Method of determining ages of hard clam meretrix lamarckii along the kashimanada coast, Ibaraki, Japan

Abstract

Edible hard clam Meretrix lamarckii is an important fishery resource along the Kashimanada coast, Ibaraki Prefecture, Japan. The aim of this study was to establish a method of determining age by verifying whether two types of growth patterns, growth lines seen in shell cross-section and "rings" visible on shell surface, are formed on an annual basis or not. To this end, we examined the shells obtained from a mark-recapture experiment that enabled us to specify growth periods after a release. Shell cross-sections were prepared using a hand-held grinder equipped with a thin diamond grinding disc and polished. High-resolution pictures of the cross-section surface were taken using a commercial photo scanner. The number of growth lines in the cross-sections was consistent with the number of winters the shellfish had experienced after release, suggesting that the growth lines were formed in winter on an annual basis. On the other hand, the "rings" on the shell surface rarely matched the expected growth periods. This study shows that the number of growth lines recognized in the shell cross-section is a more reliable method of determining the age of the hard clam.