CHAPTER IV
BROODSTOCK REARING OF SEABASS

T. Tattanong and S. Maneewongsa*

A. COLLECTION AND SELECTION OF SEABASS SPAWNERS

1. Introduction

Before controlled spawning of seabass was discovered we tried to produce fertilized eggs by using spawners from natural waters. We had to study and investigate about spawning seasons, spawning grounds, and spawning time of seabass in the natural grounds. If we can get all of these information clearly, then success in seabass growing can be assured, This will be of prime importance in promoting the seabass culture industry.

Fortunately, this has been successful in Thailand with test trials started in 1971. Success in producing consequential number of seabass fry (200,000) was first achieved in 1973. This triggered and encouraged the establishment of hatchery facilities.

2. Conditions of the Spawning Ground of Seabass on Songkhla Lake

The spawning ground of seabass along Songkhla Lake is at the mouth of the lake. Spawning begins from April and lasts till October or before the rainy season. Salinity during the spawning season is high at about 28 to 32 ppt and water temperature ranges from 28 to 34 degrees Celsius. The spawning period depends on the phase of the moon, that is, it usually occurs from full moon to six days after full moon during low tide at 1800 to 2100 hours or in the evening hours.

3. Gears Used to Collect Spawners

Along the mouth of Songkhla Lake, gears used to catch mature seabass are enumerated below:

1. Bottom gillnets

   1. Movable bottom gillnet

   2. Fixed-bottom gillnet

2. Hook and Line

   1. Single hook and line

   2. Longline

3. Cast nets, traps

* The authors are Senior Fishery Biologists, NICA.

4. Selection of Ripe Spawner

The wild spawners obtained from the natural spawning grounds should be selected. They should be of suitable size for stripping eggs and milt; too big or too small spawners should be avoided as the survival rate of larvae and fry from such spawners seem to be comparatively lower.

Distinction of Male and Female Seabass Adult Fish

Male spawner is more slender and generally weighs less than the female spawner even with the same total length. Abdomen of the male is not bulging out like the female spawner and the depth of the body of male is less than that of the female. The scales around the anal area of male becomes thicker than the female. The female spawner has large quantities of eggs causing the abdomen to bulge out. With slight pressure by hand, the eggs will flow out. If the male is pressed on the abdomen, the milt will come out from the urogenital opening.

In general, male spawners for stripping range from 2 to 10 kg in body weight. The milt obtained from the male spawner should not be sticky and strongly concentrated, If one male spawner yields milt less than 2 ml in volume, this may not be very satisfactory. The male spawners that can give milt between 5 to 15 ml by stripping would be more suitable.

Female spawners more than 3 kg in weight can yield 1.0 to 1.5 million eggs per stripping.

In the case of broodstock raised in nursing nets and later in grow-out cages, before they are brought into the tanks for natural spawning, careful selection is made for both the female and male spawners. The characteristics of good female and male spawners are the following:

1. Active
2. Strong
3. Complete, no missing parts of the body
4. Disease-free
5. No wounds or loss of body parts
6. On examination, have good sperms or eggs
7. Fins normal, without abnormalities

7. Age of male and female spawner about the same

B. CARE AND MAINTENANCE OF SEABASS BROODSTOCK

1. Introduction

The broodstock can be obtained from two sources: 1) broodstock collected from the spawning ground during the spawning; and 2) broodfish obtained from grow-out cultures especially from netcages.

Broodfish collected from the spawning grounds are used only for artificial fertilization method by stripping. The fish taken from grow—out cages are usually used for spawning in captivity.

At present, seabass fry are in demand among the fish farmers in Thailand and also in some other countries such as Malaysia, Singapore and Taiwan. The production of seabass seedling should not be conducted without enough broodfish that are properly maintained.

2. Care of Broodstock During their Earlier Age Stages

Broodfish are raised from juvenile stages in cages. The healthy and fast growing fish with size 1.5 to 2.0 cm in total length are selected out from the rearing tank to be raised in netcages of 1 × 2 × 1 m in natural open water for use as future spawners.

About one month later, they become 3 to 3.5 cm in length and again approximately 50 percent of them are selected from those that appear healthy and fast growing to be kept for culture for up to two years. After the first year rearing period when the fish are within their third or fourth month, they are transferred from cage of 1 × 2 × 1 m to a bigger cage of 5 × 5 × 2 m until they are mature. After two years, another selection of 50 percent of this stock is made and the remaining stock are then raised up to three years. At this time, the fish will have attained 3.5 to 4 kg in their body weight and are ready to mature. The gradual reduction in number of selected broodstock is shown in Table 1.

During this period, the fish are fed approximately 5 to 10 percent of their body weight daily.

3. Care of Maturing and Mature Broodstock

One month before the spawning season (April-September), broodfish are transferred from the cagenet to the spawning tank. These are round-concrete tanks with 10 m diameter and 2 m depth, used as spawning tanks. Twenty-four fish are kept in each tank with female to sex ration of 1.1. The spawners are fed with sardine after the head portion and the intestines are removed. The feeding rate is approximately 1 percent of their body weight daily.

The water in the spawning tank should be in good quality, utilizing the running water system. Everyday, the total amount of water changed by this system is about 30 percent of the volume of water in the tank. Every few days, the tank should be cleaned and 80 percent of the water in the tank changed for a new fresh seawater, Enough aeration is always supplied in the spawning tank.

The fish spawn in the tanks without any inducement. After spawning, the fertilized eggs are scooped out from the spawning tank to the hatching-rearing tanks. Since the spawners are kept in the tank for a long time, they are sometimes damaged by bacteria that cause finrot disease. If this occurs, they should be treated with 1 ppm KMn04 solution for 10–15 minutes and anitbiotics such as tetracycline hydrochloride at 50–60 milligrams per day for 3–4 days continually.

The age of the fish which one can continue to use as spawners each year is under study. At NICA 6-year old female fish is still usable as good spawner. Each year, a number of seabass fingerlings should be selected for future spawners in order to replace the old one.

At the end of the spawning season, the broodfish are moved back from the tanks to the netcages in natural open water.

4. Further Information Needed on Seabass Broodstock

Studies on genetic selection and the use of artificial diets are needed in order to improve the technique and production of seabass fry.