Posts Tagged ‘Trout Selectivity’

Vulnerability, A super stimulus?

March 7, 2016



Is frailty a key trigger for trout?

Sometime back I published a post “The Cuckoo and the Trout” based on the genetic considerations of “super stimuli” as discussed in Richard Dawkin’s exceptional book “The selfish Gene”..
The basic premise being that some stimuli override other considerations such that in this instance a tiny parent wren “ignores” the obvious fact that its parasitical baby is far larger than makes sense.

I think that the concept that some stimuli override other considerations might go a long way to explain some of the rather perverse considerations of fly tying and fly fishing. Why would a trout ignore the hook sticking out of a fly or the tippet tied to its head? And why would it make sense to make close copy imitations of bugs when we all know too well that the best efforts are going to be let down by these necessary limitations of design?

Certainly I know anglers from the States who claim that the bodies of their PMD’s need to be a little more red on the upper reaches of a particular stream and a little more yellow lower down. They will swear on the Bible this is true and I have no reason to doubt their assertions, but surely it is daft to consider such a minor variation of import when the trout can easily see the hook sticking out of the imitation.

Could one suggest that some factors override others when making an assessment and that we all do this at one level or another. It is a case of simplified abductive reasoning “if it looks like a duck, swims like a duck and quacks like a duck, it most probably is a duck”.(I could add: even if there is a hook sticking out of its bum)

For animals, including ourselves, to operate in a complex and ever changing world we cannot in reality assess every possible piece of information available before making a decision, and for a predator, such as a trout, the window of opportunity to make a decision as to “eat it or not eat it” is limited before the potential prey item has been whisked away by the current or flies out of reach.

In reality then one need not actually undertake any cognitive gymnastics to be able to come up with a quick strategy in terms of assessing information. General rules which hold true most of the time will suffice.

A Lion on the plains of the Serengetti, need not consider why a particular Wildebeest is slow or limping, it only need recognize that a slow or limping prey animal is a better bet requiring less effort and smaller risk in terms its capture. So I would suggest much the same holds true for trout in a stream.

Firstly we all recognize that it is on average much easier to deceive fish in faster flowing currents, they have less time to make the decision; I would suggest that every fly angler across the world recognizes this simple truth. I would then further hypothesize that the less time available the more one relies on key information.

So of all the information available to a fish to assess the validity of a potential food item the less time it has to make that decision the fewer bits of information it can use to reach a conclusion

In this, admittedly arbitrary, diagram below, the idea is that of all the possible clues to the validity of selecting a potential food item as real some will take precedence depending on the amount of time available. i.e. Faster currents allow for less time. Not to mention previous positive and negative experiences of the fish.

AssessmentOf all the possible considerations in assessing potential food items how many does a fish actually use and is it possible to induce a shortcut?

If this were true one would expect that the faster the water the less specific one’s imitation would have to be and even the less important the presentation, this would seem to be borne out by much on stream experience.

So what if one could “beat the system”? What if even when the fish had all the time in the world we could find a way to shortcut the selection process and increase our chances of deception?

Obviously one might expect that plenty of other factors , some of which we can’t imagine are potentially at play. To hypothesize further then, one might expect that the more hungry the fish the more likely it would be to make an erroneous snap decision. Equally where there is a massive opportunity of lots of food in a short time, (The classic duffer’s fortnight of Ephemera Danica on the English Chalkstreams for example), the fish may be rather more “Gung-Ho” than normal. It is perhaps equally worthy of consideration that most of the time in nature an erroneous assessment isn’t overly problematic to the fish, a waste of a little energy and spit out the offending item. It is only the machinations of the angler which make an erroneous selection potentially fatal or at best inconvenient.

It strikes me that one of the significant triggers to predatory behavior is apparent vulnerability, the lions on the Serengetti sitting about under a tree, chilling in the afternoon sun; but should a limping Wildebeest wander past the whole game changes and predatory instincts kick in. The pack is on the hunt, keyed into the possibility of easy prey.

