You have to be really careful.
Bobby Glennon a biologist for Malone and Sons told me they are very careful when culling fish by sex. He says if they can't actually get gametes from the fish (milt and eggs) they don't do any guesses no matter how educated.
This even goes for chickens. Earlier this year I bought 10 one-day-old "sexlink" pullets (females) from a reputable chicken dealer. A "sexlink" is a mix between two varieties of chickens that supposedly makes sex identification very reliable. Wrong!
(In my case, these were the offspring of white rock hens and Rhode Island red roosters.)
When they hatch, foot color and feather color are supposed to reliably identify their sex. They sure all looked the same to me.
When they were about a week old, I thought that one female was kind of dominant. At about two weeks old, I thought she was kind of masculine. At about four weeks old, I introduced the young ones to my older ones. They knew -- I still wasn't sure.
At about five weeks there was no mistaking the young cockerel for a pullet. He really changed colors and markings.
Are fish much different?
