A Detailed Explanation of the Living Environment and Habit Characteristics of Koi
I. Koi's Requirements for Their Living Environment
Like goldfish, koi are not strict about water temperature and water quality. The water turbidity range for koi is 2-30°C. Although koi are highly adaptable to the environment, they have a weakness in that they cannot resist sudden changes in water temperature. If kept under long-term artificial rearing, they can still tolerate a water turbidity rise or fall of 2-3°C, but when the range of temperature decrease or increase exceeds 2-3°C, the fish are prone to illness. When the temperature rise or fall increases to 7-8°C, the fish lie prone on the bottom, neither eating nor moving. If the range of sudden temperature change becomes even greater, koi may even die immediately. The most suitable water turbidity for koi is 20-25°C. In water at this temperature, koi swim actively, have a strong appetite, are robust in constitution, and have bright colors. Thanks to scientific development, hobbyists with the means can use thermostats and coolers to regulate water temperature in winter and summer respectively, controlling it to the water temperature most suitable for koi growth, so that the fish can have a comfortable environment and grow more rapidly, which is even more ideal. Koi living in water rely on their gills to absorb oxygen dissolved in the water, sending the oxygen into the fish body where it combines with absorbed food nutrients to produce energy to sustain life. Therefore, whether there is sufficient oxygen in the water is the key to raising fish well. According to research results, the oxygen requirement per kilogram of koi body weight per hour is 35 mg/L at a water temperature of 5-6°C. This proves that the higher the water temperature, the greater the oxygen consumption.
Koi are also suitable for living in slightly alkaline water environments with low hardness.
II. Koi's Feeding Habits
Koi are omnivorous. Generally, mollusks, fragments of higher aquatic plants, benthic animals, and even tiny algae are all their food. Because koi have a wide range of adaptation to food and are also not very strict about other living conditions, they have strong vitality. As the fish body grows and the seasons change, their feeding situation also changes accordingly. In summer, koi have the greatest feeding intensity, while in winter they hardly eat at all. Newly hatched fry mainly feed on rotifers and small cladocerans; juvenile fish over 3 cm feed on benthic organisms, insect larvae, shellfish, snails, and fragments of higher aquatic plants. In addition to being able to swallow feed, koi can also extract food from pond bottom mud. Koi have no teeth on their upper and lower jaws, and often use their well-developed pharyngeal teeth to chew hard food.
III. Koi's Growth and Development
(I) Growth In addition to water temperature and the abundance or scarcity of bait affecting the growth rate of koi, there are also great differences in growth between female and male fish. A koi with a body length of more than 90 cm can be said to be quite large, but at a judging event in Japan there once appeared an Akamatsuha koi with a body length of 125 cm, and even more extreme is the record of a super giant koi with a body length of 150 cm and a weight of 45 kg. Koi have a very long lifespan, generally reaching 70 years. According to records, in Japan there was a red koi named "Hanako," born in 1751 and died in 1977, living a total of 226 years, with a body length of 77 cm and a weight of 9 kg, making it worthy of being called the "longevity star" among koi. The age determination of koi is the same as that of most fish: counting the number of annual rings on the scales indicates the age of the koi.
(II) Development The age of gonadal maturity in koi is 2 years for male fish and 3 years for female fish. After the gonads mature, they begin to lay eggs and reproduce. After that, the gonads periodically mature once and reproduce once each year. The spawning period is generally from April to June each year, but in southern Japan there is also autumn spawning and reproduction. This is due to the climate; in these areas, the ovaries of koi can mature twice a year.