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Ethology and the design of effective trout flies: The myth of the "Educated" Trout
bullet head hoppercoparadun For hundreds of years anglers have been using 'models' of natural insects to catch trout. These models are often very elaborate:
  • Do they need to be so complex? 
  • Do they need to be close imitations of the natural insect?
  • Do successful trout flies combine essential with non-essential elements?
  • Can we design simple trout flies that still catch fish?


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This page explores the application of 'ethology' to the design of trout flies in order to address these questions
  • Ethology is a branch of biology which studies animal behaviour
  • Ethology rejects excessively anthropomorphic interpretation of animal behavior
  • Lloyd Morgan's Canon is a formal statement of this rejection:
    "In no case is an animal activity to be interpreted in terms of higher psychological processes, if it can be fairly interpreted in terms of processes which stand lower in the scale of psychological evolution and development" 
  • In other words a simple explanation is better than a more complex one
  • One of the founding fathers of ethology - Konrad Lorenz - identified Fixed Behavioural Patterns 
  • Fixed Behavioural Patterns are instinctive behaviours that occur in the presence of identifiable stimuli called sign stimuli or releasing stimuli
  • A well-known example of sign stimuli eliciting fixed behavioural patterns is Tinbergen's study of courtship and aggression in sticklebacks
  • You may be interested in these articles which discuss ethology and sign stimuli from a more academic perspective

"Ethology and fish behaviour"

In Spring male sticklebacks change colour, establish a territory and build a nest. They attack male sticklebacks that enter their territory, but court females and entice them to enter the nest to lay eggs.

stickleback courtship and aggressionThe ethologist Tinbergen used crude 'models' of sticklebacks to investigate which features of male and female sticklebacks elicited attack and courtship behaviour from male sticklebacks.

This diagram shows Tinbergen's main findings.

Male sticklebacks:

  • attacked a model with a red belly
  • courted a model with a swollen silver belly  

The term sign stimulus or releaser was used to describe simple features (e.g. red belly, swollen belly) of a complex stimulus (e.g. male stickleback, female stickleback) that bring about a particular fixed behaviour pattern (e.g. head down attack behaviour in male sticklebacks).

"The Hare's Ear Puzzle"


Skues wrote: "At one time the late Mr. F.M. Halford was a great advocate of the Gold-ribbed Hare's Ear, 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." (italics added)

Can we use ethological concepts to explain the success of the GRHE?
  • it's difficult, but it helps to put human aesthetic judgement to one side
  • try to ignore the fact that the artificial looks nothing like a natural insect 

Remember that a male stickleback will:
  • attack this  attack model and court this court model
  • a Google image search for "gold ribbed hares ear" reveals the wide variety of tyings for this popular pattern
  • the simplest feature(s) they all share  may be 'sign stimuli' that elicit feeding under some circumstances

"The ubiquitous Pheasant Tail"

sawyer pheasant tail nymphgrey goose nymph
Sawyer Pheasant Tail and Grey Goose nymphs
We can apply an ethological approach to explore the possibility that several flies made from pheasant tail fibres incorporate sign stimuli that elicit feeding behaviour in trout.

Frank Sawyer introduced the Pheasant Tail nymph - a simple fly construced from copper wire and dark pheasant tail fibres to imitate Baetis nymphs - which is cast upstream of the trout so it sinks to trout's level. Then the rod tip is lifted so that the fly ascends in the water in front of the fish- the 'induced take' technique.

Sawyer (1979) commented:
"General shape and colouration, together with the right size is of greater importance than an exact copy. My two universal patterns, as I call them, are the Pheasant Tail and the Grey Goose. The Pheasant Tail serves for the darker coloured nymphs and the Gray Goose for the lighter ones."

Cove's pheasant tailTeeny nymph
Cove's Pheasant Tail and Teeny's Nymph
The effectivenees of simple flies constructed from pheasant tail fibres is not restricted to English chalkstreams. Arthur Cove's Pheasant Tail was developed to imitate 'buzzers' (chironomid midges) on Eyebrook reservoir. The American Al Troth based his Pheasant Tail nymph on Sawyer's original pattern but used peacock herl as thorax material. The Teeny Nymph is another example of a simple but effective trout fly which may imitate a shrimp. Size and colour variations of Jim Teeny's basic pattern have been responsible for catching 25 IGFA (International Game Fish Association) fresh and saltwater world records. Troth's pheasant tail
Troth's Pheasant Tail

The simplicity of these flies suggests several candidates for sign stimuli that elicit a trout's feeding response:
  • movement - these patterns tend to be fished with some form of movement
  • colour - colour is often varied to match the colour of the natural nymph
  • thorax - is present but construction materials vary (Sawyer and Teeny used pheasant tail; Cove used rabbit fur and Troth used peacock herl)
  • body shape - designed to represent shape of natural (Sawyer and Troth tied a straight body to reprent a mayfly nymph; Cove tied around the hook bend to represent chironomid pupae)
  • the ratio between body size and thorax may be important when representing particular insect groups 

This table presents the design elements in several 'classic' artificial flies used for sub-surface presentation to trout in rivers and stillwaters.

