Why Are My Koi Fighting?

If you notice your fish being rambunctious in the pond during the spring season, you might assume the koi are fighting with one another. In reality, just the opposite is true. Your koi are spawning and it can be quite a sight to behold!

Koi are river fish with mechanisms developed for river life, which means that when it comes to producing offspring, they scatter their eggs everywhere rather than laying them in a neat and tidy nest like other pond fish. The mother and father koi never give their fry another thought and don’t provide any after-care. In your pond, all the same stuff happens as in a river, but you get to see more of the intimate details!

Magic in the Pond

Two things begin the process of koi spawning in your pond. First, the water warms up in the spring, and second, the days grow 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 poised and ready for fertilization!

The Spawning Dance

During the spring season you might witness your koi chasing each other around the pond; usually first thing in the morning. It’s a remarkable sight because large koi can really cause a raucous in the pond with their shenanigans. They will bull around the plants in a group, upsetting pots, rocks, roots – 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!”

While you might assume this is too traumatic for the female koi, this flurry of activity has to happen because she has no ability to push her eggs out with her 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 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.

It Can Get Pretty Rough

If you have a few koi in your pond, it’s important to know about spawning behavior because in the spring, you might be shocked and appalled to see your fish “fighting” when, in fact, they are not really fighting but are rushing each other in a spawn.

It’s also good to know that 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 be able to heal quickly and properly. 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.

When you experience spawning in your pond and worry about the female, keep in mind that 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 slick. The slime on rocks is called bio-film and it’s wonderment to the fish as well as a beneficial cleansing component for your water.

Now What?

Once spawning occurs in your pond, the water can get quite foamy.  You might want to perform a partial water change at this point.

In two days, the eggs released during spawning 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 eaten by 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.  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. If you have a pond with a coating of bio-film and a thin greenish layer of algae on things, then the fry will have plenty to eat. They grow fast. Unfortunately, some of the fry will be spied by their elder siblings and parents and enjoyed as a meal. Others will survive by color or cunning, and live to join the shoal.

Are They Really That Cute?

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, cope-pod, 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 very 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 extremely 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 of Warning

A word on mixed populations of goldfish and koi – koi don’t like baby fish very much. Oh sure, in a pinch, they’ll consume them – 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 in the same pond, the surviving babies will all be goldfish babies, not koi babies.

So if you want koi babies this spring, consider relinquishing your goldfish to another party who wants them, and keep just your koi.