Fly Fishing Devon: Instruction & Guiding on Dartmoor Rivers

How does a trout recognise food?

Trout are faced with a constant stream of inedible debris. Obviously fish can usually distinguish pretty well between edible and inedible objects. But how do they do it?

We know a lot about trout vision which helps to explain how a trout intercepts a fly, but less about how food is recognisedas edible.

John Simonson wrote in his blog: "This Fall while standing in the Madison River looking upstream I could see a tremendous amount of small debris and other objects drifting on and below the surface. Yet mixed in with the plethora of junk drifting downstream I could spot a struggling Baetis dun size #20, a size #14 spent Callibaetis spinner, and several tiny winged ants drifting downstream towards me just by their shape and slight movements."

There's a scientific term for this human ability - ' parallel visual search'- that enabled John to spot and identify a range of insects at different stages in their lifecycles. Everybody uses parallel visual search in day-to-day activities; it enables us to quickly find our favourite brand on supermarket shelves, our car in a car park, a book on a bookshelf etc. It's an unconscious ability that we take for granted.

Speed is one important characteristic of parallel visual search. We don't have to examine in turn each car, face or cereal packet, the object we're looking for seems to ' pop out' from the background.

Until recently it was thought that only mammals and birds were capable of parallel visual search. But earlier this year scientists reported that fishshared this ability.

It is generally agreed by scientists studying 'visual search 'that

  • colour
  • movement
  • orientation
  • size
  • and probably shape
are attributes that enable an object to be quickly spotted or 'pop out'from background 'clutter' during parallel visual search.

It may be no accident that these are features reproduced in artificial trout flies.

In addition, previous research has shown that "Your ability to find a target in the current search is affected by what you have been searching for previously. In general, you are faster searching for a given target if you found that same target on a recent trial"

In other words, if you are a trout feeding on a particular type of natural fly, you are more likely to quickly spot the next similar one that floats towards you.

Could parallel visual search be an explanation for selective feeding?

Trout flies: Simple or Complex?

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 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?

Ethology & Fly Design

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

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.

The 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 an artificial trout fly looks nothing like a natural insect

Remember that a male stickleback will

attack this:

and court this:

"The ubiquitous Pheasant Tail"

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 sizeis 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."

The effectivenees of simple flies constructed from pheasant tail fibres is not restricted to English chalkstreams. Arthur Cove's Pheasant Tailwas developed to imitate 'buzzers' (chironomid midges) on Eyebrook reservoir. The American Al Troth based his Pheasant Tail nymphon Sawyer's original pattern but used peacock herl as thorax material. The Teeny Nymphis 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.

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 ratiobetween 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.
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 Canonwould 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 summationwhich 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 ethologisthas attempted this task

However the writings of experienced anglerssuggest 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 Baringfound 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 Kitealso 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 Christiedeveloped 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 Canonwhen searching for an answer.

In his book 'Trout Flies' Dave Hughesremarked "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.

Final 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:

    Further information on parallel visual search
  1. Parallel Mechanisms for Visual Search in Zebrafish
  2. Visual search article in Scholarpedia
  3. Inhibition of return article in Scholarpedia
  4. Underwater camera flyfishing 1080p- 2014 best fishing video winner
  5. Trout on a dry fly from an underwater camera
  6. John Simonson blog entry: What triggers a trout to eat your fly?
  7. How Does a Trout Catch a Fly?: Marinaro's "Edge of the Window Theory"
  8. Discussion of selective feeding in Nymph Fishing Rivers and Streams: A Biologist's View of Taking Trout Below ... By Rick Hafele

Fly Fishing Devon: Instruction & Guiding on Dartmoor & South Devon Rivers

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