Tuesday, 20 October 2020
RiverBum Fly Fishing Gear

How Does a Trout Catch a Fly?: Marinaro’s

anglers often asked what fly will catch a trout I want to turn that question on its head and explore how a trout recognizes and intercepts the fly drifting towards it on the surface of the water the answer may give an insight into how to design effective trout flies and why our artificial flies are sometimes ignored by feeding trout trout see the world through a skylight or circular window surrounded by a mirror Marinaro's great insight was to recognize how trout use the position of the fly in this window to make an effective rise Because of the laws of refraction fish cannot see objects which lie below an angle of 10 degrees to the water surface at the edge of their window

The red triangles in these diagrams show this blind spot extending outwards all around the edge of the trout's window this is why we adopt a stealthy approach when casting to a rising trout We often judge where to cast a fly by noting where a trout rises to take a natural fly but there is a flaw in this approach which has been explained by Vince Marinaro It turns out that trout get advanced warning of a fly long before it appears in their window trout are able to see parts of an insect or artificial fly that rests on or punctures the mirror The bodies of emerging flies break through the water surface, they hang beneath the mirror the legs of duns resting on the surface also create a light pattern that triggers the start of the rise A number of authors have provided photographic evidence that trout can see the wings of approaching insects in their window

The wings of an insect that protrude above an angle of greater than 10 degrees to the water surface are potentially visible in the window therefore a trout has two cues that an approaching object may be edible: firstly body parts that break through the mirror and secondly wings appearing in the window There are photograph in Clark and Goddard's book that show that when the insect reaches the edge of the trout's window a crucial event takes place: the wings body and legs of the insect merged together Marinaro provides similar photographs and he summed up his extensive observational studies of trout feeding behavior in these important words: it is an inescapable conclusion that the trout places the fly always at the edge of the window for all purposes viewing inspecting and taking Why does the trout keep the fly at the edge of the window? I think trout behave in this way in order to judge the exact position of the fly By keeping the fly in a precise position relative to their body the trout stands a very good chance of engulfing the insect, I'm not for one moment suggesting that trout do mathematical calculation, but I am suggesting that the trout's behavior has evolved in response to the physical laws which describe their everyday environment We know that the trout's window has a width of 97 degrees and that the radius of the trout's window is a precise function of the depth of the trout in the water, and therefore the distance between the trout and the insect can be calculated

Because this distance is a precise function of the depth of the trout in the river the trout stands a very good chance of successfully ingesting a fly if they drift downstream keeping the insect on the edge of the window The acquisition of this skill may involve learning maturation and practice The main message from this analysis is that a successful trout fly should present a primary trigger stimulus that penetrates the mirror this engages the trout's attention and initiates the rise: wings on the artificial may act as cues during a rise to maintain the trout's movement towards the fly Marinaro's Theory can also account for how a trout intercepts a natural or artificial fly swimming beneath the surface As the sunk fly approaches the fish sees two images: the actual fly and its reflection in the mirror and then a single image when the fly crosses the edge of the fish's window

By keeping the fly on the edge of the window the trout stands a very good chance of engulfing the insect

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