Not all, but many. For shy, picky trout and grayling, size, type and colour of fly can make the difference between a blank and a catch. The same in calm, clear water in the salt with sea trout feeding in the shallows. Here fly size is king - whether they’re eating gammarus, miss, small shrimp etc., only very few trout will refuse a well presented Red Tag size 12 or 14. I like the imitation game, but that’s a (slightly) different discussion.
Water clarity is an important factor to take into consideration when choosing a fly. It’ obvious to most, but the general rule of thumb of course is the clearer the water, the smaller the fly. In dirty, murky water I very rarely go for orange and other bright colours. Black is a heavy contrast and very visible and once you get deeper than the sun/light reaches, brightly coloured flies go grey.
When it comes to salmon, it’s common knowledge that late season salmon are usually best caught on small flies. But what’s a small salmon fly? Again water clarity is to be considered, but when fishing running water, another factor comes into play: Speed of current.
Most seasoned salmon anglers are of the opinion that the later the salmon sees the fly, the better the chance of a reaction. That is - if the salmon sees the fly swinging towards it 6 yards away, it’s less like to strike than if it sees it when it’s two feet in front of its nose.
I believe that the faster a fly moves, the smaller the salmon might perceive it. A long, slim fly (as in a lightly dressed Sunray Shadow) can easily be perceived as smaller than a heavily dressed #2 single hook, even though it’s 2 inches longer.
I also always try to fish my flies fast. In most conditions I find a faster fly more effective for salmon than a slower one. In slow water (as in Denmark) I usually add a retrieve of some sort to the swing. Maybe a figure-of-eight is enough to make the difference. Maybe a faster, standard retrieve will do the trick. Never do the same thing once, as Paul says. Danish salmon streams can’t be said to be fast, so I usually fish quite small flies. In later years rarely bigger then 5-6cm, even in high spring water.
Predatory fish have excellent vision, and I rarely go for a “big” fly as my first choice. If I see specific prey, I go for an imitation. If I’m “restricted” to choose fly based on experience, I usually go for the smallest fly I think the fish will spot (pike might be an exception).
Have a great weekend!
PoD: From Gaula in Norway. I didn’t land the salmon, but it was big, over 8kg. First of all the spot needed left-hand speycasting, which was why it hadn’t been fished much, and I know the spot - it tends to hold salmon. There’s a good, even current, where a cast across and a down stream mend gives the fly good speed. I had on a small Sunray Shadow, maybe 5cm long and after two swings over the spot, the salmon was on. It ended up bending out the hook.