These are also called drop nets, and you bait them and drop them for about 30 minutes then go round your run and pull them all up again and grab the crabs, they work really well.
Is this how they catch crabs? ______________________________________
This is one way, where they walk around with these scoop nets on a pool and literally scoop the crabs up when they see them. I prefer the drop nets - they are more effective but you need a boat to do it properly.