I would suggest that using the same logic it is possible, at least some of the time, to trigger that response in fish with the arrival of an apparently easily captured food item.

In a human context perhaps much the same applies when hunting (read shopping in the modern world). Yes you can research the presence of GMOs in your food, the number of calories, whether it is halal slaughtered, the sell by date and much more information all of which is readily available. But do you? And more to the point even if you are more than averagely pedantic can the offer of a bargain,” two for the price of one”,” 10% off” etc shortcut your normally extensive analysis? I would suggest that it can and that the marketing departments of most food companies fully understand that.

What would happen to our supposed decision time-line were we to add in some super stimulus, the piscatorial equivalent of “A Bargain”? Such as apparent vulnerability? After all to a predator, an easy meal is in effect a bargain, less costly in terms of effort and risk, could that result in the bypassing of normal selectivity?

Is it not likely that with the bonus of apparently “easy prey” the decision making process could be short cut, a snap decision induced in the fish?

VulnerabilityCould the trigger of an apparently easy meal short-cut the process of selection and result in more effective fly pattern?

In a recently observed example I was guiding a couple and the one angler had opportunity to cast over a clearly visible fish, not feeding overly actively but quietly taking the odd nymph or surface fly. This all in slow moving clear water (The worst case scenario for an angler in general). Casting small dries, nymphs and even more weighted nymphs elicited no response and it seemed as though the fish may have become aware of our presence. Then a cast of a diminutive and very simple soft hackle pattern, presented apparently helpless in the film. A non specific morsel that undoubtedly looked a bit worse for wear. The fly landed a fraction to the side and slightly behind the fish, it turned and ate the fly with knee jerk aggression. This after better presentations of far more perfectly constructed flies. Could it be that the “vulnerability” of the pattern was the key?

I would suggest that this and other “super stimuli” might equally short cut the decision making process (not for a moment implying that there is any great deal of cognitive behavior on the part of the fish). Most of us would accept that a negative super stimulus, for example drag on a dry fly or a splash on presentation would result in a shortcut, this time a negative selection, so why not a positive shortcut if we get the stimulus right?

Add in something that the fish particularly likes, say an Ant pattern. It is well known that trout LOVE ants and their response to ant patterns is frequently nonsensical, they expend more energy and move further to capture an ant pattern than they do other food items, real or fake. Could it be that the super stimulus of a segmented body and recollection of pleasant taste override the normal selection process? It is certainly worth a thought.

The ideas discussed here were mostly driven by a desire to consider why should very simple soft hackle patterns be so effective. Soft hackles, North Country Spiders, Emergers, Stillborns and such all lack much in terms of actual imitation but do offer up the illusion of vulnerability and/or chances of escape (in the case of emergers). Could it be that these patterns work as well as they do because they provide a triggered shortcut to the normal food selection process?

I don’t know what or even if a trout thinks, I do know that they do some things that don’t on the face of it make sense, but I would suggest that viewing their behavior in the light of this hypothesis does potentially offer some explanation.

Some examples:

The current is fast and on average the fish are less critical of fly and presentation. (decision process limited by available time).

Fish slash and burn energy during rapid emergence of caddis flies… (decision process pre-empted by the lack of time due to potential escape of the prey).

Fish in slow flat water are difficult to fool, (few limitations on the decision making process, plenty of time and increased visibility of the fly, the hook, the tippet and even the angler.)

Significant hatch of large flies (Ephemera Danica): Increased gain of calories at low effort, repetitive reinforcement of decision making (other flies have been fine to eat), limited time to make the most of the windfall.. short cut decision making and eat as much as possible..

Wild fish in remote spots: On average have never had a negative consequence and as such will eat almost anything. No evolutionary pressure to be more selective.

