When does the season start?
When does the koi breeding season start?
In South Africa the spawning season generally extends from mid-spring to late summer, with much of the spawning taking place from September to November. By that time of the year, the female fish will be heavy in egg and the males will have developed small, whitish tubercles (nodules) on their pectoral fins and gill plates (easily felt by stroking those areas) to indicate they are sexually ready… (and)… that a female koi must be exposed to elevated temperatures of at least 16.8°C for a sufficient period of time (this can be as little as eight hours) in order to ovulate and lay eggs. Living Jewels .
Such a quick answer may not suffice.
At a Western Cape koi farm, we knew that the first week of October was when the parent fish were ready for spawning. From that day onwards one could always find a breeding group ready to do the work. It was possible to breed to February, but that had its own challenges and was not so successful.
While most spawning is done in the period October to December (which translates to March to June in the Northern hemisphere.) It is possible to spawn beyond these dates, whether natural or artificially induced, but your risks of a successful harvest decreases. If too early, a sudden cold spell can kill the fragile fry while if too late the coming winter will slow the growth rate and stunt them. Also later in the season the fertility rate can be seriously affected.
Each area in the country will have its own most suitable onset date of the breeding season, which will be determined by the ambient temperature, the weather pattern and other local environmental conditions. Of cause if you breed indoors in a barn or hatchery these environmental conditions can be artificially adjusted to the point that you can breed virtually any time.
Breeding is a process, not a singular event. It starts way before the actual spawning. Females have to consume food to store in the egg yolk of all the individual egg. Without this nutritious food little eggs will develop. The final stage of fertilization happen when environmental cues like temperature and the presence of spawning material stimulate the parents for the amorous deed. Then with vigorous actions the female releases her eggs in the water and the males squirt sperm is the hope that they find their target. It is a brief affair. Once in contact with water the egg starts to swell and gets sticky leaving a three minutes window to find a willing sperm. Failing that it gets impenetrable for further fertilised. No fear however, a healthy female of average size produces some 500,000 eggs in short bursts leaving the party going on for quite a few hours…
The spawning is followed by the fry rearing and culling phases of the season. The ultimate goal is to produce salable fish for the market and select the high quality offspring for growing-on to the next season.
 Living Jewels by Ronnie Watt and Servaas de Kock, 1996, Delta Books, Cape Town.