Why do perch schools sort themselves by size?

Three mechanical reasons. Same-sized fish cruise at the same speed, so a mixed school would constantly tear itself apart; same-sized fish eat the same menu, so a size-sorted school grazes one habitat efficiently instead of arguing over two; and perch eat perch — a 10 cm (4 in) fish swimming beside a 30 cm (12 in) one is swimming beside a predator, so the small have every reason to keep their own company. The angler-facing consequence is the most useful rule in the sport: the first fish tells you what the next fifty look like, and changing your catch means changing schools, not changing lures.

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