minnow
鰷 centers on a small freshwater fish: the minnow.
鰷 combines 魚 (fish) with 條, which likely contributes the sound and may relate to slender or branch-like shapes, fitting a small slender fish like a minnow.
The left side is a fish 魚, and the right side 條 looks like a slender branch with leaves. Picture a tiny minnow darting among slender water plants.
For チョウ, imagine a minnow named Chou swimming in a tiny pond: Chou -> チョウ, and the little fish darts away.