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  1. #1

    Default Selectivity - why and how do trout become selective feeders

    This post on Matching the Hatch prompts me to post about selectivity.

    Please forgive me for this long post. I have posted on this subject on other BBs and I have found that most fly fishers do not understand the biologic basis of selectivity or matching the hatch.

    The underlying principle behind matching the hatch is selectivity as a feeding behavior. What is it and why does it occur? Why do fish sometime eat sticks and hit strike indicators, and at other times will ignore a well presented fly that imitates the current hatch?

    There are several principles that need to be understood.

    The first rule is that any behavior is population based. What I mean by that is that when we are fly fishing, it is our nature to assume that the behavior we see is the behavior of all the fish. It is not. Just because one fish takes a fly or refuses a fly, we cannot assume that all the fish will do the same thing.

    In fact, the behavior we observe may be an atypical one. This is called sampling error. When we fish, we are sampling the populations for those fish that are susceptible to the method we are using; and therefore, any success we have is based solely on statistical variation. We are more likely to be successful when using a method that targets a greater proportion of the population, but we often forget that fishing is a statistical sampling method.

    The second rule is that selectivity is a survival mechanism and is not based on intelligence.

    The third rule is that selectivity can only occur in fertile watersheds.

    The fourth rule is that larger fish must become more selective than smaller fish if they are to survive.

    So with those principle in mind, here is my view of selectivity.

    Selectivity occurs only in situations where there is abundant food. If the fish live in a stream where food is scarce, they will feed opportunistically. They cannot be selective to just a single food source because there is never a large or sustained hatch to become selective. So selective feeding occurs only in nutrient rich environments.

    Selectivity is the most efficient method of feeding. An organism can only survive if the energy it gets from food is greater than the energy it expends to catch and eat the food. Selectivity then is a biologic necessary method that optimizes survival. It is a biologic adaptation that gives the organism that uses it a survival advantage. That is the reason that the fish feed selectively. They cannot help themselves from becoming selective feeders.

    Another biological cause of selectivity is the size and age of the fish. As fish grow older and larger, their energy (calorie) requirements become relatively greater. A large fish requires more energy to chase food and yet gains fewer calories per body weight when it eats the food. Because larger fish expend relatively greater energy to catch food, but receives relatively less energy when it captures food; it must feed effectively if it is to survive. What is crucial then is not the total number of calories in a food item, it is the ratio of the calories spent vs the calories consumed per body weight.

    For large fish, selectivity is more important as a survival strategy than for smaller fish. That is why we see smaller fish chasing food and our flies but rarely do large fish chase a fly unless it is a large fly that promises a large reward in calories.

    Once a fish feeds selectively, it cannot help itself from feeding on our fly, if the fly meets the criteria for food.

    How then does a fish become selectivity? Well, they don't do it through intelligence. Trout cannot reason. What trout can do is sample; they sample what they think is food. If the item is food and it is abundant enough, the trout will feed on it often enough that the visual pattern of the food eventually becomes imprinted. Then the fish then begins to search for this food pattern exclusively and ignores most other items that could also be food. If the food is very abundant, the fish will begin to both narrow the area it searches ( the fish feeds only on food that is in a narrow "feeding" lane) and the fish will develop a feeding rhythm. The fish mechanically moves up and down, taking the food item that happens to be in its feeding window.

    The psychologist term for the development of selectivity is Operant Conditioning, Operant conditioning - Wikipedia, the free encyclopedia as first described by B. F. Skinner. The fish is rewarded when it feeds on an item that is food and if there is enough of this particular item, the fish becomes conditioned to feed only on that food. That conditioning is what we call selectivity.

    The fish does not reason, it takes whatever is in the window that meets its search pattern. It is all automatic and the fish cannot help itself from taking our fly if it meets the search pattern, and it is in the right place (feeding lane) at the right time (rhythm).

    The question then becomes, what are the fishes search criteria (triggers)? Everyone seems to agree that size, shape, behavior and color are search criteria. We know this, not because of positive evidence, but because when the fly does not meet all or most of these criteria, the fish refuses to take the pattern when it is in the right place at the right time.

