How do you figure out which bug on the water the trout are eating? It seems like I saw 5 different ones the other day. I asked this guy what bug to use and he said Prince Nymph. Does a prince nymph match them all?
Well, you've asked the $64,000 question alright. Trouble is, a complete and accurate answer would probably have to be more than 64,000 words. What a beating, huh?
When you said 'on the water" can we assume you saw fish rising and eating those bugs? If you are on a river you might be able to watch a single fish for a few minutes and figure out which bug he was eating. Of course, a lot of fish have been caught by just chucking, for example, a #16 Adams at risers. It's an "attractor" dry that looks a little like many things and yet nothing specifically.
A Prince nymph, usually fished with a bead head these days, is a general nymph pattern. Doesn't look exactly like anything - a nymph equivalant of the Adams dry. John Geirach, for one, says he won't fish Princes because he could never figure out what those white biot wings were supposed to be, but most FF'ers aren't as interested in natural materials and profiles as he is. So, if your advisor was using a Prince, especially with a BH, he was ignoring the rising fish and concentrating on fish down near the bottom. If on a lake, he was fishing somewhere between the bottom and a few feet below the surface.
There is a fly shop called St. Vrain Angler in LongMont. The owner and fly fishing expert Dale Darling (a dry fly fishing legend) - has written a handfull of information packed short booklets called "Solution Guides" - one of them is on bugs and how to identify not just the bugs in the water but a process to determine which ones they are eating- from proper bug sampling from the water to fly selection and progression.
I generally regard a Prince more as a stonefly imitation than anything else. The dual biot wings and tails are what really throw off the purists (white and brown!). Usually onlu carry them in a size 16 and they are deadly on the Arkansas next month, beadhead or not.
But how one determines just what they are eating at anytime is exactly the $64,000 question. Is it the emerger? The cripple? The dun? The spinner? The pupa? Or is it a midge that's emerging subsurface while there's a full blown caddis hatch in progress? Or, to echo John Gierach once again: "How much did you pay for that fly rod?" (Zen & Nymph Fishing)...
Yeah, anyone thats fished for awhile can tell you thats a tough question. unless the conditions are right and you can actually see the trout feeding on a particular insect, all you can do is try to narrow it down some and hope they are not being too picky. Colorado has a few good hatches that are easy to predict every year (examples would be the Mother's Day caddis hatch on the Arkansas, the Green Drakes on the Frying Pan, the Blue Wing Olives on the South Platte, the damsels emerging off of the prairie lakes in July, to name a few) but for the most part i like to do a little research before i hit a particular body of water before i head out to see what has been active lately. there are also a few patterns you can always have with you that imitate several types of insects, these are staples in most peoples boxes and should never be left at home....the Gold Ribbed Hare's ear and Adams are two examples. Finding a fly that imitates them all...well, when you do find it let me know lol
all the prior posts below are good sound advice...books, the internet, and other fishermans experience are the best bet.