sturgeon
鰉 refers specifically to the sturgeon, a large fish. The meaning is narrow and concrete, with no extended abstract senses.
鰉 combines the fish radical 魚 with the phonetic component 皇, which likely indicates the sound. The character was created to name a specific fish, the sturgeon.
The fish radical 魚 tells you it's a fish, and the right side 皇 looks like an emperor. Imagine a majestic emperor fish—the sturgeon, prized for its size and caviar.
For コウ, picture a sturgeon swimming in a koi pond: koi -> コウ, but this is no ordinary koi—it's the emperor of the pond.