Locals at Po Toi O going out setting up traps

The traps use the same concept as Eel traps commonly used in Europe.  Just bigger.  Could be used to trap fishes, crabs, even lobsters.  They are weighed down by stones and dropped onto seabed at selected places.  Sometime they are marked by floaters for easier collection in a day or two.

This Youtube link would show how the traps are made, and how to use.  Only Chinese sub-titles.  This clip cited they would not use floaters to mark the location as that means invitation to theft.