The Fly Selection Probability Curve

In last week’s post we learned that although most trout will behave in a certain way under certain circumstances, there will always be some fish that tend to buck the trend. In a similar way, given a certain set of conditions, individual trout will have a high probability of taking a fly that matches the preferred search image, but there is always a possibility that the same fish may stray from the norm and take a fly that is far from a perfect match. The fly selection probability curve is a good way to visualize this:

In this version, the fish is feeding in a somewhat opportunistic mode. There is a reasonable chance it will take most flies if presented properly (zones B & C), but there is a somewhat higher probability that it will grab flies that match the search images of bugs it has been recently feeding on (zone A). Here, most fly anglers will do well regardless of what fly pattern they fish, but for anglers who take the time to select a fly based on recent bug activity, they will tend to catch a few more fish. Unfortunately, trout feeding in the near opportunistic mode is not very common. A more likely scenario is the non-hatch feeding mode:

In the non-hatch mode, the fish is unlikely to grab a fly that does not resemble natural food items that it has been exposed to at some point in its development (zone C). Flies that have a more natural and realistic appearance will get a little more attention (zone B) but certainly not as much attention as flies that match food items that the fish has been recently feeding on (zone A). In a scenario like this, the fly you choose can mean the difference between catching a few fish and catching a few dozen fish. Even in the non-hatch mode you can see that knowing a little about bug biology and what the trout have been recently feeding on can be a significant advantage. That advantage becomes magnified when trout move to the somewhat selective mode:

In the somewhat selective mode, the fly selection probability curve starts to tighten up. Most flies are ignored (zones B & C) and the fish focuses its attention on what is actively hatching or what is most active and available (zone A). It will still sample other food items, but the tendency is definitely more focused on specific food items. The somewhat selective mode explains why a fish may be feeding on a hatch but suddenly decide to grab a fly that is not even close to what is hatching, or even your strike indicator. When trout are feeding in this mode, the average fly angler who has selected his fly based on what worked well last week, or what his favorite “go-to” pattern is, will have a tough day – while the fly angler who selects his fly pattern based on observation and what is active will do well.

During selective feeding, the fish is focused on a single life stage of a specific bug. Other flies are generally ignored but there is always a very small chance that the fish may grab something out of character. This is where you need to know exactly what bug is hatching and what stage they are feeding on if you want to have any chance at hooking a fish. Not quite hyper-selective at this point but for some fly anglers, the fish might as well be.

The fly selection probability curve explains a lot when it comes to why one fly is working well while others get little attention. It also explains why selective fish may sometimes do the unexpected. But this is not the end of our journey into how mathematics and probabilities can make you a better fly angler. Other layers to add to all of this are how individual fly patterns are aligned on the probability curve, how the curve can shift slightly as feeding conditions change, and how each individual fish may not be feeding on the same probability curve as other fish are.