmers. However, commercial culture as practised at that time was of lower intensity. This was due to the unpredictable natural supply of broodfish.
Natural spawning of seabass in controlled environment was first successful in 1973. By this method, much greater number of seedfish supply can be produced for commercial seabass culture. This operation has been conducted since that time.
2. Source, Collection and Selection of Spawners
Parent fish are raised from the juvenile stage in cages. These can come from fry originally collected from natural waters or from larvae and fry spawned in the hatchery. There is not much selection at this stage except for the regular routine grading of the growing fry. When this stock attain the juvenile stage, the healthy ones that grow very fast are usually separated to be reared as future broodstock. Second selection is done after the first year and on the second year for the healthy and fast growing individuals. All these stages are still being reared in netcages in the sea.
After the second year, some of the male individuals can be identified as males when they would extrude milt with pressure on the belly. However the females and other males cannot still be separated at this stage.
If during the elimination process for slow-growing individuals not enough stock is available, the deficiency is usually filled by selective purchase from private fishcage owners. Be sure that fish of the same age group are put together. The reared broodfish would be ready to start spawning at the end of their third year when they attain about 3.5 kg body weight. The best male milt is furnished by male 2–4 years old and female fish are not used before their third or fourth year. About one month before the spawning season, the parent fish are moved from cages into the spawning tanks.
Round tanks, 10 m in diameter and 2 m in depth are used as spawning tanks at NICA. Approximately 24 spawners are kept in each tanks at a male-female ratio of 1:1.
3. Care and Maturation of Spawners in the Hatchery
Water supply in the spawning tanks is seawater with average salinity of 30 ppt. Every other day 80–100 percent of the total volume of water is drained for new clean seawater. Fish, sardine or anchovy, with intestines and head removed are used for feeding the spawners by chopping these food into bite-sized pieces. Spawners are fed once a day in the morning using food equivalent to 1 percent of their body weight. The excess food that accumulates at the bottom of the tank should be removed siphoned out.
Generally, during spawning season, the female will appear with their big abdominal portion and swim awkwardly. Approximately 1 to 2 weeks before spawning, the female fish separate from the school and their feeding activity decreases. The male fish still keeps on eating normally, schools and swims actively,
4. Method of Spawning in the Hatchery
Natural spawning in controlled tanks takes place at the same time as natural spawning in open waters. It starts from the beginning of April to the end of September. Spawning activity always occurs between 1900–2300 hours on the first to the eight day of a full moon. As the female approaches full maturity there is an increase in spawning play activity. The ripe male and female swim together often turning laterally when swimming and then spawn. The fertilized eggs then float at the surface while the unfertilized eggs sink down to the bottom. The floating fertilized eggs are scooped out of the tank and placed in hatching/nursery tanks. The unfertilized eggs and other dirt at the bottom of the tank are cleaned out by siphoning.
5. Procedure Followed After Spawning
The spawners are free in their tank habitat while they are spawning. After the fertilized eggs are scooped out, the spawning tank should be cleaned and a volume of new seawater is supplied in preparation for repeated spawning activity in the future. It is also possible that different fish of the confine spawners in each tank spawn in different days within a spawning period,
As mentioned earlier, the fertilized eggs are placed in a separate tank for embryonic development and hatching. To safeguard proper development of the eggs, the salinity should be maintained at levels of 25–32 ppt. Salinities above or below this range can result in weak larvae. The water temperature of the big tanks is usually stable and does not fluctuate much generally at an average of 28 degrees Celsius. Radical temperature fluctuations which may occur in smaller tanks can harm the developing eggs and early larvae.
C. EXPERIENCES IN INDUCED SPAWNING OF SEABASS
1. Methods of Inducing Seabass to Spawn
At present, spawning of seabass is very popular and it has been done among the fisheries stations located along the coast of Thailand. Two methods to induce seabass to spawn have been done in hatcheries:
Natural spawning in concrete tanks.
