Light & daily rhythm
SolidLight & the crepuscular window
Trout feed hardest in low light — the dawn and dusk 'crepuscular' windows — because that's when they can hunt without being hunted.
Why
Trout are visual, drift-feeding predators, but in bright light they are also exposed to their own predators — herons, cormorants, ospreys. Low light tips the balance: they can still find prey, but are much harder to be seen themselves, so they move out and feed with confidence.
Their food obliges. Many lough and river insects emerge and drift at dusk and after dark, and the daytime zooplankton rise toward the surface as the light falls. The low-light window stacks feeding fish over active food.
Feeding timing is not fixed, though — it moves with temperature. Trout and salmon feed by day in mild water but switch to feeding at night once it drops below about 10°C (Fraser, Metcalfe & Thorpe). That's why the app anchors dawn and dusk on the real sun for the day, not a fixed clock time.
On the water
- On a bright, calm summer day the middle of the day is usually dead — save your effort for the first and last hours of light.
- In a heatwave the evening edge often beats the dawn edge: both are cool, but dawn sits at the daily oxygen low while the evening has recovered (see the oxygen card).
- A dull, overcast day is low light all day — trout will feed through it far more readily than under high sun.
The numbers
- Prime windows
- dawn & dusk
- Feeds nocturnally
- below ~10°C
How LoughLogic uses this
This is what the app's light-level scoring is built on: the outlook rewards low-light windows, marks down flat-calm bright middays, and anchors dawn and dusk on the real solar elevation for the day.
How sure are we?
Solid. Crepuscular feeding in salmonids is well documented, and the temperature-driven day↔night switch is a foundational, repeatedly replicated result (Fraser, Metcalfe & Thorpe 1993; Reebs review). The exact best minute varies with water, weather and season — the pattern is robust, the precise timing is not a formula.
Related
Sources & how we know this (2)
- Salmonid foraging switches between day and night as a function of temperature — nocturnal below ~10°C.
Fraser, Metcalfe & Thorpe 1993, Proceedings of the Royal Society B · 2026-07-16 - Fish show strong daily activity rhythms, with many species most active — and feeding — around dawn and dusk.
Reebs — Plasticity of diel and circadian activity rhythms in fishes (review) · 2026-07-16