The Feeding Mode Probability Curve

It’s funny; I became a biologist, in part, because I didn’t like math. But the more I learned, the more I realized that biology is as much about mathematics and probabilities as it is about the biology.

Take trout length for example. If you were to survey trout in a lake that all hatched in the previous spring (i.e. they have all been developing and growing for roughly a year), you would find that trout length varies within a certain size range. If you measured each trout and plotted the total number of fish at each size, you would end up with a graph that looks like this:

The graph shape is what is referred to as a normal distribution or a probability curve. If you were to catch 100 of those trout and measured each one, odds are that most of them would be in the size range near the center of the distribution (i.e. 15-17 cm). A few trout might be bigger (17-19 cm) or smaller than the average (15-13 cm), and fewer still would be out at the tails of the graph – being quite a bit bigger (19-22 cm) or smaller than expected (10-13 cm). In other words, the probability of catching a near average sized trout is quite high, but the probability of catching a one year old trout that is quite a bit bigger or smaller is very low – but still possible.

This distribution not only applies to physical characteristics, it also applies to trout behavior. Under any given set of conditions, you would expect most trout to act a certain way. But there will always be a few trout that will act somewhat different from what is expected, and fewer still that act very different from what is expected. If we place the normal distribution on our trout feeding mode continuum from last week, we get a feeding mode probability curve; it looks like this:

As feeding opportunities or conditions change, the curve will shift left or right, with most trout feeding near the center of the curve. Under average conditions most trout will be feeding in the non-hatch mode, but there will still be a few fish feeding somewhat selectively or somewhat opportunistically, and possibly the occasional fish feeding in hyper-selective or opportunistic mode. If conditions change – say in the presence of a heavy hatch, the curve may shift farther to the right, and tighten up:

Here, most trout will be feeding selectively, a few trout may be feeding hyper-selective or somewhat selective, and the occasional fish may still be feeding somewhat opportunistically. The interesting thing is that not only does the trout feeding probability curve apply collectively to groups of trout exposed to similar conditions in the same water body; it also applies to individual fish.  This leads to what I call the fly selection probability curve (yes, more graphs are on the way…next week). 

When it comes to biology and animal behavior, things are rarely cut-and-dry. The best we can do is distill things down to a mathematical model that predicts behavior under a certain set of circumstances. As you move away from the peak of the probability curve, the odds on a specific event or behavior taking place becomes lower. It turns out that fly fishing and fly selection is also as much about mathematics and probabilities as it is about biology.

Trout Feeding Modes

 Last week’s post was fairly lengthy and contained several new concepts that could benefit from further discussion. In this week’s post I will start by explaining the basic concept of the trout feeding mode continuum. Successive posts will build on this foundation, leading the fly angler to a more complete understanding of just what leads up to the moment when the fish takes your fly.

The feeding mode continuum is really quite simple. It starts with fish that are feeding opportunistically. In the opportunistic feeding mode a trout will essentially sample everything that appears to be even remotely edible. Buggy looking things and critters that move in a tantalizing way are at the top of the list but small bits of wood and bright, colorful objects are also tested to see if they might contain any nutritional value. In the opportunistic mode, trout feed without any concern of being caught and are usually found in seldom fished places where food is scarce and organized, heavy hatches are rare.

At the opposite end of the continuum are hyper-selective trout. When it comes to hyper-selective trout, there are two factors at play. The first one is that trout that are under heavy fishing pressure usually become more selective in the search image they respond to in order to avoid the unpleasant experience of being caught (sometimes a negative search image comes into play – more on this in a later post). The second factor is that in the presence of a heavy hatch, or several hatches taking place at the same time, it becomes more efficient to focus on a single life-stage of a single bug. This narrowing down of the search image allows a trout to feed without having to waste time inspecting each item in the drift and making a decision on each one – instead, the process becomes automated.

The trout feeding mode continuum then, encompasses the entire spectrum of possible feeding behaviors from opportunistic to hyper-selective. Most of the time trout are somewhere in the middle of the continuum – in what I call the non-hatch mode. Here, there are still elements of selectivity where the different search images a trout has been recently exposed to influences which flies they are most likely to take. 

