| Fly Fishing Devon | "The Heuristic Trout" A web-based collection of essays

The Heuristic Trout is a collection of essays that introduce an alternative to the popular belief that some trout are highly educated and shrewdly reasoning, as well as considering the history of fly-fishing techniques developed in South Devon.

Some of these ideas were introduced in my article  How does a trout see a fly?  in the May 2020 edition of The Field magazine. It is available to   read online

Some other ideas from The Heuristic Trout are included in my article  Are we killing the sea trout that lay the golden eggs?  published in the Wild Trout Trust's annual journal Salmo Trutta in 2022. Author's proof copy  available here.

Contents of The Heuristic Trout

  1. Introduction
  2. Essay How does a trout catch a fly?
  3. Essay Why does a trout take your fly?
  4. Essay Designing Trout Flies: Supernormal Sign Stimuli
  5. Essay The Christmas Tree Theory of Super-Stimulus Trout Flies
  6. Essay Early Roots of Fly Fishing in South Devon
  7. Essay Background Information on Soltau's book
  8. Essay A History of the Dry-Dropper in South Devon
  9. Essay Refining a Roll Cast to Improve Accuracy
  10. Essay Combining Spey and Skagit casting on narrow rivers
  11. Essay Should I Fish Upstream or Downstream?
  12. Essay Fly-fishing history as a series of paradigm shifts
  13. Essay Evolution of the wet fly: From drowned insect to emerger
  14. Essay Stealth: The importance of (in)visibility to the trout fisher
  15. Essay Should we equate fishing quality with the size of the fish caught?
  16. Essay Fly Tying for Westcountry Brown Trout and Sea trout
  17. Essay Invertebrate drift: The "Hidden Harvest"
  18. Essay Some lesser-known West Country river flies
  19. Essay Sea trout fishing in South Devon
  20. Essay Is it a sea trout or brown trout?
  21. Essay History of fly-casting instruction on the Devonshire Avon
  22. The Heuristic Trout  Index
  • Fly Casting: Key casting concepts, casting faults - diagnosis and treatment
  • Balancing rod, line and tippet
  • 2015-2021:  Upper Yealm video diary
  • About this project

    Introduction to The Heuristic Trout

    The Heuristic Trout is presented as a collection of essays that attempt to answer a question, or present a point of view, that invites further discussion.

    What are heuristics? Heuristics are 'rules of thumb', simple strategies used by humans and other animals to deal with problems in their everyday life. Heuristics accomplish the job quickly, on the basis of very little information and 'brain power'.

    In an uncertain world a rule of thumb can save the day. After a commercial airliner was ditched in the freezing waters of the Hudson River the copilot Jeffrey Skiles explained how a simple rule of thumb - the Gaze Heuristic - prevented disaster: “It’s not so much a mathematical calculation as visual, in that when you are flying in an airplane a point that you can’t reach will actually rise in your windshield. A point that you are going to overfly will descend in your windshield.”  The point they were trying to reach did not descend but rose. They went for the Hudson. Scientific research on the Gaze Heuristic focuses on much less serious events. For example, the problem of catching a ball where the same rule of thumb applies. The Gaze Heuristic is only one of a group of heuristics.

    An everyday example opens this heuristic 'Toolbox' . It doesn't take much exposure to Facebook fly-fishing groups before you come across a request for advice on what brand of breathable waders is best. John Gierach expressed the inevitability of the problem in his book title Death, Taxes, and Leaky Waders.

    You could base your decision after collecting all the information you can find about the range of waders on the market. Or, you could decide on the basis of what you, or your friends, know about waders. The first approach seems a rational, if time-consuming process. The latter strategy is an example of using a heuristic. It's called a Recognition Heuristic by the behavioural scientists who investigate human and animal behaviour (Todd and Gigerenzer 2012). Experiments revealed unexpected results; using a recognition heuristic is not only faster, but also more accurate - too much information can be an impediment to coming to the best solution - for a wide variety of problems.

