The fry rearing system of the Sg. Merbok hatchery is designed for rearing two-week-old seabass. The tank is rectangular with a 3 × 9 m flat bottom and 1 m depth (Fig.4).
Rearing the fry in confinement is subject to adverse effects of overcrowding and ecological problems inherent in the culture system. As environmental parameters fluctuate and other factors extend its adaptive response, the fry attempts to maintain or re-establish its normal physiological balance. If this process is within the adaptive range the chance of survival of the fry is high. The survival rates of seabass fry 1–30 days old are determined by salinity, stocking density, feed and feeding, nursing tank management and grading.
Salinity will affect the survival rate of seabass fry. Maneewongsa and Tattanon (1982b) showed that in nursing seabass fry aged 1–30 days, salinity of 20 ppt gave the highest survival rate (Table 5).
Stocking density in the nursery tank also varies according to the age of fry (Table 6). During the first week 40 000–50 000 fry/m3 of water can be reared. Stocking density should be reduced gradually to 15 000–20 000 fry/m3 in the second week, 5 000–10 000 fry/m3 in the third week and 2 000–5 000 fry/m3 in the fourth week.
It is important to give the right kind of food to nursing seabass fry. Each stage of larvae needs specific food of specific size. The food given to the first stage of the larvae are Chlorella and rotifer. The rotifer is given at a density ranging between 10 and 30 pieces per millilitre. Both Chlorella and rotifer are given when the larvae are 3 days old until the 14th day. Artemia is given when the fry reach the 8th day until the 20th day. The brine shrimp nauplii are fed at a density of 5 pc/ml twice a day. Daphnia or Moina is given at the 16th day until the 30th day at a density of 1–2 pc/ml 8 times a day. Acetes is also good for the fry after the 21st day. It can be given 8 times a day. Types and relative amounts of food given at various stages as shown in Table 7 are only given as a guideline. The actual amount of food required should be adjusted by the observing, eating and swimming behaviour of the fry. If the fry are swimming fast around the tank it means that more food is required. Alternatively, if food remains, the amount of food given should be reduced.
Since seabass is a carnivorous fish the fry prefer live food to inert food. After 21 days minced fish can be given to reduce the operational cost. The fish might not accept the minced fish at first but gradually accepts it once it is accustomed.
The nursery tank should be cleaned every day. The rate of water replacement in the nursing tanks depends on the feeding period of each age stage. In the period of rotifer feeding only 10–20 percent of water is replaced each day. During the Artemia feeding period approximately 50 percent water is changed while an almost complete change is made during the trash fish feeding period (Fig. 5).
First grading should be started after the fry attain 10–14 days. Second grading can be started with 20-day-old fry to separate the fast-growing fry from the slow-growing ones. To reduce unnecessary injury and stress to the fish fry, and to speed up the operation, a series of fish graders with different pore sizes can be used (Fig. 5). The pore size of graders to retain the different sizes of fish is given in Table 8.
