small kind of sturgeon, found in the Yangtze
鮰 refers to a specific small sturgeon found in the Yangtze River. Its meaning is narrow and tied to that particular fish.
鮰 combines the fish radical 魚 with 回, which likely provides the sound. The character was created to name a specific fish species.
The fish radical 魚 tells you it's a fish, and 回 (return, revolve) might suggest a fish that swims in circles or returns to the Yangtze. Picture a small sturgeon looping through the river.
For カイ, imagine a small sturgeon swimming in a circular pattern that looks like the letter K: K -> カイ, and the fish keeps circling back.