Temperature & oxygen
SolidWater temperature & thermal stress
Once water pushes past about 20°C, the honest call is to stop fishing — for your catch rate and for the fish.
Why
Brown trout are cold-water fish. They grow best at around 13°C, and by roughly 19.5°C their growth has fallen to zero even when they are eating all they can — the fish is now losing ground, not gaining it. Push past 20°C and trout actively avoid the warmth: the first sign of thermal stress is a plain reluctance to feed.
Warm water is a double squeeze. It holds less oxygen to begin with, and it raises the fish's own oxygen demand at the same time — so the fish needs more air from water that carries less of it. On top of that, a fish hooked and released in warm water is far more likely to die from the fight and handling than the same fish in cool water.
This is not just angler lore. Inland Fisheries Ireland treats 20°C as the thermal-stress threshold for salmon and trout, and runs a Warm Water Protocol that closes state salmon fisheries when temperatures cross it — reopening only once they drop back below 18°C.
On the water
- In genuinely hot weather the best fishing — and the kindest — is often no fishing. Rest the water and come back when it cools.
- If you do go, fish the cool ends of the day and keep any fish you release in the water, unhooked quickly. Air exposure and warmth compound each other.
- A shaded, spring-fed or deeper water will run cooler than a shallow, open lough in the same heatwave — worth knowing which of your waters holds up.
The numbers
- Best growth
- ≈ 13°C
- Growth stops
- ≈ 19.5°C
- Trout avoid
- above 20°C
- IFI closes / reopens
- > 20°C / < 18°C
How LoughLogic uses this
This is the reasoning behind the welfare notes on your outlook: a caution once estimated water reaches 16°C, and a hard fishing stop at 19°C — the point the app greys the day out rather than sending you to stress the fish.
How sure are we?
Solid. The growth and avoidance figures come from Elliott's brown-trout model validated against 42 river populations, and the 20°C stop line is the published position of Inland Fisheries Ireland and the UK Environment Agency. The sharpest catch-and-release survival figures come from cutthroat and brook trout rather than brown trout directly — the same direction, not a species-exact transfer.
Related
Sources & how we know this (3)
- Once water exceeds 20°C during daylight, salmon and trout suffer thermal stress; IFI asks anglers to stop and closes salmon fisheries.
Inland Fisheries Ireland — hot-weather thermal-stress warning · 2026-07-16 - Brown trout growth optimum ~13°C, falling to zero by ~19.5°C; fish avoid water above 20°C.
Environment Agency / Wild Trout Trust — thermal biology of brown trout & salmon · 2026-07-16 - Angler catch rate and catch-and-release survival both fall sharply as water temperature rises.
Meyer et al. 2023, Fisheries Management & Ecology · 2026-07-16