This bird does not breed every year due to fluctuations in food supply and conditions. Courting occurs in January or February. Laying of eggs begins as early as mid-March, when ground and trees are still covered with snow. These owls prefer nesting in hollow tree cavities in Japan and Russia. Mature forests with a mixture of coniferous and deciduous trees is usually the preferred nesting habitat. In Russia, trees selected for nesting can consist of elm (Ulmus ssp.), Japanese poplar (Populus maximowiczii), willow (Salix ssp.), chosenia (Chosenia arbutifolia), Mongolian oak (Quercus mongolica), ash (Sorbus ssp.), and stone birch (Betula ermanii). Nest height range is , but normally is at least of the ground. Reports of nesting on fallen tree trunks and on the forest floor are very rare occurrences at best and possibly untrue. Other than nest cavities, there are very isolated records of nesting on cliff shelves and in old black kite (Milvus migrans) nests. Additionally, Blakiston's fish owl have now been recorded using temporarily abandoned nests of Steller's sea eagles.Utekhina, I., Popatov, Е., & Makgrad, М. (2016). NESTING OF THE BLAKISTON’S FISH-OWL IN THE NEST OF THE STELLER’S SEA EAGLE, MAGADAN REGION, RUSSIA. Raptors Conservation, (32), 126–129.
