Gonadal maturation and spawning migration of stocked female barfin flounder Verasper moseri off the Pacific coast of northern Japan

Abstract

Commercial catches of barfin flounder Verasper moseri, an important target flatfish for stock enhancement programs in northern Japan, have recovered remarkably since the large-scale release of seedlings from Hokkaido in 2006. However, their reproductive ecology remains unclear. Our study of 2008–2012 investigated seasonal changes in the maturity of stocked female barfin flounder (n = 4,123) off the Pacific coast from Hokkaido to Tohoku (35.7°–43.4°N). Histological observation of ovaries revealed that fish undergoing vitellogenesis were observed mainly off Hokkaido and partially off northern Tohoku during August–January (40.2°–43.4°N). However, fully matured or spawning fish were not found near those areas. They were observed only in southern Tohoku. Ongoing spawning was observed off southernmost Tohoku (35.8°–37.5°N around 300 m depth) from early February through late April, when commercial landings had decreased drastically off Hokkaido. Spent fish were caught only rarely off southern Tohoku, but were observed frequently off Hokkaido during April–June. These results demonstrate that spawning grounds form at the upper continental slope off southernmost Tohoku during February–April. Relations between maturation traits and seasonal landings strongly suggest that female barfin flounder repeatedly migrate more than 700 km from feeding grounds off Hokkaido to spawning grounds off southern Tohoku.