Effects of altering dietary protein content in morning and evening feedings on growth and ammonia excretion of red drum (Sciaenops ocellatus)
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Two consecutive 8-week growth trials were undertaken to compare effects of different feeding regimes in which dietary protein content was varied on a daily or weekly basis on growth performance, protein utilization and nitrogenous waste production of red drum (Sciaenops ocellatus). Three isoenergetic practical diets were formulated to contain 30% (P30), 35% (P35) or 40% (P40) crude protein (CP) levels (dry-matter basis) and fed to juvenile fish in these two experiments. In the first trial (trial 1), the P40 diet together with the P35 or P30 diet were alternatively fed in the morning (a.m.) and evening (p.m.) to form five different experimental treatments (amP40/pmP40, amP35/pmP40, amP40/pmP35, amP30/pmP40, amP40/pmP30). In trial 2, the amP40/pmP40, amP35/pmP40, amP40/pmP35 treatments of trial 1 were repeated, and two new treatments were added by alternating the P40 diet daily (DP40/DP35) or weekly (WP40/WP35) with the P35 diet. Juvenile red drum (average initial weight of 2.98. g/fish in trial 1 or 3.44. g/fish in trial 2) were stocked as groups of 15 fish in 110-L aquaria containing brackish water (7. g/L) and operated as a recirculating system. Fish in triplicate aquaria were fed each dietary treatment at a rate approaching apparent satiation for 8. weeks. Nitrogenous waste was evaluated by monitoring water ammonia concentration (g/kg body weight) 6-h postprandial at the end of the first growth trial.Replacing the P40 diet with the P30 diet in the morning (a.m.) or evening (p.m.) feeding significantly (p<0.05) reduced fish growth performance; whereas, weight gain of red drum fed the P40 diet both at a.m. and at p.m. showed no significant (p0.05) differences compared to fish fed the P35 diet at a.m. and the P40 diet at p.m. or fish fed the P40 diet at a.m. and the P35 diet at p.m., or fish fed the P40 diet and P35 diet alternating on a daily or weekly basis. Indices of body condition as well as protein intake, protein efficiency ratio, protein productive value and muscle ratio did not vary remarkably among experimental treatments. Plasma thyroid hormones triiodothyronine (T3) and tetraiodothyronine (T4) as well as ammonia concentrations also did not show significant (p0.05) differences among experimental treatments; however, water ammonia production 6-h post a.m. or p.m. feeding from fish fed the P40 diet were significantly (p<0.05) higher than those of fish fed the P35 or P30 diets. 2014 Elsevier B.V.
