snakefish
鮦 centers on a specific fish: the snakefish. It is a narrow, concrete term for this animal rather than a broad conceptual category.
鮦 combines 魚 (fish) with 同, which likely contributes the sound. The character was created to name the snakefish specifically.
The left side 魚 means fish, and the right side 同 looks like a container. Imagine a snakefish being the same (同) shape as a container, or a fish that looks like a tube.
For トウ, picture a snakefish swimming through a narrow tube: tube -> トウ, and the fish's long body fits the tube shape.