A trout's eye view

Just how much can a trout see, compared to ourselves? I have spent time in the past discussing colour vision in trout since I have been much involved with colour technology during my working life. Recently, however, a friend raised a question about another aspect of vision: acuity. Can a trout see as much detail as we can?

It is easy nowadays to find information on the internet and research has shown that acuity in all fish is less than ours. Acuity is measured as the smallest angle subtended to the eye at which two objects can be seen as separate. In humans that angle is one minute of arc (i.e. one sixtieth of a degree). In trout it is fourteen minutes of arc (at one year of age, prior to which it is even more coarse). A considerable difference.


It would be interesting to visualize just how a trout sees the finer detail of an insect's appendages and at what distance. But by what contrivance could we ever see through a trout's eyes?

Acuity is largely a result of the packing density of receptors on the retina, each receptor providing a single area of colour on the field of vision, the area being (in the case of the trout) that which represents an angle subtended to the eye of 14'. It occured to me that this could be visualized as a pixel matrix such as we now commonly use in digital photography and our television and computer monitor screens. Given that this is a valid parallel, it is a simple matter to process an image on the computer using bitmap format to reduce to any required level of pixel resolution. The following calculations and images show the result of such a process:

First we need a correct calculation of the height of our and the trout's 'pixel' at any given distance. So:

Our angle of 1' = 0.017degrees
      Height of our pixel @ distance d = d x Sin(0.017)
      = d x 0.000297

Trout's angle of 14' = 0.233degrees
      Height of trout's pixel at distance d = d x Sin(0.233)
     = d x 0.00407

Next, using a picture of a fly with a body length of about 20mm:
  1. Calculate as above the height of a visual pixel at a given distance for ourselves and for a trout.
  2. Divide the fly's body length by the height of the pixel to obtain the number of pixels which the fly's body crosses at that distance.
  3. Resize the image in Paint so that the body crosses that number of screen pixels (approximately)
  4. Resize back to the original to display the detail that is visible.

So at a distance of 1m we see the 20mm fly across 20/(1000 x 0.000297) pixels
     = 67 pixels
[image Our~1m]

The trout sees it across 20/(1000 x 0.00407) pixels
      = 4.25 pixels
[image Trout~1m]

Similarly calculating for a distance of 250mm, we derive [image Our~250mm] and [image Trout~250mm]


It is tempting to suppose as a result of these images that at one metre distance, and particularly since it is such a large fly, the trout can see nothing recognisable. But the above images have been brought to the same size to display the comparative detail. In reality, the view of the fly at 1m occupies only one quarter linear of that occupied at 250mm. So putting the images back into perspective, we have the following view:


It is apparent that in spite of the low resolution, at distance (i.e. when reduced in size) the image forms an identifiable pattern, albeit a shadowy pattern with only the bulkiest part of the fly's body appearing solid.

Now, you might well ask, "What use is this to us?". It immediately brings to my mind the times I have seen trout in clear, still water rise to a dry fly, inspect it from a few inches and reject it. The above images confirm the need for such close inspection – when there is opportunity. But perhaps of more significance is the inference that a pattern needs to be constructed so as to arouse the trout's interest from a distance, without which no activity will ensue. So as any manner of lure approaches close enough to the trout to arouse awareness, its bulk profile must be favourably suggestive. Hence the importance, for example, of a prominent thorax where relevant. Approaching more closely, legs and wings become more significant but not in terms of exact number – which perhaps explains why a many-fibred hackle is acceptable as a substitute for three pairs of legs. In the above images it is the general pattern that identifies the insect, not the precise detail. Where circumstances allow time for close inspection, perhaps the trout will count the legs. Who knows?

The difference between surface (meniscus supported) and sub-surface presentation must be considered. Where a dry fly is concerned, probably the silhouette is of greatest importance (when inside the trout's window of the outer world) and it should be borne in mind that acuity is enhanced by high illumination. The sub-surface environment offers lower light intensity and lower contrast between the approaching potential food item and the background so perhaps colour becomes more important and small detail less so. This is supported by the success of some well known and very simple nymph patterns. However, I leave further extrapolation to the imagination of the reader.

John Bernard Sunderland January 2014