Lava has always been plentiful in the Goto Islands. The earliest settlers here were surely familiar with the black rocks they saw everywhere.
These ancients developed an ingenious way of using lava rocks to catch fish. They waded out into the shallow bays and used small rocks to build walled-in spaces among the subsurface lava. The walls were designed to be near the waterline at high tide. When the tide came in, a few fish might swim into the walled area. All the fishermen had to do was wait for the tide to go out again. The fish would be trapped inside, with less and less water to swim around in, and could be caught easily by hand or with primitive nets or spikes.
People in the Tomie area called this “shallows fishing.” If you would like to try it for yourself, the SanSan Tomie Camp Village nearby has a program to give visitors a taste of shallows fishing.