As commercial grow-out is still in the early stages, there is only limited information available. However, early trials have shown that turbot young can be grown to market size with high levels of survival. Marketable-size fish can be raised in 12–14 months with proper feeding and environmental control, especially of temperature. The optimal temperature appears to be 15 to 19°C. Though heated water may have to be used in growing tanks, the quantity of water required is not large as turbot are reported to require tanks with a surface area only equivalent to their own.
In tanks fed with cooling water from a nuclear power plant in North Wales, UK, a stocking density of 25–55 kg/m3 of 10–1500 g fish has been maintained, but this required continuous re-oxygenation using special equipment (Jones, 1981). Several foodstuffs and compound diets have been used for on-growing.
Chopped or whole trash fish and industrial fish are well accepted by turbot, but it is believed that when such feed is used for a long period the survival rates are adversely affected. So moist compound pellets are recommended. However, fish like mackerel, sprat or mysids stimulate the appetite of turbot and so incorporation of these fish in a ground form to the extent of 20–25 per cent dry weight (as suggested for juvenile growing) has been recommended to improve feeding and growth rates, especially at lower temperatures.
Experimental grow-out of young turbot collected from the wild and raised in hatcheries in floating sea cages of 2.9 m3 size in Scotland (Hull and Edwards, 1979) has shown that they can be grown successfully in cages, with very high survival rates. To obtain high growth rates, the juveniles had to be reared in land-based nurseries over the first winter period, to a size above 50 g, and then transferred to the cages. The maximum density tried was 41 kg/m3 or 240 fish/m3 and this did not depress growth. Moist pellet preparations with a protein content of about 36–39 per cent were used as feed. In the ambient temperature ranging from 6 to 16°C, a market size of 450–500 g was reached in 18 months of on-growing and after another 12 months a weight of 1.2 kg could be attained.
Vibriosis is the most common bacterial disease recorded for young turbot and this can cause high levels of mortality. The antibiotic oxytetracyclin, administered through feed at a concentration of 75 mg/kg body weight per day and also as a bath at 53 mg/ has been found to be effective. In addition, infestation by Trichodina spp. has been observed. This is controlled by one-hour baths of formalin at a concentration of 1 : 6000. In rearing facilities using power-station cooling water, gas bubble disease can occur, but is controlled by changing to ambient sea water.