How does a trout catch a fly?

Understanding how a trout recognises a fly and then intercepts it is key to your choice of fly – and success

- PAUL KENYON

Before guiding anglers on South Devon rivers I am often asked, “What fly should I use?” I advise them to start with a fly they have confidence in. This short answer avoids a long explanation but, if pressed, the following is my rationale.

I turn the angler’s question on its head to focus on the trout rather than the fly. Let’s explore how a trout recognises and intercepts a fly drifting towards it on the surface of the river. The answer may give an insight into how to design effective trout flies and why our artificial flies are sometimes ignored by feeding trout.

Traditionally, angling writers have talked in terms of deceiving a trout into taking an artificial fly. I put a slightly different slant on the angler’s task. I want to work with the trout rather than trying to deceive it. After all, fish want to eat. If we understand how a fish first recognises and then catches an insect we can select and present an artificial fly so that it can be caught easily by trout.

How does a trout catch a fly? It’s a deceptively simple question about something many of us take for granted. But catching a fly moving quickly downstream is no simple feat, especially for trout lying beside popply water near the head of a pool. The trout’s problem is similar to ours when we try to catch a cricket ball. It looks easy until you try to do it. Outfielders are expected to make a catch but it’s clearly more difficult for infielders in the slips who – like trout at the head of a pool – must react quickly.