bullhead
鰍 centers on a specific type of fish: the bullhead, a small freshwater fish. The meaning is narrow and concrete, referring directly to this animal.
鰍 combines 魚 (fish) with 秋, which likely contributes the sound. The character was created to name a specific fish, the bullhead.
The left side is a fish 魚, and the right side is autumn 秋. Imagine a bullhead fish swimming among fallen autumn leaves in a cool stream.
For シュウ, picture a bullhead fish being shown off at an autumn festival: show -> シュウ, and the fish is the star of the show.