Should I fish a dry fly or a nymph?

What is the actual difference between a dry fly and a nymph?

A dry fly floats on the surface film and imitates an adult insect — something that has already hatched, or fallen in. A nymph sinks and imitates the immature, underwater stage of the same insect, which is where nearly every aquatic insect spends nearly all of its life.

That life-cycle asymmetry is the whole argument. A mayfly might live underwater as a nymph for a year and as a winged adult for a day or two. A trout that eats mayflies therefore spends almost all of its feeding time eating nymphs, and only rises to duns during the brief windows when duns are actually available.

Everything else — the tackle, the takes, the difficulty — follows from that one fact.

Dry fly vs nymph — how they compare

The honest summary: if you want to catch the most fish, nymph. If you want the moment where a trout tilts up out of the current and takes something you can see, fish a dry. Both are legitimate reasons to be on the water, and most good anglers do both on the same day.

How do I decide which to fish when I arrive at the river?

1. Watch the water for two minutes before you rig up

   Do not cast. Stand back from the bank and look at the surface across a riffle, a seam and a flat. You are looking for rings, noses and fins. This is free information that decides your whole approach and almost nobody collects it.

2. Look for insects in the air and on the water

   Duns riding the current, caddis fluttering over the surface, or spinners in a column above the riffle all mean fish are likely to be looking up. No insects at all, in any life stage, means they are almost certainly not.

3. If fish are rising, fish a dry

   Match the approximate size and silhouette of what is on the water. Size is the variable that matters most — a size 16 that is roughly the right shape beats a size 12 that is the right species.

4. If nothing is rising, fish a nymph

   Set your depth so the fly drifts within a foot of the bottom, and cover the water systematically. This is the default state of a trout river and where most of the fishing happens.

5. If you cannot tell, fish a dry-dropper

   A buoyant dry with a small nymph on 18 to 24 inches of tippet beneath it answers the question for you. Whichever one they eat is the answer, and you get it in ten minutes instead of an hour.

When does the dry fly genuinely outfish the nymph?

During a real hatch, and it is not close. When a hatch is dense enough that trout lock onto duns drifting in the surface film, a nymph passing a foot below them can be ignored entirely. Fish that are keyed to one stage of one insect become extremely literal about it.

The other reliable dry-fly windows:

• Terrestrial season. Mid to late summer, with wind, along grassy banks. Fish sit tight to the bank waiting for hoppers, beetles and ants, and they will move a long way for one.
• Evening spinner falls. The last hour of light on a warm day, in flat water, when spent adults drift flush in the film.
• Freestone pocket water in summer. Broken, oxygenated water with opportunistic fish that will take a well-drifted Elk Hair Caddis without any insect being on the water at all.

When is the nymph the only sensible choice?

Cold water. Below roughly 50°F, insect activity drops off and trout hold deep in slower water, conserving energy. They will not move far or upward for a meal, so the fly has to arrive at their depth. This is most of winter and much of early spring on most trout rivers.

High or colored water, for the same reason plus visibility: a fly on the surface of a turbid river is invisible to a fish holding two feet down. Bright water, dead-drifted deep, is the answer.

And any time you simply have no surface information — midday, flat light, nothing hatching, nothing rising. A Pheasant Tail Nymph drifted through the bottom third of a riffle catches fish on days when the surface tells you nothing at all.

What about streamers — where do they fit?

A third method, and worth knowing you have it. A streamer imitates something swimming — baitfish, leeches, crayfish — and is retrieved rather than drifted. It targets larger, more aggressive fish and covers water quickly.

A Woolly Bugger swung across and downstream is the least technical way to catch a trout that exists. It requires no drag management and no depth control, which is exactly why it is a good thing to fall back on when the drift-based methods are not working and you want to fish rather than fiddle.

Which should a beginner start with?

Start with a dry fly, for a reason that has nothing to do with catch rates: you can see everything. You can see the fly, you can see whether it is dragging, and you can see the take. Every mistake you make is visible, so you learn drift management — the skill that underpins all of it — in a way that watching an indicator does not teach.

A Parachute Adams in #14 or #16, fished upstream on a 9-foot 5X leader, on broken water in the middle of the day, is a genuinely good first exercise even when fish are not rising. Then move to nymphing once you can hold a drag-free drift for three seconds, because at that point the indicator is telling you about a drift you already understand.

Where to put that drift is a separate skill and the one that pays best. How to read water for trout covers it.