At Windsor fish hatchery our source of knowledge is experience.
You may blame the warmer weather activity in your pond on koi fighting if you see your fish pushing other fish around. The reverse is true, though.The koi are spawning and its an awesome spectacle to watch. This can also happen when you add new fish to your pond in summer.
I have had customers call me at this stage, worried that it's too stressful for the female koi. She can utilize her ab muscles to push the eggs out on her own, but to have viable eggs she needs the males, so all this commotion is necessary. by moving the female fish around the pond, usually against a rock or some plant material, the eggs are essentially released and then fertilized by the males. Males help females ovulate by bumping their heads into her flanks, giving her the extra push she needs to release the majority of her eggs.
There is no malice intended, but females may be injured or wear out in the pursuit.
She may experience extreme anxiety due to the constant stress of being chased. She might not be eating because she's stressed out and afraid of the male, or because she's hiding from him.
If you keep an eye on your koi and notice this happening, you may wish to remove the female from the pond. If you don't have access to a second pond, you can give her a break by enclosing her in a space where the males aren't allowed to enter.
You can use shade cloth, or netting, to create a barrier in the pond that allows the water to flow through but keeps the male fish away from any female that looks stressed. floats can also be utilized on top of the net, and or tent pegs at the sides.
Fish like koi lay hundreds of eggs when they spawn, but only a fraction of them will develop into adult fish. Koi scatter their eggs on all available surfaces within the water, plants, rocks and the bottom of the pond. You may notice your water is quite foamy. Do a water change of around 20%.
Many of these eggs will be devoured by the koi themselves and other pond inhabitants such as tadpoles & goldfish. Just as many will not be viable.
When a number of the eggs hatch a few days after they are laid the babies are tiny a millimetre long , you may not have even noticed they have hatched. The babies will start feeding in a few days on algae & food in the pond, tiny little organisms on the plants and in the water, they will grow quite quickly in a few months you will be able to see them & feed them on crushed koi food.
The babies that survive will continue to grow and put on bulk for the rest of summer, only slowing once the water temperatures start to drop. You can feed a high protein koi food over summer while they are growing.
If you bring them inside for winter, you will need a large tank with a very good filter. If the tank is heated, they will continue to grow.
Seeing what colours emerge from a spawning is a fascinating element of the hobby. You can expect most of the "homemade" offspring in your pond will be plain red,white or dark, due in part to the fact that dark or less colourful are favourable survival colours for koi fish, and that their respective genes are extremely common. Consequently, these are the prominent offspring you'll have, there will also be a lesser number of nicely coloured and patterned koi.