Stocks and fisheries of asari in Japan
Asari clam is one of the most important fishery resources in Japan. The clam has been utilized as not only a food but also a recreational harvestable species. Main habitat of the asari extends from tidal area to shallow subtidal area of sandy soft bottom. Asari is harvested by hand raking or short handled basket raking in tidal areas and long handled basket raking, dredging, helmet diving in subtidal areas. Major areas of asari harvest are Tokyo Bay, Mikawa Bay, Ise Bay, Suo Nada and Ariake Bay. Annual harvest of the asari in the major areas, however, decreased largely after late 1980s except Mikawa Bay. Stock decline of natural asari is remarkable and seriously problematic for coastal fisheries in Japan. Along with application of the various countermeasures intended to restore the natural stocks of asari, a number of factors have been discussed to be responsible for the asari decline. Some of these are decrease of suitable habitat areas, deterioration of water and substrate quality, erosion of the clams by strong wave disturbance, new predators and parasites, oligotrophication, and excess harvesting. We still have not determined significant factors responsible for the asari decline in Japan. In the discussed factors, we compared the effect of five factors; blue tide, river flood, strong wave disturbance, infection of sea spider, and hypoxia on clam larvae, based on the studies in Tokyo Bay. The impacts (mortalities) of the factors for the total asari stock by the blue tide in Sanbanze in 2008, the river flood in Sanbanze in 2007, winter wave mortality in the three areas in 2009 2010, and infection of sea spider in Kisarazu in 2009 were calculated to be 0.16,0.07,0.95,0.30, respectively. Among these factors, winter wave mortality showed the most significant effect on the asari stock in Tokyo Bay. Comparing the properties of the estimated factors, the factors that affected wider areas and longer period with higher mortality were supposed to give severer impact for the asari stock. In order to take effective countermeasures for the decline of asari stock in Japan, further assessment of the impact of the suspected factors is required to be progressed.