Metabolic responses to dietary protein/carbohydrate ratios in zebra sea bream (Diplodus cervinus, Lowe, 1838) juveniles.

This study aims to evaluate the effect of diets with different protein to carbohydrate ratios (P:C) on the omnivorous zebra sea bream (Diplodus cervinus) juveniles growth performance, feed efficiency, N excretion and metabolic response of intermediary metabolism enzymes. Four isoenergetic and isolipidic diets were formulated to contain increasing protein levels (25, 35, 45 and 55%) at the expense of carbohydrates (43, 32, 21 and 9%): diets P25C43, P35C32, P45C21 and P55C9. Growth performance, feed efficiency (FE), N intake [(g kg(-1) average body weight (ABW) day(-1))], N retention (g kg(-1) ABW day(-1)) and energy retention (kJ kg(-1) ABW day(-1)) increased with the increase of P:C ratio. The best growth performance and FE were achieved with diet P45C21. Ammonia excretion (mg NH4–N kg(-1) ABW day(-1)) increased as dietary protein level increased. Alanine aminotransferase and glutamate dehydrogenase activities increased with the increase of dietary P:C ratio. The opposite was observed for malic enzyme activity. Aspartate aminotransferase, hexokinase, glucokinase, fructose-1, 6-bisphosphatase and fatty acid synthetase activities were unaffected by dietary treatments. Response of key amino acid catabolic enzymes and N excretion levels to dietary P:C ratio supports the metabolic adaptability of this species to dietary protein inclusion levels. Overall, zebra sea bream seems capable of better utilize dietary protein rather than dietary carbohydrates as energy source which may be an obstacle for using more economically diets and thus for reducing environmental N loads in semi-intensive aquaculture of this species.