| Fly Fishing Devon | "Why does a trout take your fly?"

This page explores in greater depth some of the ideas introduced in our article in the May 2020 edition of The Field magazine. It is available to   read online

Another article How does a trout catch a fly?  was written during the first COVID-19 lockdown.

The UK and the USA: Two countries united by a common misunderstanding of trout behaviours - Selectivity

The title refers to the transatlantic sharing of fly-fishing techniques during the 20th Century. In the early years the British sent the Americans these dainty high-riding dry flies to tempt fastidious trout. At the end of the century, the Americans sent the British low-riding dry flies to catch ultra-selective trout.

At times confusing, but ultimately a worthwhile exercise for anglers starting to fly fish for wild brown trout on rain-fed freestone rivers. For example, Dartmoor rivers such as the East and West Dart, the Devonshire Avon and River Yealm.

The article focuses on fishing with a dry fly and introduces a little-known book - published in 1914 - by an American George La Branche (The Dry Fly and Fast Water) which offers a better foundation for fishing on fast-flowing freestone rivers, than some of the classics written by authors who fished mainly on placid UK spring-fed chalkstreams.

Halford's 1889 book Dry-Fly Fishing in Theory and Practice enjoys an international reputation for developing and promoting a method of catching trout by casting upstream to a rising fish with a dry fly tied to be an exact imitation of the insect that the fish has been seen to take from the surface. Halford's method transformed fly-fishing for trout into a scientific endeavor (Gibbins, 2018, Schweibert, 1979).

This certainly has the hallmarks of a scientific approach: an underlying theory, a method that can be used to test the theory, and like all useful theories, ways to refute or extend the theory.

Skues and Halford

The first, and most famous, challenge came from Skues who pointed out that trout could be caught on a nymph presented a few inches below the surface. This 'minor tactic' generated more heat than scientific light. In order to gain acceptance from dry-fly purists, Skues retained the essential elements of Halford's theory: he cast upstream to a feeding trout with an artificial fly tied to represent an insect that autopsy confirmed had been consumed shortly before capture. Skues (1914) simply modified Halford's technique by altering the position of the artificial fly in the water column.

Gingrich (1974) suggested that Skues effected a revolution and Schullery (2008) called Skues a tentative rebel. Skues himself introduced the nymph to be used as a supplement to, and in no sense to supplant or rival, the beautiful art of which Mr. F. M. Halford is the prophet. (Skues 1914).

From a scientific point of view Skues' ideas are no threat at all to Halford's theory, but it enraged Halford and his disciples. For example this quote from Halford "Those of us who will not in any circumstances cast except over rising fish are sometimes called ultra purists and those who occasionally will try to tempt a fish in position but not actually rising are termed purists... and I would urge every dry fly fisher to follow the example of these purists and ultra purists." Quoted by Bark (1992) in " A History of Flyfishing",

Skues borrowed heavily from Halford to present a parallel method of catching trout. Explanation of the heat generated by the debate is of interest to students of human interactions, but the debate itself threw no light on the veracity of independent theories that seek to explain the reaction of trout to food on (Halford) or below (Skues) the water surface..

Replication and extension: Doubts from abroad

For a scientist it is important that observations can be replicated by independent laboratories. And that is just what happened next when Halford was approached by two Americans: Theodore Gordon and George La Branche for advice on dry-fly fishing.

Theodore Gordon

Theodore Gordon imported English fly-fishing tackle and flies, but needed to alter the English flies to precisely match the insects hatching in the Catskills region. This was to be expected; insects vary between regions and this alone does not challenge Halford's theory of precise imitation.

George La Branche: A fresh pair of eyes and a logical mind

In 1914 George La Branche published The Dry Fly and Fast Water. La Branche had spotted a logical flaw in Halford's theory. He wrote: When fish are feeding upon some particular species of insects it is quite logical to assume that an imitation of that species will appeal to them more readily than an imitation of any other. But when insects are numerous, as they are on occassions, and the fish are moving about, the chance of the artificial fly being selected from among the great number of naturals on the water is one to whatever the number may be. (La Branche 1914 p60 emphasis added).

The logical problem - of a trout selecting an imitation fly amongst a large hatch of identical insects (selectivity) - spotted by La Branche exists to this day on rivers with substantial fly hatches.

Furthermore, in contrast to Halford and dry-fly purists, La Branche advocated fishing the water rather than casting to a specific rising trout. This was necessary because of the river conditions he faced - few rising trout and fast water. These conditions will be familiar to fly-fishers on Dartmoor and other spate rivers.

La Branche did not consider himself to be a dry-fly purist. He once remarked: I consider that the real purist wastes countless joyous and active hours waiting, according to theory, for the fish to rise before he starts. That was standard behaviour in England. La Branche, not one to waste minutes, let alone hours, learned to fish dry flies in fast water, casting to trout he knew were there. but which had not revealed themselves by rising. (Betts 2002)

La Branche challenged Halford's theory in another fundamental way that remains a live issue to this day.

La Branche rejected Halford's insistence on using a precise imitation of the natural fly. Today we use the term selectivity to refer to this central tenet of Halford's theory.

La Branche quotes, with approval, Professor of Zoology James Rennie who wrote in 1833: I have used the phrase "pretended imitation" as strictly applicable to by far the greater number of what are called by anglers artificial flies, because these rarely indeed bear the most distant resemblance to any living fly or insect whatever, though, if exact imitation were an object, there can be little doubt that it could be accomplished much more perfectly than is ever done in any of the numerous artificial flies made by the best artists in that line of work. (Rennie 1833, p 137-8)

La Branche's attitude prompted a debate with Louis Rhead in the 1920s on exact imitation in tying trout flies. It was held at The Anglers' Club of New York. The confrontation was described in a letter written by Eugene V. Connett 111. Louis Rhead is described as, a conceited little fellow, a very delightful artist, and the worst fly designer that God ever put breath in. La Branche chewed poor Rhead into small pieces and spit them out. It was quite cruel and I always felt a bit guilty about having arranged the debate:' Connett wrote. (Belknap, 1992)

La Branche's deviation from Halford did not go unnoticed by Theodore Gordon. Paul Schullery (1987 p119-120) refers to a letter donated to the American Museum of Fly Fishing by La Branche's daughter written by her father probably in the 1950s in which he describes Gordon as ... A great friend and companion. But then reveals that Gordon ... told me that I was ?belittling? (word unclear) the theory of dry fly fishing. He (Gordon) agreed with Dewar and Halford that what I was doing was an affectation and that the dry fly should be used on slow flowing water over rising fish only. I (LaBranche) was upset more than a little, but persevered with my idea.

But La Branche was not alone in finding that Halford's approach - developed in relatively sedate English chalk streams - was not an applicable technique in faster-flowing freestone (spate) rivers.

Baigent: An independent Yorkshire voice

La Branche corresponded with Dr William Baigent (1862-1935) a medical doctor in Northallerton, a market town in North Yorkshire (UK). He is remembered for Baigent’s Brown, and his influence on the Catskill tradition of fly-tying through correspondance with American authors George La Branch and Preston Jennings. (Rob Smith, 2018).

Like La Branche , Baigent had struck out on his own. He did not follow the local fly-fishing culture. Born and living his life in North Yorkshire he fishing dry flies in an area of England where fishing soft-hackled wet flies reigned supreme. He fished dry flies at a time when the Halfordian doctrine of precise imitation was de rigueur. He eschewed precise imitation, instead devoting many years to breed Old English Game Cock to supply hackle to tie his unique 'variant' style of flies including this long-hackled Baigent's Brown. Asked what the Baigent Brown was tied to represent, Baigent replied: "It is not tied to represent any fly, it is tied to catch trout.(Roberts, 1994, p 215)

The influence of Baigent is clear in the four dry flies in the corners of this picture of a collection of La Branche's flies held by the American Museum of Fly Fishing (Bett, 2002)

It's surprising that La Branche did not adopt Baigent's two-fly technique: If trout are nymphing, a nymph or wet fly could be mounted on the point, whilst a dry fly could be mounted on the dropper. Nicholas Fitton (1992) gives a detailed description of the history, and his experience using this almost-forgotten technique which is very effective on local rivers whether or not fish are rising.

Baigent's influence on American fly designs live on; Swisher and Richards (1975 p.72) recommended searching the water with "heavily hackled" spiders, variants and bivisibles.

The Gold Ribbed Hare's Ear: Halford's Achilles' Heel ?

In Halford's time the Gold Ribbed Hare's Ear (GRHE) was, and continues to be, a very effective pattern. It doesn't look like any natural fly. Halford added wings to make it acceptable to dry fly purists, without any deterioration in its attractiveness to trout. In his books Halford regarded it as the most successful [fly] of modern times .

