The behavior of sinking residual feed pellets in a fish (yellowtail) aquaculture farm was investigated using a numerical model. The measured sinking rates of dry pellets (DP) varied from 12.7 to 19.2 cm/s, and were higher than those of moist pellets (MP), which was from 0 to 11.0 cm/s. One MP did not sink and remained suspended, keeping its shape, after two hours of measurement. The results revealed that less than 10% of uneaten MP persisted in the water column one hour after feeding if the residual feed sank faster than 8.0 cm/s. However, the single pellet that remained suspended drifted 300 m during the observation period. Recently, a traceability system has been introduced in fish products, and it is very important for aquaculture operations to monitor the effect of feed and medication used in neighboring fish cages. Our results indicated that the effect from neighboring fish cages could not be ignored if the residual feed did not sink.