Tunicate food chain: a key process of high fishery production in oligotrophic Kuroshio ecosystem

The Kuroshio is the western boundary current of the North Pacific transports nutrient-depleted subtropical water to the north along Japan. The name of Kuroshio comes from its water color: in Japanese “kuro” means black or dark, and “shio” means current. The name clearly indicated is low phytoplankton concentration relative to greenish coastal water. Despite the oligotrophy, the Kuroshio region is good spawning and nursery grounds of various fishes and squids, and fishing grounds are formed along the Kuroshio. Saito (2019) named the inconsistency of high fisheries production in an oligotrophic condition as “Kuroshio Paradox”. In a series of field studies and laboratory experiments, we found that tunicates, especially for appendicularians and doliolids, play an important role transport pico- and nanophytoplankton production to larval/juvenile fishes through poecilostomatoid copepods such as Oncaea spp. and Sapphirina spp. These poecilostomatoid copepods preferred discards of appendicularian house and doliolids, and are preferred prey for various fish species. Okazaki et al (2019) proposed the prey-predator relationships as “tunicate food-chain”. The tunicate food chain may present and play an important role in other oligotrophic environments such as Gulf Stream region and Bay of Bengal.