Remember reading about it in a salmon magazine years ago, can't remember if the idea came from Norway or Scotland? But in a pool in a spot where you know that a salmon might usually be sitting, put a white rock on the bottom! When you get to that pool, if you can't see the white rock? There just might be a big fish there! Thought about doing it at my local river, but the water is normally the colour of coffee, colour of chocolate milk when blown out & the clearest it gets is the colour of tea, but at that time the water is so low it's not worth fishing! Just thought I'd pass the idea on!