I have a very mature 32 000 liter pond and the coldest it ever gets in winter is 19C and the warmest it ever gets is 28C so the definition of "winter" in SA might be different to elsewhere. In heating my spawing pond I was trying to take them from their normal environment to a sudden drop in temperature. I have heard that this often results in triggering the spawning.
You might be right regarding the develoment of the ovaries - from a prize winner at our local Koi show she has become fat over the last three months, together with my other female Koi and looks very much full of eggs. I have now put her and the male back into my main pond and will try a different pair. I have 13 VERY high grade Koi with 11 being females and only 2 being males.
I have heard of Koi changing sex - could it be that my two males are/have changed and do not have the urge? They are still producing milt and under the microscope the sperm is active but they may potentially not have the urge to merge
