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
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?
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.
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).
Remember that a male stickleback willattack this:
and court this:
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:
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:
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.
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!