    What about trout and the problems they face in capturing food? Speed and accuracy are just as important for trout. This line in Todd and Gigerenzer's book sparked my interest in using heuristics to explore that question:"Some teleost fish catch their prey by keeping a constant angle between their own line of motion and that of their target; " (Lanchester& Mark, 1975).

    Clip from "Floating Flies" (Nylander 2020)

    This led to an essay on the question How does a trout catch a fly?

    A comprehensive review of the literature leads to an alternative explanation for 'drag', 'refusals' and 'inspection', and suggests that the merged image of the body and wings on the edge into the window allows trout to judge and maintain distance from the fly.

    This enables a trout to use a Gaze / Tracking Heuristic to control its rise.

    I think it's time to reexamine the status of inspection and refusal in the fly-fishing literature. I now think in terms of alignment rather than inspection, and view a refusal during a rise as a failure of a trout to be able to align itself with the natural or artificial fly.

    I then turned to a question of more immediate importance to the majority of fly-fishers: "What fly should I use?". There is a vast literature on this topic. There are whole books devoted to fly patterns, and even more on how and why you should use them to deal with educated selective trout.

    But increasingly there are dissenting voices against this omniscient view of trout. Datus Proper in What the Trout Said (1989) proposed that trout flies should be designed, rather than tied according to a pattern like a recipe in a cook-book.

    Bob Wyatt's 2013 book title What Trout Want echoes Proper. But Wyatt is much more forthright in his attack on fly-fishing's myths: "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."

    My essay  "Why does a trout take your fly?"  examines these challenges to remnants of Halfordian dry-fly dogma that still exist within recent fly-fishing books. I suggest that a trout takes an artificial fly because it is recognized by a heuristic that compares a trout's 'search image' with the 'prey image' presented by an artificial fly. I have resisted the temptation to include a essay describing 'new' flies. It wasn't difficult. Some very talented people have designed, and then improved innovative flies that qualify , in my opinion, as prey images.

    One ethological concept - the supernormal stimulus - has received relatively little attention in the fly-fishing literature. The essay  Designing Trout Flies  includes an appreciation of Frank Sawyer's Pheasant Tail Nymph because it shows how observation, and insight into the structure and behaviour of insects and trout, created an outstanding trout fly. Sawyer's approach exemplifies an 'ethological' approach to fly design. It's not possible to reach firm conclusions about what features in a trout fly would confer super-stimulus properties. But it is worth describing ethological studies that have studied super-stimuli as pointers to features to consider when designing trout flies.

    The next essay The Christmas Tree Theory of  Super-Stimulus Trout Flies  starts with a question: Is there a special property of hare's ear fur that is responsible for its long history in the construction of artificial trout flies? A similar question was asked by Skues 100 years ago, but we still don't have a clear answer.

    In the second half of the last century, V.S. "Pete" Hidy put forward a plausible explanation, that has since been supported by photographic evidence, which suggests that hare's ear fur may endow trout flies with super-stimulus properties.

    Many years ago I came across a book written in 1847 by a local (Plymouth) author G. W. Soltau,  Trout Flies of Devon and Cornwall and How and When to Use Them.   The essay  Early Roots of Fly Fishing in South Devon  examines a popular misconception that fly fishing on the rivers running off Dartmoor in South Devon (UK) was restricted to the wet fly until the dry fly was adopted here in the late 1880s and 1890s, as a result of the influence of the chalk stream angler F. M. Halford.

    Soltau cast upstream with two flies, a dry fly above a wet fly, in an arrangement now reinvented, and referred to by various names: Dry-Dropper, 'Klink and Dink', Duo Method, or New Zealand style.

    Soltau's book has been overlooked in the history of fly-fishing literature, and consequently there is very little by way of commentary on its content.

    The essay asks to what extent did Soltau's fly-fishing technique anticipate:
  • the Dry-Dropper rig rediscovered in the 20th Century ?
  • Leonard Wright's (1972, 1975) advice that rather than being presented 'dead-drift', an artificial dry fly could be 'twitched' to represent the movements of the natural insect ?
  • And,
  • To what extent was Soltau using a combination of the fly-fishing techniques that were intentionally broken apart by Halford and Skues at the end of the 19th and early 20th centuries ?

