My friend told me last week that there teacher told her that all or most fish start out as girls then grow in to boys. I didn't beleve this but is it true?
The sex of quite a few fish is determined at birth based on water quality... temp, Ph, etc.
There are also fish that can switch sex later on in life. Many Saltwater fish do this. They can change from male to female or female to male based on what they need to do to reproduce.
She might have meant that young fish are female in morphology. This is generally the case. The ones that will become males, in the majority of species, have this set from conception.