Influences of food particle size on growth, size heterogeneity, food intake and gastric evacuation in juvenile Nile tilapia, Oreochromis niloticus, L., 1758

This study aimed at determining the food particle sizes (diameter) that produce the fastest growth (S-opt) in juvenile Nile tilapia, and modeling the growth penalties for particles departing from S-opt. Three 14-day experiments were run with 3,7 and 11 g juveniles, which were raised in 120-L flow-through cylindro-conical tanks at 29 +/- 1 degrees C, fed at the optimum feeding level (R-opt) and weighed at weekly intervals. In each experiment, four different sizes of pellets (diameter) were evaluated (0.5-mm intervals, 1.0-3.5 mm; three replications per pellet size). At the end of each experiment, food intake and gastric evacuation rate were determined by radiographic methods following the use of ballotini labelled feed. The results indicate that i) in all experiments, specific growth rates (SGRs) are significantly depressed for the largest particles; ii) S-opt increases with increasing fish size (1.4 mm at 3 g, to 2.5 mm at 20 g); iii) the optimum particle size relative to mouth width (MW) varies only slightly with fish size (28 to 25% MW from 3 to 20 g); iv) growth heterogeneity is proportional to food particle size; and, v) food intake and gastric evacuation rates are inversely proportional to food particle size. A growth model (SGR) was constructed with a stepwise multiple-regression analysis against fish size, feeding level and food particle size (R-2 = 0.853, df = 71). For a particular fish size, the growth-to-particle size relationship is a log-log second order polynomial, which implies that the growth penalty is more severe when fish are fed small than large particles. By contrast, application of food particles slightly smaller than S-opt tends to minimize growth heterogeneity. As fish size increases, the width of the SGR-to-S relationship broadens; so, the penalty for distributing particles departing from S-opt is lower, and feeding strategies are facilitated