The role of farming practice in nutrient assimilation in small-scale tilapia farming

Abstract

Subsistence freshwater fish farming is rapidly expanding in developing countries to address food and income security concerns. Despite this significant growth, fish farming practices are often inconsistent, and subsequently many farmers still struggle with low farm productivity. This study compared the growth performance and profitability of two subsistence-level fish farming practices: a common practice in many developing countries (pelleted feed and sweet potato as a supplementary feed) and an alternative practice (pelleted feed and pond fertilisation with NPK) in a small-scale, earthen pond farming setting. We investigated whether farming practice significantly impacts the growth, gastrointestinal microbiota, and the stable isotope profile of Genetically Improved Farmed Tilapia (GIFT) strain Oreochromis niloticus. GIFT reared using NPK were significantly larger (weight: 51.64 g v 24.9 g) and in a better condition (Fulton's K: 1.86 v 1.78) than those reared using vegetables as a supplementary feed. Using a source mixing model of stable isotope results we show that GIFT reared using a common farming method (pellets and sweet potato) acquire the majority (92.9%) of their nutrients from a low protein source, the sediment organic matter (protein = ∼1.25%), possibly explaining their poor growth. In contrast, we found that GIFT reared using an alternative practice (pellets and NPK) assimilate 53.9% of their nutrients from commercial feed pellets, suggesting that the addition of NPK improves their ability to utilise this resource. The microbiota of GIFT reared with NPK could be responsible for this improved growth performance, with its composition correlated positively with fish length, weight, and δ13C. In addition, we identified three bacterial zOTUs; Bacillus, Romboutsia and Patescibacteria whose relative abundance correlated positively with farm profitability, indicating they may represent naturally occurring probiotics. Overall, our results highlight the complex relationship between nutrient inputs, the gastrointestinal microbiota composition, fish growth performance and the profitability of GIFT farms. These findings are important not only for the health status of subsistence fish farmers, but also for improving the livelihoods of wider communities in Papua New Guinea.