Perhaps a little cruelly,  Skues  wrote: At one time the late Mr. F.M. Halford was a great advocate of the Gold-ribbed Hare's Ear [GRHE], but I believe that latterly his enthusiasm for precise imitation induced him to give it up, successful pattern though he knew it to be, because he could not explain its success to his satisfaction.

Halford discarded the GRHE because it simply did not fit into his theory that dry-fly fishing required presenting a fly tied to imitate an identified floating dun.

A scientist would have stopped at this point and asked what it was that made the GRHE such an effective artificial fly.

Like La Branche before him, Marinaro (1970 p 62) was a critic of Halford's strict insistence on precise imitation "Every detail of the natural fly's anatomy must be included, no matter how absurd the result. For example "... the segments of the natural fly were counted and the same number of turns of ribbing were included in the artificial"

Marinaro uses a strong word, 'absurd', to describe this "out of many, one" approach E pluribus unum  . The angler is casting an artificial fly during a hatch of naturals - the statistical odds are stacked against the angler.

The myth of the educated selective trout?

Nevertheless Halford's central tenet - using an artificial fly to imitate the natural fly that a trout is feeding on - lives on in concepts such as 'selectivity', and 'the educated trout'.

Originally published in 1971 as 'Selective Trout' , and republished in 2018 as 'Selective Trout The Last Word on Stream Entomology and Aquatic Insect Imitation', the front cover proclaims that Doug Swisher and Carl Richards have written The book that changed fly fishing in America.

Wyatt (2013) commented: "Selective Trout, possibly the largest selling fly-fishing book of all time, directly and indirectly provides the theoretical background for much of what has been written since."

Wyatt wrote 'What Trout Want: The Educated Trout and Other Myths' in response to 'Selective Trout' .

You don't have to read beyond the dust covers to know the authors opinions: Doug Swisher and Carl Richards have written the last words on imitating aquatic imitation; Bob Wyatt regards 'selectivity', 'matching the hatch' and 'the educated trout' as myths.

Swisher and Richards do seem to perpetuate Halford's attitude to fishing with a dry fly. For example they wrote: The fly fisherman who knows what is hatching and has realistic imitations will consistently be more effective than the angler relying on trial-and-error methods....The right fly is one that resembles the natural so closely that the fish seem to prefer it to the real thing.. The still image is taken from Doug Swisher's video Strategies for Selective Trout in which he describes the characteristics of the right fly as being : size, shape and colour in that order.

The fast flowing spate (freestone) rivers you fish are not the main focus of Swisher and Richards' book: 'The types of streams that are conducive to the dry fly and selective rising trout are: 1. Slow pools interspersed with riffles or rapids ... 2. Uniform flow, unbroken water ... ' i.e. placid limestone and freestone creeks (Chapter 3 The Need for Realistic Imitation). This attitude of implying that dry-fly fishing is more effective in slow flowing water echoes the views of Halford and his followers - dry-fly purists.

Do Swisher and Richards have any relevance for fly fishing on freestone rivers?  Yes. "There are periods, though, especially during low water in quiet pools, when delicate casting and realistic imitations are needed."  [emphasis added] (Swisher & Richards, 1975)

We saw above that La Branche was criticised for dry-fly fishing in rapid water :He (Gordon) agreed with Dewar and Halford that what I (La Branche) was doing was an affectation and that the dry fly should be used on slow flowing water over rising fish only. (Schullery, 1987)

Swisher and Richards explain that trout in slow flowing water become ultra-selective because in placid fertile rivers they enjoy long inspection times that enable them to discriminate between small insects and flotsam drifting towards them. To further complicate dry-fly fishing on these rivers, smaller insects hatch in greater numbers than larger flies, and trout become more selective as the size of the natural insect decreases. "When fishing a #28 (i.e. hook size=28) hatch, for example, a 1-mm variation from the natural means at least a 30 per cent dimensional error - which is disastrous and results in nothing but refusals from the trout." (Chapter 3).

An important feature of the first edition (1971) of Selective Trout was the introduction of No-Hackle flies with a body that rests in the surface film.

This represented a significant departure from the conventional Halfordian dry fly at that time tied with hackles wound around the hook so that the fly would land with just hackle and tail touching the water.

Schullery (1987, p 101-2) considered that Marinaro's redefinition of dry flies is far more useful than the narrower Halfordian concept: "We must begin with the proposition that no matter how dry the fly is, it must touch the water and be exposd to the air at the same time. If this idea is carried out to its logical conclusion, all of us must agree that if the smallest portion is exposed to the air no matter how deeply submerged the fly may be, it is still a legitimate form of the dry fly," (Marinaro 1970)

You might think that Marinaro's definition of what constitutes a dry fly is just "dancing on the head of a pin". But it has the practical effect of extending dry flies to encompass   'emergers' and 'cripples'  (partially emerged insects) that are represented by No-Hackle patterns.

The No-Hackle approach was based on Vince Marinaro's observation that the wings and body of the natural fly trigger the rise. However Swisher and Richards No-Hackle dry flies proved unpopular because the wings were difficult to tie in the correct position on the body and lacked durability. Consequently they fell out of favour. But the No-Hackle design remains popular because of the effectiveness of the design concept.

We now have a choice of no-hackle flies with - "big-wing-and-soggy-bottom" - features that trigger a trout's rise (Kenyon 2020). For example, Fran Betters Haystack and Usual - impressionistic flies designed to handle the rigours of the AuSable's fast moving water - inspired Caucci and Nastasi's Comparadun, that in turn inspired Craig Mathews' Sparkle Dun as well as the wing and spiky body in the Wyatt's Deer Hair Sedge. These are especially important design features for dry flies in rain-fed fast-running (freestone) rivers running off Dartmoor, and those fished by George La Branche in the early years of the last century.

Devon based author, and Wild Trout Trust founder, Mike Weaver introduced UK anglers to comparaduns in his widely acclaimed 1991 book "The Pursuit of Wild Trout". He continues to demonstrate the tying technique at the annual Snowbee Open Day each Spring. Mike fished with American angler Sid Neff (Agro 2000) over many years -  "who has influenced my fly fishing for river trout more than any other angler" (Weaver, 1991, p 5).  Their friendship enabled an influential positive transatlantic transfer of fly-fishing innovations - such as the use of deer hair - to the benefit of anglers fishing UK freestone (spate) rivers.

Trigger #1 ...

or Trigger #2 ...

or Trigger #3

The problem faced by trout in these British and American spate (freestone) rivers, are summed up in this passage from John Bett's (2002) essay on La Branche. For a trout in six inches of water, the window above him is twelve inches across. Slow water moves at one foot or less per second; fast water at about four feet or more in the same period of time. In the swifter currents a fly will cross the front half (six inches) of the window in one-eighth of a second or less, depending on the speed of the water. Even for an animal conditioned to these circumstances over millions of years, a tiny fraction of a second is not much time to go through everything needed to discriminate and decide whether to intercept or refuse an object. So the trout has to make the best guess he can.

In summary, No-Hackle flies were invented to cope with selective trout on slow-moving limestone / chalkstreams (UK) rivers. Their design was influenced by Marinaro's insights into factors that trigger the rise. No-Hackle flies were found to be effective on both types of river - fast and slow moving - whereas Halford's precise imitation hackled dry flies are relatively ineffective on fast-flowing freestone rivers.

Is selectivity still alive and well in the UK ?

I detect two forms of selectivity in the UK fly-fishing literature: 'selective trout' and 'selective-educated trout'. The more reasonable form is expressed by Pat O'Reilly in the Preface to his excellent book "Matching the Hatch" (2006): "Matching the hatch ... can greatly increase your chances of success. That is not to suggest that it is every day on every river and lake that the trout feed in a selective way; sometimes all that the fish are looking for is food, ..."

The 'selective-educated trout' makes a brief appearance under the subtitle "The hyper-cautious trout". "On heavily fished waters the largest trout may be battle-scarred survivors of many campaigns, continually on the look out for suspect flies. These hyper-cautious fish even reject a proportion of the natural flies that that drift past, perhaps because of a crumpled wing or a missing tail. And when large, succulent mayflies first appear they are often viewed with suspicion and shunned for quite some time. To catch an educated trout you will probably need a very close imitation of the natural creature it is feeding on at the time, ..." (O'Reilly 2006, p 18)

Other British writers remain wholeheartedly committed selectionists.

This quote is from British authors Peter Hayes and Don Stazicker in a Kindle book published in 2019: "Some writers have denied selectivity in trout, and criticised Swisher and Richards (“Selective Trout”, Crown Publishers 1971), notably Bob Wyatt (in “What Trout Want”, Headwater Books 2013). Our work puts us firmly among the believers in it."  (Page 44) [emphasis added]

Bob Wyatt's book referred to by Hayes and Stazicker is a detailed critique of the terms 'the educated trout' and 'selectivity' as used by some British and American fly-fishing authors.

