“Ebisuki” is a unique fishing method for shrimps carried by strong ebb currents at night, using scoop nets and lights on small boats in Lake Hamana, Japan, a semi-enclosed brackish lake connected to the Pacific Ocean. We examined temporal changes in the body size of Penaeus japonicus in the summer of 2020 and 2021 by “Ebisuki”. P. japonicus was predominant in the five penaeid species collected. Carapace length (CL) ranged from 14.2–33.1 mm, and temporal differences in size were not observed. CL of both sexes on the same sampling day were identical, and genital organs were almost incomplete. In the mark-recapture experiments in 2022 and 2023, only one individual was recaptured at the same position of the release after two days. These results indicate that P. japonicus was carried by the tidal ebb and flow around the research site until ultimately carried out to the Pacific Ocean in summer before maturation. Almost all the specimens were estimated to be from the late spawned group, which spawn in autumn and recruit in the following summer. This indicates a scarcity of large adults producing the early spawned group, which spawns in early summer and recruits in autumn of the same year.