Nematode Size Fraction Data Improves Explanatory Power of Multivariate Model of Macrobenthic Assemblage in Clam ( <i>Ruditapes</i> ) Fishery Grounds

ABSTRACT In Japan, fishery grounds for Ruditapes philippinarum are located in estuarine and tidal flat habitats and occur over an extensive geographic range. In these habitats, due to increasing risks from environmental stressors, there is a need to develop rapid and reliable methods for inferring compositions of macrobenthic assemblages and their respective relationships with environmental and biological factors. This study examines spatial variations of macrobenthic assemblages on the fishery grounds and evaluates size‐fractioned nematode data as a predictor of the macrobenthic assemblages. Multivariate analyses were carried out on the macrobenthic assemblage at 34 sites in Japan, using nematode density of three size fractions and sediment environmental variables as explanatory parameters. Four groups of assemblages were recognized by cluster analysis. Ruditapes philippinarum was the indicator taxonomic unit of one of the assemblages (Group 2) which occurred mainly in northeastern Japan, reflecting the recent decreasing trend of the population in southwestern Japan. Comparisons of seven models by distance‐based redundancy analysis (dbRDA) revealed that the model with the density of medium and large‐sized nematode (≥ 0.125 mm), in addition to sediment environmental variables, best explained the variation in the composition of the macrobenthic assemblages. The dbRDA result indicated that the increase in nematode density of these size fractions related to the occurrence of the Group 2 assemblage. Therefore, density data of medium and large nematode (≥ 0.125 mm) contain useful information for modeling macrobenthic assemblages on Ruditapes fishery grounds.