fish
鯈 centers on a specific kind of fish, likely a small freshwater fish. The meaning is narrow and concrete, with no clear abstract extensions.
鯈 combines 魚 (fish) with 攸, which likely contributes a phonetic element. The exact historical development is uncertain, but it is a phono-semantic compound for a type of fish.
The bottom part is 魚 (fish), and the top part 攸 looks like a person beside a stick. Imagine a person using a stick to point at a specific small fish: that's the fish 鯈.
For ユウ, imagine a person saying 'you' while pointing at the fish with a stick: 'you' -> ユウ, and the fish is the one being pointed at.