Three species of hammerhead sharks are distributed in the southwestern waters of Japan.They are Sphyrna mokarran (Rüppell), S.zygaena (Linnaeus), and S.lewini (Griffith and Smith).A key to the Japanese species of Sphyrna is presented.In the surveyed areas catch records show that S.lewini is most abundant whereas S.mokarran is least in number.The density distribution expressed as hook rates by small statistical quadrates indicates that the distribution of these species is almost confined to the west of lines connecting southern Kyushu, the Nansei Islands, and Formosa.They rarely occur far out at sea.The density distribution also shows that they do not make south-north migrations on an extensive scale.Main populations of these species appear to dwell within the East China Sea.
