Dietary protein requirement and protein-nitrogen flux in the greenback flounder, Rhombosolea tapirina Gunther 1862

The overall objectives of this study were to estimate protein requirement of and to study nitrogen flux in juvenile greenback flounder, Rhombosolea tapirina Gunther 1862. Six isoenergetic diets (- 17.5MJ Kg\\(^{-1}\\)) containing graded levels of protein ranging from 30-55% (19.24 - 34.70 gDP MJ\\(^{-1}\\) DE) were fed to juvenile (0.45 g) greenback flounder with the aim of estimating its dietary protein requirement. Food consumption decreased with increasing levels of protein in the diet while protein intake increased. Fish feeding on the 45% and 50% protein diets had the highest percent increases in body weight (%BWI), specific growth rates (SGR), productive protein values(PPV) and protein retention efficiencies (PRE). A second degree polynomial (quadratic) model of the form y = a + bx + cx\\(^2\\) was fitted to dose - response data (SGR, % BWI, PPV and PRE). The dietary protein concentration required for optimal growth ranged between 44-48% and the optimal digestible protein to digestible energy ratio was estimated to be 28.14-30.41 g DP MJ\\(^{-1}\\) DE. The daily protein requirement at the maximum specific growth rate of 2.01 % day' was calculated to be 5.84 g kg\\(^{-1}\\) day\\(^{-1}\\). Endogenous and exogenous nitrogen excretion rates of juvenile( 12g) greenback flounder were estimated at 16°C and nitrogen budgets constructed. Fish were fasted for 48 hours to determine endogenous rates and fish were fed (on a diet containing approx. 46% protein) 1% body weight once a day (Treatment A), 0.5% body weight once a day (Treatment B) and 0.5% body weight twice a day (Treatment C) to determine the effect of ration size and feeding frequency on nitrogen excretion rates. The mean endogenous excretion rate of total nitrogen(TN) was found to be 23.36 mg N kg\\(^{-1}\\) day\\(^{-1}\\) with total ammonia nitrogen (TAN) and urea nitrogen (UN) constituting 82.5 and 10.4% of total nitrogen respectively. The mean exogenous total excretion rates were 402.52 mg N kg\\(^{-1}\\) day\\(^{-1}\\), 236.60 mg N kg\\(^{-1}\\) day\\(^{-1}\\) and 398.34 mg N kg\\(^{-1}\\) day\\(^{-1}\\) for treatments A, B and C respectively. TAN and UN accounted for 83-85% and 8-9% of total nitrogen excreted in all treatments. Fish from treatment B excreted 64.48% and retained 30.52% of nitrogen consumed, while fish from treatments A and C excreted 54% and retained 40% of consumed nitrogen. Excretion rates, retention rates and percentages of consumed nitrogen excreted and retained were significantly (p<0.05) affected by ration size but not by frequency of feeding.