This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鰧 combines 魚 (fish) with a phonetic component that likely contributes the sound. The exact historical development is uncertain.
The fish radical 魚 suggests a connection to fish, but the overall meaning is unclear. Imagine a mysterious deep-sea fish whose name is written with this kanji.
For トウ, picture a fish with a toe-like fin: toe -> トウ, and the fish radical 魚 reminds you it's a fish.