Elucidating the spawning migration and core reproductive duration of male flatfish using sperm duct volume as an index for better fishery advice and management

The conservation and management of heavily exploited fish stocks require reliable data on various aspects of reproduction. In particular, information about spawning migration and spawning period of both sexes is essential for protecting important spawning habitats. However, studies that focus on male spawning ecology are very limited. We elucidated the male spawning migration, core duration of reproduction, and seasonal changes in the maturity of the barfin flounder Verasper moseri, an important target flatfish for stock enhancement programs in Japan. We conducted histological observations of the testes and utilized the sperm duct volume index to facilitate accurate determination of the male spawning period. Fish before spermiation were mainly observed off Hokkaido and partially off northern Tohoku (40.2-43.4°N) during August-December. However, fish undergoing spermiation and sperm release were not found near these areas and were observed only off southernmost Tohoku (35.8-37.5°N approximately 300 m deep) from late January to April. Very few spent fish were caught off southern Tohoku, but were frequently observed off Hokkaido during May-June. These results demonstrate that male V. moseri repeat widespread spawning migrations over 700 km between feeding grounds off Hokkaido and spawning grounds off southern Tohoku. Moreover, a combined analysis related to seasonal variations in the sperm duct volume and the proportion of spawning adults identified that the core duration of male reproduction was during March. Until now, reproduction of the next generation in the wild has not been confirmed, despite the continual release of seedlings and local fishery restrictions around Hokkaido. Our results underscore the necessity of a more widespread fishery management plan for V. moseri that encompasses the waters off Hokkaido and Tohoku to more effectively enhance reproduction.