Why do perch slide deep in high summer?

Because the surface layer overshoots their comfort band. When the shallows push past about 23–24 °C (73–75 °F), every degree costs extra metabolic overhead, and the fish simply follow cooler water down — typically to flats, shell beds, and basin edges in 6–12 m (20–40 ft), as long as the deep water still carries oxygen. The slide is strongest in big fish, which run hotter metabolisms per meal, and in clear stratified lakes; in shallow, wind-mixed lakes there is nowhere cooler to go and the depth swing compresses. At northern latitudes the "slide" may be only a few weeks long; in the southern part of the range it can define the whole summer.

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