Catching a crab @ Tokyo bay = 0 yen

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. 

rockey area

beach

crab

hook rig for fishing

hermit crab