In next week’s post I will super-impose a normal distribution on the feeding mode continuum and explain more fully what a search image is. Then you will start to see what drives an individual trout’s behavior when it is “deciding” whether or not to take your fly.

Matching the Non-Hatch

    I have to admit that when it comes to fly fishing I am not much of a gambler, so when it comes to deciding what fly to use I prefer to play the odds. During a typical bug hatch, that means selecting a fly to match the insect and life stage that the fish are keyed in on. But what do you do when there is no hatch? You still play the odds – and match the non-hatch.

Male and female Golden Stones (Hesperoperla pacifica) in copula. Mating activity usually follows close on the heels of an insect hatch. 

   Most fly anglers are aware of the fact that during a hatch trout will feed selectively on a specific insect, often to the exclusion of everything else. This selective feeding mode is an evolutionary adaptation that allows trout to maximize their calorie intake by ignoring other potential food items (which would require more time to inspect), in favor of a known food source that can be quickly recognized using an instinctive visual trigger known as a search image. It is important to note that the search image is not a mental picture of the entire prey item (that much information would take too long to process) rather, it is made up of one or more specific characteristics that the trout uses to define an object as the targeted prey item. If trout are in a moderately selective feeding mode they may respond to a general search image that includes size, shape and color (sound familiar?). Hyper-selective fish (those that have seen a lot of fishing pressure) may focus on one or two very specific details that separate the naturals from the artificial. These search images are not static. They change as the feeding situation changes; sometimes day by day, sometimes hour by hour, and in some situations from fish to fish.  In many cases a search image is retained in the trout’s memory for a period of time after the food source is no longer present. Think of a non-hatch as the intervening period between hatches where trout maintain a search image for certain prey items, and are more inclined to selectively feed on those prey items, even though there is no active hatch.

   The development of non-hatch search images can come from two sources. The first would be search images formed during a recent hatch. This situation is fairly straight forward; selective feeding results in the creation of search images for the different life stages that the trout have been feeding on. Remember that a hatch can go through several phases where the fish may key in on migrating or drifting nymphs during pre-hatch, rising nymphs or pupae early in the hatch, emergers or cripples during the heat of the hatch, and duns or adults later in the hatch. This can also be followed by feeding on spent spinners or egg laying adults in the day or so following the hatch. The non-hatch search image is therefore not always a single entity. It can be a group of search images retained from the different phases of the hatch. The second source results from sporadic (opportunistic) feeding on the prey items that are most commonly encountered. During extended periods between hatches, trout will tend to forage on whatever prey items are available. Less commonly encountered food items are more likely to be inspected prior to ingestion, and are more likely to be rejected. Feeding on the most commonly available food source can become automated – the search image takes over and the fish spends less time inspecting that particular prey item and instead responds to the instinctive visual trigger to grab the fly. In either case, when responding to a search image the trout instinctively recognizes the item as food, knows exactly what to expect, and there is little or no hesitation in taking the fly.

A male winter stonefly (Isocapnia integra) along with several exuvia on a stream-side rock.
Adults and exuvia together – always a good sign of a recent hatch. 

   When fishing during a non-hatch there is a clear advantage in selecting a fly that matches one of these search images. This point was driven home one summer when I was fishing a freestone cutthroat stream and having a tough go of it. It was a hot, sunny day; there was nothing hatching, and not a single fish rising. I had tried just about every nymph in my box and hadn’t hit a single fish. Noticing the low, clear water, and thinking the fish were leader shy, I decided to start dropping my tippet size. An hour or so later I was down to 7X and still fishless. Finally I remembered that there was a short Green Drake hatch during a thunderstorm the day before. I switched to a green Drake Nymph and picked up six fish in the same run that I had just covered. Not wanting to risk stressing the fish with an extended fight on a light leader, I went up to 4X tippet and continued to take fish after fish in the crystal clear water for the rest of the day. Tippet size had nothing to do with it. It was the search image of a fly that the fish had been recently feeding on that was the ticket to success.

A Green Drake dun (Drunella doddsi) resting on a rock shortly after emerging. Seeing mayfly duns (with dusky wings) are a good indication of a hatch in progress or one that is less than a few hours past. 

