Effects of translocation on the asari clam Ruditapes philippinarum at small spatial scales in Matsukawaura, Japan

Abstract

The asari (Manila) clam Ruditapes philippinarum is a commercially important bivalve that lives on tidal flats, but the clam landings have decreased in many estuaries in Japan. To increase the production of the clam, we conducted pilot translocation experiments at small spatial scales within the estuarine system of Matsukawaura Lagoon, Japan. Two scales were used: one was a horizontal translocation from an area with low clam productivity to an area with high productivity, and the other was a vertical translocation from the upper area to the lower area of the tidal flat. Our caging experiment with the horizontal translocation revealed that the growth rate and somatic condition of the clams translocated to favorable conditions were significantly better than those remaining in the original habitat. On a sandy fishing ground, vertical translocation of clams from the upper intertidal zone to the lower intertidal zone resulted in higher growth rates and better somatic conditions for the clams. Moreover, the clams from sandy ground showed larger growth increases than those from muddy-sand ground, indicating that the origin of clams affected translocation effectiveness. We found that clam biomass increased by 42%– 53% with the vertical translocation of small (<30 mm shell length) clams during the fishery off-season from August to April. These results indicate that translocation of local clams to favorable habitats within an estuarine system can increase clam production and yield practically during the off-season.