How many eggs do Koi lay? How many and how often

As you Koi starts to mature they will enevitabily start to breed in your pond as long as the conditions and timings are right. Spawning is a sight to behold and it is likely that your female koi looks fit to birth while she swells up in a short-lived ‘pregnancy’.

There are many things that you can do to encourage Koi fish to spawn and often this is as easy as providing good quality water conditions.

But just how many eggs do koi produce? Read on because the numbers you will be dealing with are truly remarkable.

How many eggs will my female koi spawn?

Koi carp are not viviparous and rely on the high stakes method of egg scattering to reproduce. The numbers involved are amazing with females producing around 100,000 eggs per kilogram of body weight. Female koi spawning upwards of 400,000 eggs is common and large ladies can push out over one million eggs in a single spawning, truly breathtaking numbers which may be hard going on your pond and cause a feeding frenzy for your koi and other inhabitants. Each small bead-like egg is about the size of a pinhead with milky or light yellow-green coloration.

Predation is one of the key reasons that koi produce large numbers of eggs to reproduce. A million eggs does not lead to a million fin-babies to overwhelm you! The vast majority simply aren’t going to survive and are seen as quite a delicacy to other pond creatures.

If you leave your mating koi with the eggs they will eat them too, with many pond owners reporting devastation of their pond vegetation as hungry koi search every leaf and crevice to gorge on the released eggs and fry.

Once released the eggs are quickly fertilized by the attendant male, of course in water, precision is less likely so the huge numbers of eggs increase the chance that there will be successful fertilization.

How often does koi spawning take place?

Mating is usually an annual event, but some koi will spawn twice per year. In these cases, the first spawning produces a much larger number of eggs. Spawning usually takes place in late spring when the water temperature hits between 15 and 21 degrees Celsius (60 to 70 degrees Fahrenheit).

Conditions need to be just right with clean balanced water as well as factors like day length and nutrition being in place. Males play their part too, with earnest attention and proddings that will stimulate the female to release get eggs.

Can any female koi produce eggs?

Ladies need to reach an adequate size and maturity to be able to spawn, so if your pond is stocked with youngsters you may be waiting a while. Most spawning females are 3 years old and over, after which you can expect an annual spawn. If a fish is under 10 inches in length, it is unlikely to be able to spawn.

Where in a koi are the eggs produced?

Eggs are produced way down in the abdominal parts of the fish with paired or fused ovaries nestles between the intestines with the air bladder nearby. A secondary vent near the anus connects the reproductive organs to the outside of the fish and is handy for identifying females.

When the time is right and under the influence of hormones, egg production rapidly expands with the girth of a gravid female rapidly increasing in size as she becomes filled with eggs. The egg production process literally overtakes the female koi with an increase in body size of up to 70 percent – no surprise that this rotund state is known as ‘koi pregnancy’.

This gestation can last anything between a few days or a week or more. The precise moment of egg release may be difficult to predict. A swollen female will have distinctly increased girth and the eggs within form irregular lumps, like a bag of marbles. After spawning you are likely to see froth on the surface of your pond.

koi fish

Fast expanding abdominal girth can cause obstruction of the gut or swim bladder

Koi pregnancy does have its risks and it is well worth keeping a close eye on the gravid females in your pond. The sudden enlargement can cause painful obstruction of the intestines or swim bladder and produce dropsy symptoms on your fish. Affected fish may have their fins sticking up on end and be gasping or floating upside down.

Sometimes things can get all too much and a koi can become egg-bound. This condition, also known as dystocia, is caused by a build-up of eggs in relatively immature ovaries. In this situation, the eggs cannot be easily ejected leading to a painful obstruction in the fish.

Smaller impactions may be absorbed by a koi but you may need to help alleviate the pressure with gentle massage or even hormonal injections that can be obtained from a veterinarian. If untreated it can quickly lead to death.

How can I support a koi with roe on board?

Both female and male mating koi will experience enlargement. These are significant physical, physiological, and behavioral changes that require supportive care. A high protein diet with smaller and more frequent feedings is advisable, perhaps up to four times per day.

Also, water cleanliness should be reviewed regularly with testing for chemistry and adjustment or water changes as needed. Ammonia is an absolute no-no, especially if you intend to breed. A fry mat, ropes, or helpful aquatic plants like hornwort or water hyacinth are ideal for getting spawning underway.

Be prepared for lots of drama in the pond as males vie for the female and nudge and butt her to stimulate egg release. Torn fins and injuries are possible so it is well worth having a backup tank to offer a worn-out female some respite if needed.

How will the koi eggs develop?

These tiny fertilized koi eggs only take 4 days to hatch into young fry. Once released the sticky eggs cling to mats, stones, or foliage they find as they are distributed in the water. Check on them in a few hours and you are likely to spot two black dots, which are the developing eyes! Once hatched, fry lives off the remaining yolk sac and then send for themselves as opportunistic feeders while avoiding being eaten themselves.

Rounding up

Mating koi are an exciting part of keeping these beautiful fish and a pregnant lady in your pond will certainly keep you on your toes. Unless you’re a breeder the thousands of eggs released may not lead to an abundance of baby koi, but supporting your koi through this important part of their lifecycle is vital for their health and perhaps you may have one or two new koi to raise at the end of this intriguing process.

Carl Broadbent
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