anchovy
鯷 centers on a specific small fish: the anchovy. The meaning is narrow and concrete, referring to this particular type of fish.
鯷 combines 魚 (fish) with 是, which likely contributes the sound. The character was created to name a specific fish, the anchovy.
The left side 魚 means fish, and the right side 是 looks like a net or a container. Picture a net full of tiny anchovies: 鯷 is the anchovy.
For イ, imagine an eagle (イーグル) swooping down to catch an anchovy: eagle -> イ, and the fish is the anchovy.