The phrase 'selective feeding' is often portrayed as leading inexorably to 'educated trout' . For example, Hayes and Stazicker (2019) provide this description of the relationship between learning / education, databases, selectivity and fly-tying: "Given that we know that trout can learn from experiences ... it seems sensible to assume that every single successful and unsuccessful food capture all the way up from their being 1 inch long to the mature length at which we are trying to catch them, forms part of a database of experience that they use all the way through life."

Now, that database of food items will, by the time we are trying to catch our trout in its maturity, amount to millions. And it is that database that we are trying to fit our imitation into in such a way that it matches the images of successfully eaten items, and does not jar.

Maybe this hyperbole is just artistic licence. Trout can learn through experience (Johnsson & Kjällman-Eriksson 2008, Brown et al (2013), Reebs (2008), Kloepper 2016). But the claim that "every single successful and unsuccessful food capture...forms part of a database...they use all the way through life."  creates a false impression of the capabilities of the 'educated trout'. And leads to the - quite unnecessary - need for precise imitation in artificial flies. "The fly tyer needs to pay attention to the construction and materials of an imitation to get these matches right." (Hayes and Stazicker 2019, p63).

Interim summary

The prominence given to so-called 'selective or educated trout' can create the impression that selectivity becomes their  modus operandi - a permanent habit ingrained by learned experiences.

Wyatt (2013) concludes that:
  • "For fly fishers, 'selective' and 'educated' trout are the same thing; the educated trout becomes more and more selective." (emphasis added).
  • "The term 'selective' is invoked to explain just about every reason a trout refuses or ignores a fly.. After all, if a trout has become too educated and choosy for you, there is no shame in not being able to catch it. "
  • "Most of what has been written on fly fishing for trout is based on a single premise: Trout are intelligent, suspicious, even capricious creatures that are wise to our tricks."
  • This may explain the apparent disconnect between current fly-fishing theory and contemporary behavioural sciences.

    The popular view of 'PhD trout'

    These beliefs about the cognitive abilities of trout are not new. They were ridiculed 70 years ago by H.B. McCaskie in his book with the apt title 'The Guileless Trout' (1950 p 78)  : "The belief, or delusion, that the trout is a highly intelligent creature is of comparatively modern origin, since it is a by-product of the development of the dry fly". McCaskie explains how followers of Halford found that when ever more precise imitations "..failed something had to be done in defence of injured pride, and the thwarted angler evolved the theory of a highly educated and shrewdly reasoning adversary ." McCasky realised that the trout's brain is incapable of these cognitive feats based on anthropomorphism. H.B. McCaskie's older brother, Norman, was a close friend of G.E.M. Skues so they were well-aware of Halfordian ultra-purist views.

    The popular view of 'PhD trout' is a profound misconception on many levels that are explored in the rest of this article. Briefly, it implies that selective trout retain search images in long-term memory for use over their lifetime. In other words selectivity becomes the modus operandi  for some trout. But this is very unlikely, a search image is a process employed to focus attention for a relatively short period of time whilst foraging for cryptic prey. To forage optimally predators need to be able to rapidly discard and replace search images to reflect changes in prey density (Ishii & Shimada 2009). Search images are probably stored in limited-capacity   visual short-term memory  (for example, Dawkins, 1971. Experiment 3, Fig 3). Short-term memory is short-lived (usually from seconds to minutes) and labile; it can rapidly decay and can be easily erased by competing information.(Ishii & Shimada 2009).

    In fact retaining 'selectivity' for a particular food item may be counter-productive: "Limited memory for food items may be related to the fact that in some environments, food sources can be quite variable, both in space and in time. There might be a trade-off (perhaps in terms of how many neurons can be allotted to each task) between remembering about old food sources, and learning to handle new ones. " (Reebs 2008)

    The growing influence of ethology on fly design

    Halford transformed fly-fishing for trout into a scientific endeavour (Gibbins, 2018, Schweibert, 1979). But there has been a tendency in the last 50 years for some to decouple the science from the endeavour. It's about time we faced up to that issue.

    Many years ago Dr William Baigent (1862-1935) pointed out that advances in the scientific study of behaviour can provide important clues to the design of effective trout flies.

    In a letter to W. K. Rollo, Baigent wrote : "... the Baigent's Brown ...is based entirely on what the trout think themselves... It was made by finding out what combination of dry fly hackles would easily stimulate what Pavlov calls the trout's "investigating reflex", ... The fly will start such ocular reflexes when properly presented which will more often than not get the other reflexes in motion, and so the fly is taken in lamb-like fashion without suspicion and fuss." (Rollo, p 91-2, 1944 reprint).

    Dr. Baigent may have read in the medical literature about Pavlov's (1849-1936) award of the 1904 Nobel Prize in Physiology or Medicine for his research on the physiology of digestion.

    Bob Wyatt (2013) pointed out that ethology - the study of animal behaviour - can make an important contribution to our understanding of trout feeding behaviour in terms of an adaptive evolved response, and showed how this can inform the design of effective trout flies.

    In his book The History of Fly Fishing in Fifty Flies Ian Whitelaw describes the thinking behind the Deer Hair Emerger: "For Wyatt, the goal of good fly design is not exact imitation but the presentation of one or more key triggers (shape, size and posture that form a general 'prey image') to elicit the feeding response...The Pheasant Tail Nymph and the Gold-Ribbed Hare's Ear clearly possess one or more of these key triggers .." (Whitelaw, 2015, p 207-8)

    During the 1950s-70s one of the founders of ethology Nikolaas Tinbergen developed concepts - such as sign stimuli / 'triggers'/ key stimuli, search image, supernormal stimuli and fixed action patterns that give us a scientific framework for understanding the success of fly-tying innovations such as the 'triggers' used by Wyatt, as well as older simpler patterns such as the Gold-Ribbed Hare's Ear and Frank Sawyer's Pheasant Tail Nymph.

    In 1973 Tinbergen won the Nobel Prize for his discoveries. His work was made available to a wide audience through the book and film Signals for Survival made with fly-fishing author Hugh Falkus.

    "Tinbergen found, it is often the case that quite crude tricks suffice, itself perhaps a reflection of animals’ greater reliance on simpler rules of thumb." i.e. heuristics (Hutchinson & Gigerenzer, 2005)

    The American writer of "A Modern Dry Fly Code", Vince Marinaro expressed it thus:"I am continually astonished by the fact that the most killing flies in fly-fishing history are of very simple construction" (Schullery, 1987 p232). This begs the question "Why did American fly-fishing authors remain wedded to using artificial flies that aim to be precise imitations of the natural insect especially when faced with large or mixed hatches?" The answer may lie in a difference in the attitude of American and European scientists to the roles of Nature and Nurture in the control of animal and human behaviours.

    Ethologists were European zoologists who focused on evolved, species-specific, innate / instinctive behaviours (i.e. Nature). They studied a wider range of animals - birds, fish and insects - usually under natural / field conditions.

    At that time research on human and animal learning was dominated by the American behaviourist Frank Skinner (1904 –1990). He is considered to be the most influential psychologist of the 20th century. In the 1960s and early 70s ethologists and behaviourists clashed over their very different approaches to recording, analysing and interpreting behaviour. This tension is summed-up by the phrase Nature versus Nurture .

    Behaviourists were psychologists who studied learning in rats and pigeons under controlled laboratory conditions and stressed the role of Nurture.

    Behaviourists rejected the notion that instinct played a role in the development and expression of behaviours. Instead, they stressed the flexibility of behaviour shown by individuals, rather than the evolution of behaviour in species.

    The aim of behaviourism was the discovery of general laws of learning that could be applied to all species including humans.

    Behaviourists viewed the brain at birth as completely empty, a blank slate (tabula rasa). Learning / experience fills the void. There were no limits on what an animal could, or could not, learn about stimuli, responses and consequences. This so-called 'principle of equipotentiality' was at the heart of Skinnerian behaviourism but was challenged in the 1970s.

    The prominence of behaviourism in the 1960s may explain why best-selling fly-fishing authors such as Swisher and Richards in the USA, and Clark and Goddard in the UK reached the conclusion that 'educated' trout were difficult to catch because of a lifetime's learning experiences.

    Skinnerian behaviourism is still a popular explanation of trout behaviour. For example, Hayes and Stazicker's conclusion (2019)  [see above] that trout construct a ".. database of experience that they use all the way through life".

    Are there limitations to behaviourism ?

