Diet source of Euphausia pacifica revealed using carbon- and nitrogen-stable isotopes in the Yellow Sea Cold Water Mass in summer

Abstract

Euphausia pacifica (E. pacifica) is an important food source for fish and other organisms in marine ecosystems. We analyzed the carbon- and nitrogen-stable isotope ratios in potential diet sources to determine the feeding characteristics of E. pacifica in the Yellow Sea Cold Water Mass (YSCWM), a habitat for oversummering. E. pacifica was collected in the spring and summer from the central and coastal parts of the Yellow Sea from 2012 to 2014, and potential prey were collected in summer 2014. Although the C-13 values of E. pacifica differed significantly in spring each year, they narrowed to a specific range in summer. The N-15 values were always 2 parts per thousand higher in summer than in spring, suggesting that the diet of E. pacifica inhabiting the YSCWM was limited. In coastal stations, the contribution of any major food source calculated using an isotope mixing model was not more than 20%, suggesting that E. pacifica is omnivorous. However, at two stations in the YSCWM, 2.0-5.0-mm and ˃5-mm plankton contributed 42 and 22% of their diet on average, respectively. These results indicate that E. pacifica inhabiting waters affected by cold water feeds on relatively large zooplankton such as the copepods Calanus sinicus. Therefore, the YSCWM likely plays a role in the trophic shift of E. pacifica during the summer.