This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鯹 combines 魚 (fish) with 星 (star), but the exact historical development is uncertain. It may have originally referred to a fishy smell or a type of fish, but no clear meaning survives in modern use.
A fish 魚 under a star 星 might suggest a fish that smells strongly, but the meaning is unclear.
For セイ, imagine a fish named Sei that glows like a star: Sei -> セイ.