Things to consider:
All migratory fish will need written permission to fish for, so fishing on a burn off the main stem for salmon is just the same as fishing the main river. On Donside, fishing for trout on and off the main stem without written permission will cause you problems due to the protection order, Hamish will elaborate i am sure.
Generally fishing rights to an estury include about 3 miles from the mouth and this goes back to the salmon netting permissions / liscenses.
Why dont you just join the club/Association?