Simulation of optimal harvesting strategies for small-scale mixed-sex tilapia (Oreochromis shiranus Boulenger 1896) ponds using a bio-economic model

Abstract

A cohort-based bio-economic biomass growth and economic model, validated with data from experiments conducted in Malawi, was used to identify an optimal harvesting strategy for mixed-sex tilapia ponds. Three harvesting scenarios (baseline, economic optimum time +10 days and economic optimum time) were used. In each harvesting scenario four options were explored: (i) no further harvest, harvest every (ii) 60 days, (iii) 90 days and (iv) 120 days after initial harvest. The lowest simulated yield (487 kg ha−1 year−1) was obtained when no partial harvesting was carried out and fish were harvested after 365 days. Maximum yield (4416 kg ha−1 year−1) was obtained when partial harvests were carried out every 90 days starting with a first harvest of fish weighing 60 g or more at day 90. Maximum financial returns (US$2561 ha−1 year−1) were obtained when partial harvests were carried out every 120 days starting with the first harvest at day 90 and removing all fish ≥60 g. The model simulations indicate that mixed-sex tilapia culture may be profitable for tilapia farmers in Africa where markets accept small (60–150 g)-sized fish. The study further shows that a cohort-based population growth model can be reliably incorporated in tilapia production models to simulate fish yields in mixed-sex tilapia production systems. However, incorporation of intergenerational competition effects could improve the model's utility as a decision support tool for managing mixed-sex tilapia production.