Maturation control of the short-spined sea urchin, Strongylocentrotus intermedius, by low temperature rearing using deep-sea water, with the aim of extending the market season

The short-spined sea urchin, Strongylocentrotus intermedius, is a valuable commercial species and one of the most popular and expensive seafoods in Japan. Serving local short-spined sea urchin to visitors during the summer tourist season in Rausu, located in a world natural heritage site 'Shiretoko,' has long been desired. However, it has not been feasible during the sea urchin spawning season (July to September), resulting in quality degradation in gonads, the edible part of sea urchins, due to maturation. Therefore, the Hokkaido Research Organization and the Rausu Fishery Cooperative Associations cooperatively investigated the possibility of suppressing gonadal maturation and maintaining high quality sea urchin gonads by low temperature rearing using deep-sea water. Unripe sea urchins captured before the spawning season were reared under two temperature conditions from June to September. In groups reared at ambient temperatures (2-18 degree C), gametogenesis in both sexes progressed rapidly with increased temperature, and almost all sea urchins reached full maturity by late July. Whereas in groups reared at low temperatures (2-5 degree C), gametogenesis progressed slowly and over 60% of the sea urchins had not reached maturity even by early September. This result suggests that the progress of gametogenesis in the short-spined sea urchin is effectively suppressed by rearing under low temperature conditions. Additionally, we also examined the effects of feeding on gonadal development in sea urchins reared under low temperature conditions, and revealed that feeding with live Saccharina diabolica, which were cultured as food in Rausu, could increase the gonadal volume efficiently to commercially preferable size, while suppressing the progress of gametogenesis. Moreover, the results of chemical analysis and tasting test proved that the quality of gonads were very excellent in sea urchin fed with live S. diabolica. Consequently, we demonstrate that low temperature rearing, supplemented with feeding live S. diabolica, is effective in suppressing gametogenesis to allow for the harvesting of high quality sea urchins during the summer tourist season. At present, this aquaculture method is being put to practical use by Rausu Fishery Cooperative.