  • The essay Background Information on Soltau's book discusses several outstanding research questions including the influence of Blacker's (1842) flies on the design of Soltau's (1847) flies. William Blacker's short 48 page book Blacker's Art of Fly Making was first published in 1842, five years before Soltau. It is unclear what, if any, influence Blacker had on the flies used by Soltau.

    To my untutored eye there are similarities between the two sets of flies. For example, if one were to take individual flies from the hand-coloured lithographs in their books, and swap them around they wouldn't look out of place. They are clearly different to the later delicate dry flies favoured by Halford, but Blacker and Soltau's flies are more suitable for Devon's freestone rivers.

    The essay  A History of the Dry-Dropper in South Devon  begins with a YouTube video of Simon Kidd and Dave Groves catching wild brown trout on the Upper East Dart with a Dry-Dropper rig in 2020.

    Most people think of the Dry-Dropper as a recent innovation that was invented overseas and came to this country under various names, for example:New Zealand Dropper, 'Klink and Dink' or Duo Method.

    Despite appearances, the Dry-Dropper is not a modern development. It's roots go back over a hundred years in Devon. It was described in 1847 by George William Soltau who lived in Plymouth, and fished rivers in South Devon. This essay describes its early development, and how and why it later fell out of favour.

    The essay refers to books written by several Devonian authors: George Pulman's early definition of dry-fly fishing, and his use of the Dry-Dropper; Soltau's description of the Dry-Dropper in 1847; Rabley's 1910 book Devonshire Trout Fishing that anticipates Sawyer's induced-take technique, and H.S.Joyce's  A Trout Angler's Notebook  (1948) that describes using a Dry-Dropper on Dartmoor.

    By the second half of the 20th century, Halford's and Skues' use of a single dry fly or nymph had replaced the Dry-Dropper in Devonshire. The contents of Skues' library confirm that he had an extensive knowledge of the fly-fishing techniques described by Devon authors that combine the use of wet and dry flies, including the use of what has been described as a Dry-Dropper rig. The essay asks why Skues did not refer to these Devonian techniques, and why Halford's dry fly method took over from older more effective technique in Devon.

    The essay Should I Fish Upstream or Downstream? explores the British tendency to favour casting sub-surface flies upstream, whereas Americans enjoy a range of techniques to fish sub-surface flies downstream.

    Anglers fishing on freestone West Country rivers can benefit from the innovations developed on chalkstreams for upstream presentation by Halford, Skues, Sawyer and Kite, as well as the development by American anglers - Leisenring, Hidy and Nemes - of more effective techniques for downstream fly presentation.

    A theme runs through the work of these innovations - movement of an artificial fly - a topic touched on in previous essays in The Heuristic Trout

    The essay explores the strengths and weakness of casting flies (dry, wet and nymphs) upstream and downstream. The fundamental difference between up- and downstream presentation involves the extent to which the angler can control the direction of movement of a fly on, or beneath, the water surface. Upstream presentation effectively restricts the angler to imparting vertical movement to a sub-surface fly (wet or nymph). Downstream presentation opens up the possibility of imparting lateral, as well as vertical movement to a sub-surface fly.

    The essay, Fly-Fishing Paradigm Shifts, explores why Halford's circumscribed method of dry-fly fishing did not sweep aside local methods of wet and dry-fly fishing for wild brown trout in South Devon. Privately-funded stocking of English chalk streams has been carried out from the 19th century. Halford's internationally adopted method of fishing a dry fly may be particularly successful at catching some of these stocked trout.

    The essay   Evolution of the wet fly: From drowned insect to emerger  may be of particular interest to local anglers. At a time when North Country writers were advocating thinly dressed soft-hackled flies such as the Partridge and Orange, the North Devon author Cutcliffe (1863) advocated stiff-hackled wingless wet flies for freestone rivers in North and South Devon, as well as those on Dartmoor and Exmoor. A chapter in Cutcliffe's book is a masterclass in fishing local rain-fed Devonshire rivers.

