Mudskipper (Boleophthalmus sp.)
I think that these are Great Blue Spotted Mudskippers (Boleophthalmus pectinirostris) ムツゴロウ.
They are common in the mudflats of the Ariake Sea where these pictures were taken. They also have blue spots, and the dorsal fins look like right. They live in the mud flats and hop around at low tide out of the water. They can respire through their skin and hide in burrows for protection. Ariake Sea 2014
Male Mudskippers fighting. Ariake Sea 2014
Mudskipper. Ariake Sea 2014