Males also develop breeding tubercles on the head and pectoral fins in the breeding season.
If left in the pond most of the eggs will be eaten, and any fry that do hatch will usually be eaten too. If you want to save some fry to grow on you will have to move them to a tank or pond with no adult koi. It is very difficult to collect eggs that have been randomly distributed in the pond so it is a good idea to use spawning brushes which the koi will spawn on. It is preferable to add an airstone as the developing eggs need plenty of oxygen. The day before hatching the eyes in the egg will have a shine to them, soon the koi will begin to wriggle and in a few hours break out of the egg.