04. Spawning koi – Hatching eggs to fry.

You will have been planning and working towards this day all year and from experience, depending on how it goes, can prove to be one of the most infuriating and rewarding times of the koi farming calendar. This is when you can see whether your preparations to this stage have been successful. If you have overlooked or forgotten to carry out any part of the preparation for this event over the preceding months then your koi will not perform (and you will have an empty koi farm this coming year!). But if your koi have responded to your preparation techniques then you will be rewarded with many thousands of viable eggs that have the potential to go on and produce a farm full of valuable koi.

Just as you are about to spawn, you should be filling your nursery ponds because you will be looking to stock out your fry in 3 weeks – just the time it takes for your ponds outside to reach the bountiful daphnia stage.

It will be early May and your broodstock will have experienced 1000oD in their indoor ponds. Consequently, the females will be ready to release their mature eggs, but to prevent a flock spawning, the males and females will be kept separate. In fact the next time the sexes will see each other again will be when they are returned to their own broodstock mud pond once they have spawned. But before spawning, you will need to know each of your brood fishes’ vital statistics – their weight in Kgs.

As you are going to prevent your koi from spawning naturally, you will need to intervene with an injection of hormone that will be the final trigger for both females and males to release their eggs and increase sperm production respectively. Once each fish has been weighed and logged, you will then know how much hormone will be required for both males and females. The hormone that will be used is Carp Pituitary Extract (CPE) which contains gonadotropins which are hormones that that target – you guessed it – the gonads. The females require 3mg/Kg of bodyweight while the males require half that – 1.5mg/Kg. To be able to weigh out such minute quantities of hormone you will have had to have invested approx £1000 on a suitably sensitive digital milligram balance.

The females require their hormone to be injected in two doses. The first one is 20% of their required dose (called a primer) and is given 24 hours before you intend to spawn. They are given their remaining 80% as a final dose (called a resolver) 12 hours before the intended spawn. True to form, the males just require a single shot of hormone, 12 hours prior to spawning.

So for a 10am spawn tomorrow (Running the broodstock ponds at approx 22C)

1. 10am today – inject your selected females with 20% (primer)

2. 10pm tonight – inject females with 80% (resolver)

- inject males with 100% of their required dose.

Each fish is lightly anaesthetised and then injected and placed in a floating cage for easy retrieval in the morning. Prior to injecting any broodfish, you would select plump females and males that are already producing sperm when squeezed gently. The hormone makes the males very frisky and flighty and very prone to jumping so ensure that their floating cage net has a secure cover.

10am the next morning (Time to spawn).

The signs you are hoping to see when you open the hatchery doors and enter the building are:

a. Frothing water. Evidence that females have started to release some eggs.

b. Evidence of a small number of eggs stuck around the perimeter of the pond and floating cage.

This shows you that at least one female has responded to your injections and that you will be spawning very shortly.

Spawning – Females First.

Now’s the time to put on some waterproof clothing. You will probably not stop handling your broodstock, spawning and fertilising eggs for the next 4-5 hours.

The females are stripped first as this will show you how they have responded to their hormone injection and whether they need a few more hours for the hormone to fully work. Only when you have collected your eggs will you know how much sperm to collect (and from which males).

Gently does it.

For spawning, one female at a time is removed from the cage. It is wise to guide her gently into a fine-meshed ‘saddle’ (the kind an angler may use to weigh his catch) and then lift it out of the water into a bowl of anaesthetic. The support provided by the saddle against the female’s abdomen reduces the likelihood of her stripping herself under gravity.

Once anaesthetised, your assistant then turns her upside down and places her head first in your lap which has been covered in a dry towel. The towel is then wrapped around the motionless female to ensure she is completely dried off.

Box out. Why dry the females and males before spawning?

The ‘dry’ method of spawning is used worldwide as the recognised method of obtaining high quality eggs and sperm that will go on to produce a high fertility rate. When koi release eggs and sperm, they are ‘inactive’ until they are activated by contact with water. In a natural ‘wet’ flock spawn, the pond water ‘activates’ both eggs and sperm so that fertilisation can take place. The effect the water has on the eggs is to cause them to hydrate immediately, closing the microscopic micropyle through which the sperm must fuse with the egg. It only takes 30 seconds after contact with water for the micropyle to close – preventing any fertilisation thereafter to take place. So when hand stripping, it is vital that the eggs are removed ‘dry’ and inactive as it may be several hours before you mix them with the sperm. Knowing how short a period a sperm has to fuse with an egg naturally (and how diluted sperm can become in a large pond) it is not surprising that natural fertilisation rates are likely to be <10%. That is why koi release so many eggs – which is excellent news for the koi farmer as we have literally hundreds of thousands of eggs to play with. Using the dry method, we can expect fertilisation rates in excess of 90%.