    Behaviourism's 'principle of equipotentiality' began to crumble in the 1970s. "The fact is that psychologists of learning have essentially ignored biological contributions to learning phenomena." (Schwartz, 1974)

    In 1970 the prestigiuos journal Psychological Review published Martin Seligman's article On the generality of the laws of learning . It became a  Citation Classic . Reflecting on its significance in 1980 Seligman wrote:

    "I intended it as an experimentely based attack on the tabula rasa principle. This was highly unpopular at the time, and Garcia, Paul Rozin, I, and others took considerable flak from traditional learning theorists. One major learning theorist said of Garcia’s findings, ‘They are no more likely to be true than you would find bird droppings in a cuckoo clock.’ The claim that natural selection might have influenced associability itself did not fall on wholly deaf ears, however. Younger learning theorists, ethologists, and cognitive psychologists under Chomskian influence found the claim congenial, if a bit ill-defined."

    Seligman's theory of 'Preparedness of association' was a major theoretical breakthrough, it .. bridges a gap between the American learning tradition and the European ethological tradition, ...(Schwartz, 1974)

    In my experience, by the mid-late 1970s ethologists and behaviourists had became less divided by the Nature - Nurture debate. Both groups came to accept that genetic and environmental factors interact to control the development and expression of behaviour (Schwartz, 1974).

    The importance of evolution is now increasingly recognized in the American fly-fishing literature. But now evolution is presented as the explanation for selective educated trout. For example, "Evolution has made trout an extremely finicky, wary, and intelligent fish species, even though instinct, acute senses, and innate genetic programming are what trout possess, rather than humanlike intelligence." (Supinski 2014 p 61).

    But - apparently - they only display these talents in chalk streams, spring creeks, and tailwaters. In addition, if you  "Combine these characteristics with catch-and-release and you get some of the most selective trout in the world" (Supinski 2014 p 61).

    Supinski comments "Based on my personal experience, I find that for tough, selective trout on today's pressure waters, even the color of the thread and hook matter." (Supinski 2014 p 68).

    I'm left wondering if selective trout are selective because of their evolutionary heritage, or because they learn to be selective in rivers that are popular with anglers.

    What are trout 'prepared' to learn ?

    Learning is important for an animal's survival. As well as structure, evolution has shaped what an animal can and cannot learn. It is now recognized that innate factors control learning especially when it has survival value (Garcia, 1955. Seligman & Hager, 1972. Schwartz, 1974. Pearce, 1997).

    According to the theory of 'Preparedness' / 'Selective association' :
  • 'Prepared' refers to what an animal can learn about stimuli, responses and consequences. Animals have evolved to pay attention to, and hence learn readily about, stimuli that are good predictors of significant events such as food or danger. There is some evidence that 'preparedness' is present from birth indicating a genetic predisposition. (Pearce, 1997 p 71-4)
  • 'Learned irrelevance' refers to an animal's ability to learn that certain stimuli are not good predictors of significant events such as food. These stimuli are ignored when they are subsequently paired with food (Pearce, 1997 p 71-4). This will come as good news for ultra-selectionists who might ponder how the presence of the hook does not interfere with precise imitation of the natural fly.
  • A human example of 'preparedness' is the sauce bearnaise effect. Seligman (1980) gives this description:  “Sauce bearnaise, an egg thickened tarragon-flavored sauce, used to be one of my favorite foods in the world. One evening, in 1966, I had sauce bearnaise on filet mignon. About six hours later I began to throw up and spent the next several hours retching. After that, sauce bearnaise tasted foul to me.“ 
  • Seligman knew that a bug was going around and that it wasn't the sauce that caused his illness.
  • Nevertheless, his aversion to the sauce lasted 13 years.
  • The learning was selective. He was only averse to the sauce.
  • The effect cannot be explained in terms of classical conditioning
  • The same effect was observed in rats under controlled laboratory conditions
  • It might be tempting to conclude that 'selectivity' in trout observed on popular rivers where anglers practice catch-and-release is an example of 'prepared learning'. Maybe these trout become selective as a result of learning that eating a fly is followed by being caught. But the problem with invoking 'preparedness' as the explanation, is that a defining feature of prepared learning is 'selectivity'. Only a particular fly would be avoided in the future. Changing the fly should overcome the problem. Trout that have been caught and released continue to eat, and could be caught again. And, of course, only trout that have experienced catch-and-release would be selective.

    Research has shown that catch rates do decline on catch-and-release fisheries (Askey et al.2006; Young and Hayes 2004). Askey et al. suggest that "the population contained a group of highly catchable fish that were quickly caught and then learned to avoid hooks" (the same two fly patterns were used throughout the study ). The remaining fish in the population were deemed less vulnerable to angling effort. A similar distinction between more- and less vulnerable trout was made by Gubbins (2018) to explain the demise of dry-fly fishing for stocked trout in Michigan rivers. The costs could not be justified at the equivalent of an eye-watering $16-$62 per fish caught across several rivers. The problem arose because a lot of the stocked fish were not caught by anglers. Gubbins explains this in terms of the behaviour of stocked fish that were deprecated by Halford writing in 1913: "When first thrown on their own resources they will take any fly offered to them...and a large proportion soon succumb to the wiles of the dry-fly fisherman.".

    In a somewhat acerbic comment Gubbins (2018) wrote [Michigan] "State fish managers schedule stocking operations for these rivers in May, thus offering the dry fly hero the opportunity in June to impersonate Halford on the Test thereby reliving the myth"

    Halford understood that stocking chalkstreams is done for financial reasons. Supinski (2014 p 3) relates how he and a friend caught 60+ trout on a stocked Test beat that resulted in the beat being closed for the rest of the season because the fish were uncatchable thereafter.

    On the two New Zealand rivers examined by Young and Hayes (2004) fish were either not seen after being caught and released or "were more likely to be scared by anglers or required smaller, low-profile flies before being caught than naïve trout. ".

    I think learned avoidance of a predator (anglers) is a simpler explanation than that fish "learned to avoid hooks" (Askey et al.2006). It is revealing that both Supinski and O'Reilly mention the need for stealth, concealment and careful presentation as ways of dealing with selective trout. This is consistent with the suggestion that selective trout on popular venues have learnt to associate anglers with danger. Of course trout can also learn to associate wading anglers with dislodged food, the so-called  "San Juan Shuffle". Not all trout respond in this way to catch-and-release, Schill et al. (1986)reported that cutthroat trout were re-captured an average of 9.7 times during the 1981 season. But I've heard that they do have a reputation for being angler-friendly.

    Background matching & prey detection

    On fast-flowing freestone Dartmoor rivers trout consume a wide variety of aquatic and terrestrial insects (Elliott, 1967). In  optimal foraging theory terms   these trout are generalist predators.

    Cryptic (camouflaged) prey are hard to detect. They are thought to have evolved coloured markings that match their background to avoid detection.

    Predators that hunt for cryptic prey learn "search images" for particular prey types (Dawkins 1971).

    Are trout similarly 'prepared' to enable them to learn to detect and consume insects with cryptic colours ?

    Using the apparatus shown here, Johnsson & Kjällman-Eriksson (2008) investigated foraging by brown trout parr.
    They comparing foraging:
  • for brown-coloured maggots (Calliphoriade) on a cryptic (brown) background
  • or
  • brown-coloured maggots (Calliphoriade) on a conspicuous ( green-coloured) background

  • They found that:
  • Conspicuous prey were easier to find than cryptic prey.
  • Search time for cryptic and conspicuous prey decreased with experience at the same rate indicating that trout learn to detect cryptic and conspicuous prey by forming a search image.
  • Cryptic prey remained more difficult to locate than the conspicuous prey throughout the experiment.
  • These results suggest that trout are generalist predators that form search images to survive in an environment with changing feeding opportunities such as diurnal variation in   invertebrate drift.

    The results are a good example of the so-called 'evolutionary arms race' between predator and prey.
    "Our results suggest that colour matching  [ i.e. background matching ] may confer relatively long-term survival advantages [ for prey ]  in natural systems as predator perceptive constraints seem to limit search image formation and / or search rate and, thus, foraging efficiency on the colour-cryptic prey." ( Johnsson & Kjällman-Eriksson, 2008) [emphasis added]

    The earliest and most famous study of 'background / colour matching' is Kettlewell's experiment in the mid-1950s to study the evolutionary mechanism of industrial melanism in the peppered moth (Kettlewell 1959, Rudge 2005, Martinowksy 2007 larger version of image ).

    "H. B. D. Kettlewell's work on the phenomenon of industrial melanism is widely regarded as the classic demonstration of natural selection and one of the most beautiful experiments in evolutionary biology." Not suprisingly it came under attack from .."advocates of creation and intelligent design".. (Rudge 2005).

