A critical problem in the production of Nile tilapia Oreochromis niloticus in intensive and hyperintensive systems is the heterogeneity of body sizes as it influences the final production and economic yield. The objective of this study was to calculate the bioeconomic effect of size heterogeneity on the production of Nile tilapia at a commercial level and to determine the optimum harvest time (OHT) considering four minimum marketable sizes target (MMS = 350, 400, 450, 500 g).
A trial of 330 days was conducted in a hatchery of a commercial farm, Nile tilapia fingerlings were reared at the same stocking density (40 ind m-3) with different fish sizes: homogeneous size (HM) and heterogeneous size (HT). The initial mean weight ± standard deviation (SD) of the organisms was HM = 100.17 ± 5.91g and HT = 96.55 ± 24.58 g, with three replicates for treatment. The initial variance of HM and HT was σ² 34.91 and σ² 604.19, respectively.
The optimal harvest time (OHT) corresponds to the time at which the quasi-profit of the variable costs (understood as the profits obtained after paying for the variable costs of production) are maximized. The large variation in the size observed in the tilapia of the same cohort illustrates a high propensity of the organisms to develop and grow at different rates. The biometric data of discrete 15-day periods, for the 330 day study period was used for the bioeconomic analysis.
The seeding of homogenous organisms is highly recommended to the farmers, because the heterogeneity at seeding negatively affects the system performance by lowering biomass, in addition to reducing the quasi-profits of the variable costs.