Environmental cost of farming 1 kg of yellowtail fish: phosphorus (P) mass balance. Yellowtail (amberjack) is the most farmed fish in Japan. To produce 1 kg of yellowtail, approximately 8 kg of trash fish is required. The 8 kg of trash fish provides around 38.4 g of P (0.48% P per wet weight, based on Shearer’s 1984 data for rainbow trout). The digestibility of P in trash fish or fish meal has not been reported for yellowtail, but it is estimated to be around 53%, based on recent data from a high-fish meal diet (Horstmann et al., 2023). Thus, 8 kg of trash fish would provide approximately 20.4 g of available P (38.4 g * 0.53 = 20.4 g). However, cultured yellowtail retain only 4.8 g of this P, excreting the remaining 15.6 g as soluble urinary P. These values are in fair agreement with Islam (2005) on a total P basis.