Strike indicator

A strike indicator is a small buoyant object, often foam, yarn or a plastic bubble, clipped or tied onto the leader above a nymph. It floats on the surface and acts like a bobber, dipping, pausing or twitching when a fish takes the fly below, which is otherwise very hard to feel through slack line and current.

Indicators also help control depth and drift, since the distance between the indicator and the fly roughly sets how deep the nymph rides, and watching the indicator's speed against the current on the surface shows whether the fly is drifting naturally or dragging unnaturally underwater. Adjusting that distance is one of the fastest ways to fix a slow day of nymphing.

Sizing the indicator to the water matters. Too large an indicator in slow, clear water can spook fish or fail to register a light take, while too small an indicator in fast, choppy water can be impossible to see or can sink under the weight of the rig. Many anglers switch to a tight-line, indicator-free approach like euro nymphing in technical water for exactly this reason.