When the water temperature drops to around 8°C (koi typically stop feeding at 10°C), the fish exhibit behaviors like lining up and resting at the bottom of the pond. Simultaneously, their respiratory rate slows, and their metabolic activity decreases significantly—marking the onset of hibernation. Koi hibernation is a non-memory-based conditional reflex, meaning that the behavior is triggered solely by low water temperatures and ceases when temperatures rise.

During hibernation, koi enter a state of dormancy to conserve energy. Their metabolism slows to a minimum, reducing the need for food and oxygen. This physiological adaptation helps them survive through cold winters when food is scarce and environmental conditions are harsh. In this state, koi remain nearly motionless, with minimal gill movement and greatly reduced responsiveness to external stimuli.

Hibernation is a natural survival strategy for koi in temperate climates. It is important for pond owners to avoid disturbing hibernating koi, as sudden disruptions can cause unnecessary stress and energy expenditure. Additionally, maintaining good water quality and ensuring the pond does not freeze completely are essential to support koi through this dormant period. Providing slight aeration and avoiding sharp temperature fluctuations can further aid in their safe overwintering.

It is also worth noting that hibernation behavior can vary based on the koi’s health, age, and local climate conditions. In regions with milder winters, koi may not enter full hibernation but simply remain less active. Proper winter pond management—such as using a pond heater or de-icer in freezing conditions—can help create a stable environment, enabling koi to transition smoothly into and out of hibernation as temperatures change.