Trophic state-dependent distribution of asari clam Ruditapes philippinarum in Japanese coastal waters: possible utilization of asari stable isotope ratios as a production indicator

Adult asari clams Ruditapes philippinarum were widely collected from 22 sampling sites including 20 clam fishery grounds in Japan in 2013 and 2014. Their δ15N and δ13C values were suggested to be an effective indicator of environmental trophic state: the stable isotopic signatures showed significant positive correlation with the 5-year (2010–2014)-mean values of total nitrogen (r = 0.691, r = 0.716) and chlorophyll a (r = 0.643, r = 0.789) calculated from the monitoring data collected by the Ministry of the Environment. Annual asari catch per unit area (CPUA) calculated from fisheries statistics scaled per unit area of each fishery ground showed significant positive correlations with trophic state parameters such as total nitrogen (r = 0.721), chlorophyll a (r = 0.800), δ15N (r = 0.732), and δ13C (r = 0.742), indicating a close relationship between asari catch and the trophic state of waters. Most clam fishery grounds in the Seto Inland Sea showed low trophic states and CPUA, indicating that recent poor asari production in these areas may be related to poor nutrient supply to the fishery environment.