Just One Week: Blinded by the Light

Dark Shrimp's First Swim 22nd August
Swimming and visibility trials

It had been almost 3 months since I had been on the Ure, so I was developing a serious case of salmon blues. The MCX Dark Shrimp was looking at me reproachfully from its box,begging me to take it for a swim. Each morning, clad in lycra for the 10 miles to work, I could sense the jealousy of my unused waders, jacket and rods, as they were left in the garage and the bike got the outing. To make matters worse, they were suffering the undignified and dusty consequences of my annual August refurbishment of a couple of sash windows. One wonders what grumpy conversations take place at night between the unused fishing tackle, the bikes and the joinery tools. The tackle would be greatly reassured by a quick word from my wife - "don't worry, he thinks of nothing else, insofar as he thinks at all" - but on the other hand, it wouldn't pay to upset the bikes, or least of all the power tools, which have the capacity for very painful vengeance.

To make matters worse, it's been raining frequently and hard in the Vale of York. Indeed, the remnants of Hurricane Bertha brought our 8 week summer to an abrupt and very chilly end, whilst depositing 65 mm of rain in the process. There's now a real autumnal feel in the air, which of course triggers every cell in my body to think salmon (it's not just fish that have things embedded in their cellular DNA). However, Bertha seemed to skirt Wensleydale, probably as a result of the over-stocking of Yorkshire's collective prayers for the Tour de France to have a Grand Depart Sec. And they were answered! Anyone who didn't believe in our special relationship with the Almighty should have been convinced by the television coverage. As a result we've only had one limited spate of about 1.5 m, which didn't persist long enough to bring many fresh salmon up from the Humber. That put the kybosh on my New Year resolution to catch a salmon on a dry fly, which requires fresh fish and warm water. On the other hand the sea trout didn't feel so constrained and the river's packed with them (of which more shortly).

I could take this torture no longer: water boarding without water so to speak. So when a day's rain promised to put the river up by a few inches, I made my break for freedom. Even though the possibility of catching a fish was very low, a day on the Ure would lift my spirits whilst fulfilling the serious business of conducting the MCX Dark Shrimp's swimming trials. In addition, the accumulation of large numbers of underwater photos would give me more data to calibrate my theoretical Photoshop models. So this post records the day whilst also exposing the visual challenges that confront salmon in a combination of bright light and coloured water (and satisfying my interest in the underwater visual aspects of our sport).

Trial Context

MCX Dark Shrimp #9

At #9 the DS was probably a little larger than the ideal for the water conditions. With the river level at +5"/12cm, bright sunlight and a water temperature of 14C, the MCX score was around 7. The biggest variable was the sub-surface clarity. At the edges,or when viewed in shadow, the water appeared very clear. The situation below the surface was markedly different.

Frodle Dub looking downstream

This shot shows the bright conditions and the clearly visible bottom in the foreground, out to a depth of 12-15"/30-40cm. Taking this view at face value you would be reaching for a #12 Blue Charm or Stoat's Tail. The photos that follow are all taken with a Panasonic FT20 14mps rugged submersible camera mounted on a tilt-head extending monopod. Although the FT20's nominal focal length is 7mm, in practical terms it is almost double that of a salmon's eye, with a correspondingly narrower field of view. There's a fuller discussion of the salmon's visual capabilities in 'Here's Looking at You', but you can take it from me that the conditions illustrated in the succeeding photos would have presented a challenge to the salmon's vision and target detection. This is for 2 main reasons. First, unlike the human, the salmon doesn't have a fast-acting iris in its eye to control the amount of light entering. Instead it relies on a relatively slow-acting chemical pigment on the retina (in effect a bit like putting on dark glasses). And second, the 160 degree field of view of the salmon's eye will encompass everything from the dark brown bottom to the brilliant sunlit surface circle of Window 1. Resolving the huge range of contrast between and within those extremes inevitably involves compromises in resolution or sensitivity, or both.

Health Warning. Once again, please note that I am not claiming that these photos faithfully represent what the salmon sees, because we just don't know. We do know the optical characteristics of the salmon's eye, but not how its brain processes the inputs, resolves colour, forms an image or employs the product in its situational awareness and decision making (conscious and/or unconscious). Certainly the salmon's eye has evolved to meet different purposes to ours in a markedly different environment, so it's a reasonable supposition that they don't 'see' like us. For those reasons the purpose of the succeeding photographs is purely relative within the context of the visible red-yellow-blue spectrum: A is more readily detected than B; under one set of conditions detection is easier than in another; one scene is brighter than the next; and the comparative luminance or transparency of the 3 Windows that comprise the salmon's visual environment.

