Effects of ration and temperature on growth, fecal production, nitrogenous excretion and energy budget of juvenile cobia (Rachycentron canadum)

A 4 × 3 factorial experiment was conducted for two weeks to determine the effects of ration level ranging from starvation to satiation and water temperature at 21, 27 and 33 °C on growth, fecal production, nitrogenous excretion and energy budget of 10-g-size cobia in this study. Over the temperature range, 21–33 °C, maximal ration (Rmax, % per day), optimal ration (Ropt, % per day) and maintenance ration (Rmaint, % per day) all increased with temperature (T, °C), described as a quadratic equation Rmax = − 0.046T2 + 2.906T − 35.97 (R2 = 0.989), a simple equation Ropt = − 0.533T − 8.001 (R2 = 0.993), and a quadratic equation Rmaint = 0.028T2 − 1.350T − 17.18 (R2 = 1), respectively. Both fecal production (f, mg g− 1 d− 1) and nitrogenous excretion (u, mg g− 1 d− 1) were affected significantly by ration and temperature and increased as ration and temperature increased. Feed absorption efficiency (FAE, %) varied small over the whole ration and temperature ranges though the effects of ration and temperature were significant in some data. Juvenile cobia grew fastest at 33 °C when fed at satiation but the growth rate was equal or better at 27 °C when food was restricted, whereas the fish showed overall significant lower growth rates at 21 °C except for the starved treatment. Among three temperatures specific growth rate in wet weight (SGRw, % per day), dry weight (SGRd, % per day), protein (SGRp, % per day) and energy (SGRe, % per day) all increased with ration, showing decelerating growth–ration relationships described as logarithmical equations at 27 and 21 °C and a linear growth–ration relationship described as a simple equation at 33 °C. Apart from starvation ration with a negative linear growth–temperature relationship growth all increased with temperature, described as quadric functions. Two-way ANOVA showed that ration and temperature had an interaction on growth. By using multiple regression analysis the relationships between specific growth rate (SGR, % per day) and ration level (RL, % per day) as well as temperature (T) took the forms: SGRw = − 11.97 + 1.23ln(RL + 1) + 0.91T − 0.02T2 + 0.16Tln(RL + 1) (R2 = 0.962), SGRd = − 17.04 + 0.72ln(RL + 1) + 1.11T − 0.02T2 + 0.12Tln(RL + 1) (R2 = 0.968), SGRp = − 18.25 + 0.20ln(RL + 1) + 1.28T − 0.03T2 + 0.15Tln(RL + 1) (R2 = 0.972) and SGRe = − 20.83 + 0.85ln(RL + 1) + 1.40T − 0.03T2 + 0.15Tln(RL + 1) (R2 = 0.969). Feed conversion efficiency in wet weight (FCEw, %), dry weight (FCEd, %), protein (FCEp, %) and energy (FCEe, %) at 27 and 33 °C was much higher than that at 21 °C, and the maximal FCE occurred at sub-satiation (i..e. feeding group 3) and 27 °C. All the relationships between FCE and temperature were described as quadric equations. Energy budgets of juvenile cobia at satiation ration were: 100C = 7.0F + 7.7U + 69.0R + 16.4G (or 100A = 81R + 19G) at 33 °C, 100C = 6.8F + 7.9U + 68.0R + 17.3G (or 100A = 80R + 20G) at 27 °C and 100C = 6.3F + 8.4U + 77.2R + 8.2G (or 100A = 90R + 10G) at 21 °C, where C is food energy, A is assimilated energy, F is feces energy, U is excretion energy, R is metabolism energy and G is growth energy.