Age and growth of the Japanese ark shell <i>Anadara broughtonii</i> in Osaka Bay, Japan

Abstract

The age and growth of the ark shell Anadara broughtonii in Osaka Bay were investigated by observation of the cross-sectioned shell and growth model estimation using samples collected from 2016 to 2022.The cross-sectioned shells showed a translucent zone each year from August to November, the period after reproduction that was estimated by seasonal variations of condition factor. In wild specimens, the translucent zone at age one was not always observed; the opposite was true in released specimens that had been cultured until juvenile. In addition, released specimens always had one or two more translucent zones than their actual age, which were possibly created by physical stresses at the time of release. Shell growth patterns were divided into four groups: (1) reproductive period including less-than-3-year-old specimens,(2) non-reproductive period including less-than-3-year-old specimens, (3) reproductive period including more-than-3-year-old specimens, and (4) non-reproductive period including more-than-3-year-old specimens.The ages were estimated to be from 1 to 8 years old. The age-length key showed that approximately half of the captured shells were estimated as 2 years old. For sustainable fisheries management, we propose to shift the main age of the fishery to 3 years old (i.e., setting the catch limit size to 70 mm or larger).