sole
鰨 centers on a specific type of flatfish: the sole. It is a narrow zoological term without broader abstract extensions.
鰨 combines the fish radical 魚 with a phonetic component that likely contributed the sound. The exact historical development of the right side is uncertain.
The left side 魚 tells you it is a fish. The right side looks like a flat, compressed shape, like a sole pressed against the sea floor.
For チョウ, imagine a sole fish named Chou, lying flat on the ocean floor: Chou -> チョウ, and the fish is a sole.