maria hayward
DAVE WOLFENDEN
Koi fry in spawning tank
airstones in the system will help to oxygenate the water and provide currents and vital circulation to the developing eggs and fry. The maintenance of a suitable temperature is also a very important factor in the success (or otherwise) of rearing a batch of fry. The eggs should be incubated at temperatures of at least 18°C, but ideally no higher than around 25°C, otherwise an increased number of developmental abnormalities can result. Because koi are, as with all fish,
ectothermic, meaning that their body temperature is dictated by external conditions, the temperature at which the eggs and fry are maintained will affect the speed of their development. Higher temperatures result in more rapid hatching and growth rates, although excesses are to be avoided.
hatching At 20°C, the eggs should take around four
days to hatch. During the development of the egg, noticeable and rapid changes will take place, including the appearance of eyes and a tail. Eventually, the fry break free from the egg and become free-swimming. Newly-hatched fry (which measure up to 3mm in length) don’t actually feed at first, as
Fry in spawning tank
they initially don’t possess any mouthparts (these won’t develop for three days or so). They respire through the blood supply surrounding the yolk sac, and their nutritional requirements are also fulfilled courtesy of the yolk sac. Keeping the spawning ropes in the tank
can be a good idea, as it gives the secretive fry somewhere to attach and hide. However these should be removed as soon as possible to reduce the pollution risk from any infertile or otherwise unhatched eggs rotting. The fry possess a tiny sucker-like structure which is used to attach themselves to the sides of the tank, or indeed any other suitable feature, such as the spawning ropes, in the environment.
feeding After a few days the fry venture to the
water’s surface and inflate their newly- developed swimbladder. It’s at this point that the tiny fish will start to feed – so what do we feed them? Obviously they’re extremely small, with
tiny mouthparts, so the food particles need to be suitably minute. The crumbled yolk of a hard-boiled egg is frequently used as a feed for the first couple of days (the particles of
Fry in spawning ropes
After a few days the fry venture to the water’s surface and inflate their newly- developed swimbladder
JULY 2010 51
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