    When discussing selectivity, or selective feeding, we must realize that we are discussing a biologic system. In any biologic system there is variation in the population and there will be a variation in behavior. So although the discussion above treats a population as a single unit of identical behavior, the reality is that there will be a variance in both behavior and timing, so that what we find is similar to a bell curve distribution. Although the majority of fish may be feeding on a given stage of emergence, some fish may be still feeding on an earlier stage or may have progressed to a later stage.

    Combine this variation in behavior with overlapping or simultaneous multiple hatches, and the possibilities become confounding to the angler who is trying to "match the hatch".

    This confounding behavior, however, is not a conscious attempt by the fish to fool the angler. It is just the result of an efficient feeding method superimposed on a fish population with biological variance.

    Why do some fish take other forms of food when most are selective? Again it is biological variance. Fish are not automatons. There is room for variance, and in fact, if there was no reported variance in a biologic system we would suspect that the data was flawed or manufactured. It would be too good to believe.

    So a fish breaking out of selectivity can mean either biologic variance in a large fish or a juvenile fish that has no need to feed as selectively to optimize its caloric intake.

    It is also apparent to me that selectivity is not an all or none proposition.

    What I mean by that, is that at any given moment you may have both selective and and non selective feeding. Even more confounding, the selective fish may be feeding on different stages of the hatch; or if there are multiple hatches, there may be trout that are feeding selectively on different stages of different hatches.

    If you fish in waters where multiple dense hatches occur often, you will find yourself in such a situation. To say that the theory of selectivity is not valid because a fish takes a Royal Wulff during a hatch does not disprove selectivity. What it proves is that some fish will take a Royal Wulff when some or most of the other fish are feeding selectively.

    Biological systems have variant behavior. To observe such behavior and state that that observation disproves a theory is simply not correct in my view. The question is not how a single individual trout may behave at a single instant in time, but how the average population behaves over a length time. An individual fish may break a selective feeding pattern to take a Royal Wulff. But my view is that this does not negate the overall behavior which is selective feeding.

    Here are two Venn diagrams that illustrate what happens during a hatch and how selectivity develops.

    The vertical Venn diagram below demonstrates what happens during a hatch with the nymphs turning into emergers and emergers into adults. Note that the size of each of the portions is relative to the timing of the hatch. In the diagram below, most of the nymphs that will emerge during that hatched have already turned into adults. At any given time the number of emergers is relatively small compared to the number of nymphs and adults BUT the emergers are trapped in the film so they may be small in absolute terms but they are huge relative to the number of the insects that can be taken by the fish. Most of the adults are already in the air and the majority of the remaining nymphs have not yet begun to emerge.

    The longitudinal Venn diagram below shows the development of selectivity in the fish as they go through a transition phase from opportunistic to selective feeding. Before the hatch they are feeding opportunistically and as the hatch develop they become selective.

    If we look at the population of trout as a population distribution of feeding behavior and graph the number of fish in the Y Axis and selectivity along the X axis we get an approximation of a bell curve. The blue curve is when most of the population is feeding opportunistically and the center of the curve gradually moves to the right to the pink scale as they become selective.

    The graph below demonstrates what happens if the hatch last a long time and is regular and heavy. The population begins to cluster and feed more alike as virtually all the fish center around a core of selective behavior. There are relatively fewer and fewer outlier fish that exhibit no selective behavior. Again they move from a wide spread blue distribution to a more consistent pink bell distribution where the behavior is more uniform across all the feeding fish.

    What the graphs demonstrate is an idealized situation. For this to occur in the real word, the macro and micro environment of each fish would have to be identical. Every fish would have to have access to the same number of nymphs, emergers and adults as every other fish. That cannot and does not occur in the real world.

    So as you move from place to place in a river, the fishes behavior will mirror the change in environment from where your were to where you are.

    Isn't that why we move from a place we are not catching fish in to a place where we hope we will catch fish in? The graphs and explanation above is the scientific underpinning of why we do what we do to catch fish as selectivity develops during a hatch.

