I started my worlds with the intention to build in and around the coral reefs, where I had spotted many aquatic mobs. However, after a few hours of playing the game, these stopped spawning. Coincidentally, on both of these worlds, I traded 'tropical fish in a bucket' from a wandering trader. After doing so, I never saw a wild fish mob, or squids and dolphins, again. Turtles don't seem to be affected by whatever is happening. I have had two survival worlds where this has occurred. One was on normal setting, the most was on peaceful