    In summary:
  • The bark on trees in heavily polluted forests is dark. These trees harbour dark-coloured moths.
  • The bark on trees in unpolluted forests is light. These trees harbour light-coloured moths.
  • Kettlewell placed light and dark moths on the trunks of trees in polluted and unpolluted areas.
  • More light than dark moths were eaten by birds in polluted forests
  • More dark than light moths were eaten by birds in unpolluted forests
  • It's worth making several points about these experiments involving birds (Kettlewell) and trout (Johnsson & Kjällman-Eriksson ) because they relate to our attempts to design effective trout flies.
  • More conspicuous prey were eaten than inconspicuous (cryptic) prey.
  • Novel colour of the prey in Kettlewell's study did not prevent birds consuming the moths.
  • This suggests that the birds' initial search image did not rely on colour, but relied instead on other factors such as shape and / or size.
  • The moths novel colour did not prevent them being eaten. In fact, it had the opposite effect. Consumption of moths with the novel (conspicuous) colour exceeded that of moths with the cryptic colour that matched the colour of the trees.
  • The evolution of background matching in aquatic insects may protect them from predation ( Johnsson & Kjällman-Eriksson, 2008). If so, Halford's dictum of precise imitation of the colour, shape and size of natural insects is likely to result in artificial flies that are effectively camouflaged from a trout's point of view. This problem was identified by  George La Branche  in his 1914 book The Dry Fly and Fast Water.

    'Background matching' may have implications for designing artificial flies. This comment by Swisher & Richards jumped out at me: "The right fly is one that resembles the natural so closely that the fish seem to prefer it to the real thing.. " [emphasis added]. Maybe the enduring success of some artificial flies is because they are more conspicuous - and therefore preferred to the natural fly they represent. The question remains: What can be added to the search image to increase the conspicuousness of the artificial ?

    Hunting by search image

    What is a 'search image? This example of human search images may help. It was developed in the 1940s to train members of the The Royal Observer Corps Club to identify friendly and hostile aircraft. Recognition is based on a group of simple features - WEFT (Wingshape, Engine configuration, Fuselage shape and Tail type)

    It is worth exploring the scientific origins of these terms, what features they may contain, how they enable animals (and humans) to pick out an object without checking it closely, as well as the current limits in our understanding. See (Hunting by Search Image ,Kenyon 2020).

    'Prey image' was originally called "hunting by search image" by the ethologist Jacob von Uexkull in 1934. It enables an animal to concentrate attention on one prey item at a time.

    Predators develop a search image based on the prey that is encountered frequently. It can be replaced by a different search image as the density of prey changes and the animal shifts from one type of prey to another. Selective feeding in trout is probably the result of adopting a search image.

    Most importantly, a search image is short-lived. It has a temporary existence. This distinguishes a search image from the persistent genetically-based response to a trigger / sign stimulus / sign stimuli (Ishii & Shimada 2009).

    What is the difference between a 'sign stimulus', the plural 'sign stimuli, and a 'search image'? The differences are subtle . In trout species-typical behaviour (e.g. a 'rise') can be elicited by either sign stimuli, or a search image. In the absence of a search image, behaviour is controlled by sign stimuli. There is an important and fundamental difference between these mechanisms.
  • A single sign stimulus or a group of sign stimuli are "... effective [in eliciting the behaviour] for all members of a species"
  • In a search image"... the stimulus characteristics may be effective [in eliciting the behaviour] for only one animal. 
  • (Hinde 1970 p123) [emphasis added]

    Hinde points out that there is wide overlap between the concepts 'search image' and 'selective attention'

    How does 'selective attention' / search image control a trout's behaviour?

    'Selective attention' refers to the process animals and humans use to filter information in their environment. Selective attention enables animals to focus their attention in order to accurately detect stimuli that, for example, signal food, and ignore irrelevant stimuli. Incidentally, this explains why trout don't reject a fly tied on a hook. The hook is ignored.

    In humans it is called "The cocktail party effect" or'selective hearing'. It refers to the brain's ability to focus selective attention on a particular stimulus while filtering out a range of other stimuli. For example, focusing on a single conversation in a noisy room.

    Luuk Tinbergen  (Niko Tinbergen's younger brother )  (1960) was the first to suggest that prey images worked because of selective attention. This generated a great deal of subsequent research that has been reviewed in detail by Kamil and Bond (2006). Kamil and Bond review laboratory studies that  "have firmly established the existence of the searching image effect" that develops after "multiple successive encounters with a single prey type" resulting in selective attention .

    In my opinion there are three "take-home" messages :
  • Search / prey images are learned as a result of experience by individual animals.
  • Prey images are not stored in the trout's brain ready to be retrieved from memory when a hatch starts. They are created anew after consecutive encounters with insects in a hatch.
  • Prey images are individually separate and distinct. Prey images are discrete. Several prey images do not operate simultaneously. Selective attention makes it very difficult to attend to two sources of information at the same time - "The cocktail party effect".
  • In addition to using prey images, trout may hunt for prey in particular locations, called 'hunting by expectation'. According to this theory animals learn to focus their foraging on an area that contains an abundance of food. This situation is familiar to fly-fishers. Insects are eaten at various stages in their life cycle from nymphs to spinners at different locations in the water column.

    "If a forager has learned that the most common prey type is available in a specific microhabitat, then the cues associated with that habitat will provide a basis for associative priming. And, once it is hunting in the chosen microhabitat, the forager is likely to encounter the same prey type many times in succession, providing a basis for sequential priming" (op cit).

    In other words, trout may selectively search locations within the water column where they expect to find food.

    What features are contained within a search image ?

    It's all very well me saying that trout probably use a search image to detect prey, and that we stand a good chance of catching them if our artificial fly mimics that search image, but what feature(s) of the natural should we emphasize in our artificial fly?

    We just don't know for sure. But scientists who study visual search have given us clues that it might contain:
  • One feature that makes the natural fly stand out from all the background 'clutter' in the trout's environment. Why? Because if the search image contains two or more features (e.g. color and shape) that must be present to define the prey item, and any of these features is shared by some other thing(s) in the drift, then making the correct choice between potential food and clutter takes more of the trout's time. Even humans find this a difficult task - the scientists call this a 'conjunction display' (Eckstein, 2011, Ishii et al 2009).
  • Location feature:  Performance in a search task can improve when a restricted location in the search space contains the target.
  • There is remarkable overlap between our current understanding of search image features, and the suggestions made by George La Branche in 1914 for features to incorporate in an artificial fly:
    "My own experiences have convinced me that imitation of the natural insect is absolutely necessary, and I put the forms this should take in the following order — the order of their importance :
  • 1st — Position of the fly upon the water, [i.e. location]
  • 2nd — Its action  [i.e.movement]
  • 3rd — Size of the fly.
  • 4th — Form [i.e. shape, appearance] of the fly.
  • 5th — Colour of the fly."
  • These components are often found in contemporary descriptions of features to include in artificial flies. Swisher (2016) regards size and shape as more important than colour in artificial representatives of the natural fly.

    Size and shape together with movement are listed in a standard textbook on animal behaviour.   "Most predators encounter a large number of different prey species that they have to discriminate from non-prey. The three most commonly used cues are size, movement and shape." (McFarland 1985 p. 231)

    How important is colour in a search image?

    Before reading this section, I must prepare you with this perceptive comment from American fly-fishing historian Paul Schullery (1987, p229): "you must read a fair amount before you realize that you are hearing, in all these books [on fly-fishing theory], a disjointed and almost painfully polite debate"

    Opinions differ on the importance of colour. For example, Marinaro wrote this about imitating the colour of duns on the surface:
    "The dun, riding lightly above the surface film, is never clearly defined at the point where the trout sees, inspects and takes the insect. Form and behavior are the most important elements. Any meticulous attention to color or detail is wasted effort” [emphasis added] (Marinaro, 1995 p26)."

    But Marinaro made a clear distinction about the importance of colour in representing flies lying on, or penetrating the surface:
    "Anything that breaks through the surface film is no longer obscured by the oblique rays or the diffusion above the [surface] film. Accordingly spinners, large terrestrials, emergers, and rising nymphs are extremely well defined as to color, form and parts. Special attention should be given to the remarkable transformation of the spinner wings as seen below the surface film..(op cit)

    Others go to great lengths to match the colour of the natural dun, and disagree with Marinaro.
    For example, this from Hayes & Stazicker (2019) .
    ...we are in awe of his  [Marinaro's ] groundbreaking work, but with the totally unfair benefit of 40-years-on digital full-colour underwater photography, we disagree: in the Plain Sight Zone of the window the fly is very clearly seen in colour as well as form, most of the time: frequently enough that we shall now always want to get the colour as right as we can...So the colours of flies are extremely important, since we want to maximise full congruence with prey image.” (Hayes & Stazicker Page 311-312)

    They quote this comment in “Dick Walker’s Trout Fishing”: “Conditions in which the trout cannot see the colours of the fly floating in his window must be rare indeed, if they ever exist.”

