Q. How close does a fly have to be presented to a fish before it actually sees it and will this depend on whether the fish is taking flies from the surface or subsurface? I find it difficult to intercept a trout that is surface feeding and wonder if it just doesn’t see the fly, or if I’m misinterpreting the feeding pattern.
A. When a trout is looking up, its window of vision consists of an angle of 97.5 degrees (see diagram 1). Bearing this in mind, the deeper the fish swims, the greater its window of vision becomes (diagram 2). This is in theory only as other factors come into play here. For example, turbid (or murky) water will affect the distance a fish is able to see.
To conserve energy, trout feeding on flies close to the surface or in the surface film, often cruise in the upper layers. The result is a reduced window of vision (diagram 2) and if your fly falls outside the trout’s window, then it won’t be able to see it.
A simple calculation can be made if, for the purposes of this exercise, we assume the trout’s window is 90 degrees. With this angle in mind a trout holding some four feet deep will have a window of eight feet. Conversely, a trout cruising a mere five inches beneath the surface will only have a window of 10 inches (diagram 2).
Bear in mind, too, that fish are usually looking forward and concentrating on the front half of the window in the direction they’re moving. So, in many circumstances we have to divide the window of vision in two. Thinking back to the trout holding at five inches deep, means we have a target of 2.5 inches. Of course, trout will still see potential food fall into the window behind them, but then it’s a case of whether they feel that circling round to take the offering warrants the effort.
From your question, it seems the biggest problem you encounter is targeting trout at the surface. When fishing a dry fly, try to establish if trout are cruising at depth, or near the surface. This is done by carefully studying rising forms, which can also help in determining which direction fish are heading. As a guide, trout often work upwind when feeding, especially on larger waters. So, if you see a rise under these circumstances, try casting upwind, ahead of the fish.
Trout feeding on an abundance of surface insects will often cruise close to the surface. With a reduced window of vision, landing the fly close to the fish is paramount, calling for accurate and delicate presentation. Look for head-and-tail rise forms (diagram 3), or any gentle bulging disturbances. Notice this rise disturbance often appears as oval-shaped rings with a pronounced side, indicating in which direction the trout is heading (diagram 4). Occasionally, and usually in calmer conditions, trout will adopt a more roving feeding pattern and tracking these random rises can be tricky. Generally, more splashy rises are a result of trout intercepting prey at speed and usually points to fish holding that bit deeper.
As to how far you need to cast in front of a rise to intercept it, this depends on several factors like food availability, wind strength, insect stage and of course, each individual fish. As a guide, start by giving the fish a lead of approximately three to four feet and revise it from there. Naturally, you need to cast well ahead of trout that are cruising at speed.
Conversely, if you cast too far in front of fish feeding haphazardly then there’s a good chance they’ll have changed direction before they ever reach your fly. Dimpling rings usually occur during flat clams and suggest fish are selecting insects stuck fast in the surface film.
Caenis or emerging buzzers spring to mind here and, with so many naturals on the water, trout will cruise just beneath the surface mopping them up. In such circumstances a cast that delivers your fly inches in front of the previous rise will be called for.
As for subsurface flies like nymphs and buzzer pupae this can be relatively academic, as we’re often fishing the water blind rather than visually targeting individual trout by sight. However, given clear water and the luxury of seeing trout beneath the surface, again I aim to drop the fly some four-foot in front of a fish.
Of course, the speed a fish is moving, its actual holding depth and water clarity will have a bearing here. Deep-lying trout require a cast several feet ahead, giving the fly time to sink.