Freud held that small fish represented the male semen. Medium-sized fish (logically enough) were children. Fishing poles were a symbol of the phallus.
Other dreams about fishing may resolve around seeking provision for your needs in life, desiring to find something that seems "below the surface," or looking for primordial instincts, if Darwinism is your worldview of choice.
To dream of fish may simply be a dream of nourishment, or lack thereof. It may also point to a sense of adventure or travel.