Tuesday, 22 January 2019

Sparkling Water

Happy New Year

I trust that Father (or Mrs) Christmas duly obliged and that you enjoyed the holiday.  We had a wonderful time with the whole tribe assembled.  The house was groaning at the seams with 9 adults, 4 toddlers and babies, and 4 dogs, only 2 of which were controllable.  In the latter regard, to my amazement, the rain gauge survived the residence of the omnivorous London Puggle, even if anything edible left unattended did not.  Once January arrived we collapsed in gratified exhaustion, which allowed me to think of salmon for the first time in a month (my wife doesn't believe that).

I couldn't muster the enthusiasm to write yet again about the weather.  By any standard 2018 was truly exceptional, having been drier for longer than either 2003 or even 1976, with only 16% of average rainfall in the Vale of York during the salmon season.  Moreover it capped a sequence of 6 non-average years, 5 unusually dry (2013, 14, 15, 16 & 18) and one exceptionally wet (2017).  In comparison the 6 previous years comprised 2 wet, 3 average and one dry.  It's no wonder I caught far more fish in 2007-12 than more recently.  But I'm not hard to please: a nice average 2019 will do very nicely thank you.  However, if it's going to be average it had better get going as Dry January is taking on another meaning round here, with only 6mm of rain to date.

The Significance of Light

So without any enthusiasm for writing about the weather and noting that traditionally I've been a bit educational in the midwinter, I'm revisiting one of my favourite subjects, the behaviour of light in water.  It bears directly on whether the salmon can detect and see our fly at a respectable distance, or in the extreme case, see it at all.  

There's plenty of evidence of the effects of light on salmon behaviour.  The chart shows my own records from 10 years of our September week at Tomatin and highlights the marked mid-morning peak of hen fish taking times.  There is a lesser peak in the early mid-afternoon.  These peaks directly correlate with the house's record books, ruling out the coincidental effects of my patterns of behaviour.  Barometric pressure, water temperature, oxygen level and so forth do not vary consistently with the time of day.  Only the sun does that, which suggests a study of light is a worthwhile exercise.

As we wish to profit from that observation it helps to have a basic understanding.  If you have a GCSE or A level in Physics you will have learnt this at school, but no doubt it will have lain dormant and forgotten in the years since, and a little reminder may be useful.  As always I apologise to those whom I am teaching to do clever things with eggs.

Salmon and Light

Before starting we need to remind ourselves that the salmon's eyes are fundamentally different to our own.  In particular they have:
  • No eyelids to shut out unwanted light.
  • No fast-acting iris to control the amount of light entering the eye and reaching the retina.  They rely on a relatively slow-acting pigment that adjusts the sensitivity of the retina's elements of rods and cones and the associated receptor junctions.  It is not known whether this can be applied selectively to a single quadrant in the field of view.  As a result salmon can find sudden increases in light level discomforting and may move to a dimmer location.  Bright light shining directly into the binocular zone where the fields of view of the eyes overlap ahead and above (i.e. straight down the pool) will cause an overall dimming of vision and reduce the probability of your fly being detected.
  • Almost double our field of view, which allows unwanted light to interfere over a much wider arc.  Imagine driving a car and being dazzled by light coming from your left or right side rather than ahead.
  • Superb low light level performance, which helps them to survive and thrive at sea in high northern latitudes in winter.  In the ocean salmon dive to seek prey, notably juvenile squid, at the limits of light penetration around 2-400 feet.
  • Lenses that are positioned off-centre, which gives them longer sight to the sides and shorter but more acute sight forwards in the binocular zone.  Your fly is much more likely to be detected at distance when it is presented to the side of the fish.
  • Much shorter sight than humans.  Their capabilities are matched to the limitations imposed by water, and nature doesn't sustain unnecessary redundancy.

If you're interested you will find fuller explanations in Windows on the World and Here's Looking at You.

