Why do brook and lake trout spawn in fall but rainbows in spring?

Two lineages, two bets on the same prize: having fry hatch into spring abundance. Brook and lake trout — both chars, the ancient cold-water branch — spawn in October–November as water falls through 4–10 °C (39–50 °F); their eggs incubate slowly in cold gravel or rubble all winter and hatch at ice-out, first in line for the spring bloom. Rainbows, on the Pacific-salmon branch, spawn in April–May at 6–12 °C (43–54 °F) as flows rise; warmer water incubates the eggs in weeks instead of months, landing the fry in the same spring window. Photoperiod — day length, not temperature — is the master trigger for both, which is why spawn timing is so consistent year to year at a given latitude.

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