Effect of size heterogeneity of Nile tilapia (Oreochromis niloticus) on the optimal harvest time: a bioeconomics approach

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). Two seeding strategies were evaluated: homogeneous seeding (HIM) with a 96.55 +/- 24.51 g initial body weight and heterogeneous seeding (HT) with a 100.17 +/- 5.91 g initial weight. Fish from both treatments were stocked at 40 fish m(-3) in triplicate using a randomized design. The calculated quasi-profits of variable costs showed an inversely proportional relationship with the minimum market size in both groups. The smaller size dispersion in HM generated higher profits than HT. The OHT for Mms [350, 500 g] of HM population was 180 days, with a mode of 641 g. The OHT was also 180 days for the HT population but only for the Mms [350, 400 g] and a mode of 578 g. In terms of quasi-profits, the HM produced 19.93% more quasi-profit than the HT in the market size of 350 g at 180 days (HM = 0.50 USD kg(-1); HT = 0.44 USD kg(-1)). The simple bioeconomic model presented here can help producers manage a series of economic decisions associated with OHT, when targeting different market segments requiring different Mms.