Sexing koi

Even professional breeders find it challenging to sex small koi. Generally koi that are smaller than 25cm (10") are understood to be sexually immature and that their sexual organs develop after this stage. Mature males tend to be narrow bodied and slender when viewed from above, the pectoral fins are usually longer and more pointed in shape than the females. Mature females are generally plumper in the belly region. Around spawning time the males develop fine, white, raised spots on the head, gill plates and front edges of the pectorals, these are called tubercles. They look like grains of salt and to the inexperienced koi keeper can be thought to be - and are often treated as - white spot (Ichthyophthirius), therefore it is vital to be aware of the difference. The tubercles are most abundant on the pectoral fins, here they appear in rows and have a slightly rough feel to them. They are used by the male during breeding in early spring when he is constantly chasing and nudging the female, it is this activity that encourages spawning and leads to the female shedding her eggs.