Design Element
fished with movement body material thorax material body shape tail hackle

Sawyer yes pheasant or goose pheasant straight pheasant or goose absent
Cove yes pheasant rabbit curved absent absent
Troth yes pheasant peacock straight pheasant absent
Teeny yes pheasant pheasant straight absent pheasant
Most frequent element fly moved body made of pheasant
thorax made of pheasant
straight body shape pheasant tail =absent tail absent

All of these successful classic trout flies have the following design elements in common:
  • body made of pheasant tail fibres
  • thorax made of pheasant tail fibres
  • movement imparted by the angler
Application of Lloyd Morgan's Canon would suggest that an artificial fly constructed with a straight body from pheasant tail fibres and some form of thorax which is moved in the water should catch trout. It is interesting that the flies constructed by Sawyer, Cove, Troth and Teeny are more elaborate than this simple pattern. For example, Sawyer's nymph has a tail. However this analysis does not consider the possibility that the tail is a sign stimulus when trout are feeding selectively on mayfly nymphs. Likewise, the curved body in Cove's fly may be a sign stimulus when trout are feeding selectively on chironomid pupae. Thus selective feeding may be the result of the operation of a "search pattern" consisting of several sign stimuli.

Thus several different sign stimuli may act together to trigger the trout's feeding behaviour. Trout may 'add-up' sign stimuli to determine if an object is edible. This would be an example of the law of heterogeneous summation which predicts that incorporating several sign stimuli into an artificial fly could increase its attractiveness to trout. In fact, the law suggests that these artificial flies could be more attractive to trout than the natural insects they are supposed to represent. 

  • If an ethologist was asked to investigate which of these features (body, thorax, movement) elicit the feeding response, they would construct even simpler flies which incorporated just one element
  • No ethologist has attempted this task
However the writings of experienced anglers suggest that the 'sign stimuli' involved may be:
  1. the outline of a nymph represented by a thorax composed of a few wire wraps
  2. movement of the 'model' in the water - the 'induced-take' technique
  • Raymond Baring found that a Pheasant Tail nymph increased in attractiveness as it became more and more bedraggled and finally lost all of its original dressing
  • Ed Zern (1979) described how he caught trout on a pheasant tail nymph that was "a bare size 18 hook with three turns of fine copper wire  around its short shank and nothing else - no fur, no feather, no silk, no tinsel." 
  • Oliver Kite also reported success with his 'bare hook nymph' which consisted of a few turns of wire wrapped around the hook shank. He was also able to catch trout whilst blindfolded by using the 'induced-take' technique 
  • Inspired by Kite's success, Roy Christie developed his Copper Wire Hare's Mask fly with which he has "..spent many hundreds of hours using this system and caught thousands of trout with it." But he adds:" Does it always work? Well, no."

The question remains "Why not?" Is the answer because the trout are 'educated'? We need to bear in mind Lloyd Morgan's Canon when searching for an answer.

brassieIn his book 'Trout Flies' Dave Hughes remarked "Many folk think that if fish aren't taking their nymphs, they need to change flies. Most often all they need to do is change depth. Fish a foot or two deeper, and suddenly the same fly is remarkably effective." The Brassie (shown on the left) is an example of a simple fly that will sink quickly.

simple pheasant tail 
Roy Christie's Copper Wire Hare's Ear Mask
(image courtesy of Hans Weilenmann)

Snowbee-Waldon viceRim Chung forceps viceFinal thought

Despite all the above, I continue to accumulate yet more fly-tying materials and expensive tools and avidly consume information on new and old fly-tying fads and fancies. I still tie over-elaborate flies. I use a Snowbee Waldron vice which is almost a work of art even though I know that master fly tyers such as Rim Chung use a simple pair of forceps to hold the hook.  I suspect all this complexity gives me a sense of confidence. The rational side of my brain whispers that it matters not one jot to the trout!

References and online resources:
  • Sawyer "Nymphing in the Classic Style" in Migel & Wright (Eds) "The Masters on the Nymph", 1979, published by Ernest Benn, London.
  • Zern "Bye-Bye, Wet Fly?" in Migel & Wright (Eds) "The Masters on the Nymph", 1979, published by Ernest Benn, London.
  • Roy Christie's article on the Copper Wire Hare's Ear
  • Skues, "The way of a trout with a fly" published by A and C Back, 4th edition, 1949, p 91
  • Sawyers nymphs
  • Bob Ireton article on tying and fishing the Teeny nymph
  • Jim Teeny's flies. Look under 'Fishing Tips' for Jim's video showing how to tie his Teeny nymph
  • An appreciation of Jim Teeny and his nymph by Peter Cockwill originally published in "Flydresser" contains clear and detailed tying instructions
  • The IGFA (International Game Fish Association) website
  • Loren Williams's clearly illustrated  article on tying Al Troth's American Pheasant Tail nymph
  • Martin Cottis' article on tying Cove's Pheasant Tail Nymph
  • In a 'spoof' article (B J Britton, J Grimley Evans and J M Potter,"Does the fly matter? The CRACKPOT study in evidence based trout fishing", British Medical Journal, 1998) the authors compared the GRHE against other dry flies on a well stocked chalkstream. The authors are cordially invited to replicate their findings on a population of  wild brown trout!
  • Rim Chung's website with tying instructions for his RS2 nymph and using forceps as a vice
  • Snowbee-Waldron fly tying vice
  • A collection of flies tied by Al Troth including his pheasant tail nymph

"The great charm of fly-fishing is that we are always learning." - Theodore Gordon
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