Temperature & oxygen
SolidCold water & the winter slow-down
In cold water a trout's whole metabolism drops right down — it feeds little, briefly, and often only in the warmest part of the day.
Why
Trout are cold-blooded: their body temperature, and with it their metabolic rate, tracks the water. In cold water everything slows — digestion, movement, the urge to chase. A winter trout needs far less food and spends most of the day sitting still, saving energy, not hunting.
The daily feeding window narrows and shifts. Below about 10°C, salmon and trout switch from feeding by day to feeding at night — hiding up in daylight and emerging after dark (Fraser, Metcalfe & Thorpe). For the angler the practical upshot is the mirror image of summer: the productive spell is the middle of the day, when a weak winter sun has lifted the water a degree or two.
There is a floor. Growth stops around 3–4°C, and in genuinely cold water fish may barely feed at all — no retrieve is clever enough to change that.
On the water
- Fish slow and deep — the fly has to come to the fish, not the other way round. A fast strip through 5°C water rarely earns a pull.
- Wait for the warmest hours — late morning to mid-afternoon — rather than the dawn-and-dusk edges that rule in summer.
- A mild, overcast day after a cold snap, when the water has nudged up a degree, beats a bright frosty one.
The numbers
- Growth stops
- ≈ 3–4°C
- Feeds nocturnally
- below ~10°C
- Best window
- warmest midday hours
How LoughLogic uses this
The engine's water-temperature band drives this: a cold band marks the day down and steers away from the low-light edges that only pay in warmer water.
How sure are we?
Solid on the biology — the metabolism-tracks-temperature relationship and the sub-10°C day-to-night feeding switch are well established (Fraser, Metcalfe & Thorpe 1993; Elliott's growth work). 'Fish the middle of a winter day' is the widely-held practical reading of that biology rather than a single measured result.
Related
Sources & how we know this (2)
- Salmonids switch from diurnal to nocturnal foraging below ~10°C, driven by temperature independent of daylength.
Fraser, Metcalfe & Thorpe 1993, Proceedings of the Royal Society B · 2026-07-16 - Brown trout growth falls to zero at the cold end around 3–4°C.
Environment Agency / Wild Trout Trust — thermal biology of brown trout & salmon · 2026-07-16