Effects of aquaculture sludge and duckweed on redworm (Eisenia fetida) productivity, nutritional composition and feed safety

Abstract Aquaculture is the fastest‑growing agricultural sector. However, improving this industry’s waste management is vital to ensure long-term sustainability. This study assessed the use of aquaculture sludge and duckweed ( Lemna sp. , grown in aquaculture wastewater) as a feed substrate for culturing redworms ( Eisenia fetida ). The study examined how the different substrate mixtures influenced redworm performance and nutrient utilisation. Five feed substrate treatments were tested: 100% sludge (100 S); 75:25 sludge: duckweed (75 S:25D); 50:50 sludge: duckweed (50 S:50D); 25:75 sludge: duckweed (25 S:75D); and 100% duckweed (100D). Each replicate received 10 g of feed substrate each week, and the trial ran for 72 days. The results showed that survival remained high in all groups, but growth and fecundity varied significantly. The 75 S:25D, 50 S:50D, and 25 S:75D mixtures led to the highest weight gain and specific growth rate, while the 50 S:50D and 25 S:75D mixtures showed the greatest reproductive output. The redworm protein content was highest when reared in 25 S:75D substrate, at 15.6% (wet weight), while lipid and ash contents were higher in the 100 S treatment. Calcium, magnesium, and sulphur levels in the redworm showed a linear decrease with increasing sludge replacement by duckweed, while calcium, magnesium, and sulphur contents were significantly higher in redworms reared on 100% sludge than in those reared on 100D. Analysis of the potentially toxic metals in the redworms showed that arsenic concentrations had exceeded the EU maximum level for undesirable substances in feed materials (2 mg/kg total As; Directive 2002/32/EC) in all treatments. Rearing redworms on aquaculture sludge combined with duckweed grown on wastewater shows promising results as a waste valorisation strategy. However, further optimisation, particularly to reduce the arsenic bioavailability and accumulation, is required to meet current regulatory compliance and commercial viability.