Overview

  The feeding pattern, and thus the growth rate, of koi depends on many factors, such as water temperature, water quality, stocking density and genetic background Koi feed most actively at temperatures in excess of 15c(59F), thus sexually immature fish can grow rapidly during the summer months when the temperature is warmer. Once koi are mature, their growth rate slows considerably; in sexually mature fish, most of the food eaten is utilized in producing eggs or sperm in preparation for breeding.

Growth Cycles

  However, unlike many other vertebrates, fish continue to grow throughout their lives and it is easy for pampered koi to reproduce and continue to grow because of their artificially high feeding rates. The cycle of rapid growth in summer and retarded growth in winter produces rings on the fish's scales (much like the growth rings of a tree) and a koi's age can be determined by counting these rings. Some koi-keepers, however, who heat their pools in the winter, continue to feed their koi throughout the year. If a koi has continued to feed during winter its rings will not be clearly defined and it will therefore be difficult to age.

Growth Factors

  Water quality affects the rate of growth because koi lose their appetites and may even stop eating if their environment is poor. Poor water quality can also affect the fish's metabolism, thus hindering digestion of food.

  The stocking density of the pond can also have marked effects on koi growth rate. In a lightly stocked pond, koi will become sexually mature while still of a relatively small size (25-3Ocm/10-l2in) and once mature, growth is retarded. Although koi in a densely stocked pond will mature at a much larger size (50-60cm/20-24in), competition for food will slow the growth rate, food will be more scarce and the 'battle' for it can stress some koi. You will have to decide on optimum stocking levels for your own koi pond bearing this in mind. In fact, these considerations are really most important for koi farmers.

  The genetic background, too, influences the size to which koi will grow; as the children of tall parents tend to be tall, and short parents produce small children, so the same is true of koi. This is obviously a simplified view of gene action, however, because of the significant role the environment plays in influencing size.

  Water temperature affects fish more than any other single factor. Fish are ectothermic - their body temperature fluctuates in accordance with the temperature of their environment, usually remaining 10C higher. As the temperature drops, the ability of the koi to digest and assimilate food decreases. In the winter months, therefore, at temperatures below 10C(50F), it is a good idea to feed cereal diets that the koi can digest quickly and easily and that do not stay in the gut too long. High-protein diets linger in the gut and can cause severe problems; the bacteria found in the fish's gut, which play a role in breaking down some less readily digestible substances, such as cellulose, may become pathogenic if food is retained in the gut too long.

Food Needs