I went Jonanjima Kaihin seaside park with my son. We did crab fishing in the rocky area. There is no charge to enter the park. We made a fishing device like the below. It consists of a string and bait (dried squid). I dropped it between the rocks, waited until a crab pinched the bait, then pull up the crab. My son caught crabs twice. After the catch, we released crabs shortly.
hook rig for fishing