This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鰪 is a rare character combining 魚 (fish) with 盍, which likely contributes the sound. The exact historical development is uncertain.
The left side is 魚 (fish) and the right side is 盍, a shape that looks like a lid over a dish. Picture a fish under a lid, but its meaning remains unclear.
For コウ, imagine a fish under a lid making a hollow sound: koh -> コウ, like a deep echo from the covered dish.