    Hayes & Stazicker (Page 103) conclude:
    "Both he [Dick Walker], and we, are sure that Marinaro’s conclusion that the flydresser can be content with imitating a) the footprint in the mirror and b) the wings, is wrong. Thorax hackle and tall wings are very far from sufficient to secure its commitment to eat the artificial. Our fly needs a body, and its body can and will be inspected closely, well lit, in the window, and therefore requires the flydresser’s full attention." (Hayes & Stazicker Page 103)

    That's a lot of the accumulated knowledge in the American fly-fishing literature to throw out of the window! I'm reminded of   Pascal's wager.

    My approach to colour in artificial flies is perhaps idiosyncratic:
    1. Cryptic colours have evolved to protect insects from predation. Therefore mimicking these colours of the natural insect in an artificial fly may be counterproductive.
    2. Kettlewell's experiment on colour / background matching showed that birds captured more conspicuous moths than cryptic moths. Conspicuous insects were preferred over the cryptic version.
    3. Conspicuously coloured daphnids face a higher risk of predation from three‐spined sticklebacks (Raveh et al, 2018)

    4. These results suggest that:
      1. Conspicuous colour did not disrupt bird and fish search images.
      2. Conspicuous colour could be incorporated in an artificial trout fly without disrupting the prey image.

    Of course, whilst colour may not be an essential part of a search image, there is no evidence that meticulously mimicking the colour of the natural insect will harm an artificial's compliance with the search image (i.e. Pascal's wager).

    What can be added to the search image to increase the conspicuousness of the artificial?

    We are now in a better position to answer that question. Can we increase the conspicuousness of the artificial?

    Why would we want to do this? For two reasons:
    1. La  Branche pointed out that when an angler casts a traditional dry fly during a hatch of naturals - the statistical odds are stacked against the angler.
    2. Swisher and Richards described the desirable feature in an artificial dry fly: "The right fly is one that resembles the natural so closely that the fish seem to prefer it to the real thing" [emphasis added]. Swisher and Richards are describing what ethologists describe as a 'supernormal stimulus' - an artificial stimulus that are more effective than the real thing in eliciting a behavioural response. Would a conspicuous feature allied with a trigger / search image meet their requirements?

    Unlike conventional trout flies, it's very difficult to design a supernormal artificial trout fly from scratch. Conventional flies are designed with the natural fly as our model. So-called 'attractor flies' may be the closest we have come to incorporating supernormal stimuli in a dry fly. It is interesting that some of these attractor flies have evolved at the hands of one or more fly tyers. The results in the next section warns that a supernormal fly may look nothing like a natural.

    Could it be that the enduring success of some artificial flies is because they contain a search image with an additional factor that makes them more conspicuous - and therefore preferred to the more subdued / cryptic / camouflaged natural fly?

    The Royal Wulff attractor pattern resembles no particular natural insect. Sometimes referred to as a 'hatch-busting fly' to cope with selective feeding. It is the result of developments made to a drab UK dry fly - the Coachman - that became popular in the USA. The modern version retains the double-hackle bi-visible introduced by E.R. Hewitt . The hair-wing was added by the American Lee Wulff. An example of UK USA co-operation to produce a prey image with a conspicuous red band that may account for its long-lasting success.

    Tup's Indispensable remains a popular and effective local (Devon) fly, with an international reputation. According to Whitelaw (2015), it inspired Skues' series of nymphs as well as American fly tyers: Leisenring, Hidy and Nemes. Precisely what natural fly it represents is unclear.

    The prescription for the mixture of colours (cream, orange, lemon, yellow, and crimson) and their proportions used to construct the thorax remained a commercial secret for several years after the inventor's death in 1911 (Courtney Williams 1979, Whitelaw, 2015, Kašpar, 2017).

    Maybe the colour of the thorax makes the artificial stand out from natural cryptic-coloured flies of a similar size and shape. Local (Devonshire Avon) expert Cedric Potter even caught sea trout during the day on Tup's Indispensable. He fished it as a dry fly or nymph. It went through several versions with increasing amounts of red / pink dubbing. I should explain. It's not just that sea trout are shy, fussy eaters. Sea trout generally don't eat anything on their return from the sea. They won't eat your, or anyone else's fly, even those created by the good Lord. Cedric's fly achieved supernatural status, one step beyond supernormal!


    Supernormal stimuli is the phrase used to describe artificial stimuli that are more effective than the real thing in eliciting a behavioural response.

    Herring gull chicks peck at a red spot on their parents's bill to induce their parents to regurgitate food. Chicks will also peck at a model consisting of a red spot against a yellow packground.

    However it is possible to construct a model that is even more effective than a real head by using a red pencil with three white bars at the end. This is an example of a supernormal stimulus.

    In this experiment the supernormal stimulus (the Stick) received about 25% more pecks from gull chicks than the natural head, a model of an adult head, or a model of the adult's bill (Tinbergen and Perdeck, 1950)

    Are trout born with the ability to hunt prey ?

    Trout are probably not born with the ability to recognize and differentiate between chironomids, mayflies and stoneflies. This is not surprising given geographical variation in available food items across salmonid habitats and the size of juvenile fish. However, young salmonids do exhibit a set of probably innate feeding behaviours that enable them to approach objects in the drift, suck them in, spit them out, or ingest them (Neuswanger et al 2014). This section describes evidence that these behaviours are controlled by a set of sign stimuli. The interrelationship between sign stimulus, sign stimuli and search image is  subtle, but important. Put simply a sign stimulus / sign stimuli control(s) the behaviour of all members of a species. The effects of search images vary across members of a species because they are formed as a result of an individual's experience.

    Sign stimuli are like sheets of paper with a title heading. Search images are these sheets of paper with pencilled notes that can be erased.

    The rest of this section describes recent research into identifying sign stimuli that control prey hunting in fish. It starts with an early classic experiment using toads. This is useful because it conveys how simple sign stimuli are, without involving baggage from the fly-fishing literature.

    The scientific discipline   neuroethology  (Zupanc 2010) explores how sign stimuli are processed in the brain to control animal behaviour. The next video is based on an early classic study in neuroethology. The research involved several steps:
  • Establish the sign stimuli that can, and cannot, elicit the behaviour.
  • Identify neurons in the brain that respond to these sign stimuli.
  • Suggest ways in which these cells interact to control the behaviour.
  • This study serves to introduces the steps followed in recent research showing that several sign stimuli act together to control feeding in fish.

    This video   shows the visually guided prey-catching behaviour of the common toad. It demonstrates how the toad selects prey objects from non-prey on the basis of their size, movement and shape. The main message here is the simplicity of the sign stimuli that release a complex behaviour (Ewert 1993).

    Recent research shows that fish are born with an innate connection between sign stimuli and species-typical stereotyped behaviours that enable them to hunt and consume prey from very shortly after birth. The main aim of this research was to examine brain cell activity in real-time during prey capture. A first step was to select a stable behavioural baseline. The results of this element provide valuable insights into sign stimuli and stereotyped hunting behaviour.

    When they start to swim zebrafish exhibit an innate ability to use simple sign stimuli to hunt and capture live prey (paramecium). After a successful capture, the larva assesses if it is food or not. If it is not judged to be food (e.g. an air bubble), the larva spits it out and swims away from it images here (Akira et al. 2013).

    Bianco et al (2011) found that small moving spots evoke eye movements and J-turns of the tail, which are defining features of natural hunting in zebrafish at this age. Larger spots elited turning-away avoidance behaviour.

    These effects have been replicated. Approach behaviour was elicited by small moving rectangles and avoidance behaviour when the size and velocity of the stimulus were increased. (Trivedi & Bollmann 2013) These results suggest that several sign stimuli   movement, size and possibly shape control the behaviour.

    The next question is "Are these features independent of each other? Is each feature capable of eliciting the hunting response on their own, or do they work in concert with each other?

    Like previous research in toads, zebrafish hunting responses are triggered by a combination of visual features. "Size, contrast polarity [dark or light] and speed of motion interact, such that stimuli that are large, dark, and fast are most effective in triggering hunting responses." (Bianco & Engert 2015). The results that support this conclusion are available here 

    In the simple terms of our sheets of paper analogy for a sign stimulus, we have 3 sheets of paper labelled:
  • size
  • contrast polarity [dark or light]
  • speed of movement
  • Thus several sign stimuli are more effective than a single sign stimulus in eliciting hunting behaviour in a fish. This is probably an example of the   law of heterogeneous summation: "The independent and heterogeneous features of a stimulus situation are additive in their effects upon behaviour."

