Seasonal variation of euphausiid life stage and taxonomic composition near the upwelling site northeast of Taiwan

Euphausiids are commercial fishery targets and one of the primary preys of an important component of marine food webs. However, knowledge about the natural life history of euphausiids in the East China Sea is limited. To describe the seasonal variation of euphausiids abundance, diversity, life stage, and taxonomic composition, we visited a site northeast of Taiwan (25.5°N, 121.9°E) in 21 cruises from May 2010 to September 2016. This site is known for an all-year-round upwelling due to the impingement of Kuroshio onto the continental shelf break. We found that the overall euphausiids abundance and diversity tended to be higher in summer to autumn (June to November) compared with spring. In terms of life stages, we found that larval stages (furcilia and calyptopis) predominated in spring when upwelling was prominent, while post-larval stages (juvenile and adult) dominated in summer and autumn when upwelling was weak, and stratification was strong. Moreover, we found that, in spring and summer, Euphausia nana was the dominant species accounting for 90% of total euphausiid abundance, whereas in autumn, Pseudeuphausia latifrons was the dominant species occupying 36% of total abundance. This study provides the foundation to better understand the ecological roles of euphausiids under the influences of upwelling northeast of Taiwan.