    For example, say we are faced with almost all the fish feeding super selectively as in the pink graph above. Frustrated with not being able to "match the hatch" we employ the "hatch breaker strategy" of using a fly that is completely different that the hatch but familiar to the fish when they are feeding opportunistically. So we put on an ant or a beetle or we strip a big streamer. Unfortunately this does not work and with our knowledge of what is going on with these "Red Zone" super selective fish, we decide to move.

    We move and find that our hatch breaker strategy is picking up fish. Why? It is because we have found a population that is in the "Green Zone" and those at the lower end of that green bell curve are still open to feeding opportunistically to the hatch breaker fly we are presenting.

    As a physician with an undergraduate degree in chemistry, I know that theories are propositions that seem to correlate with reality. By that I mean that theories do no have to be absolutely true for them to be of usefulness. All they need to do is to predict or explain behavior such that we can use the theories to help us remember how the world works and to predict what will occur.

    What do I mean by the above? As an example we can use Newtonian physics which explains and predicts the motion of planets. Newton believed that gravity was due to the attraction of heavenly bodies. We now know that Newton was wrong. Einstein showed that gravity is actually a wave that is pushed out from an object that distorts space. Well so what?

    Well, it is apparent to me that even if Newton's theory was wrong, we can still use Newton's laws of gravitation to predict the motion of heavenly bodies. It may be wrong but it correctly predicts what happens and what will happen.

    Therefore, the theory of selectivity does not have to be absolutely true. If selectivity explains the behavior of a fish and predicts what will happen during a hatch, it gives us useful information and gives us a framework with which to understand and predict behavior.

    Selectivity and any other "theory" of fishing does not have to be absolutely true. It just has to explain and predict behavior better than a competing theory.
    Last edited by silver creek; 06-06-2018 at 11:18 PM.


    "Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy

  2. #2

    Default Re: Selectivity - why and how do trout become selective feeders

    Well it is clear how you chose "Silver Creek" as your moniker!

  3. #3
    Join Date
    Apr 2009
    Languedoc/near montpellier
    Blog Entries

    Default Re: Selectivity - why and how do trout become selective feeders

    Very interesting post Silver

  4. #4

    Default Re: Selectivity - why and how do trout become selective feeders

    Great information Silver! You are spot on with your analysis describing these "outliers" during selective feeding. In medical research we refer to these as "standard deviations from the mean". As with all biological & physical systems, there is no such word as "absolute". The vast majority of a population will fall on or near the "mean". However, you will always have approximately 10-15% fall within 1 SD (standard deviation) and as much as 1-3% fall with in 2 SD. When conducting research we are always well aware of these deviations and have to account for them in our data analysis.

    Again, great post! Very informative as your posts always are!

  5. #5
    Join Date
    Sep 2013
    North Central Montana

    Default Re: Selectivity - why and how do trout become selective feeders

    Another well crafted, scientifically informed, relevant post. You truly make a contribution to the forums here, Silver! Thank you.
    "Sometimes the least important thing about fishing is fishing." --Roderick Haig-Brown

  6. #6

    Default Re: Selectivity - why and how do trout become selective feeders

    That's a lot of data; more than I can absorb right now. But I wanted to float a theory by you just based on my observations and some understanding of where trout come from.

    It seems to me that trout fishing difficulty, in order of species from easiest to catch to hardest to catch, looks something like this:

    (1) Cutthroat (notoriously stupid; great beginner fish);

    (2) Brook trout (will eat anything provided you don't spook them);

    (3) Rainbow trout (genetically homogeneous in the modern United States, easily fooled by generic flies unless they have 'gone native' and re-wilded in rivers like the Henry's Fork);

    (4) Brown trout (commonly caught only on small nymphs and tiny tippet, especially in highly-pressured environments. Brown trout on dry flies especially rare).

    In support of this ranking I offer up two observations: first, creel surveys in many waters record a roughly 10:1 catch rate of rainbows to browns.