    The authors discovered 'feature-analyzing' brain cells that they suggest are responsible for detecting prey. These included neurons that responded to large, dark, moving spots - i.e. the most effective stimuli combination for eliciting hunting behaviour." We identified NLMS [Non-Linear Mixed Selectivity] neurons that preferentially respond to large, dark, fast-moving spots, which are the visual stimuli that were most effective in evoking hunting responses. These neurons are therefore candidates for mediating the perceptual recognition of optimal prey-like visual objects."

    In addition, they found that dark spots had the greatest effect on eliciting hunting behaviour of any of the other single features they tested, and this was accompanied by finding "neurons that were highly selective for dark spots". Similar neurons in frogs and dragonflies have been described as "bug perceivers" by previous researchers. Bianco & Engert (2015) suggest that because dragonflies and larval zebrafish attach prey from below "selectivity for dark targets might represent an adaptive feature of the visual system for discriminating prey against a relatively bright background." This is consistent with Goddard and Clarke's suggestion that the body of a dun in the trout's window is seen as a 'dark silhouette' rather than in colour. The importance of colour in trout flies is  hotly debated.

    The next question is "Do zebrafish continue to rely on a simple set of sign stimuli, or do they form a search image?". Predators develop a search image based on the prey that is encountered frequently. A prey image will increase hunting efficiency. This has been demonstrated by Lagogiannis et al (2020). They studied the development of hunting behaviour in larval zebrafish by comparing hunting behavior (prey pursuit) and performance (prey capture) between larvae that had been reared with or without live prey.

    They reported that:
  • 32% of the 651 hunts initiated by zebrafish that had prior hunting experience resulted in prey capture compared with a 21% capture rate for zebrafish with no prior hunting experience; a statistically significant difference.
  • Zebrafish with no prior hunting experience managed to capture prey
  • Experience of feeding on live prey increased capture efficiency.
  • The authors discuss the possibility that refinement of a heuristic 'rule-of-thumb' is responsible for development of improved hunting efficiency. " We cannot exclude the possibility that larvae posses an  adaptive toolbox  (see Todd and Gigerenzer, 2012; Todd and Gigerenzer, 2007) of distinct, preset, hunting strategies. This would imply that the role of learning is to utilize experience to find the best match between the set of available hunting behaviors and the particular foraging environments a larva encounters." Lagogiannis et al. (2020) [emphasis & link added]

    Lagogiannis et al. (2020) are referring here to a startlingly simple way of explaining complex animal and human behaviours - heuristics [ Gigerenzer (2001);Hutchinson & Gigerenzer (2005); Gigerenzer & Gray (2017); Hamlin(2017) ] Heuristics enable "fast, frugal, and computationally cheap decisions. Heuristics are composed of building blocks that guide search, stop search, and make decisions."  (Gigerenzer 2001)

    In this video mathematics philosopher Gregory Wheeler explains how humans and animals use the Gaze - or Tracking - Heuristic to intercept  moving objects without solving differential equations. And how the co-pilot Jeffrey Skiles used the Gaze Heuristic in the Hudson River incident

    I have recently proposed that heuristics / rules-of-thumb are ideally suited for enabling prey capture by adult brown trout. (Kenyon 2020). I agree with Lagogiannis et al. (2020) that fish may also use a heuristic rule-of-thumb search image to recognize prey

    Interim summary

  • A small set of features - movement, size, background-matching / contrast and possibly shape - control feeding in fish from birth.
  • Size, shape, colour and movement are also found in the  fly-fishing literature  as features of varying importance in trout flies.
  • Selective feeding by trout after multiple successive encounters with a single prey type could be an example of a behaviour controlled by a heuristic search image, rather than the result of an 'educated' trout comparing potential food against a database containing a record of every insect ever eaten.

  • What do mistakes tell us about prey images?

    Fly fishing depends on trout making mistakes, often and persistently !

    In freestone rivers : "... all offerings must be quickly devoured and captured, since the trout's window of interception is small, and opportunity comes infrequently. It is interesting to stomach-pump a fish and investigate the ingestion process of high-gradient freestone stream trout. Often twigs, plastic/rubber bands and gravel might be found alongside aquatic invertebrates. The trout’s need to quickly intercept and capture food far outweighs the selective process." (Supinski 2014 p.10)

    Heuristics may offer an explanation for this impetuous behaviour. Trout may be using a simple Recognition Heuristic to survive in their fast-moving environment. The Recognition Heuristic is part of an  adaptive toolbox  of distinct, preset, hunting strategies. The simple rule-of-thumb is to choose a recognized object. Searching is stopped whenever potential food is recognized. No further information is looked up about the recognized object (Goldstein and Gigerenzer 2002, Hutchinson and Gigerenzer 2005).

    I describe how trout may use a Recognition Heuristic to identify prey here in an earlier article.

    Trout need to react to potential food very quickly. Betts (2002) calculated that a trout lying 6 inches below the surface had one-eighth of a second or less, depending on the speed of the water, to react to food. It is possible that trout recognize an edible object when they perceive an incomplete prey image missing some element(s) size, shape, movement, or colour contrast. Bianco & Engert's (2015) finding that hunting was elicited when a dark large stimulus was presented to larval zebrafish supports the suggestion that a sub-optimal set of sign stimuli is still capable of eliciting behaviour. Data available here. The brain cells responding to this feature were described as "bug percievers" in frogs and dragonflies. This may account for 'mistakes' by trout. Furthermore trout have an innate mechanism for dealing with mistakes - their sense of taste.

    Young salmonids exhibit a set of innate behaviours: approach objects in the drift, suck them in, spit them out, or ingest them (Neuswanger et al 2014).

    Neuswanger et al, (2014) used video cameras to record in 3D the feeding behaviour of juvenile Chinoook salmon during a four month period after they emerging as fry in an Alaskan river with potential trout food familiar to fly-fishers: chironomids, mayflies and stoneflies.

    The fish displayed the ability to ingest, visually inspect and reject items in the drift. But the fish often made mistakes. They ingested a suprisingly large number of non-food items. The research revealed the importance of considering the impact of non-food debris is understanding salmonid feeding strategies.

    Neuswanger et al (2014) reported: Among all potential food items fish pursued:
  • 52 % were captured and quickly expelled from the mouth
  • 39 % were visually inspected but not captured and
  • only 9 % were ingested
  • The most interesting finding was the high percentage (52%) of items captured and quickly spat out after capture (see frequency chart), compared with the low percentage (9%) retained. Interestingly 39% of items were visually inspected and rejected.

    This finding is confirmed by Jason Randall's (2014 p. 160) observation  "During underwater filming, I have seen trout expel items faster than you can blink. " ...The actual time the trout held some of the objects in their mouths was less than a second, which would make a hookset impossible even if the strike detection was immediate."

    Randall offers a plausible explanation for this behaviour: "the gustatory [taste] sense determines whether the trout will swallow the item or expel it. Even in visually oriented predators such as trout, a meal may be rejected based on taste once it is in the mouth. ... Younger fish with less experience will experiment with objects by taking them into the mouth only to immediately expel them. " Randall (2014 p. 160).

    All anglers miss fish. This is usualy put down to angler error; lack of attention or delay in striking. But it is possible that no matter how fast an angler strikes, the fish will have spat out the fly before any angler has time to react.

    The average reaction time for humans to a visual stimulus is 0.25 seconds. Given the time taken to raise the rod, take slack out of the line, and the speed of a trout's reaction time, it's not surprising that many fish are missed. The fly-fishing literature rarely includes estimates on the number of fish missed by anglers. Nicholas Fitton is a rare exception. He estimates that on freestone rivers in the North and West of the UK, rising 30 fish for every 10 caught is "pretty competent performance." Fitton (1992, p 144-6)

    What do prey images tell us about 'inspection' and 'refusal'?

    In addition to a simple rise, Marinaro described two additional rise forms - compound and complex that were interpreted as evidence of the trout's close inspection of natural and artificial flies.

    Marinaro makes the important point that compound and complex rises are seen in response to natural as well as artificial flies. And both types of fly can be refused. It's always surprised me that - having invested effort - a trout would refuse a natural fly. Is a simpler process involved? Recent neuroethological research may provide an answer.

    Compound rise

    "The compound rise is essentially a simple rise wherein the trout takes more time to examine the food source. It will continue to drift downstream with the fly and may even turn a little before it either takes or refuses it." Description from Fishing Small Flies By Ed Engle

    Complex rise

    "The complex rise occurs when the trout has considerable doubt about the food source. As the trout drifts downstream with the fly, it will stay further away from it and take longer to inspect it, it will either turn and quickly pursue the fly or refuse it. Vince Marinaro says that once the trout decides to turn downstream and chase the fly, he will never refuse it" Description from Fishing Small Flies By Ed Engle

    In his description of the Compound rise Marinaro writes: "... in the compound rise the trout will stay with the insect (or artificial fly) throughout the entire drift, constantly inspecting while deciding whether to take or refuse." (Marinaro, 1995 edition, colour pages following text page 8)

    I recently offered an alternative, albeit novel, explanation of a trout's apparent refusal after a period of inspection (Kenyon 2020).

