Used to go crabbing for bait when I lived in the UK over forty years ago. No need to dig for them, just turn over manageable sized rocks and stones on the mud where they hide underneath. Remember to replace the rock as you found it so the next inhabitant can take up residence. Also run your fingers through any weed or kelp attached to the seabed. This is another favourite spot for them to hold up. If you have a harbour wall with seaweed attached and hanging, check that out as well, another favourite spot for crabs.

If you can find them in the peeling or soft stage (just after they have shed their shell) so much the better. They are likely to be bigger than the size you are looking for, flounders are really attracted to a cut, well presented crab, bait elastic is necessary.

Another advantage of crab bait is they keep really well. Put your bait in a box well filled with seaweed, put this is the bottom on your fridge and they will keep for at least a week.

Hope this helps.