Changes in the distribution and abundance of the Great Cormorant in Hokkaido Japan

Abstract

I investigated changes in the breeding colonies of Great Cormorant Phalacrocorax carbo in Hokkaido from 2001 to 2022. Through literature research and field surveys，a total of 18 breeding sites were identified, including 12 colonies using natural objects and 6 colonies using man-made structure. In 2022, there were a total of nine breeding colonies, including four natural breeding colonies and five artificial breeding colonies. The number of nests increased from 16 in 2001 to 2,558 in 2022. The number of nests in three breeding colonies located from the Sarobetsu Plain to the coast of the Sea of Okhotsk, Penke swamp, Yubetsu river, and Abashiri port, accounted for 92.9% of the total number of nests in Hokkaido. These three colonies have many lakes and marshes within 15km from colony. Natural breeding colonies were more likely to be abandoned than artificial colonies. This was thought to be due to the invasion of terrestrial predators and the death of nesting trees. Great Cormorant in Hokkaido are thought to spend the winter mainly in Honshu. Therefore, in order to effectively manage the cormorant population against fishery damage, it is necessary to continuously and accurately grasp the number of nesting birds in Hokkaido.