Depth 3'/1m
Sun Out
Horizontal
Direction - South East

Blinded by the Light. In contrast to the pretty photo above, this was the reality 3' down, looking across the pool when the sun was fully exposed at 2 pm. Windows 1 (opaque, bottom), 2 (reflective, middle) and 3 (clear, top) are all in the frame. The dominant features are the extreme brightness of the direct sunlight in Window 1; its consequential effect on the contrast in the lower regions; and the backscatter caused by fine, highly reflective mineral particles suspended in the water. The net effect is that a fly 6'/2m away is rendered invisible.

Depth 3'/1m
Sun In (cloud)
Horizontal
Direction - South East

And this was 10 minutes later when the sun went behind a passing cloud. Brightness, contrast and backscatter are all markedly reduced. By the way, there's no fly present in this shot.

Depth 2'/60cm
Sun Out
Oblique
Direction - South East

If we raise the angle of sight to the salmon's 45 degree attack angle, things become rather clearer when the fly is viewed in Window 2 at a range of 2'/60cm. This is what a fish closing in to take would see. It is interesting to note that the silver banding on the fly's body is invisible under these conditions.

Depth 2'/60cm
Sun In (cloud)
Vertical

However, if we increase the angle towards 90 degrees, the fly appears in Window 1, and is highly visible as a silhouette provided there isn't direct sunlight and rippled water, as shown below, which hides a fly in the dark stripes. These pictures give a distinct clue to the merits of using floating and hitched flies in high summer.

Dark Shrimp Impressions

The DS swims beautifully - stable, level and entirely without rotation - at all water speeds. It also flies nicely: well, you don't imagine I spent the whole day without casting! Owing to its understated dressing it swims somewhat deeper than the equivalent sized Cascade or Ally Shrimp, but not so much as to dispense with the need for sink tips in faster water.

As described above, the trial took place in extremely challenging conditions for detectability - the capacity to catch the salmon's attention - for a fly designed for lower levels of brightness. I shall need to take more trial shots in the lower light levels prevailing in September and October, which as you can see from the graph are about half those in August, and much better matched to the salmon's visual characteristics and the Dark Shrimp's design brief.

Depth 3'/1m
Sun Out
Direction - South
Low oblique, range 6'/2m

Despite the very challenging conditions of contrast and backscatter, the DS fared well. This shot shows the results at an exceptionally demanding near 2 metre range within Window 1 (the Ally Shrimp was lost at just over 1.3 metres). There is faint flash from the sparkle in the tail, but more black in the main and tail dressing could be beneficial. A salmon would detect this faint target relatively easily, provided that it was moving.

Depth 3'/1m
Sun partial cloud
Direction - South
Oblique, range 4' 6"/1.5m

Up in Window 2 (at midday), at a range of 1.5m the detectability and colour differentiation are good. I judge that the orange content is about right for the conditions.

2.5 lbs

And here's a second opinion on that! Having set out to design a salmon fly, on this day at least, the Ure's sea trout found the large Dark Shrimp irresistible in bright conditions in broad daylight, and amidst a consistent hatch of sedges that they were taking on the surface. This was the first and smallest of the three that stayed on to come to hand. Its successors were 3 and 4 lbs respectively and nicely silver-grey, comprising a most pleasant but wholly unexpected reward in the absence of a salmon.

Conclusions

Overall I judge that the DS provides an excellent basis for further development. The key test will come in the duller days and lower sub-surface light levels of September and October, which framed the design brief that I gave to Darragh for tying the prototype. Even at this stage of development I shall fish the Dark Shrimp with some confidence.

Prototype 2 will involve an increase in the black component of the body and tail, and possibly a reduction in the silver banding to a narrower thread. The orange fore hackle and red head look perfect, so no changes there.

One other thing you may have noticed (or rather not) in the photos, is the near invisibility of the Seaguar fluorocarbon leader in almost every instance.

It's that time of year, I can't wait to get out again, but a very nostalgic trip to France for the commemoration of the 100th anniversary of the outbreak of the First World War and the Battle of Nery on 1st September will take precedence. Thereafter, with no Tomatin week this year, the Ure will feel the full force of my enthusiasm and energy. Anticipating your own enthusiasm, I wish you tight lines.

End of the day, job done, DS takes a breather in good company

Comparator Photos

Just for interest I attach a selection of photos of the classic low water flies - Blue Charm and Stoat's Tail - taken during the test, without analysis or comment.