The fishing on foot is allowed all around the Arcachon Bay, except in the oyster parks.
You can fish all species good to eat, except savage oysters.

You have to respect the minimal size of shells to preserve the environment :
Clams and mussels 4cm,
Cockles 3cm,
Razor shell 10cm.
Throw the very small shells back.

Advice

Before leaving, check the tides timetable. Go back on the beach, 1 hour after the law tide time.
Stay on the sandy area or follow the steams because there is a lot of mud.
Tides timetable

Every 6 hours, it is different

Law time, the water recedes entirely : perfect time for fishing on foot
High tide, the water go back, go and swimm before the water go away again !