Growth, faecal production, nitrogenous excretion and energy budget of juvenile cobia (Rachycentron canadum) relative to feed type and ration level

Growth, faecal production, nitrogenous excretion and energy budget of juvenile cobia (initial body weight ∼ 28 g) at different ration levels (RL, % per day) ranging from starvation to ad libitum for three feed types, i.e., natural sardine fish (NSF), commercial eel formulated feed (CEFF) and commercial marine-fish formulated feed (CMFF), were investigated in this study. Both feed type and ration level affected significantly faecal production (f, mg g− 1 day− 1), and nitrogenous excretion (u, mg g− 1 day− 1). Feed type, not ration level, for the NSF-fed and CEFF-fed groups and both feed type and ration level for the CMFF-fed group affected significantly feed absorption efficiency (FAE, %). For each feed type group, faecal production and nitrogenous excretion increased whereas feed absorption efficiency in dry weight (FAEd, %), protein (FAEp, %) and energy (FAEe, %), with a small variation, tended to decrease as ration increased. Specific growth rate in wet weight (SGRw, % per day), dry weight (SGRd, % per day), protein (SGRp, % per day) and energy (SGRe, % per day) for the NSF-fed and CEFF-fed groups was much higher than that for the CMFF-fed group, and the growth–ration relationship was a decelerating curve described as a logarithmic equation for the NSF-fed and CEFF-fed group, and a linearity described as a simple equation for the CMFF-fed group. There was a significant difference of the slopes in the regression equations among three feed type groups by analysis of covariance. Multiple regression analysis showed that the relationships between specific growth rate (SGR, % per day) and ration level in dry weight (RLd, % per day) as well as feed type (D1 D2) were SGRw = − 2.226 + 4.022ln(RLd + 1) − 0.895D1 − 2.705D2, SGRd = − 2.686 + 4.422ln(RLd + 1) − 1.014D1 − 2.969D2, SGRp = − 2.481 + 4.316ln(RLd + 1) − 1.122D1 − 2.943D2 and SGRe = − 3.239 + 4.972ln(RLd + 1) − 0.954D1 − 3.053D2. Feed conversion efficiency in wet weight (FCEw, %), dry weight (FCEd, %), protein (FCEp, %) and energy (FCEe, %) for the NSF-fed and CEFF-fed groups was much higher than that for the CMFF-fed group, and with increased ration FCE increased or first increased then decreased for the NSF-fed and CEFF-fed groups and increased significantly for the CMFF-fed group. Energy budgets of juvenile cobia at satiation ration were  100A = 59R + 41G for NSF-fed group, 100A = 67R + 33G for CEFF-fed group and 100A = 83R + 17G for CMFF-fed group, where A is assimilated energy R is energy spent in metabolism and G is energy stored as growth.