William Anderson wrote:Otter, you've opened an enourmous can of worms. This thread is really interesting. I know I have always tried to tie flies based on species, or at least I've made the stretch to say...this could be a sulpher, maybe.
Having reflected a bit on all of this I have to agree William, it is an enormous can of worms that one could ponder on for a lifetime and still be unable to prove anything.
My main river trout experience is on a single river, wild trout and abundant fly life. Water levels are controlled by a dam and are therfore pretty consistent for most of the season.
Let me outline a scenario last July, dusk on a single pool.
10pm : Trout rising in riffle. Some taking BWO emergers and adults, others taking caddis pupa and emergers some taking both. I throat pumped each one I caught and can verify that the predomiance of fly life in each trout varied from a right mix to definable concentrations of BWO's and of Caddis. More had concentrations than mixes. Several trout were caught after fly type switch, some notably on first drift after change.
10:30pm Riffle: Bwo dun and emerger imitations being ignored, riffle gone quiet except for odd trout slashing at Caddis.
30 yards downstream, large trout , some in 3lb bracket sipping a huge concentration of Sherry Spinners in very slack water- impossible to cover with
even a reasonable chance of success - a more skilful caster and angler may have been able to tempt one or two.
Slightly slower water at end of riffle far bank, a few risers - couldnt tempt them and could not identify what they were taking - it wasnt BWO's or
Caddis, supspect some smaller olives/spinners.
10:45pm Sherry Spinner sippers have nearly all stopped rising.
11pm: Riffle comes slightly alive again, numerous fish taken on caddis emergers, notable here is that some good fish , probably some of the sherry sippers that had moved up to riffle.
11:45 Bats are out, river is quiet - time to go home.
This type of scenario is very common on my river, the actors will change but the trout will apply themselves accordingly.
Trout are IMHO very much creatures of habit, pre-programmed to generally react in quite specific ways to the floatsam of food that comes their way. In the above example only a small number of the larger trout moved to the riffle, on other nights when a fall of spinners did not occur a lot more would have moved to the riffle and probably sooner. To maximise the effeciency of their food intake, they will at times key into specific food items, why one trout does this and another a few yards away is not quite so keyed in can only be a matter of conjecture though I suspect it may be part of their evolution to ensure maximum intake of available food for the species and not just the individual.
Had I fished a P&O for example I have no doubt I would have taken trout but not near as many as I did by switching between caddis pupa, caddis emergers, BWO emergers, BWO duns and egg laying caddis. Certain suggestive patterns may also have worked well and taken quite a few trout but once again would have failed to entice many many more. I suspect on that night though I cannot prove it to be so that a suggestive pattern would mainly have enticed trout that were happy to take a mixed bag of food.
A really important point in all of this is at times, that certain trout require a fly that is pretty precise in its design and presentation, anything less than that and it will be at worst ignored , at best snatched at with few hookups. The real interesting thing here is that within a relatively small section of water, when there are many food items available different trout can at times choose to dine in different ways.
If you fish with the belief that the water in front of you will possibly require several presentations of different food forms then you allow yourself to the oppurtunity to catch a lot more fish than a less open minded angler fishing the same water.
Once you accept that the best fly to fish is the one that the trout want and not the one you want to fish then your catch rate will really improve - I know mine has. Now how do you know which one they really want - he he
, thats the fun bit !!!!!!!!!!!!
Personally, I think there are two many variables to have too much faith in that method, and you can find patterns in anything if you look hard enough. But, if you and your mates keep records like that you presumably fish a lot, and if you fish a lot you are going to gather a lot of experience, and if you have a lot of experience you are going to catch more fish, otherwise you would presumably have changed to a different pastime! 
Ain't that the truth! lol You know him too?