What does it mean when you dream of dead fishes?

That depends on the severity and how often you dream them. In this case the dead fishes would be known as Latent Dream Content or the things we don't understand about a dream that often have some deeper meaning. The mob used dead fish to convey the sense that someone was about to be killed and thrown into the ocean hence the saying "Sleeping with the fishes". Your dreams probably mean nothing serious...unless of course you disrespected the mob in some way.