The Purple Peril feather wing single hook streamer wet fly is one of those strange flies that should not work but does.

STREAMER WET FLY PATTERNS. Hook size 10 - $US each

The Purple Peril Streamer fly pattern does not look like any natural insect or bait fish. It is one of those 'piss them off' flies. When it sail past the front of a salmon, steelhead, bass or large trout's nose it catches their attention and they snap at it. It is technically called an attractor fly pattern. It produces fish when on a three foot leader fished on its own when you can see your target trout. I have also had success with this fly on the middle dropper of a team of three flies that are dead drifted with the current having been cast up stream.

When Brown trout change their spots

I have noticed on my travels that trout in one river or stream will differ in markings to similar sized fish in a nearby water system. Why is this? Is its Darwin's law of evolution having an effect; the fish with the best camouflage markings that match the local environment survive predation so it is their young with the similar inherited markings that you see in that river or do fish have a way of adapting their markings to suit their surroundings?

Most fly- fishermen have noticed that the appearance of brown trout varies greatly even between individuals from the same stretch of river or stream.

Is the reason for this difference just camouflage? Certainly the chameleon-like ability of fish to match its back to the colour shade of the river-bed makes it less visible when seen from above. In clear water the dark shadow cast by the fish is often more visible than the fish itself.

Marine biologists have found indications that the scales on the trout's belly are shaped to deflect light, making the shadow itself less easily seen than it would have been otherwise. Such obvious camouflage ploys are in contrast however to features such as juvenile Parr marks, the eye-catching thumb prints which are involved in a quite different function.

Parr marks are found on small trout, usually but not always young fish, in densely populated water where they are in constant rivalry for territory. These vertical marks, their effect enhanced by the black and white leading edges of dorsal and anal fins, also found on small fish, are known to play an important part in threatening territorial behaviour.

Horizontal stripes have an opposite or appeasing function. Those marine biologists spent hours watching parr trout spar with others as they marked out feeding zone successfully or unsuccessfully chasing the competition away.

I have seen this happen with young rainbow trout in a large aquarium marina fish tank. The dominate one had bright parr marks, while the subordinate ones maintained smaller, duller and more numerous marks joined along the tops by a dark tone which created a lateral stripe effect along the upper flanks.

These adaptations are lost when growing fish move into rivers, lakes or down into the sea, where their flanks become lighter, more silvery and more appropriate to the non-territorial, fee-swimming life-style of the sea trout. Pale or silvery flanks are also a feature of brown trout in some large open stillwaters.

The prominent spots so typical of brown trout are less easy to interpret in terms of function. To the human observer they have an almost hypnotic quality which may have a similar effect on non-human predators.

The common rounded spots with haloes resemble a collection of eyes arranged in subtle, wavy lines with red spots predominating towards the tail, the opposite end to the real eyes of the trout. Such configurations may well confuse predators and if so, can be classified broadly as deceptive camouflage. It has also been argued that the spots serve as recognition signals between the fish.

Sea-trout retain their spots to some degree in salt-water, while in rivers they acquire additional rusty brown spots which form a distinctive part of their breeding dress. It is possible that the enigmatic spotting patterns serve two separate but compatible functions.

Marine biologists have put forward the theory, supported by breeding studies, that indicates the size and number of spots are inherited features, which every trout starting life with its own genetic blueprint.

The environment, however, appears to determine how the spots manifest themselves. Their purpose is obviously so important to the fish that they may develop in all sorts of ways, often bearing little resemblance to the original blueprint. The quality of light in the water is also significant.

In one section of chalkstream I fish in Hampshire, there is very poor light. It is dark and densely overgrown but the fish like it because the water is very oxygenated as there is a watermill weir a few yards upstream. The small trout in this stretch of the river have no spots yet half a mile downstream as the water passes through watercress beds in open fields, that only have a few bushes to offer shade, the young trout have spots.

I have also noticed that sparsely spotted trout seem to frequent slow deep river pools and in deep lanes in some stillwaters. Heavily spotted trout seem to prefer broad shallow river pools with a bed of moderately sized stones. Do the fish know what the pattern on their back looks like? I doubt it.

Does this situation come about because sparsely spotted trout are easy to see in pools with a stony bottom and trout with densely spotted backs are more noticeable against the silt bottoms of deep water sections of rivers and lakes thus making easy targets for predators?

I do not have all the answers. From my experiences I believe that Darwin's 'survival of the fittest' theory is at work in our rivers, streams and lakes; those adult fish with the best camouflage for a particular stretch of water seem to survive longer than others with different markings.

They are the most difficult to see. They are normally the largest as they have had longer time on the water to feed. These are the fish you want to target so how does this knowledge help the fly-fisherman? Do not look for these fish as I just mentioned they have survived because they are difficult to see. Look for their shadows on the stream or river bed.

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