type of fish, mullet
鰡 is a specific fish: the mullet. The general "type of fish" gloss simply identifies it as a fish species.
鰡 combines 魚 (fish) with 留, which likely contributes the sound リュウ. The character was created to name the mullet fish.
The left side is a fish 魚, and the right side 留 suggests something staying or remaining. Picture a mullet staying in coastal waters, easily recognized by its fish shape.
For リュウ, imagine a mullet leaping in a river (リュウ sounds like "ryuu" in "ryuukou" but here use a simple hook: "rew" as in "rewind"). Picture you rewind a video of a mullet jumping: rewind -> リュウ.