    During the downstream drift the trout's window is very small because the trout is close to the insect on the surface, "literally nose-to-nose with it" (Randall, undated). I suggested that breakdown in the trout's Tracking Heuristic may lead to so-called refusals.

    Neuroethological research enables us to be more specific, and suggests that complex and compound rises are caused by trout adjusting their distance from prey, rather than 'inspecting' ot 'refusing' natural or artificial flies.

    Bianco et al's. (2011) findings on eye movements during hunting by larval zebrafish are consistent with this suggestion.
  • Every zebrafish hunting episode starts with eye convergence.
  • Eye convergence is maintained throughout the stereotyped hunting sequence.
  • " We propose that eye convergence allows larval zebrafish to use binocular vision to position themselves precisely with respect to their prey. " Bianco et al. (2011)
  • "In several cases we observed larvae making fine adjustments to their final position prior to the capture swim, including cases where they used pectoral fin movements to “back-up” and slightly increase the distance to their prey "
  • This is remarkably similar to the trout's behaviour when it makes compound and complex rises.

    In a literature review Linton (2020) concluded "that vergence is one of our most important absolute distance cues for near distances. The consensus seems to be that “as targets get nearer, vergence information plays an increasingly important role in distance perception”, with vergence providing “critically important information” in reaching and grasping (Quinlan & Culham, 2007)."

    Diagram from Puig et al. (2010) Schematic explanation of the angle of eye vergence.

    Author of "Selectivity" Matt Supinski (p63) suggests that "Compound rises usually exist when the trout has been hooked before, has received substantial pressure on catch-amd-release waters, or there is a combination of phases in the hatch activity..." Alternatively if you are fishing on Dartmoor rivers you may encounter compound and complex rises when trout need to adjust their position prior to ingesting your fly.

    Are search images modified through experience?

    The answer to this question is Yes. Fish are born with a combination of sign stimuli that enables them to locate food items, and stereotyped movements that enable them to hunt and ingest prey. It may be convenient to regard this combination of sign stimuli as the basic search image. Obviously the prey that is hunted changes as fish grow. The basic search / prey image is 'plastic' to enable this change.

    "it would appear here that learning is constrained (i.e.  prepared) to expecting specific information about the environment to instruct the parameters of particular behavior of the developing animal (Bateson, 1981; Todd and Gigerenzer, 2007). In this case [larval zebrafish], early foraging experience could allow larvae to learn and adapt their behavior to the food sources available in their environment. "  Lagogiannis et al. (2020) [link added]

    Throughout its life, an individual trout's basic search image may be altered as a result of maturation and experience of food in a particular environment. For example, as a trout grows it needs to eat larger tems of food. We saw above that larval zebrafish approached small moving spots, larger spots elited turning-away avoidance behaviour. It is reasonable to suggest that the approach and avoid dimensions will increase with growth.

    In addition the stereotyped hunting behaviour of a young fish could be described as a fixed action pattern (FAP). But in an adult fish it is best described as a more fluid action - a Modal Action Pattern.

    The basic search image can be relied on to enable feeding in the absence of an abundance of a particular insect i.e. between hatches. This cribbage score board represents how I envisage the innate basic search image. The basic search image starts with a few pegs in place that together represent a basic food search image consisting of movement, size and possibly shape.

    If the trout encounters a hatch,and the trout engages in selective feeding, extra pegs could be added to the board to form an acquired / learned hatch-search image. When the hatch subsides these additional pegs would be lost via extinction of the conditioned response. Foraging for food continues based on the basic search image. When the next hatch is encountered pegs are added to the board to form a new hatch search image.

    I suspect that trout may develop a search image based on 'vulnerability'. Vulnerability is signalled by a lack of movement. A vulnerable insect is one that is unlikely to take off from the surface of the water for a variety of reasons identified by generations of fly fishers: an emerging dun, a dun trapped in its shuck, a spinner etc. The common feature here is 'lack of movement'. Sensitivity to movement is one of the features recognised from birth by  larval zebrafish. This will present the trout with a vulnerable search image.

    William G. Tapply sums it up for me: "We fly fishermen often marvel at the strange effectiveness of scraggly, bedraggled flies. An unused, beautifully tied imitation fresh from the fly box catches nothing until a wing falls off or the hackle starts unwinding. Suddenly trout cannot resist it. The reason is simple: Trout often key on scraggly, crumpled insects–cripples and stillborns and dead spinners–because they taste as good as the pretty ones and are easier to capture."

    Is selectivity ubiquitous?

    I have never understood why the term 'selectivity' in the fly-fishing literature is reserved for 'educated' trout in slow-moving placid chalkstreams during a large hatch of a particular insect. In freestone rain-fed rivers where food is less abundant trout are said to feed opportunistically.

    I don't think it's helpful to draw such a sharp distinction between the mechanisms controlling the feeding behaviour of trout living in environments that vary in insect densities.

    Are trout in freestone rivers selective feeders? Almost certainly. Why? Trout in freestone rain-fed rivers employ the basic search image to select edible items. In all rivers not everything in the drift is edible. I prefer to use the terms 'hatch-search image' and basic search image to describe the feeding strategy adopted in the presence and absence of large hatches of a particular insect.

    Conclusion: The future is full of possibilities

    In the early 20th Century Halford launched dry-fly fishing on a scientific basis.

    The sciences of (neuro)ethology and heuristics offer a way of understanding how trout select and catch their food, and potentially identifying the characteristics of effective trout flies that have stood the test of time.

    Wyatt's (2013) criticism that "For fly fishers, 'selective' and 'educated' trout are the same thing; the educated trout becomes more and more selective." (emphasis added) seems justified in light of the contrast between some fly-fishing literature, and what is known about the inherent simplicity of the mechanisms controlling fish feeding behaviours.

    Many years ago Vince Marinaro realized the limits of precise imitation and expressed it thus: "I am continually astonished by the fact that the most killing flies in fly-fishing history are of very simple construction"

    The ethologist Tinbergen pointed out that animal behaviours often turn out to be controlled by simple rules of thumb - heuristics. The 2002 Nobel Laureate Daniel Kahneman described heuristics as cognitive shortcuts or rules of thumb that simplify decisions, especially under conditions of uncertainty. There is a growing body of evidence that simple heuristics are used by human and animals (Gigerenzer and Gray 2017) and possibly trout to select and catch their prey  (Kenyon, 2020).

    And Schullery offers this advice on what he calls 'fly theory': "Rule One: Don't insult the past. You may never realize the extent to which you are a product of it, ..."

    Why are simple flies so effective, and why are some commercially available flies so complex? Maybe it's because a lot of flies have the characteristics of a spandrel: twiddly bits added around triggers. Evolutionary biology borrowed the term spandrel from architecture to describe a structure or behaviour that is not particularly advantageous to have, but is retained because it is not particuularly harmful.

    About the author

    Paul guiding ITV News reporter in June 2019

    with sea trout in camera range ...

    Paul Kenyon lives in Ivybridge on the southern edge of Dartmoor about 6 miles from the Upper Yealm Fishery.

    Paul devotes more time than is reasonable to his love of all things associated with fish, fishing, instruction and guiding on Dartmoor rivers.

    He retired in 2006 from the Department of Psychology, University of Plymouth where he lectured in behavioural neuroscience and evolutionary psychology.

    email paul@flyfishingdevon.co.uk


    In this video Bob Wyatt ties his Snowy Shoe Hare Emerger

    Bob uses this material in place of CDC because he has found that CDC tends to be "a one fish fly" which is an absolute no-no for guides on local rivers.

    This article would not have been possible without the help and encouragement of Bob Wyatt. Bob is an artist, author, Certified Fly Casting Instructor and long-time angler. Born in Canada, he fished the freestone streams of southwestern Alberta in the late 1950s. He now lives on New Zealand's South Island. His articles have appeared in Fly Fishing & Fly Tying(UK), Gray's Sporting Journal, Fly Rod & Reel, and Flylife Magazine (AU). He has published two books: Trout Hunting: The Pursuit of Happiness (2004) and What Trout Want: The Educated Trout and Other Myths (2013). In this interview by April Vokey he discusses his “prey image” theory, trout fishing and the early days of steelhead fly fishing.

    Thanks to DASH: "A central, open-access repository of research by members of the Harvard community." for free open access to Bianco, I. H., Kampff, A. R., & Engert, F. (2011). Prey capture behavior evoked by simple visual stimuli in larval zebrafish. Frontiers in systems neuroscience, 5, 101.


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