Bull. Jpn. Soc. Fish. Oceanogr. 76(1), Page 24-30, 2012   Vulnerability of juvenile fish to piscivorous fish predators increases during nighttime in a seagrass bed in the central Seto Inland Sea, Japan Hikari Kinoshita1, Yasuhiro Kamimura1, Kotaro Kirai1, Ken-ichiro Mizuno1, Yuji Iwamoto1 and Jun Shoji1† 1 Takehara Fisheries Research Laboratory, Hiroshima University, 5-8-1 Minato-machi, Takehara, Hiroshima 725-0024, Japan † e-mail: jshoji@hiroshima-u.ac.jp To test the hypothesis that vulnerability to predation of small-sized fishes increases during nighttime, the fish species composition, abundance, biomass and stomach contents of potential predators, and the mortality rate during tethering experiments were compared between daytime and nighttime at a seagrass bed in the central Seto Inland Sea, Japan. A total of 494 fishes belonging to 30 taxa were collected using a round seine on 28 and 30 August 2007. Abundance and biomass of potential fish predators (piscivorous fish >82.5 mm in total length) were higher in nighttime than in daytime. Sebastes cheni juveniles and Rudaris ercodes were predated on by Sebastes inermis and Lateolabrax japonicus. Predation rate of juvenile S. cheni, a dominant species in the fish community, estimated from the ratio of the number of juveniles predated to that of population (sum of surviving and predated juveniles) during nighttime (1930-2100 h) was 5.1%. Tethering experiments for 6 h at the seagrass bed indicated that the mortality rate of juvenile S. cheni in nighttime was significantly higher than that in daytime. It is plausible that seagrass beds contribute as foraging habitats for piscivorous fish during nighttime although they are important nurseries for a variety of fish species. Key words: seagrass bed, fish nursery, predation, day-night comparison, tethering experiment, Seto Inland Sea