Here is a cheap bushcrafty crab trap that can be made entirely from natural materials or from recycled and scavenged material from beachcombing. Its called a bintur and is used extensively in the islands of South East Asia.
It can be made in about 30 minutes (including photos) once all the material is ready.
You need:
2 lengths of springy wood about 1-1.5cm thick (bamboo is ideal) about 28-30 inches long.
A piece of netting that is about 25 inches on the diagonal or slightly less than the wood
A piece of string about 1.5 metres long
Weave the string in and out of the perimeter mesh of the netting as in the photo
Make sure the ends come out of the same mesh
Notch the sticks at either end on OPPOSING sides
Start by slipping a buntline hitch or a similar knot over a notched end
Make a clove hitch for slipping over the end of the other stick
Now do the same for the other end of the first stick. Make the second clove hitch about the same distance as between the first two knots. Then make another hitch on the end of the second stick. Finish off with a couple of round turns and a slip hitch on the end of the first stick where you started.
The bintur is unlikely to be square the first few times. If you are hungry it doesnt matter. (Once you get the hang of it you can tie the first sticks end first then do the second but that requires a good estimate of the length needed)
If you have time adjust the clove hitches or re-tension the slip hitch till you are happy with the shape. What is more important is that both sticks are arched and that there is sag or a belly in the netting so that a crab will not be able to climb out when you haul it up
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You can secure the bait (chicken bones etc.) with string or wire on the sticks or the center of the net. I have used cat food in cloth successfully
If you want to make the trap collapsible just loosen 2 of the hitches.
Depending on your bait and materials, you may have to add a weight in the net to make the bintur sink. As the wood dries if you don't use it for a while a larger weight is desireable
I check the traps every 15 minutes or so.