The dried, anaesthetised female is then rolled over and held over a bowl into which her eggs will flow, requiring minimal pressure to expel the eggs. If only a few eggs are released (in relation to the size of the female) or eggs are only released under significant pressure then the eggs have not been released from the ovaries. Simply return the underperforming female to the floating cage in her pond and give her 2-4 more hours for the hormone to take full effect. Once you have completely stripped a female, label her bowl and cover it with a waterproof lid to prevent any water from splashing into the bowl and activating the eggs – A nightmare scenario.


We perform the same task with the males to collect their sperm. Drying them ensures that no water activates the sperm. The hormone injection increases the quantity of sperm produced and released by the koi and gentle pressure applied to either side of the male’s vent will enable you to collect it in a small container. Even though the sperm appears to be ‘wet’ – it is in fact inactive until it touches water. Remember to label each container so you know which male produced each sample of sperm.

Once you have stripped your chosen males and females you will have rows of labelled bowls and containers full of eggs and sperm. The crosses you wish to make will determine which sperm you mix with which eggs. The eggs and sperm at this stage are still dry and inactive, but this method ensures that adjacent to each egg will be millions of sperm in a confined space. This puts the odds firmly in your favour for a high rate of fertilisation.

Then….we add water to cause fertilisation to take place and stir the eggs gently. After 1 minute, any eggs that have not been fertilised will be infertile. Carp eggs are naturally very sticky (so that they adhere to submerged vegetation and other surfaces in a natural pond). This stickiness is a real threat to a koi farmer as it could lead to all of the many hundreds of thousands of eggs to stick together into an irretrievable mass of eggs. This would soon prevent them from developing to the hatching stage as they would die through lack of oxygen. So to prevent them from clumping together we stir the eggs in a solution of water, salt and urea. The eggs are stirred in this safe fertilising solution for 1.5 hours while the eggs hydrate (take up water), swelling to 3x their original volume. After 1.5 hours of stirring, the sticky layer must now be removed completely before they go into their hatching cones (otherwise they will clump together).

The outer sticky layer is removed using several washes of mild tannic acid. This is a real heart-in-the-mouth period because too much acid or a prolonged exposure will kill the whole batch of eggs (and your income for the next year). If you are too cautious and rinse the eggs in insufficient tannic acid they will retain a degree of stickiness, clumping easily and catastrophically when they are placed into the hatching cones (Zouger Jar).

Box out. What’s a Zouger Jar?

Once the sticky layer has been successfully removed, the eggs are placed in incubating (or Zouger jars). A Zouger jar is a versatile piece of koi farming equipment. It consists of a 25 litre Perspex jar with a tapped bottom inlet and a spout on the rim. The eggs are placed in the jar where a gentle upwelling current of oxygenated water from the hatchery system turns the eggs over. Once the eggs have hatched, the fry swim up and out of the jar, over the spout and into the adjacent hatchery tank. The eggs take between 75 and 100oDays to hatch. The jars are then removed and set up on a separate racking system as stand-alone brineshrimp hatching cones. These will now supply the fry with their first food for the next 2 weeks until they are ready to be stocked out into the daphnia-rich mud nursery ponds outside.

Once the koi fry have hatched, they will spend 2-3 days simply hanging on to the side of the tank while they absorb their yolk sacs. During this period, the fry do not require any food which gives you the opportunity to start hatching brineshrimp. Four cones are set up, each taking 24 hours to be ready. The fry are then fed on brineshrimp 4 times a day and each time a cone is emptied of brineshrimp, it is cleaned, disinfected and set up again for one of tomorrow’s four feeds.

During this period in the hatchery, it is important to keep an eye on the development of the live food in the nursery ponds outside. Once large densities of daphnia are evident in the ponds, the fry can be netted from their hatchery tanks, bagged up and transferred to the mud ponds. Split your fry up so they are stocked evenly into your mud ponds – and then relax!

You can now switch off and clean out your hatchery systems as they’ve done their job for another year. The fry are no longer at the mercy of a power cut and will now grow phenomenally in your well-prepared mud ponds.

By now it is likely to be early June, so you will need to maximise your fry’s growth (and hence their value) between now and September (when they will stop growing). Next month in the final part of this series we will see how we grow these fry up to fingerling koi, market and sell them.

Box Out

The 10 stages to the spawning process

1. Choose likely female spawning candidates that have obviously swollen during the preceding 1000 Degree Days.

2. Select compatible males

3. Weigh both sets of broodfish to determine the doses of hormone required.

4. Prepare the hormone, anaesthetise and inject the females and males accordingly.

5. Next day, anaesthetise, dry and strip broodstock to collect sperm and eggs

6. Mix compatible eggs and sperm and fertilise using fertilising solution

7. Stir the eggs for 1.5 hours until fully hydrated.

8. Treat with mild tannic acid to remove the sticky layer before transferring to Zouger jars.

9. Incubate in Zouger jars for 75 – 100 Degree Days until they hatch

10. Remove the Zouger jars and set them up ready to hatch brineshrimp.

1 comment to 04. Spawning koi – Hatching eggs to fry.

Kill blanketweed and string algae.