What actually drives the perch year — calendar, temperature, or light?

Two clocks running together. Photoperiod — day length — is the coarse clock that stages the year: it cues gonad development through fall and winter so fish arrive at ice-out ready to spawn, and its shrinking days trigger the fall feeding urgency. Water temperature is the fine clock that executes: spawning fires at roughly 7–12 °C (45–54 °F), the comfort band around 17–23 °C (63–73 °F) sets summer depth, and cooling water times the fall slide to the basins. The calendar is only a shadow of these two — which is why dates shift weeks between latitudes and between years, and why reading a thermometer beats reading a datebook.

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