If you think about it Luca keeps a certain area of fish in a trench and in Nemo they say not to leave the reef. This could be because if they leave the reef Luca cannot protect them in fear of being seen. Also you can see in Luca that there is a patch of fish that cannot go out of a area with some grass under them which could be the coral of Nemo's home.Plus Nemo has verious batches of coral that Luca protects from the fisherman.
Theory by Jacob Thor