    The text of Cutcliffe's book, and high quality photographs of John Shaner's collection of Cutcliffe's flies tied by Roger Woolley, are now available in a book published by Paul Gaskell in 2019.

    I have included further essays containing material written over the past 10 years. There is a focus on what I consider to be a relatively neglected aspect of 'presentation' - casting, specifically roll casting. The roll cast is often presented as a way for beginners to extend fly line out from their fly rod prior to making an overhead cast. I have seen it described, by very knowledgable authors, as inaccurate, and limited in its ability to allow the angler to change the direction of the cast. I disagree with both criticisms. I hope to show that these criticisms can be overcome, and that the roll cast deals with problems encountered when casting overhead on narrow rivers with overhanging vegetation. I've watched with interest the recent emergence of single-handed Spey casting. There is evidence that a South Devon river may have been - just after the First World War - the birthplace of single-handed Spey casting.

    Not far away on Dartmoor is the site where data was collected for one of the ‘pioneer-studies’ on invertebrate drift. I have included an essay on this because Elliott's findings generated significant research interest from freshwater ecologists, but are not widely discussed in the British angling literature. We don't have large 'hatches' of fly on Dartmoor rivers, but Elliott's research shows that there may be periods during the day when increased insect food is available to trout in the absence of clearly visible surface activity. Hence my use of the term " Hidden Harvest"

    I've included some thoughts on sea trout. Sea trout were my all-consuming passion when I moved to Devon in the 1970s. Very gradually attitudes towards sea trout changed - for the better - over the years. The majority are now returned to provide the next generation of trout. We now have a greatly improved understanding of their remarkable lifestyle, and importance for maintaining healthy stocks of brown trout.

    The Appendix is a vade mecum for heuristics that I do not remember, but need to use from time to time such as: the "Rule of 3" to calculate the size of tippet to use with a fly, or the "Rule of 11" to convert X to diameter in inches, as well as ways to overcome relatively rare casting problems.

    About this project

    The Heuristic Trout   is an online resource for fly-fishers.

    There are several advantages to this publication route:
  • Hypertext links within each essay enable the reader to check my interpretation of the sources I used to reach my conclusions. This was one of the original aims of the World Wide Web. The web was developed with within-document and between-document links to enable information-sharing between scientists in universities and institutes around the world. To some extent this reduces the need to hunt down original sources - a lengthy and potentially expensive process.
  • Some of the essays involve evidence from academic subjects outside my area. For example, the history of fly-fishing. I wasn't surprised to find that historians, like scientists, rely on evidence: Historians interpret the past, scientists interpret the present.
  • Hopefully, this web-based approach will encourage access to the current debate about selective-educated trout, and challenge some accepted fly-fishing creation myths.
  • I can quickly correct mistakes, and add bits-and-pieces in an approach I call internet gardening
  • The internet is a relatively cheap way of sharing information with people around the world.
  • Additional "Essays" can be added from time-to-time
  • A conventional index is replaced by an online Search Box
  • Index Search of The Heuristic Trout


    1. Lanchester, B. S., & Mark, R. F. (1975). Pursuit and prediction in the tracking of moving food by a teleost fish (Acanthaluteres spilomelanu-rus).Journal of Experimental Biology, 63,627–645. Available online
    2. Proper, Datus What the Trout Said, Swan Hill Press, 1989.
    3. Todd Peter M. and Gerd Gigerenzer (2012)."What Is Ecological Rationality?". Chapter 1 (p3-32) In Ecological Rationality: Intelligence in the World. Peter M. Todd and Gerd Gigerenzer (Eds). Print publication date: 2012 Print ISBN-13: 9780195315448. Published to Oxford Scholarship Online: May 2012 Available online
    4. Wyatt, B. (2004). Trout Hunting: The Pursuit of Happiness. Stackpole Books
    5. Wyatt, Bob. (2013). What Trout Want: The Educated Trout and Other Myths

    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.

    Paul constructed his first web page in Autumn 1993 as a way of distributing lecture notes to undergraduates.

    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

    The author's  YouTube channel


    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.

    These articles 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.