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

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 = similar to 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 delta C-13. 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 perfor-mance and the profitability of GIFT farms. These findings are important not only for the health status of sub-sistence fish farmers, but also for improving the livelihoods of wider communities in Papua New Guinea.

Automated summary

Subsistence freshwater fish farming in developing countries is growing rapidly, but many farmers still struggle with low productivity. This study compared two farming practices and found that using pelleted feed and NPK fertilization resulted in larger and healthier fish compared to using vegetables as supplementary feed. The addition of NPK improved the fish's ability to utilize commercial feed pellets, and the microbiota of fish reared with NPK positively correlated with growth and profitability. These findings have important implications for the health of subsistence fish farmers and the livelihoods of communities in Papua New Guinea.