Environmental impact assessment of fish feed for aquaponic systems to introduce higher phosphorus and potassium in value-added fish sludge

Summary

Potassium (K) and phosphorus (P) are often deficient in fish sludge and must be supplemented by inorganic fertilizers in aquaponics. The present work evaluated aquaponic fish feed formulation strategies that can do value-addition of fish effluents in terms of P and K for plants, reduce inorganic fertilizer use, and draw down overall environmental impact of integrated fish-plant food systems. Three similar protein (41 ± 2 %) and energy (342 ± 3 kcal 100 g⁻¹) feed with graded P and K levels were validated in a tilapia-based aquaponic facility for 35 days, followed by a life cycle impact assessment (LCIA) per unit fish yield and plant nutrient fertilization during fish yield: (1) low KP feed (6.9 g P, 10.5 g K kg⁻¹ feed; trophic level 1.30, feed-food conflict 88.9 %), (2) medium KP feed (14 g P, 13.1 g K kg⁻¹ feed; trophic level 1.18, feed-food conflict 63.8 %), and (3) high KP feed (18.6 g P, 17 g K kg⁻¹ feed; trophic level 1.35, feed-food conflict 44.6 %). The final body weight, TGC, yield, and FCR did not significantly differ between low, medium, and high KP feed (p > 0.05), but palatability was elevated (p < 0.05) in low KP feed. Significantly higher (p < 0.05) removable solids were generated by the medium and high KP feed. The best settleability of solids was observed in high KP feed. A high proportion of dissolved P, K in process water, as well as higher P, K bound to solids, under medium and high KP fed groups confirmed value addition of fish effluents (+290 % for P and + 33 % for K) for plant fertilization. For conventional aquaculture, use of low KP feed is the wise choice, bringing least environmental impact. But for aquaponics, in addition to fish yield, high KP feed created lowest environmental impact of K/P fertilization, compared to either low KP, medium KP feed, or inorganic P/K fertilizers. A high KP feed comprised of food system by-products/co-products (rich in P and K; identified) and low use of inorganic fertilizers in aquaponics may draw-down planetary health boundaries transgressions, most notably the climate change, biogeochemical cycles (N, P flows), land cover change, and freshwater use.