Text: Alan Bulmer         Featured image: Auckland Freshwater Anglers

I have just finished reading a book by Reed Curry entitled “The New Scientific Angling – Trout and Ultraviolet Vision”. The ISBN number is 9780984086306. It is thought provoking read that will probably change forever the way you think about trout fly design.

The book looks primarily at UV reflectance and how this is likely be used by trout to select insects and other fish that are suitable to eat. Essentially each prey item has a unique reflected UV fingerprint that trout recognise. Reed hypothesises that it is this fingerprint which trout lock onto during the hatch of a particular type of fly, especially after dark, and this could explain what drives trout to feed selectively.

Unfortunately humans cannot see in the UV spectrum whereas birds, insects and trout can. This makes it impossible for us to see what they are seeing. Obviously if we cannot see reflected UV then it makes it impossible to see with the naked eye whether our fly has the correct highlights to match the natural. Despite this, Reed suggests anglers need to consider reflected UV light in fly design as trout almost certainly use the UV reflected images to help identify what to eat and the book systematically takes us through what to consider.

Special photographic techniques are required for humans to see reflected UV in the same way as other creatures and these are described in detail in the later chapters. Those with an interest in photography will no doubt relish this detail.

It is important to note that Reflected UV is not UV fluorescence. Humans can see UV fluorescence as it occurs when ultraviolet light strikes a material and triggers and emission of light at a longer wavelength – usually a wavelength that is visible to humans. Reed argues that trout fishermen should not be concerned too greatly with discernible fluorescence as it does not often occur in trout stream insects or baitfish. Fluorescent hot spots “stimulate other instincts in trout that are unrelated to reflected UV – aggression, curiosity, or territorial behaviour – rather than imitating a natural food item”.

This statement directly contradicts the common trend to include “hot spots” on flies. It is unlikely that trout zone in on fluorescent hot spots in the same way that they do with the reflected UV fingerprints which characterise bait fish and insects. Perhaps the success of hot spots may have more to do with increased angler confidence than any overall heightening of interest on behalf of the fish?

The photographs below have been taken using special technique which allows humans to see reflected UV light in the same way that trout do. You can see how on the wings of the natural insect there are shards of light and how these shards are replicated in the artificial flies. Every fly has a unique reflected UV fingerprint which helps trout lock in on a specific food source, especially in low light.

This goes some way to explaining selective feeding behaviour as it is perhaps just the UV fingerprint that trout hone in on. Fortunately it is possible to create flies which closely match these unique UV fingerprints by selecting the right fly tying materials .

When it comes to bait fish the same applies. The scales of each species reflect UV in a defined way to present a unique reflected UV fingerprint. Some metallic tinsels and Mylar foil reflect UV in the same way as scales which is why they are so effective. Reed points out that “some areas of a baitfishes’ body, particularly white under bellies, as well portions of the bodies of many insects, provide diffuse reflectance” and knowing this helps the fly tier select materials to more closely match the reflective UV image of the target species.

Interestingly UV wavelengths can penetrate to depths up to 55 metres. Thus, fish are able to see objects, either through UV reflectance or UV absorption, at greater depths or in more turbid water than with light in the visible spectrum. UV scattering backlights prey even during high, turbid water conditions allowing trout to continue to forage effectively.

Reed also states that at dawn and after dusk fish are feeding heavily using UV triggers so it is best to use flies which provide maximum contrast in UV reflectance at these times of the day. It is probably more important to match the approximate size and shape of the reflected UV image at this point to attract attention rather than to try to match the UV fingerprint precisely.

The reflected UV theory eloquently explains why some historically successful fly patterns which bear no resemblance to any natural insect, such as the Royal Coachman, work. It is all down to the highly UV reflective wing matched to the UV absorbent iridescent peacock herl. This is a very generic UV match to many insects that trout feed on. This is clearly shown in the photograph above.

White hackles, Marabou, Mallard and Turkey feathers are highly UV reflective whereas iridescent feathers are not. White furs or tips on certain types of fur reflect UV strongly as do lighter rabbit and hare fur. Dyeing the fur can either enhance or diminish UV reflectance depending on the colour used. Synthetic dubbing, especially the lighter shades of products such as Ice dubbing and Antron, are often highly UV reflective and should be used sparingly or in combination with darker furs.

The book has loads of images of flies, fly tying materials and other fly fishing paraphernalia all photographed in visible light and with UV filters so the reader can see the reflected UV and note the difference. This is invaluable reference material for use in fly design.

One thing that Reed illustrates near the end of the book is how it is possible to improve the UV reflectance of any fly pattern simply by dusting it with a coating of microfine Magnesium oxide in a fly shaker. Apparently the Magnesium oxide is not only highly UV reflective but also a wonderful fly floatant. This fact really piqued my interest and I’ve already got some high purity Magnesium oxide to experiment with this later in the year. I’ll run a controlled experiment or two and let you know how whether it works.

Reed is careful not to offer reflected UV as a fly tying panacea. He merely provides the dots and leaves it to the individual angler to join them. It is a fascinating theory that is well worth further consideration.