Well I don't know where you are, but if you put a gill net out up here, and set at 90degrees to the beach, you would fill the net with salmon, which is not allowed. Plus baliffifs would swoop on it possibly, and the fines for taking salmon aren't good.
Set parrallel to the beach would mean more weed and stuff in the net but much less salmon.
If you're looking for gill nets, if you pm me, I can rig you something up depending on what you want.