This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鮏 combines 魚 (fish) with 生 (life, raw). The exact historical development is uncertain, but it likely originally referred to a type of fish or a fishy smell.
A fish 魚 that is raw 生 might smell fishy, but since the meaning is uncertain, treat this as a tentative visual mnemonic.
For セイ, imagine a raw fish being served as a 'say' (セイ) in a dish, but the fish is so fresh it still smells of the sea.