Controlled spawning and hatchery production of juveniles
Two- to three-year-old female turbot (of 2 kg weight) have been observed to reach sexual maturity and spawn in the wild from May to August. Males reach maturity in the second year (1 kg body weight). The annual fecundity is about a million eggs per kg body weight. The diameter of the eggs varies from 0.9 to 1.2 mm. Mature fish will spawn naturally in tanks. Circular indoor tanks up to 2.7 m3 in size, supplied with warm water and continuous high-intensity illumination (up to 3000 lux), have been successfully used for spawning and larval rearing. The size and colour of the tanks do not seem to have any significant effect. Eggs and milt have also been obtained by manual stripping and artificial fertilization has been achieved. Through proper management of brood stock, it is now possible to produce eggs all year round. Temperature is maintained between 10 and 15°C, and the photoperiod is adjusted to obtain spawning at any time of the year. The eggs are incubated at about 12°C in filtered sea water treated with antibiotics. Newly hatched larvae are reared at densities of 30–45 larvae per , in 60–450 tanks. Temperatures between 18 and 20°C are maintained with 90 per cent water exchange every day.
The comparatively small size of turbot hatchlings (3.11 mm length and 0.10–0.15 mg weight) makes it necessary to handle them with special care and to feed the right sized live foods to obtain reasonable survival rates. The rotifer Brachionus plicatilis and nauplii and metanauplii of the brine shrimp Artemia salina are the most commonly used larval foods. It has been suggested that the algae used to feed the rotifer affect the growth and survival of the larvae
(Howell, 1979). When fed on Isochrysis galbana rather than Dunaliella tertiolecta, the larvae grew better and mortality was lowered. From the first day of hatching to about the eighth day, Brachionus is the preferred food; Artemia nauplii are then added and the rotifer reduced, usually terminating by about the eleventh day. Naupliar feeding is continued until the larvae develop into metamorphosed juveniles at a size of 25–30 mm in about 30–40 days. From about the eighteenth day, larger metanauplii of Artemia are used.
The juveniles, weighing 55–105 mg, are weaned to artificial diets in less than two weeks. Dry pellet crumbs (400 mm size) give satisfactory results, but greater success has been achieved by the use of moist pellets, when survival rates have been increased to about 50 per cent. Experimental work (Person-Le Ruyet etal., 1983) seems to show that expanded pelletsenriched with inosine (a chemical attractant) increase food intake during the beginning of the weaning period, but the economics have yet to be determined. After weaning, dry pellets are normally used for feeding, and in about three months after hatching the fry attain a weight of around 2 g.
In areas where the temperature is much below the optimum range of 18–20°C, the fry are initially reared indoors, in heated water, and then transferred outside. If they are to be transferred during the summer months, they are grown indoors up to a weight of 5 g, but if they are to be transferred during winter, they are grown to at least 20 g size. Survival below 5–6°C is very low. In well-oxygenated water they can tolerate temperatures between 25 and 30°C, and salinities ranging from brackish to 40 ppt. For rearing up to 20 g size, dry pellet feed is commonly used, but for growth beyond that moist pellets (containing 25 per cent trash fish, as well as fish and meat meal, wheat middlings, brewer’s yeast, cod liver oil and a vitamin premix) are recommended. Moist pellets made from a mixture of industrial fish and dry compound meal have been used to feed 5 g fish at the rate of 2.6 per cent of body weight at 10°C and at 4.5 per cent at 15°C (Jones, 1981). Juveniles of 50 g weight are fed at 1.1 per cent of body weight at 10°C and at 2 per cent of body weight at 15°C. Dry pellets are not easily accepted by larger fish, but trash fish forms an excellent feed. After a period of one year’s growth under favourable conditions, turbot attain weights of 175–350 g in indoor tanks in areas with higher water temperatures, while a weight of about 120 g only may be reached in outdoor tanks.