Effects of salinity and nutritional status on growth and metabolism of Sparus sarba in a closed seawater system

A re-circulating seawater system employing biological filtration was set up to assess the growth potential of sea bream (Sparus sarba) under the combined effects of salinity (7, 15, 35 p.p.t.) and dietary protein level (30, 35, 40, 45, 50, 55%). The re-circulating system was able to sustain growth of sea bream at high stocking density (3 gl(-1)), possibly by minimising stress. Typical stress indices such as serum cortisol glucose and lactate levels were reduced in sea bream cultured within the recirculating system. Growth rates and protein efficiency ratios of sea bream cultured at 15 p.p.t. were consistently higher than those at other salinities. Growth enhancement at 15 p.p.t. was accompanied by a reduction in oxygen consumption, ammonia excretion rates,liver lipid levels and hepatic glucose-phosphatase activity. However, hepatic hexokinase activity was stimulated at 15 p.p.t. Most of these effects were more evident in fish fed high protein levels. These data suggest that growth enhancement at 15 p.p.t. may be explained by (1) reduction of metabolic cost of osmoregulation, and (2) reorganisation of metabolism, which would allow protein sparing in favour of a shift towards preferential utilisation of carbohydrate and lipid.