pomfret, stromateoides argenteus
鯧 refers to a specific type of fish: the pomfret (Stromateoides argenteus). The meaning is narrow and concrete, centered on this particular marine species.
鯧 is a phono-semantic compound: the left side 魚 (fish) indicates the meaning, while the right side 昌 likely contributes the sound. The character was created to name the pomfret fish.
The fish radical 魚 on the left tells you it's a fish, and the right side 昌 (which looks like two suns) can remind you of the shiny, silvery body of a pomfret glinting in the water.
For ショウ, imagine a pomfret fish being displayed in a fish market show: show -> ショウ, and the shiny fish is the star of the show.