Light on the Water

In a blinding glimpse of the obvious, the light arrives at the surface of the water by 3 routes:
  • Directly from the sun in almost parallel rays.  Its intensity varies with the sun's altitude, which is affected by the latitude, season and time of day.
  • Passing through cloud, which diffuses and scatters, and thereby reduces brilliance
  • Reflecting from the surface of clouds, which absorb some energy and scatter the rest

This chart gives an indication of the effects of season and time of day on the amount of light reaching the water on a clear cloudless day.  In the UK the influence of latitude is less pronounced: the peak June figure on the Hampshire Avon would be around 106, and on the Findhorn about 94.

Things can change markedly once you start applying the effects of cloud cover.  At 60% cover on an October afternoon at 3pm you will be down below 20% on the intensity scale.  In those conditions you will need to present your fly well above the salmon's sight line in Windows 2 & 3 to have the best chance of detection and hence a take.

September sunrise
Cloud reflection can be an important factor in certain conditions and at some times of the year.  In this photo there is insufficient cloud to reflect the sun rising behind the mountains.  As a result the water in the foreground remains pitch dark because there is very little light emanating from the otherwise deep blue sky.  However, in the next 10 minutes everything will change as the sun broaches the crest; the light level in the pool goes from zero to 25% of its midday September value; and the salmon react.  There's a fuller explanation in the post Morning Glory - Sex and Flies.

Later in the day cloud diffusion and reflection can enhance underwater light levels significantly.  This is looking down the same pool on a showery and overcast September day.  Although the sun is at a relatively shallow angle (from the left) there is a visibly good light penetration into the water from diffusion and reflection.  As a result you should be considering both the depth of presentation of your fly and its size to match the conditions.

At the beginning and end of the day the direct sunlight will strike the water obliquely. At an angle of arrival of below 10 degrees on smooth water almost all the light will be reflected.  Above 10 degrees it will generally enter the water.  As salmon anglers we rarely encounter truly smooth water and so will experience partial penetration below 10 degrees and a great deal of reflection at the sun elevations that are normal at our latitude.

The effects of differential reflection is to present the salmon with an endlessly changing kaleidoscope of light 'tiles' where the light has penetrated, and dark 'tiles' where it has been deflected and the reflected image of the bottom predominates.  In this photo you can see the effects of small water wave-forms in the light, mid and dark tones.  The lower half of the photo is in effect an image of the bottom of the river projected onto a wavy screen.

Most fishing photography of the underwater world has been done on chalk streams with crystalline shallow water and high light levels.  It presents a smooth mirror in Window 2 and a clear forward view in Window 1.  Those views do not represent the reality on the great majority of UK salmon rivers, which are generally well-dosed with bio-materials - especially peat in Scotland - and mineral particles.  As an aside, the water in snow melt Norwegian rivers has a very different character.  It is therefore essential to understand the effects of those particles on light underwater and hence on what the salmon sees.

Light Under the Water

The first phenomenon is Refraction.  As the light enters the water its path is changed by the increased density compared to air.  The salmon is not where it seems!  Once the sun's elevation is above the reflection-critical 10 degrees the light is refracted quite strongly downwards providing respectable levels of illumination (note the steep rise in the earlier hours of the Sun Intensity chart above).  In contrast, between 1030 and 1500 the light intensity changes less rapidly and remains within a narrower band, which makes life easier for the salmon.

Refraction also cuts the other way for the salmon.  Whereas the critical reflection angle from air to water is 10 degrees, from water to air it's around 41 degrees elevation above the salmon's horizontal sight line.  In smooth water everything below that elevation is reflected on the underside of the surface to form Window 2.  You can see the Window 3-2 boundary clearly in the photo above.

