Abstract
The occurrence of the salp Thetys vagina was observed in the Japan Sea during spring 2004. Catches up to 187 kg wet weight (WW) per 2.18 × 105 m3 (equal to ∼0.9 g WW m–3) were collected with 10-m diameter surface-water otter trawl nets. The horizontal distribution indicated that the high biomass was related to the area with high chlorophyll a (Chl a) concentration, which was located around the subarctic front with the warm Tsushima Current. Five prey taxa were identified from the gut contents of individuals from the high Chl a area. The diatom Coscinodiscus spp. (13–55 µm in diameter) dominated numerically. Another significant prey was the large diatom Coscinodiscus wailesii (219–313 µm) that is an indicator of the spring bloom in this area. The mass occurrence of T. vagina thus appears related to phytoplankton availability, though the mechanisms remain uncertain.
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