This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鯯 combines 魚 (fish) with 制, but the exact historical development and original meaning are uncertain.
The fish radical 魚 suggests a connection to fish, but the overall meaning is unclear. Think of it as a mysterious fish-related character.
For セイ, imagine a fish saying 'say' as it swims: say -> セイ, though the fish's message remains a mystery.