mullet (fish)
鯔 refers specifically to the mullet, a type of fish. The meaning is a single concrete animal rather than an abstract concept.
鯔 combines 魚 (fish) with 甾, which likely contributes the sound. The character was created to name the mullet fish specifically.
The left side 魚 means fish, and the right side 甾 looks like a fish trap or net. Picture a mullet caught in a woven trap.
For シ, imagine a mullet swimming in the sea: sea -> シ, and the fish is a mullet.