kind of sea turtle, surname
鼂 refers to a kind of sea turtle, and is also used as a surname. The two meanings are separate; the surname usage does not derive from the turtle meaning.
鼂 combines 旦 (dawn) and 黽 (frog/turtle-like creature). It likely originally depicted a type of turtle, with 旦 possibly contributing a phonetic element. The exact historical development is uncertain.
The top 旦 looks like the sun rising over the horizon, and the bottom 黽 is a creature with a shell and legs. Picture a sea turtle emerging from the ocean at dawn.
For チョウ, imagine a sea turtle named Chou slowly crawling onto the beach at dawn: Chou -> チョウ.