Determination of trophic levels of marine fish in the Yellow Sea and northern East China Sea using nitrogen stable isotope (δ15N) analysis of otoliths

Fish otolith δ15N (δ15Noto) is a demonstrated source of information of dietary history for marine fish as it is available in otolith archives and sedimentary deposits unlike white muscle tissue (WMT). WMT and stomach content data are insufficient for trophic level (TL) data of past fishes which is important for the changes of marine fishery resources over long time scales. To determine the correlation between δ15Noto and fish WMT δ15N (δ15Nwmt) and the feasibility of using δ15Noto in characterizing the TLs of marine fishes, we conducted nitrogen stable isotope analysis (SIA) in the otolith and WMT of 36 marine fish species sampled from the Yellow Sea and northern East China Sea in 2011–2014. Both δ15Noto and δ15Nwmt were analyzed using an elemental analyzer coupled with an isotope ratio mass spectrometer (EA-IRMS). Multiple otoliths were combined to make each otolith measurement and were analyzed as-is without a carbonate dissolution pre-processing step. δ15Noto and δ15Nwmt comparisons for species in the Yellow Sea and northern East China Sea are currently lacking and would be helpful for both regional studies and for increasing the number of species for which δ15Noto and δ15Nwmt have been compared. Additionally, to determine the relative accuracy of trophic level calculated using δ15Noto, we compared TL calculated from δ15Noto to traditional trophic level metrics calculated using δ15Nwmt. The results showed a positive and highly significant correlation (R=0.780, P<0.001) between δ15Noto and δ15Nwmt. Trophic level estimation using WMT (TLwmt) and otolith (TLoto) showed congruence in our study, which is not entirely surprising given that δ15Noto was regressed against δ15Nwmt and the resulting regression coefficient was used to convert δ15Noto to δ15Nwmt prior to calculating TLoto. This conversion was required in order to be consistent with previous δ15Nwmt-based calculations of TL for comparison. TLoto calculations resulted in TL values that were largely within 5%–10% of TL values calculated with δ15Nwmt. Our findings show that δ15Noto is a feasible technique for characterizing the TLs of marine fish and can also assist in food web and marine ecosystem studies.