Koi are basically river fish with mechanisms that were developed for river life. That's not to say they cannot live in a pond. Actually, they do fine in ponds - but they're bodies and their behavior is all "river-y."
Why is this important in this conversation? Because they are engineered to scatter their eggs everywhere without parental care afterward rather than laying them in a nest like a pond fish would. The mother and father koi never give the fry another thought. In your pond, all the same stuff happens as in a river, but you get to see more of the gory details … and they matter, so listen up!
Do you know why fish spawn? Ostensibly, to lay eggs and make babies, right? But I meant do you know why, biologically they actually spawn? What causes it?
Well, it's two things – first, the water warms up in the spring, and then the days get longer. These two changes in the environment cause hormones to be released by the fish, causing the female to become full (gravid) of eggs and the male to dress them up for hatchin’ (fertilization)!
During this time you will likely see koi chase each other around the pond; usually first thing in the morning. It's a remarkable sight because large koi can really, really wreck a pond with their shenanigans. They will bull around the plants in a group, upsetting pots, rocks, roots – and all for spawning. The female, usually a larger rounder fish, is "driven" by one or more males, and in their blind excitement, even females will join the pursuit of the female laying her eggs. They all get together going in one direction like, "Hey, it's a Conga line!"
All of this has to happen because the female koi has no ability to push her eggs out with abdominal muscles. Instead, the eggs are basically leaked out of the fish from the passive pressure that comes with pushing the female fish around the pond, usually against something like a rock or some plant material. The males bump their heads (rather hard) into her flanks to provide the extra oomph needed to expel most of her eggs.
If there are no shallows, obstacles, or plants for the female to push into, it is unlikely, to almost impossible, that a female koi will spawn on her own. In plain liner ponds with no decorative elements, rocks, plants or shallow areas, the fish have no obstacles to spawn against and they may require artificially induced spawning hormones.
The reason I want you to know about the spawning rush is because in the spring, the unprepared will be shocked and appalled to see their fish fighting when, in fact, they are not really fighting, but instead are rushing each other in a spawn.
The second reason I wanted to mention this is because of what some people refer to as breeding injuries. When the female koi gets rushed into the side of the pond, the shallows or the rocks, she may endure some abrasion of her face and/or flanks. These will quickly heal under two conditions:
1. Be alert to the number of females in your pond. Ideally, there should be about two males to every female. If there is a higher ratio of males to females, she becomes basically the only gal in the pond, and is pretty much rushed all day. When this happens, she can get pretty beat up and severe injury can occur. Remove any female that gets run for more than four hours.
2. If the water quality is healthy and the important nitrogen numbers are all zero or nearly so, then she should go on and heal up fine as well. If the water is high in ammonia, nitrite or nitrate, or if the pH is sagging low, the female will not heal well and infections are inevitable.
People talk about sharp rocks in the pond, cutting up the fish. There are several problems with this hypothesis. First, sharp rocks should never be used in the pond. Your liner is far more likely to get cut by (the weight of) sharp rocks than your fish are. So if you have sharp, gashy rocks in the pond, don't gnash your teeth for the fish, gnash them for the long-term integrity of your liner! Ouch!
Secondly, rocks under water are not abrasive! Give any rock three weeks under water and, unless it's a foamy piece of lava rock, it's going to be too slick to stand on in that short span. I dare you to try! The slime on rocks is called bio film and it's a wonderment to the fish as well as a beneficial cleansing component for your water.
So, now you have all these eggs, and spent fish. What now? Well, if the water gets quite foamy, a partial water change would be recommended. Then get in the pond and set up all the plants for tomorrow, because chances are there will be another female ready to pop and you can do it all over again … orrrrr simply put your plants in more protected or plant them in rock crevices in such a way that the fish can’t knock them over.
In two days, the eggs will hatch but they are so small that you really can’t see them. If you have gravel in your pond, it makes a great place for them to hide out, away from the danger of being snarfed up by the bigger fish. They will hide there for another day or so using their yolk sac for energy then, when they are 24 to 36 hours old they will swim up into the foliage of the pond. Of course, if you have no foliage, it's a short story of delicious fry sushi and no babies the following day.
The fry eat microscopic plants and animals at this point. If you have a pond with a coating of biofilm and a thin greenish layer of algae on things, then the fry will have plenty to eat. They grow fast. Of course some of the fry will be spied by their elder siblings and the parents and, shall we say, be taken as food. Others will survive by color or cunning, and live to join the shoal.
Baby fish (and young koi in general) grow an inch per month in the first year, especially in biologically filtered ponds with an abundant plant, copepod, nematode, rotifer, crustacean, molluscan, and protozoan-rich gravel bed to sustain them. In clean, liner-bottom, drained ponds, few fry live.
Of the babies that live, a small, small percentage of fish will have colors of any appreciable pattern or brilliance. Fish of collectible quality are very rare, and are hand-selected from a hundred thousand babies by talented breeders in Japan who recognize good fish nearly at birth and discard all the rest. In your pond, of the hundred thousand offspring, a thousand will hatch and a hundred will live to even be seen by you. Of that hundred, 10 will get big enough to catch with a net and be examined, and of that 10, maybe one will be tolerable as a "keeper."
The vast majority of spawned "homemade" babies in your pond will be grey or brown. This is partly because the genes for that color are very, very common, and that grey and brown are good survival colors for koi ponds. So these will be the dominant babies you're left with.
A word on mixed populations of goldfish and koi – koi don't like baby fish very much. Oh sure, in a pinch, they'll take some … probably more accidentally than intentionally. Goldfish, on the other hand, love baby fish – especially the big, chunky koi eggs and babies. When you keep koi and goldfish together, the surviving babies will all be goldfish babies, not koi babies. I'm sorry to tell you.
So if you want koi babies this spring, consider relinquishing your goldfish to another party who wants them, and leave the koi to the koi.
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