This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鮅 combines 魚 (fish) with 必, which likely contributes a phonetic element. The exact historical development is uncertain.
The left side is a fish 魚, and the right side is 必. Without a clear meaning, picture a fish that must (必) be identified, but its name remains a mystery.
For ヒツ, imagine a fish that hits (ヒツ) the water with a splash, but you still don't know what kind of fish it is.