If in classic trout and grayling fly fishing the chance of catching fish foresees some sort of deterministic approach: a good hatch, a drift of nymphs, the fish are active during a specific time of the day, in salmon fishing there is not much that one can rely on. Trout and grayling fishing actively search nutriment (not always of course). It is an open system, where the behavior of the individual fish is determined by internal and external situations that we can evaluate at times. Or that at least we pretend to.