Fish in heavily fished Catch and Release waters, a history of negative consequences for poor decision making. More evolutionary pressure to be increasingly careful, fish more difficult to deceive.

The classic, “induced take”. Is the decision making process short-cut through the apparent risk of escape of a food item?

The overly large fly, could it be that the promise of very high calorie food easily obtained can circumvent the normal selection process and induce a snap decision from the fish?

I think that in all of these examples there is enough subjective evidence to suggest that much of the time this hypothesis holds true and that the angler can use this to become more effective at deceiving his quarry.

I would suggest that in most of the cases I can consider the idea that the decision making process is varied and that thinking in these terms many apparently aberrant behaviors could have a logical explanation. It also suggests that “exact copy” fly tying may well be one of the least effective strategies for the angler.

Something worth thinking about ?


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Thoughts on selectivity.

March 3, 2014


Thoughts on selectivity:

Much is made of a trout’s selective feeding in a great many angling publications, in fact it comes up so frequently that one would have to imagine that it is a fact, and if not fact at least commonly accepted wisdom based on subjective observation. Certainly although I don’t fish alkaline waters with strong hatches of insect I most definitely have seen fish apparently eat nothing else but flying ants for example, or become seemingly fixated on egg laying spinners that are hovering just above the surface. So selective feeding must be the thing right?

Well to play Devil’s advocate I have also just finished looking through (I am not sure that one could call it reading) a book by Jerry Hubka and Rick Takahashi called “Modern Midges”, published by Headwater Books. There are over a thousand midge patterns in there, all displayed in glorious Technicolor. A thousand different patterns of every possible interpretation of midges, from larvae to emergers, pupae to drowned cripples and to be honest half of the time I am struggling to see the difference between one and another, I have to question if the trout could. That doesn’t mean it isn’t a fascinating book.  Equally if the trout did when eating midges really require only one of the patterns in that book our failure rates on the water would be staggering. You couldn’t carry a thousand different patterns even if you wanted to and even supposing that you could find the one you wanted when necessary. That would particularly be the case when you consider that you might have to carry similar numbers of caddis flies, mayflies, stoneflies etc etc. So therefore by deduction selective feeding by trout can’t be true can it?

Two virtually diametrically opposed viewpoints based on the observation of either the fish, the angler or both. There are arguments that the trout are selective not because they are smart but because they are dumb and become preoccupied, there are those who believe the fish have such a discerning pallet that they will pick only one bug out of the drift. Which is right? Is either school missing the point?

Well let me say that I personally believe that all fish are feeding selectively all the time, the question isn’t about whether they are or are not being selective, it is more a case of how selective. They are simply being more or less selective than each other.

As a further adjunct to the equation, we tend to think of selectivity as being a “fly pattern” issue, but I would put it to you that much selectivity is a “Presentation issue”. The trout on my local streams will for the most part eat any reasonably small and dead drifting fly pattern, but no matter the fly, if that delicately feathered tid-bit should twitch in the current they won’t take it. Hell they won’t even take a real fly that twitches in the breeze. Here the fish are more “selective” in terms of presentation than they are in terms of pattern. I would venture more selective in terms of fly size than fly pattern too for that matter.

So my current views go along the lines of this:

Every trout you encounter is somewhere along a line of selectivity where at one end they will eat anything from Bananas to drowned Elephants (i.e. virtually none) and on the other they will only take a size 16 pale morning dun emerger pattern on a curved hook with silver rib and a genetic hackle of medium dun cock hackle, (equally virtually none).

SelectivityLineIt seems apparent to my way of thinking that pattern selectivity is going to be primarily a function of the prevalence of a particular insect or stage of insect at any given time. Such that selectivity itself is going to become more apparent as the density of the hatch, spinner fall or whatever increases. Even then though one might expect a distribution amongst the population of fish that some will be ultra selective and some not as picky, it is a normal Gaussian distribution found in all things in nature.

