In this study, we estimated prey availability in the early life stages of Pacific bluefin tuna (PBF) based on their mouth gape size, collected in two spawning grounds, the Pacific Ocean around the Nansei Islands and the Sea of Japan. The increase in mouth gape sizes was investigated using laboratory-reared PBF larvae and juveniles. Thereafter, we compared the mouth gape size of PBF with the body size of prey larvae, including PBF, collected in field surveys in the spawning grounds of PBF conducted in 2017 and 2018. We defined the collected larvae smaller than the mouth gape, mouth width, and 1/3^rd the body length (BL) of PBF as potential prey larvae (PPL), which have the potential of being captured and ingested by PBF. The number of PPL in each sampling station was estimated for PBF of 8, 10, 15, 20, and 30 mm BL size classes as predatory PBF. The density of PPL increased for PBF larger than 20 mm in BL, suggesting that prey availability could change drastically for PBF larvae and juveniles larger than 20 mm BL.