I have read a number of threads where hobbyists attempt to sex tosai or young nisai. There are threads where 1/3 says that it can't be sex, 1/3 says male, and 1/3 says female. There's even a difference in opinion among dealers in the U.S. as well. Is it possible to sex tosai or young nisai? I know that this question is very helpful, especially for beginners who don't have the skills or finance to acquire female nisai or sansai.
I believe that the Japanese breeders know how to sex tosai, especially since most tosai that are sold here in the U.S. are males. My question is, what is it that they look for?
There is a great article in the AKCA's book of Judging and Buying Koi written by Brady Brandwood on sexing koi. In this article, Brady comments that it's possible to sex koi if you look at siblings from the same hatch. He says that if you compare siblings, the colors of females generally tend to be more bland. He then went on to explain how to sex koi that are two years or older.
I have a dealer who is also a very good friend who says that he can sex tosai just by looking at the body conformation. In particular, he says that he looks for the broadness of the shoulders and the roundness of the pectoral fins. I have bought a few tosai from him. As far as friendship is concerned, that remains to be seen in a couple of years!
As far as sexing tosai is concerned, here is what I have gathered. Please correct me or comment on any of these criteria:
1. Body conformation: females have broader shoulders and are more stout-looking compared to males.
2. Pectoral fins: females have rounder fins with rays that are even in length. The fins usually have less colors and are generally opaque.
3. Color: females have blander colors, especially beni.
4. Anus: female tosai have nonprotruding anus when pressed whereas males have protruding anus.
5. Belly: a dealer said that the bellies of males tend to be harder whereas females tend to be softer.
These are what I have gathered in this topic. Please feel free to dissect, comment, or add to this list.