Detailed Explanation of Natural Breeding Methods and Technical Key Points for Koi

Selecting excellent broodstock is the key to successful koi breeding. To use natural breeding, the first step is to select broodstock that meet the standards. In the north, koi previously kept as breeding stock are moved to outdoor ponds in mid-to-late March after spending the winter indoors. At this time, broodstock should be further carefully selected to obtain excellent offspring.

(1) Selection of broodstock: Broodstock used for breeding should generally have the following: a strong physique, bright and crystalline coloration, pure lineage, distinct breed characteristics, clearly defined color patch margins with no blurred edges, no blemishes, smooth and neat scales, steady swimming posture, and complete fins without defects.

(2) Methods for distinguishing male and female fish: When koi reach maturity, differences in the morphology of female and male fish become apparent. Generally speaking, the female fish has a short, thick, and plump body with a swollen abdomen, and the closer to spawning time, the more pronounced the abdominal swelling becomes, with the head slightly narrow and longer; the male fish has a leaner, more elongated body, with a broad and short head and a slightly protruding forehead. The tips of the pectoral fins of the female fish are rounded, while the tips of the pectoral fins of the male fish are slightly pointed. During the reproductive season, the differences between female and male fish become even more obvious: the first fin ray of the pectoral fins and the gill covers of the male fish develop several white granular small protuberances, called "breeding stars." After the reproductive season, the breeding stars disappear on their own. The male fish has a small and recessed genital pore, and when gently pressed by hand, milky white semen flows out; the female fish has a slightly wider and flatter genital pore, slightly protruding outward, and when the abdomen is gently pressed by hand, eggs are released. When koi breed, whether the number of female and male broodstock is properly matched has a great relationship with the fertilization rate of the eggs. To ensure sufficient sperm count and improve the fertilization rate of eggs, the matching ratio of female to male fish is 1:3 (1 female fish with 3 male fish forming a group, or 1:2 or 2:3 also works well. If the number of male fish is small, a female-to-male matching ratio of 1:1 can also be used. The reproductive glands of female koi aged 2-3 years have just matured, and reproductive function is not vigorous, so the number of eggs carried is small, the eggs are also small, and the hatched fry are prone to poor development, so they are generally not selected as broodstock. Broodstock of appropriate age for breeding: female fish of appropriate age for breeding are generally 4-10 years old; male fish should also be 3-5 years old or older. Such broodstock have a strong physique, full reproductive glands, strong vitality of eggs and sperm, and relatively high fertilization and hatching rates, making them the most ideal broodstock.

(3) Preparation of the spawning pond: A cement fish pond is used as the spawning pond. A square pond with an area of 4 m x 4 m or a rectangular pond of 4 m x 5 m is more suitable. The pond should not be too large, as it is inconvenient to manage if too large, and if too small it affects the spawning activity of the broodstock. The water depth is 30-40 cm, and water quality with sufficient oxygen content, clean water, a hydrogen ion concentration of 39.81-63.09 nmol/L (pH 7.2-7.4), slightly alkaline, and low hardness is preferable.

(4) Spawning nest: The spawning nest is used to attach the eggs, and it must be prepared before spawning. Generally, hornwort, palm bark, willow root fibers (which must be boiled multiple times), and similar materials are better for making the spawning nest. Tie them into small bundles and they are ready for use.

(5) Spawning: In northern regions, late April to mid-June is the koi spawning season. When the water temperature rises to 16-18°C, the broodstock can be moved into the spawning pond. When rapid chasing behavior is observed among the broodstock, the spawning nest can be placed into the spawning pond. When the water temperature rises to 20°C, large-scale spawning can occur. Spawning begins at around 4 a.m. at dawn and continues until 10 a.m. or noon. If the weather changes suddenly and the water temperature drops sharply, spawning will be interrupted. Generally, one koi 30-40 cm long produces 200,000-400,000 eggs, and some will release remaining eggs that were not laid the previous time about 1 month after spawning. Its eggs, like goldfish eggs, are also externally fertilized and are adhesive eggs. The size of the eggs varies with the size and age of the mother; a larger mother produces larger eggs, and conversely, the eggs are smaller. The egg diameter is generally 2.1-2.6 mm. After fertilized eggs absorb water, the egg diameter becomes somewhat larger than that of unfertilized eggs. After removing the spawning nest covered with attached eggs from the spawning pond, soak it in a 5-7% salt solution for 5 minutes, or in a 1/10,000 malachite green solution for 10-15 minutes, or in a 3 ppm methylene blue solution for 15 minutes for disinfection, and then move it into the hatching pond for incubation. The use of these drugs has a certain effect on preventing the occurrence of water mold.

(6) Hatching: First, prepare the hatching pond, using a cement pond with an area of 3 m x 3 m and a water depth of 30 cm. The water temperature of the hatching pond must be the same as that of the spawning pond. If the water temperature is 5°C lower or higher than that of the spawning pond, the hatching rate will decrease, and a further increase in the temperature difference will cause the eggs to die. The length of the incubation period is related to the water temperature. When the water temperature of the hatching pond is 20°C, fry can hatch in about 4 days. The higher the water temperature, the shorter the incubation time required; conversely, the longer the incubation time. Generally speaking, at a water temperature of about 18°C, fry can hatch in 4-5 days; at 20-22°C, fry can hatch in 3-4 days; at 25°C, fry can hatch in 3 days. The newly hatched fry do not eat or move, relying on the nutrients in the yolk sac to maintain the energy needed for survival. They attach to the spawning nest using adhesive organs on the body. When the nutrients in the yolk sac are completely absorbed, the fry begin to swim and seek food, and at this point feeding can begin. The types of feed and feeding methods are the same as those for feeding goldfish fry. In addition to the above methods, flow-through incubation can also be used. With this method, attention must be paid to the flow rate. The water flow can be somewhat larger at first. Place the spawning nest with attached eggs into a small-mesh incubation net cage placed in the flowing water. Higher oxygen content in the flowing water is beneficial to the incubation of the eggs. After the fry hatch, the flow rate should be reduced, or they should be placed in a still-water net cage to continue developing. Otherwise, the fry may be injured by the current, or the large amount of swimming by the fry may exhaust them and be detrimental to development.