The propensity for such “selective” feeding is equally likely to be enhanced on waters which are rich, alkaline and produce regular opportunities to feed on specific occurrences of high density food availability, in effect the fish can “select” not to feed at all during periods of low food availability, something that fish in less nutrient rich waters probably cannot to do.

SelectivityCurvesYou can see larger versions of all these graphics by simply clicking on them.

One might well posture that the pattern selectivity curve would move more towards the right in the attached graphic when certain insects were prevalent and move to the left when the hatch was over or there was no hatch in the first place.

Selectivity Curve Animated

One would expect the selectivity curve to move to the right when there is a prevalence of specific insects available to the fish and to the left when there is no hatch on.

However I would equally add that “selectivity” is generally viewed as a function of the close copying of the prevalent insect or stage of insect at the time, and has given rise to the notion of the “imitation versus presentation” schools of thought as though they were mutually exclusive. To my mind selectivity combines both at the same time, a trout may well not select a fly because of its presentation but it most certainly can and will “deselect” a pattern that behaves inappropriately, here I am mostly thinking of dragging and unnatural movement of the fly. More so on waters which see good amounts of angling pressure and that sensitivity to presentation is all the more prevalent on catch and release water.

Then again there are other parts to the presentation situation, for example the presentation depth, were it the case that the fish were feeding on a specific and concentrated food source occurring at a specific depth it would make sense that presentation of the artificial occur at that depth such that perhaps the successful fly pattern is effective more due to its sinking properties than its actual construction. Much the same would hold true of presenting a floating fly in the drift where the naturals are occurring as opposed to the back eddies where they are not.

So whilst “pattern selectivity” is most likely a function of specific food availability so “presentation selectivity” could be expected to be more closely linked to angling pressure. Thus with increased angling pressure (particularly associated with catch and release fishing) one would expect the sensitivity of the fish to move to the right in the attached graphic and to the left in remote and unfished waters. This is something that is pretty much accepted as the rule for most anglers. It is probably why some have a tendency to cough up large quantities of cash to get to remote and unfished spots, very simply the fishing would be expected to be easier.

Presentation Selectivity

Presentation Selectivity is more a function of angling pressure and enhanced on catch and release waters.

To me, “selectivity” isn’t really a singular concept of close imitation of specific bugs, that is only part of it. Fish may well be selective in terms of “what they eat”, “the behaviour of what they eat”, “the position in terms of depth or location of what they eat” and perhaps a good deal more. When considering selectivity one needs to look at the overall picture. There are various pressures on the fish to be “more or less selective” based on food availability, angling pressure, quite possibly a lot else,  and in some instances one pressure will tend to outweigh another. So for example:

On a relatively infertile stream where large hatches are not the norm but where there is considerable angling pressure and catch and release fishing one might well expect fish to be highly sensitive to presentation but far less so in terms of pattern.

On waters where regular significant hatches occur the bias would tend to be towards pattern itself.

I think that this dynamic is best seen as a variable quadrant of behaviour under the influence of different “selectivity pressures”.


There is an additional, well documented and interesting variation in a situation such as “Duffers fortnight” on the chalk streams of England, where the prevalence of Ephemera Danica adults, combined with their large size (and consequently high calorie value) seem to cause the fish to  give into the pressure of making the most of the food source over a short duration such that presentation selectivity pretty much disappears, even pattern selectivity can become less pronounced simply as a result of the need to make the most of a highly nutritious food source that is only available for a very short period of time. It is as though despite the high food density and the expectation of pattern selectivity the sheer value of the feeding opportunity makes the fish “throw caution to the wind”.

So when considering “selective trout” one should perhaps look at a wide number of variables, which may well include the presentation side of the equation. “Imitation and presentation” are then both parts of the same discussion, both linked to some form of selective behaviour on the part of the fish and they cannot simply be broken into two different approaches, but rather seen as a continuum of variable factors and responses which provide a near infinite variety of situations and fish behaviours.