Thoughts on selectivity.


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.





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7 Responses to “Thoughts on selectivity.”

  1. iheartthefly Says:

    Some thought provoking words here. Thanks for sharing!

  2. paracaddis Says:

    Thanks for reading and commenting.. glad that you enjoyed it.

  3. grant Says:

    g’day from australia,
    an i wont mention the cricket, ..or at least not the second test 😉

    top stuff, enjoyed reading it, i always enjoy your writings, ive delved into your archives during office hours and found myself with much work left to do by the end of the day !
    thank you for sharing your thoughts


  4. trutta99 Says:

    Tim: your clarity of thought on the subject is brilliant. Thank you for putting this together.

  5. Gray Beard Says:

    I think the thing to remember is that the book by Takahashi and Hubka is a compilation of patterns compiled from many tiers. Interesting in that one can see how different tiers use varying techniques and materials to imitate a rather simplistic little creature. One can pick and choose the look they’d like to achieve! Then again those of us that are fanatics about fishing the waters where we may see several midge hatches come off over the course of a day often settle for simple sewing thread bodies that match the color and size of the naturals. Fly tying is such a subjective thing.

    • paracaddis Says:

      Thanks GB: This wasn’t meant to infer that all those magnificent patterns in “Midges” or any other tome of similar genre are wasted or not valuable, but simply to highlight that in the end we all seem to have our own favourites in which we have faith. I love fly tying books and am fascinated by the variations, adaptions and personal thought that goes into all the patterns out there. I just think that most of the time it is more important to us than it is to the fish.. 🙂 Thanks for dropping by and leaving a comment.

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