What does brown trout eat?

A trout in a river is running an energy calculation, continuously.

The fish holds a feeding station in the current and intercepts what drifts past it, and the position is chosen so that the food delivered exceeds the cost of holding station there. That economics is precise and observable: a trout will not move far for a small item and will move a considerable distance for a large one, and the same fish will abandon a station that stops paying.

The two verified relationships are what the drift mostly consists of. Aquatic insect larvae — mayfly, caddis, stonefly and chironomid — are the staple across the range and across most of the fish's life. Gammarid amphipods are the second, and they matter most in clean stony rivers and in winter, when insect larvae are less available and the shrimp population is not.

Surface feeding is a minority of the total effort despite being the part everybody photographs. Most of what a river trout eats, it eats below the surface.

Stillwater trout patrol rather than hold, which is a different strategy for the same food, and large individuals in both environments become substantially piscivorous — a shift that takes the fish out of the drift economy entirely.

Vision dominates, and the restricted cone through which a trout sees the world above the surface is the reason approach angle matters so much to anyone trying to present something to one.

The drift economics is the single most productive idea in river trout angling, and it converts into both location and presentation: find where food is delivered, and put something there that behaves as though it is being delivered. Everything about dead-drift presentation follows from it, and it is practitioner reasoning of great depth resting on well-observed behaviour.

The dominance of subsurface feeding is the reasoning behind the emphasis on nymphs, and the widely repeated observation that the visible surface activity represents a small share of what the fish is doing.

The visual window explains the approach conventions — low, from behind, off the skyline — which are convention derived from optics.

The shift to fish prey in large individuals is the reason a different approach reaches a different class of fish, and it is inference from the recorded diet rather than a demonstrated selection effect.

Both relationships are group-level and neither establishes proportion. Diet varies enormously between chalk streams, freestone rivers, upland burns and stillwaters, between seasons, and between sizes of fish — and the piscivorous component in large trout is not represented by a verified relationship at all. Nothing here describes any particular water.