The seventeen locks on the island of Oléron still in operation are a legacy of traditional fishing. There were over two hundred of them a century ago. Most of these locks are located on or near Chassiron Point in the commune of Saint-Denis d'Oléron (Madame Naud, Le Grand Sabia, etc.). The locks named Les Jeunes Pointes and Les Vieilles Longes are located at the tip of the point and are next to the lighthouse. Their shape is clearly visible from its platform. Guided tours allow visitors to discover the fish locks, the rocky foreshore, and the edible seaweed that grows there. These tours are organized from the Chassiron lighthouse.
These fixed fish traps consist of a wall 500 to 700 meters long. They have the shape of a horseshoe, more or less symmetrical depending on the configuration of the foreshore on which they are built. On the Oléron coast, wherever the coast was rocky and there were stones, there were locks. They are built with coastal stones stacked in such a way that they can withstand the assaults of the sea. These enormous masses of stone are made without any connection and become fragile as soon as one of their stones is moved. Their main role is, of course, to catch fish, but they also serve as breakwaters by mitigating marine erosion on cliffs and beaches. Their system is simple: the fish enter the lock at high tide and find themselves trapped there at low tide, surprised by the rapid flow of water through the lock trap. All that remains is for the fisherman to retrieve the trapped fish. Various species of fish can be caught here: sea bass, sea bream, garfish, mullet, mackerel, and conger eel. The first locks date back at least to the Middle Ages, with texts proving that they existed as early as the 14th century. There were once two rows of locks, the upper locks closer to the shore, the lower locks further away. At low tide, the lower locks located in the second row received water from the upper locks located in the first row. There were as many as 237 locks in the mid-19th century, including 45 in Saint Denis. Today, only 17 locks remain on the entire island of Oléron. The locks are fragile; it is strictly forbidden to move the stones from the walls and to fish within fifty meters.