Gonad stage: Percentage (from a montly sample of 30 clams) of male and female clams classified into four gonad stages: inmature, mature, late maturity, spawned, regression. Classification was made after histological examination of Bouin-preserved gonad samples. Tissue embedded in paraffin after dehydration, sectioned at 7 µm and stained with standard haematoxylin-eosin stain before examination.