Nitrogen, phosphorus, and energy waste outputs of four marine cage-cultured fish fed with trash fish

Trash fish used as aquafeeds in marine cage cultivation sometimes induced environmental risk. In this study, nitrogen, phosphorus, and energy waste outputs were measured in four marine cage-cultured carnivorous fish species – Sciaenops ocellatus, Plectorhynchus cinctus, Epinephelus coioides, and Rhabdosargus sarba – fed with trash fish for 6 weeks in laboratory. Waste outputs of cultured fish were divided into soft tissues in uneaten feed, bones in uneaten feed, scales in uneaten feed, soluble uneaten feed, solid faeces, soluble faeces, urine, caducous scales, and other parts. The results showed that an increase in 1 g of fish mass produced: 1) 34.4–67.2 mg nitrogen in urine, uneaten feed, faeces, and caduceus scales; 2) 3.4–10.9 mg phosphorus in uneaten feed, faeces, and caduceus scales; and 3) 12.0–23.8 kJ in metabolism, uneaten feed, urine, and caduceus scales. In the nutrient budget, urine was the most important component of nitrogenous waste and accounted for 7.8–14.4% of total nitrogen input. Bones and scales in uneaten feed carried the bulk of phosphorus waste, and these accounted for 4.1–13.8% of total phosphorus input. Metabolism was the largest energy consumed and it accounted for 12.0–20.8% of total energy input. The ratio of energy:nitrogen:phosphorus in waste outputs was 1.0–1.4:3.9–4.5:1. The information on nutrient and energy in waste output of the four fish in this study would help to assess and predict pollution of trash fish used as aquafeeds.