    Secondly, on my 30th birthday I caught 30 fish on the White River in Arkansas, and then cut myself off. I was fishing a two-fly rig of an egg pattern with a Trout Crack dropper behind it. 15 of the fish were rainbows and 15 were browns. ALL of the rainbows came on the egg, while the browns all took the (much smaller and more naturalistic) Trout Crack.

    My theory to explain this is that humans have imposed artificial selection on these species in exactly the order you would expect based on their catchability. Brown trout have by far the longest history of interaction with anglers by virtue of coming from Europe. The American species were encountered in this order: brook trout, then cutthroat, then rainbows (which is all mixed up). But in terms of propagation, rainbows proved to be the easiest by far and thus were supplemented most heavily into waters subject to angling pressure. Brookies were largely left in place, while cutthroat did not really see heavy angling pressure until the early 20th century. Thus in terms of "angler hours per fish species," the rank is exactly as set forth above; it correlates perfectly with catchability.

    My bottom line from this is that trout are likely to grow harder and harder to catch over time. Our great grandchildren may all specialize in micro-midges on 8X tippet!


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  8. #7

    Default Re: Selectivity - why and how do trout become selective feeders


    When you go fishing, you are sampling the behavior of the fish in the watershed that you are fishing. There are at least 2 variables which are the type of fish and the watershed. Then add the amount of fishing pressure, the time of day, the location in the river that you are fishing, whether there is a hatch or not, and we can see that there are multiple important and less important variables that affect selectivity.

    Brook trout generally live in freestone streams and in smaller rivers/streams that are less fertile. Brookies in this type of environment are non selective and can be easy to catch. So fly fishers may think that brookies are genetically programmed to be easier to catch, but I believe that the truth of the matter is that the environment also makes them easier to catch.

    Similarly, before the population crash of Cutts in the Yellowstone River in Yellowstone National Park, they could be very difficult to hatch. Buffalo Ford in the YNP was one spot that was heavily fished and the cutts were very selective.

    Rainbows generally favor the faster areas of a river and Browns the slower sections. Fish are easier to catch in faster water because they have less time to decide whether to take a fly, we can get closer to the fish, and it is harder to get a close look at flies when the water surface is broken.

    There are other factors besides selectivity that make fish difficult to catch. For lack of a better word, I'll use shyness or spookiness. Some species may be spookier than other species. I don't know for sure but most fly fishers generally rank brown trout as the spookiest.

    I think that your list is probably very close to how most fly fishers rank difficulty in catching fish by species

    However, I think this ranking is due to many factors besides the just the fish species. I think there is variance due to the type of water the fish are in, and that fishing pressure affects how difficult the fish are to catch. The truth is that unless multiple trout species are in the same location in the same population distribution, it is difficult to separate species differences from the differences that result from the environment in which the species prefer to live.


    "Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy

  9. #8

    Default Re: Selectivity - why and how do trout become selective feeders

    I place environmental conditions first. Spring Creeks and rich tail waters with similar characteristics invariably produce the most selective trout, whether Rainbow or Brown, particularly if they are managed for natural reproduction rather than stocked. These fish, as illustrated in Vince Marinaro's wonderful, "In the Ring of the Rise", get a good look at their food from further away and with less pressure to grab it before its gone than their riffle dwelling counterparts. They can afford the time and waste less energy rising through the water column for nothing by being selective feeders. On technical waters that are inhabited by both Rainbows and Browns, their behavior can be quite similar. I fish three habitats that offer terrific opportunities to observe both species behavior; Silver Creek and the Missouri in the West and the Delaware in the East. The Delaware is a little different in that it had an actual wild strain of Rainbow planted in the 1880's from non-migratory McCloud River stock from California...they were called California trout at that time, the moniker "Rainbow" had yet to be coined. These are slow sexually maturing, long lived fish unlike the domesticated hatchery developed strains planted in the Montana rivers (with the exception of the Kootenai where they are native), Silver Creek and the Fork. Though it is hard to breed the "white leghorn" out of the chicken, I rarely complain about genetics when some Missouri River slab shows me the colored string (as long as it has all its fins and didn't wash over out of Hebgen). With heavy Hendricksons or PMD's emerging, both species will take up their feeding lies according to their dominance in their micro habitat hierarchy and feed similarly. Given equal size, Browns almost always will dominate being more pugnacious by nature. Under less fecund feeding conditions a variable is common; a sparse spinner fall on Silver Creek in the early evening will have lots of young Rainbows feeding out in the open, fewer of the mature rainbows, and they will be on the banks or tucked into a weed bed Browns seem to be in evidence. Then the Sun goes beyond the golden hills and the next good head you a Brown. Browns are more prone to nocturnal feeding than Rainbows but given Drakes at dark or midnight stonefly emergences, Rainbows will join right in. I do not believe, but I can not know, that this is in response to humans who go home for dinner. Trout have been around for millions of years and instinctually adaptive behavior is more likely an evolutionary response to Pterodactyl, Heron and Raptors feeding from above than new comer, Homo sapiens.