   To tip the odds in your favor it helps to know what the potential non-hatch search images are. When considering likely candidates there is nothing better than first-hand knowledge of the hatch activity over the preceding couple of days. If you were there and witnessed exactly what the trout were keyed in on, it’s just a matter of selecting the right non-hatch fly pattern based on this knowledge. Things get a little more complicated when you show up on a lake or stream for the weekend, or it’s a quick day trip to a local trout lake. Here, the time spent doing a little detective work greatly outweighs the time wasted experimenting with random fly patterns, or sticking with a pattern that is catching the odd fish but not living up to the full potential of the day.

   Clues as to which non-hatch search images are in play can be found through careful observation. Your first priority should be to look for signs of a recent hatch. Do you see any adults fluttering over the water or hiding in the bushes? Are there any adults crawling along the bank? Are there any freshly shed exoskeletons (exuvia) clinging to the rocks or shoreline vegetation? Look for exuvia floating along the windward shore or in back eddies where they tend to accumulate. If there are no signs of a recent hatch, try to determine what the most available food items are. Are there any bugs active in the water? Are there any nymphs crawling on the rocks or congregating along the shore? If nothing is active, flip over a few rocks or run a dip net through the aquatic vegetation to see what is present. In all cases pay close attention to the smaller and usually more numerous bugs, and then consider which bugs are more likely to be exposed to feeding trout. These are all simple tactics, but many anglers skip over them, or give it nothing more than a cursory consideration. Do not make this mistake. The clues are always there, but you will need to take the time to find them.

Several stonefly exuvia (Claassenia sabulosa) on a stream-side rock. Stonefly exuvia can sometimes remain intact for some time after a hatch; if they look fresh, you’re in business. If they appear to be dusty and old, the hatch likely occurred many weeks ago. 

   Of course knowing a little bit about the bugs and their life-cycles will be needed to make the connection between the clues you have found and the appropriate non-hatch fly pattern. You don’t need a PhD in entomology, but any knowledge you can gain will go a long way in enhancing your ability to match the non-hatch. If you spot several adults in the stream-side vegetation it helps to know what the nymphs look like, and the habitat they prefer; if you see a bunch of exuvia on the rocks it’s good to know how they got there, and what insect group they came from; if you find active nymphs in the water, knowing their life-cycle and habits will enable you to more accurately imitate them. This is the kind of additional information that will allow you to take full advantage of a non-hatch.

A newly emerged (teneral) damselfly (Enallagma sp.) along with exuvia on lakeside vegetation.
Teneral adults (those that are pale and not ready to fly) indicate a hatch is in progress. 

   Matching the non-hatch is all about playing the odds. Trout feeding modes form a continuum from opportunistic, where anything that appears even remotely edible is sampled, to selective, where their attention is singularly focused on a specific life stage of a particular bug. But most of their time is spent somewhere in the middle – in the non-hatch mode where search images do not necessarily control their feeding behavior, but certainly influence it. Matching the hatch even when there is no hatch is a way to take full advantage of this feeding mode. Random fly patterns and attractor flies will continue to catch their share of fish but in the long run, my money will always be on matching the non-hatch.

Thinking Like a Trout

Last week I introduced the importance of having the right flies in your fly box, and understanding aquatic insect life cycles and how trout relate to and capitalize on them. The other part of the equation is how trout make the “decision” to eat your fly or not. The successful fly angler will have a good understanding of these three concepts and how they all mesh together.

These are all topics that I am passionate about, and topics that influence my approach to fly design and on-the-water fly selection. I am looking forward to sharing details about these three topics in future posts.

Next week I will introduce the basics of trout decision making and how a search image influences trout feeding behavior (this will be a more lengthy posts). The concept of a search image explains a lot when it comes to why a fly will work well one day but not the next, and why while several fish may not hesitate to take your fly, others simply ignore the same offering.

Here is a short video that shows cutthroat trout feeding behavior from an underwater perspective. Most of the fish are feeding on drifting nymphs or adult bugs on the surface. What I find interesting is how the trout can spot tiny bugs in the drift far better than us, often from several feet away.