Eggs and Fry
Care of Eggs
Once released into the water, a koi egg must be fertilised within 30-60 seconds (sperm remain viable for 1-2 minutes). Often, up to 40% of a spawning can be infertile. To try reduce this, use at least two males to each female during spawning.
Following spawning, the eggs absorb water and swell, in a process called hardening. This is achieved by osmosis, water entering the relatively concentrated egg from the dilute surrounding medium.
In waters which are excessively hard, with high levels of dissolved salts (such as certain spring waters). the difference in the levels of dissolved substances in the egg, compared with the surrounding water, may not be sufficient to induce osmosis and, hence, normal hardening.
Other causes of egg mortality and low hatchability include
1. Low oxygen levels
2. Incorrect water temperature. Fatal below around 16 degrees Celsius, above 25 degrees Celsius stimulates development but may produce weakened and deformed fry. Rapid temperature changes will also cause embryonic death.
Care of Fry
After two-six days, the fry hatch out. At this stage, they show marked light avoidance, seeking darkened areas in which to lie to absorb their yolk sac. After 48 to 72 hours, fry development has progressed to the stage where they can become free-swimming. Following a quick dash to the surface to take in a gulp of atmospheric air which is forced into the swim bladder, the fry alter their behaviour and seek out light, which usually draws them towards the higher densities of their food.
Factors to consider
1. Excessive levels of illumination or inadequate provision of shaded areas can be stressful to fry.
2. Very high or low temperatures can cause stress.
3. Following hatching, fry need a dramatic increase in oxygen.
4. Poor water quality can also cause loss of fry.
5. Gross overcrowding of the fry will lead to stunting.
6. Insufficient food can affect the growth of fry.
7. Young fry do not possess a fully functional immune system.