Yes you do. 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.

I know this spring when I plant my brood gills I will be using a catheter and probing for gametes along with all the visual characteristics.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.