LAUSR

Many studies have used stable isotope ratios in marine organisms to infer ecology, but few studies have considered potential temporal changes of baseline isotopic values. Such changes can reflect natural and anthropogenic changes in regional nutrient cycling and food-web ecology. This study evaluated changes in carbon and nitrogen stable isotope ratios (δ13C and δ15N respectively) of three small pelagic forage fish species (sardine (Sardinops melanostictus), anchovy (Engraulis japonicus), chub mackerel (Scomber japonicus)) in the Sea of Japan and East China Sea over 22 years (1998–2019). We calculated annual means of both stable isotope ratios with a generalised linear model (GLM) and time-series analysis (multivariate autoregressive state-space, MARSS) to account for environmental and anthropogenic effects. The highest and lowest least-square means of δ13C on the basis of the GLM were –17.2‰ (in 2005) and –18.5‰ (in 2013) respectively, and values declined drastically from 2005 to 2006. δ15N values decreased gradually on the basis of MARSS time-series, or were stable on the basis of the GLM analysis. Trends of δ13C and δ15N of forage fish species can be affected by the changes in baseline values, and changes in baseline values should, therefore, be taken into consideration when stable isotope ratios are used to infer food-web structures.