The thing here is still permissions, you need permission to set the traps.
Fyke net (or not), Crayfish trap (or not) the Salmon and Freshwater Fisheries (Consolidation) (Scotland) Act 2003 states (Part 1, Methods of fishing, Section 2):
"In any inland waters an owner or occupier may fish for or take freshwater fish, other than trout, by net or trap."
So it is legal, but only if the owner or occupier does it or gives written permission to do so and only if the target species are other than Trout (and obviously Salmon...).
My tuppence.