sheat
鰋 is a rare kanji for a type of sheatfish (catfish). Its meaning is narrow and specific to that fish.
鰋 combines 魚 (fish) with 匽, which likely contributes the sound. The exact historical development is uncertain.
The left side 魚 means fish, and the right side 匽 looks like a shelter or enclosure. Picture a catfish hiding in a rocky shelter underwater.
For エン, imagine an eel-like catfish ending with the sound 'en': eel-en -> エン.