sturgeon
鱣 refers specifically to a sturgeon, a large fish. The meaning is narrow and concrete, with no abstract extensions.
鱣 combines 魚 (fish) with 亶, which likely contributes the sound. The character was created to name the sturgeon specifically.
The fish radical 魚 on the left tells you it's a fish, and the right side 亶 looks like a complex, armored body — picture a sturgeon with its bony plates.
For タン, imagine a sturgeon getting a tan while sunbathing on a riverbank: tan -> タン, and the fish is a sturgeon.