    I frequent Spring Creeks and tail waters because I enjoy the selective trout, match-the-hatch challenge. I do not have to catch a lot of fish, I did that in my youth. I will walk past a nice rising trout because its lie does not interest me. My passion is the hunt for the big, difficult trout tucked into a hard to get a good drift to, convoluted current, bank recess. I can suspend time fishing to that fish.

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  11. #9
    Join Date
    Sep 2013
    Eastern Iowa

    Default Re: Selectivity - why and how do trout become selective feeders


    Don't let my question derail your thread in a irrelevant direction, but it would seem to fit the scientific experiment model anyway. I found your post very interesting, and my first reaction was how I might usefully apply it to my fishing situation. 75% of my trout time is spent fishing spring creeks primarily stocked with rainbow trout. Entire life spent chasing protein pellets. Diet conditioned to a level a native trout would never know. Eat pellets, and eat them the moment they are available of go to bed hungry.

    Generally they will be at least somewhat receptive to live or artificial baits immediately upon stocking. Sometimes they all seemingly agree to commit suicide as a group, and you can literally watch 75% of them harvested (by bait dunkers, not by me), in just a few hours . These fish have not seen a nightcrawler, meal worm, Panther Martin, wooly bugger or a Copper John in their entire life. They will usually hit these new food sources within a few days, if not 30 minutes after stocking. The fish that survive a few weeks, and quite likely a catch and release near death experience (or two), appear to get some stream smarts. They'll hover off the rapids picking off nymphs etc. Get spooky in clear water on a bright day, ignore flies if your presentation is not perfect (even live bait). Point being they often start acting like real trout.

    Any thoughts on what I am observing here?

  12. #10

    Default Re: Selectivity - why and how do trout become selective feeders


    Great job, both in content and format.

    The one additional thing I would offer for discussion regarding trigger mechanisms is perhaps behavior/movement of the prey . Certainly in still water conditions, each species of zooplankton has a very distinctive pattern when moving thru the water column. The movement may be more readily detectible than any physical characteristic, certainly from any sort of distance. I'm sure it makes as much sense for fish to feed selectively on zooplankton as any other food item, so it may also make sense for them to key in on a particular pattern of movement to identify a targeted species.

    It may not fall under the heading of selective feeding, but I find presentation for trout often times is the key between take and rejection. Best example I can think of is an egg laying caddis. That up and down dapple certainly could be very recognizable to a fish. I think the same may be true with the swimming style and speed of nymphs and other prey.

    I think movement may be a powerful trigger. It seems like there is no more positive identifier that a thing is actual alive than the ability to move in a manner other than dead drift with the current. Of course the same can be said if the fish are keying in on a prey which does indeed try to keep still on the water, movement would raise a red flag.

    Once again, great job, very thought provoking.

    The more you know, the less you need.

    Tenkara Fly Fishing

    Tenkara Fly Fishing Blog

    "People tend to get the politicians and the fishing tackle they deserve" - John Gierach, Fishing Bamboo

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