Next is Attenuation.  As light passes through water its component photons collide with the water molecules, which absorb them as warming energy and generally impede their progress.  As a result the strength of the light diminishes exponentially with depth, at a rate determined by wavelength.  The longer wavelengths - red and orange - shed energy faster than the shorter - green and blue.  As a rule of thumb, in non-spate Findhorn water, about 35+% of the red wavelengths are attenuated per metre traveled through the water.  However, we need to bear in mind that mid-morning in September at Tomatin the sun is at about 30 degrees, so by the time it reaches a depth of 1 metre it has travelled through two of water.  There only 40% of the red/orange energy remains to be reflected off your fly to travel to the salmon's eye a further 2 metres away.  Only 20-25% of the red/orange energy arrives, and bear in mind also that the fly is not a perfect reflector, so more energy is lost in the transfer.  

Optically your fly now has markedly different characteristics to what you saw in the shop, with lots of attractive strong colours.

Like this.  Everything has gone rather grey as 75% of the red and orange shades have been sucked out and the overall brightness reduced for depth.  You do, however, notice how the black bits stand out: contrast is the key to detection (ask any fighter pilot).

Things are a bit more cheerful at about half the depth (0.4 m), but remember that this is in normal clear, non-spate water.

Once the river rises Absorption comes to the fore.  At all wavelengths the photons hit a lump of peat in the water with a dull (inaudible) thud and stop dead.  Very little energy escapes.  Underwater light levels fall dramatically as bio-matter content rises.  It's always there but spates stir up the biomass as tiny particles, which being about the same density as water hang around for extended periods.  Lots of light gets absorbed and you get some interesting shades.

A favourite in this regard is the Deveron, seen here on a bright early September afternoon in 2015, in falling and clearing mode after a small spate, showing the tones of a fine, well-aged burgundy.  In this situation an orange or red fly is very hard to see, except as an outline against the lighter upper quadrant.

Making your flies fished at 0.4m/15-18" deep look like this when viewed at 2 metres range from below (i.e. the normal salmon point of view) (flies not range-corrected for size).

When the sun was obscured, down in Window 1 it was like old Madeira and your flies were invisible.

Another form of bio-absorption is that created by growing green matter in the spring.  This photo was taken during a very sunny week on the Dee in late April 2015 at a shallow camera depth, hence the high light level in the foreground.  The important element is the mass of fine green particles in the water, which otherwise looks wholly clear when viewed from above.  Live green biomass doesn't absorb light other than green as efficiently as dead matter, so doesn't reduce overall light levels as rapidly. Nonetheless, when it's growing strongly in the spring it does reduce visibility.

Bio-absorption by peat is of course markedly different to the blocking effects of bulk silt.  This photo was taken at the same place on the Deveron in the early stages of the spate 24 hours before the shots above, when silt predominated over biomass.  The visibility is close to zero and therefore hopeless for fly fishing.  The shade shows that this is mostly very fine agricultural loam, characteristic of the farming on that section of the Deveron.

The reason for the lighter appearance of the silt photo above is Backscatter.  Unlike biological material, silt is made up of uniform particles with faceted sides that deflect and scatter light in almost all directions.  The effect spreads light throughout the water in incoherent, chaotic and dazzling forms, which dramatically reduce visibility in Windows 1 and 2.

This is especially the case in bright conditions, as in this example from the Ure.  The source here is very fine near-white clay particles washed down from forestry clear felling operations.  The orange shade arises from the mixture of backscatter and residual peat coloration.  The brightness and dazzle of the backscatter was extraordinary, even in Window 1 at a depth of over a metre.  

In these conditions the salmon will only get a decent view of the fly when it is right up in Window 3, and it probably won't get much warning of its lateral approach from distance, as evidenced by the difficulty in spotting the fly in the circle.

You can catch salmon amidst high backscatter (I have occasionally) but only with difficulty and patience.  The colour of the fly is immaterial as virtually everything above the fish will appear dark if not black.  it is essential to present the fly well above the salmon's sight line.

