Review of the decline in Asari (Manila) clam (Ruditapes philippinarum) stocks and contributing factors in Mikawa Bay, Japan

Abstract

The Isshiki tidal flat in Mikawa Bay, one of the major fishing grounds for Asari (Manila) clams in Japan, has experienced a drastic stock decline, despite active fishery management by local fishermen since 2014. This study reviews the reasons for the decline in Asari clam stocks within Mikawa Bay. The findings showed that the stock depletion is likely to be associated with annual decreases in the condition factor of the clams during the fall and winter seasons. As the stock has declined, the density of planktonic larvae has also declined, and their survival rates have remained low in recent years. These changes may be linked to a reduction in phytoplankton as food for the clam. In coastal regions of Japan, including Mikawa Bay, the implementation of stringent water quality standards has resulted in marked reductions in nutrient loads (e.g., nitrogen and phosphorus) from terrestrial sources. This reduction in nutrient inputs to coastal waters could have decreased primary production, and consequently, the survival and recruitment of Asari clams in coastal areas. Under these environmental conditions, even slight increases in biomass appear to negatively affect growth rates and condition factors, suggesting that density-dependent effects have become more prominent. As a result, improving primary productivity is considered to be important for the recovery of clam stocks.