Spawning by stripping running ripe fish from the spawning ground.
Natural spawning in concrete tanks
Spawning without hormone injection. Two hatcheries, Songkhla and Satul, are successful in spawning of seabass in concrete tanks without hormone injection. This is probably because these fisheries stations are located near the spawning grounds and the spawners are kept in the netcage before putting them in the concrete tanks during the spawning season.
Spawning with hormone injection. Many seabass hatcheries in Rayong, Chonburi, Phuket and Prachuap Kirikhan have to use hormone injection for seabass spawning in concrete tanks. Even stock previously obtained from Songkhla reared in Rayong failed, for unknown reasons, to spawn naturally, but with hormone injection, they were able to spawn. These hatcheries mentioned appear to have different environment for the rearing of the spawners. Although on the spawning trials, the female spawners have ripe eggs, they failed to spawn in the concrete tanks. Thereafter, use of hormone injection of one to two doses to the seabass spawner were done and the spawners easily spawn in concrete tanks.
Spawning by Stripping
For this method, the spawners have to be collected from the spawning grounds. For the male spawner, there is usually no need to use hormone injection. In the case of the female spawner, the eggs have to be checked before stripping. If the eggs are good or in the running stage, there would be no need to use hormone injection. But if the eggs are not in running stage hormone injection has to be done on the female spawner. The dose of hormone is rather high compared with female spawner reared in concrete tanks.
2. Natural Spawning in Concrete Tanks
Some Factors that may Affect Spawning
Food. Good quality and suitable amount for spawners should be given. Only about one percent of body weight using trash fish is fed to spawners. Overfeeding can result in failure to spawn.
Water quality. Good fresh running water supply with adequate dissolved oxygen should be used, or water with not less than 6 ppm DO, and pH ranges of 7.5 to 8.5.
Salinity. Salinity in the concrete tanks should be 28–32 ppt and maintained at this range.
Clean tanks. Spawning concrete tanks should be clean.
Stress. Disturbances to spawners during spawning season in concrete tanks are not conducive for the spawning of the seabass spawners.
Water management. Usually changing of water or running water system have to be done in the spawning tanks.
Uniform size of spawners. The size of male and female spawners should not be very different,
Age. Age of spawners should also be suitable so that spawners can spawn readily.
Sex ratio. Sex ratio in spawning tank is usually 1:1 but the male spawners are often very much less in weight and their number should sometimes be increased.
Types and Doses in Using Hormone for Injection
Three types of hormones are used for injection to induce spawning:
The most popular hormone that is used is the HCG hormone. The dosage in using hormone for injection of seabass spawner in concrete tanks ranges from 50–100 IU/kg of body weight. For Puberogen, 200 IU/kg is used.
3. Spawning by Stripping
The hormone that is used for injection is HCG, the same as that used in concrete tanks. But the dosage is higher, ranging between 50–1,000 IU/kg of body weight. The interval of injection is about 12 hours. Three spots on the body where hormone is usually injected are: a) behind the pectoral fin; b) on the lateral side; and c) between the first and dorsal fins. The best spot for scaled fish such as the seabass is behind the pectoral fins.
4. Some Results After Injection
In the experiments on induced spawning of seabass by hormone injection carried out in 1974, the results are shown below:
|11.00||-||-||Dead, some parts showed ripe eggs. Sperm added and 10% fertilized.|
|20.30||-||-||Killed, found some ripe eggs.|
|3||5.0||09.45||3000 +||2 doses|
|21.45||3500 +||4 doses|
|stripped||-||All running ripe eggs. 70% fertilized eggs. Hatched out 80%.|
The foregoing experiments show some of the experiences on induced spawning, by hormone, of seabass at NICA. It should be noted that after spawning by stripping running ripe spawners, and natural spawning in tanks were achieved at the Institute, these latter methods were found much more convenient with the facilities at hand.