Fish tend to surface in dreams as symbols of emotion and the unconscious -- like water itself, they represent what moves beneath the surface of ordinary awareness. Catching a fish can reflect insight you're finally able to name, while fish slipping away might point to feelings or ideas that keep eluding your grasp. Fish are also linked to fertility, quietly forming below where you can't yet see.
The Sun answers those who are ruled by Mars, and Mars answers those ruled by the Sun. - Picatrix (Andalusia, ~1000.AD)