Influence on benthos and Asari clam (Ruditapes philippinarum) of installed cover nets on tidal flat in Yamaguchi Bay, Seto Inland Sea, Japan

Abstract

The effects of cover-nets installed to prevent the feeding of fishes on Asari clams Ruditapes philippinarum were evaluated to the environment, the dynamics of macrobenthos including clams and meiobenthos in tidal flat of Yamaguchi Bay, Yamaguchi Prefecture, Japan. Five months after net installation, the mud content and total organic carbon inside the nets have been higher than those outside the nets. The densities of major macrobenthos, with the exception of Arcuatula senhousia, inside the nets tended to be higher than those outside the nets. The densities of nematodes inside the nets also tended to be higher than those outside the nets, but the same fluctuation trend was also observed for the densities outside the nets. The densities of copepods inside and outside the nets fluctuated similarly. The densities of R. philippinarum larger than a shell length of 15 mm inside the nets hardly exceeded 50 individuals/m² until two years after the nets were installed. R. philippinarum larger than a shell length of 15 mm rarely occurred outside the nets, suggesting that juveniles of R. philippinarum did not survive to adult size outside of the nets. On the other hand, the densities of juvenile R. philippinarum smaller than the shell length of 15 mm, which peaks at approximately a shell length of 5 mm in May and June every year, were always higher outside the nets, suggesting that the nets reduced the density of juveniles of R. philippinarum.