Feeding habits of six species of euphausiids (Decapoda: Euphausiacea) in the northwestern Pacific Ocean determined by carbon and nitrogen stable isotope ratios

The nitrogen (delta N-15) and carbon stable isotope (delta C-13) values of six species of euphausiids predominant in the northwestern (NW) Pacific Ocean were investigated to elucidate species-specific feeding habits and seasonal and regional differences. The order of annual mean delta N-15 values was: Tessarabrachion oculatum Hansen, 1911 (11.2 +/- 0.8%) > Thysanoessa longipes Brandt, 1851 (10.4 +/- 0.4%) > Thysanoessa inspinata Nemoto, 1963 (9.3 +/- 0.8%) > Nematoscelis difficilis Hansen, 1911 (8.2 +/- 1.4%) > Euphausia pacifica Hansen, 1911 (7.7 +/- 0.5%) > Euphausia gibboides Ortmann, 1893 (6.9 +/- 0.9%). The trophic levels of four species based on delta N-15 values were estimated as 3.6, 3.2, 2.9, and 2.5 for T. oculatum, T. longipes, T. inspinata, and E. pacifica, respectively, in the Oyashio region in June 2011. The seasonal differences in delta N-15 values of these species were relatively small, which suggests euphausiids play the same role in the food web throughout the year. Carapace lengths were negatively correlated with delta N-15 values of five species except T. longipes. Regional differences in stable isotope ratios that might reflect regional differences in the primary producer signature were observed: higher delta C-13 values and lower delta N-15 values in the Oyashio-Kuroshio mixed water region compared to those in the Oyashio region. The biplot of delta C-13 and delta N-15 annual mean values with zooplankton in the NW Pacific shows that euphausiids have a wide range of delta N-15 values, suggesting they play diverse roles in the pelagic food web in the region.