MODELING OF DAILY WEIGHT GAIN, FEED CONVERSION RATIO, NITROGEN RETENTION AND DIGESTIBLE ENERGY AND PROTEIN REQUIREMENTS OF GROW-OUT CAGE-FARMED NILE TILAPIA FED HIGH DENSITY DIET  

Nile tilapia is one of the most important farmed fish species in Brazil. In recent years, tilapia culture has been expanded from earth pond to intensive culture in net cages. High density diets have been proposed to grow-out Nile tilapia to enhance weight gain, feed efficiency of cage-farmed Nile tilapia. Digestible energy (DE) and digestible protein (DP) have been proposed to elaborate diets based on precision nutrition concept for Nile tilapia. The present work was designed to elaborate modeling of growth performance and dietary DE and DE requirements of grow-out cage-farmed Nile tilapia using linear or second-order regression analysis.

Nile tilapia (n=6,000; 29.9 ± 4.9 g initial weight) were distributed into six hexagonal cages of 12 m³ each. The fish were hand-fed to extruded diet containing 320.2 g kg^-1 of DP, 15.6 MJ kg^-1 of DE (dry matter basis). Fish were hand-fed fed at 3.5 to 2.5% of fish biomass, twice daily, seven days per week, during 100 days. Dietary digestible energy and digestible protein were determined by in vivo digestibility assay. Twenty fish per cage were weighed at beginning and every 20 days to access the growth performance and adjust the feeding rate. The survival of fish was calculated from daily mortality and recorded in each cage. The relationship between body weight and daily weight gain, feed conversion ratio and nitrogen retention and the relationship between daily weight gain and digestible energy intake and digestible protein intake were determined by linear (Y = a + bx) or second-order (Y = a + bx + cx²) regression analysis. The relationships between body weight and daily weight gain of cage-farmed Nile tilapia was best expressed by second-order regression analysis. However the relationship between daily weight gain, feed conversion ratio and nitrogen retention were best expressed using linear regression analysis (Table 1). Coefficients of determination (R²) ranged from 0.753 to 0.859.

Nitrogen retention decreased linearly according to increases in body weight of grow-out Nile tilapia fed high-density diet. The relationship between daily weight gain and DE intake (Y= 7.1045x - 11.422, R² = 0.892) and DP intake (Y = 0.6098x - 0.9804, R² = 0.8924) were best fit by linear regression analysis. Digestible energy and digestible protein requirements increase linearly according to increases in daily weight gain of cage-farmed Nile tilapia.