Gonad stage: Percentage (from a montly sample of 50 clams ranging between 60 to 80 mm in shell length) of male and female clams classified into four gonad stages: starting, developing, ripe, spawned, recovering. Classification was made after histological examination of Bouin-preserved gonad samples. Tissue embedded in paraffin after dehydration, sectioned at 5 µm and stained with standard haematoxylin-eosin stain before examination.