Taking Advantage of Pre-hatch Migrations

We often think of the opportunity to fish nymphs during pre-hatch or hatch migrations as being limited to insect groups that crawl out to emerge. But for insect groups that emerge at the surface in open water, like many mayflies and caddisflies, there are also migratory activities in the days leading up to a hatch that increases their exposure to feeding trout and imprint search images in the trout’s brain.

As each generation of each insect species approaches maturity, and the bugs become bigger, requiring more resources to support final development, competition for prime feeding spots heats up. This competition can result in more bugs in the drift through two mechanisms; direct conflict and intentional migration.

In direct conflict, two individuals battle it out over the most productive feeding spots. The loser if often cast into the drift, where it may tumble with the current for a few meters before regaining a foothold on the bottom. I once witnessed two Brachycentrus caddisfly larvae doing exactly this. It was a hard-fought battle, but in the end, one was ejected into the current and went tumbling downstream.

Brachycentrus caddisfly larvae lined up in prime feeding stations. As the water level changes,
the best spots on the rock will change - leading to a game of "musical chairs".

While direct conflict results in sporadic numbers of nymphs and larvae in the drift, intentional migration results in consistently large numbers of bugs traveling with the current. If there is too much competition on a particular riffle, or the resources are being depleted, the most expeditious strategy for moving to greener pastures is to float downstream a ways and explore new areas. These migrations, often taking place in the late evening, are far more common than most anglers realize.

With specific species of nymphs or larvae being more common in the drift as hatch periods approach, trout are more inclined to form search images for, and feed on, the ones they are seeing most often. I often use this to my advantage by fishing the corresponding nymph of whatever insect has been recently hatching during periods when there are no active hatches.