Trout and Vision in the Ult...

Within the past thirty years we have come to appreciate that many animals - some reptiles, birds, fish, insects, and mammals - use the Ultraviolet range, particularly the long wavelength Ultraviolet (320-400 nm, also known as UVA), for detecting food, prey, danger, and mates. As fly fishermen this comes as a surprise - for hundreds of years we have been basing our imitation of trout stream insects on the visible spectrum that we, as trichromatic humans, can see. Trout, we now find, are basing some - we cannot quantify the extent - of their feeding decisions on a UVA trigger detected in the drifting food items.

We should first understand the difference between "Reflected UV" and "UV Fluorescence". When UV light contacts certain materials it causes them to emit light in a different wavelength, typically a light visible to humans. Thus a white T-shirt under a "black light" (UV emitting) fluoresces as a bright white; just as fluorescent markers, not visible under normal light, glow brightly in strong UV.

As trout fishermen we are not greatly concerned with UV Fluorescence (although West coast salmon fishermen use many fluorescent materials in their flies); instead we need to focus on UV Reflectance. While we cannot see the UV reflected from objects, the trout can, and that is highly significant because research suggests that ultraviolet inputs may contribute to prey detection and orientation. [3]

You will note in the image on the right, above, of the Baetid, that certain areas have been highlighted in red. These areas denote the presence of UV reflectance. [4] It is reasonable to assume that for the Baetid, these UV markings allow the insect to distinguish species and gender for purposes of mating in low-light conditions; just as the females of certain species of butterflies choose their mate based upon the size and brilliance of ultraviolet reflective wing markings [5] and some lizard populations maintain their genetic uniqueness through distinguishing appropriate mates by UV markings.[6] What is more to the point for the angler is that trout are also able to see the UV highlights on aquatic insects - and that this provides a ready distinction between the natural insect and our skillful imitations. This is not to say that the UV is a primary consideration - the trout see our visible spectrum as well - but it may make the difference for the fisherman between a confident acceptance of his fly, or a tail splashing rejection.

The next entry will describe the UV-reflective characteristics of some common fly-tying materials and UV enhancement of the artificial fly.

[1]"A Dictionary of Psychology", 2001, ANDREW M. COLMAN
[2]"Because most of the putative UV cones are believed to disappear in early ontogeny, their presence over a large proportion (15-20%) of the surface area of the adult retina suggests that they may be reincorporated prior to or at sexual maturity, at least in rainbow trout." -- http://www.ncbi.nlm.nih.gov/pubmed/9184985?dopt=Abstract
[3]"The Contribution of Ultraviolet and Short-Wavelength Sensitive Cone Mechanisms to Color Vision in Rainbow Trout" - David J. Coughlin, Craig W. Hawryshyn - Brain Behav Evol 1994;43:219-232
[4] The initial JPG image on the left is courtesy of Alex Wild ©2004 (see http://www.myrmecos.net/ ). I first decomposed the JPG to YCbCr_470 and discarded the luma channel. The red channel had nothing in range and was also discarded. The blue channel was reduced to 90-100 and a histogram stretch done. The result was used as a mask on the original and auto-highlighted in red.
[5]"Female Bicyclus anynana butterflies choose males on the basis of their dorsal UV-reflective eyespot pupils" - Kendra A Robertson and Antónia Monteiro - http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1559841
[6]"Evidence that ultraviolet markings are associated with patterns of molecular gene flow" - Roger S. Thorpe and Murielle Richard - http://www.pnas.org/cgi/content/full/pnas;98/7/3929