tactics

Think Vulnerability, not Hatch

By Scott Richmond

When you say the word "Hatch," most fly anglers go glassy-eyed and tie on a dry fly. But that's not always the best approach. "Vulnerability" is a better framework for thinking about trout food.

nlike fly anglers, trout pursue their quarry--aquatic food--with no sporting instincts whatsoever. Trout would take mayflies with bazookas if they could find a way to do it. To be successful, a fly fisher must understand that trout are predators. Their only ethic is survival. Eating an insect when it is helpless, with no defenses or means of escape, is not the depths of moral depravity to a trout; it is the pinnacle of good sense and the path of survival.

It's About Vulnerability, Not Hatches

Trout focus their feeding at those points in an insect's life when it is easiest to catch--when it is most vulnerable. The more vulnerable the insect is, the more likely a trout will eat it. Where, when, and how an insect is vulnerable varies with its life cycle.

For example, the March brown (the genus Rhithrogena) is a clinger type of mayfly. In its nymphal stage, it has a flat shape which keeps it from being swept away by the fast currents where it lives. That's why March brown nymphs are seldom found drifting in the river. And why they are seldom found in the stomachs of trout. Shortly before a hatch, active nymphs that are preparing to emerge may lose their grip, drift in the current, and be taken by trout. But for the rest of the nymph's year-long life cycle it is not especially vulnerable to trout. Therefore it is seldom worth an angler's time to imitate the nymph stage of a March brown. But when the nymph emerges into a winged adult and floats briefly on the surface of the river, it is highly vulnerable and may become lunch for a sharp-eyed trout. So imitating that point of vulnerability is a good fly fishing strategy.

On the other hand, the giant stonefly nymph (salmonfly; the genus Pteronarcys), lives in fast, riffly water, but it doesn't have a hydrodynamic shape. These nymphs often loose their grip, drift in the current, and are eaten by trout. They're vulnerable. And drifting a big stonefly nymph pattern such as a Kaufmann's Stone near the river bottom just below a riffle can catch a lot of trout. But unlike the March brown, this stonefly does not emerge in the water. It migrates to shore and does its hatching on land, where trout can't eat it. When they migrate, salmonfly nymphs are more exposed to the current, and so they are often knocked loose, and are often eaten by trout. Later in its life cycle, windblown adult salmonflies drop out of bankside alders and onto the river, where trout gleefully gobble them up. So imitating an emerging salmonfly will catch you very few fish, but imitating drifting nymphs, especially when they migrate toward shore, and imitating windblown adults are very productive tactics

As another example, adult female caddisflies return to the water to lay eggs. Some caddis species lay eggs by dropping them from above the river; not much vulnerability there! Other species dive into the water and swim to the bottom to lay their eggs; they are highly vulnerable. A good fly angler knows which caddis behaves which way, because one presents a fishing opportunity, but the other does not.

The What, Where, and How of Vulnerability

At each stage of its life, an aquatic insect species is vulnerable (or not) in different ways. At each point of vulnerability it has a different appearance, is vulnerable at a different depth, and behaves in a different way. From an angling viewpoint, those are the three important ingredients in a point of vulnerability:

1. The appearance of the insect, which a fly fisher matches with the appropriate fly pattern.
2. The depth at which the vulnerability occurs, which an angler achieves through proper selection of fly line, leader, fly pattern, etc.
3. The action of the insect at its point of vulnerability, which is matched by the fly fisher's presentation.

These are the what, where, and how of successful fly choice and presentation. If you don't match all three--appearance, depth, and action--you will not be giving the trout what they are looking for, and your success will be limited.

If the above discussion seems obvious, consider this: if you plow through the literature of fly fishing and hatch matching, it is very difficult to find the points of vulnerability for an insect. You find bits and pieces, but you rarely see the whole story in one place. Even detailed entomology tomes don't cover the issue very well, and it takes diligent research in several books to dig out the information. Anytime you read some article or book on fly fishing entomology, consider the entire life cycle of the insect so you can determine the what, where, and how of its vulnerability.

It's Not As Bad As You Think

There are over 600 mayfly species in North America, over 1,200 caddis species, and you don't want to know how many midge species are buzzing around. Does a fly angler need to understand how each of them differs from the other? Fortunately, no.

Points of vulnerability are generally consistent within each genus of insect. That narrows it down considerably. Further, many insect genera are vulnerable in similar ways, so they can be lumped together.

The chart shows how it falls out for the major aquatic insects of interest to western fly anglers, and the sections below give some background about the points of vulnerability for three stages of insect life: pre-adult (larval), emergence, and adult.

Over the next couple of months, fishinginoregon.com will expand its entomology and fly pattern data bases. The major points of vulnerability for each insect will be listed, and for each point of vulnerability you will find the best fly pattern, fishing depth, and presentation.

My book The Pocket Gillie focuses on vulnerability and has charts for 18 groups of aquatic insects. Each chart shows the point of vulnerability, the depth at which it occurs, the type of fly you need, and the appropriate tactic. It's a simplified approach and doesn't cover all possibilities, but it hits over 90% of the situations fly anglers come up against. Click for a sample chart.