Beyond Human Sight

Some fish have visual capabilities that go beyond the spectrum visible to the human eye in air.  Sticklebacks are a well researched example, in that their mating hue is only fully visible in the ultra-violet range.  At the other end of the spectrum certain species have well document sensitivity at wavelengths greater than 780 nm, beyond the infrared boundary, which would confer advantages in dark or turbid conditions.  Nobody seems to have done much research work on salmon vision in the IR range, apart from this article in The Atlantic science magazine.  In essence it suggests that the salmon's pigmentation process for dimming excess light can exist in an alternative form that heightens the sensitivity of its retinal rod receptors to IR frequencies.  Certain breeds of frog have this capability and lampreys probably do too, so it's entirely feasible that salmon, which are pretty wondrous in many respects, have this trick up their scaly sleeves to be able to see, even in the Deveron.

I don't have a modelling capacity for IR, but If you go to the very limit of the red boundary of the spectrum, this is what your fly selection may look like at 0.4m'15" depth in clear post-spate water viewed from 2m range. 

Nobody knows what salmon actually see because their image forming brain processes are a closed book.  Only humans see 'orange' as 'orange' and have language to describe it.  What this picture shows is that the radiation reflected from the flies in this domain, which the salmon is physically capable of detecting, forms the basis of an image entirely different to our human perception.

However, the most interesting part of this analysis is not the radiation but its absence, which is represented by black in the image above.  That intense contrast is central to the detectability of moving objects, to which the salmon's eyes are highly attuned.  The flies that appear bright red or orange in daylight may deliver sharp black contrast in the IR domain, which might be the secret of their success in the lower light levels of autumn. It may therefore be a supreme irony that the flies we select in the belief that their bright colours will attract salmon, actually work in an entirely different way through their absence of colour.

I have long held that the secret of black and yellow flies - Dee Monkeys, Posh Tosh etc - as a response to the low light levels of spring fishing in northern rivers lies in their direct presentation of contrasting parts.

Here's a Posh Cascade in daylight

In low light in clear Findhorn water

And in the same water at the edge of the IR band

In all 3 cases the internal contrast of the black/yellow combination exceeds that of the orange shrimps by a fair margin.  That should be a significant advantage when the salmon is viewing the fly against a dark background, for example in Windows 1 or 2 on a dull day.  It would, however, make no difference to visibility in Window 3.

So What?

That's a very big question and if I could answer it I'd probably give up fishing.  The mystery is an essential part of the magic.  The first great unknown is how salmon convert the physical capabilities of their eyes into usable images in the brain and in what form.  The second is what colour means to a salmon in terms of stimulus and interest.  And of course, superimposed on everything is the great mystery of why a fish that isn't meant to eat takes a fly at all.

But philosophy apart I offer the following thoughts in closing:

  • It's worth understanding how light interacts with water to help us fish more effectively.
  • The daily cyclical changes of light levels affect salmon and their propensity to take a fly, and are probably more significant than barometric pressure and most other transient factors such as oxygen levels.
  • Reading how light is behaving under the surface will help you select the right fly and present it at the most useful depth.
  • When the water is turbid with silt or bio-matter, keep the fly well above the fish.
  • The salmon will detect a fly at greater distance when presented off to the side rather than straight ahead, so bear that in mind when casting to specific lies.
  • The colour you see in the shop may well not be the colour or absence of colour that excites the salmon.  Bright in air is not bright in spate river water, especially if its orange.
  • Black and yellow flies appear to offer superior contrast and detectability in low light levels.
  • Green only seems to stand out in very clear water.
  • I was unable to experiment with blue flies owing to a lack of good images.  If you do tie blue flies, please send me some high resolution images for modelling.
  • Exploring the boundary of the IR band gives some useful insights into detectability in low light conditions.  More research in this area may lead to interesting conclusions directly applicable to fly design.


I should welcome feedback on this article, especially from anyone with a proper education in optics and physics.  My A level was almost 55 years ago, so I'm very rusty and will happily apologise for and correct any errors of science.  Anything that improves this work could be valuable.

If you found it interesting please let me know via the Comment facility below.

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