If you keep koi then you may be keen to discover when they might begin to mate. Like people, fish can reach sexual maturity at different ages and their age can be difficult to determine.
How old are your fish?
It is hard to know how old your fish are unless you obtained them shortly after birth. Most koi will be at least one year old before they are sold. This is partly because it may take several months for them to be chosen by a new owner and offered their forever pond. You cant base your estimate of age on the date that your purchased the koi as the information you were given may not have been accurate. To make matters worse, you cant estimate the age of a fish based on its size. Koi vary in size naturally and their rate of growth will be influenced by their diet. You could encounter a large fish which is quite young or an older fish which is reasonably small. Breeders and specialist vets would be able to help you. There are indicators of age but you will need the benefit of experience to interpret them. The best way to tell the age of a koi is by checking its ear bone! But you might want to leave that to the vet!
Koi may exhibit spawning behaviour before they reach full sexual maturity. Spawning behaviour can appear to be very aggressive and so is often confused with fighting. The courtship ritual involves the males chasing and nipping the females. This can cause the females to attempt to jump out of the pond, to hide and to wedge themselves between plants, rocks and other features in the pond. The intense nature of the activity when fish spawn may result in foamy, murky and even smelly water in the pond. Spawning typically occurs in May or June but will depend on the seasonal changes to the water temperature. You should not disrupt the pond environment by attempting to address the foamy or murky water. It is best to allow nature to take its course until spawning is over!
Finding the Eggs
When the koi have calmed down after their intense period of activity, take a look to see if there are eggs in your pond. These should be visible around the rocks and plants at the edge of the pond. The eggs will be clear, round and roughly the size of a grain of salt. Sometimes, however, the eggs are eaten by the fish.
Attending to Your Fish
After the activity dies down and the mating ritual appears to be over, it is a good idea to look at your fish to check their condition. You should ensure that they are being properly aerated, especially in very foamy conditions. Spawning is an interesting activity to witness, if sometimes a little disturbing! A few weeks later you could be lucky enough to find a host of fry swimming happily in your pond.