Pre-adult Points of Vulnerability

1. Swimming. Some species of nymphs swim. These include dragonflies, damselflies, and speckle-wing quills (the genus Callibaetis). They flit about searching for food, often moving from one weedy area to another. This makes them vulnerable to trout. Think about the movie Jaws and you get the idea.
2. Drifting. Many nymphs are vulnerable to being swept into the current. This is especially true of large stonefly nymphs, such as the salmonfly, discussed above. Some mayflies that live in faster water are also vulnerable, as are many caddis larvae.

Emergence Points of Vulnerability

1. Migrating. Many aquatic insects do not emerge in the water, but on land or on above-water objects such as grass stems, trees, or rocks. Clearly these insects are not vulnerable to trout at emergence, unless you see a few fish prowling in the alders (in which case, catching fish is the least of your problems). However, many of these insects expose themselves when they migrate. Damselflies, for example, swim in great hoards to vegetation, and trout slash through these migrants like Cossacks through peasants. Other species, such as stoneflies, migrate by crawling along the river bottom to shore. They are often knocked loose, and trout are waiting for them.
2. Rising. As a nymph or pupa (or sometimes even a fully formed adult) leaves the bottom and rises to the surface, it is vulnerable to trout. This point of vulnerability is more important in slow water than in fast because in slow water the insect is an easier to capture.
3. Under the film. Imagine being trapped in a room with a hungry tiger pacing back and forth. There is a door that opens outwards through which you can escape, but there is a 1,000 pound weight stacked behind it. If you are built like Arnold Schwarzenegger this is not a problem, but if you're more the Danny DeVito or Peewee Herman type, you might be somebody's lunch. So it is with emerging insects. The water's surface tension creates a springy, trampoline-like barrier they have to break through. The smaller the insect, the bigger barrier the surface film presents to it. Thus, smaller insects are more likely to be taken as emergers just under the surface.
4. Hatching. Nymphs and pupae that emerge in open water must crawl out of their shuck before they fly off. During this brief period they are helpless. Also, a small percentage of insects get stuck in the shuck and can't emerge. They, too, are helpless. Many (but not all) fly shops now carry commercially-tied flies that match hatching or crippled insects.
5. Drying. A winged insect that has emerged may need to rest on the film (which now works to its advantage as a support) and "dry" (actually, pump up) its wings before it can fly off. It is helpless, and trout know it. Larger mayflies take longer to dry their wings, and thus are more likely to be taken as duns than as emergers. For instance, the Green Drake (Drunella grandis) drifts for a long time and is a sitting duck for the trout. On the other hand, many caddisflies emerge on rocks and vegetation, and those that emerge in open water need no wing drying time; midges emerge in open water, but like caddis they need no drying time; stoneflies (except some species of the little yellow stone), damselflies, and dragonflies emerge on vegetation above the water. Thus mayflies and the little yellow stonefly are the only insects that have this point of vulnerability, and even some mayflies emerge out of the water and are not vulnerable at this stage. This is the classic match-the-hatch situation. It is a very important point of vulnerability, but it is just one of many ways in which insects are vulnerable to trout.

Adult Points of Vulnerability

1. Accident. Adult insects, especially caddis and stoneflies, are blown off trees, grass, and other vegetation and land on the water. There they are temporarily stuck. This is especially true of caddis and stoneflies. When these adults are active, trout often wait near the surface downstream from overhanging vegetation and suck down the inevitable blundering bugs. This means caddis and stoneflies provide excellent dry fly fishing unrelated to any hatch activity, and in fact you can be fishing a caddis "hatch" a couple of weeks after the adults emerged. It also means that dry mayflies are fished where the insect emerges, but dry caddis and stoneflies are best fished near shoreside vegetation until they come back to the water to lay eggs (see below).
2. Ovipositing. Female insects must somehow get their eggs into the water as their last act of life. The act of egg-laying is called "ovipositing." Some of the ways they do this are: dipping to the surface, sprawling on the surface, swimming to the bottom, crawling to the bottom, dropping eggs from above, or laying eggs on overhanging branches so hatching larva will drop in the water later. Dippers may be imitated by "dapping", sprawlers with conventional spinner presentations, swimmers and crawlers with wet fly tactics. There isn't much you can do about droppers or branch layers unless they fall into the water.

Other Factors

Weather and other conditions can affect vulnerability. A cool damp day, for instance, can increase the wing drying time for mayfly duns and extend their vulnerability. Another example is broken water, such as in riffles or on windy day, where emerging insects will have an easier time overcoming surface tension. On the other hand, rough water can drown newly hatched insects and trout can become selective on them. Observant anglers will find special situations that create vulnerabilities that trout (and anglers) will take advantage of.

Scott Richmond is Westfly's creator and Executive Director. He is the author of eight books on Oregon fly fishing, including Fishing Oregon's Deschutes River (second edition).

Uploaded 12/08/1998.

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