What is winter-kill, and why does it shape so many perch lakes?

In shallow, fertile lakes under months of snow-covered ice, photosynthesis stops, decomposition keeps consuming oxygen, and by late winter the whole lake can slip below what fish survive — that is winter-kill, a fact of life across the northern prairies and countless small Canadian and upper-Midwest waters. Perch tolerate low oxygen better than most of their predators, and their boom-style spawning refills a lake within a few years, so winter-kill waters are often perch-dominated by default. The honest catch: big fish, with more mass to keep oxygenated, die first — which is why such lakes tend to cycle between stunted swarms and, in the few years after a partial kill thins the crowd, surprisingly fast-growing survivors.

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