This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鰶 combines 魚 (fish) with 祭, likely as a phono-semantic compound where 祭 contributes the sound and 魚 indicates a type of fish. The exact historical development is uncertain.
The fish radical 魚 suggests a fish, and 祭 looks like a festival offering. Imagine a rare fish used only in special festival ceremonies, its meaning tied to that context.
For セイ, picture a ceremonial fish offering that is so sacred you must say 'say' (セイ) a prayer before touching it.