Abundance and Population Structure of the Blacklip Pearl Oyster, Pinctada margaritifera l. 1758 (Bivalvia: Pteriidae), in Coastal Kenya
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Pearl oysters are an important marine resource within the Indo-Pacific oceans. They are widely cultured for the production of black pearls, their flesh is eaten, and their shell, known as mother-of-pearl (MOP) is used in the ornament and button industry. The blacklip pearl oyster, Pinctada argaritifera L., has been harvested from East Africa for MOP for decades. A survey within nearshore habitats in Kenya showed that Pinctada margaritifera is widely distributed in shallow lagoons, bays and channels. Other oyster species found during the survey were the wing oyster, Pteria penguin, occurring in deep channels in Shimoni and Mombasa, and Pt. chinensis, within lagoons and channels in Malindi, Mombasa and Shimoni. Pinctada margaritifera was most abundant in Gazi Bay within sheltered back reefs and lagoon, and on an intertidal reef flat and back reef in Shimoni. Mean abundance and sizes declined with depth. The abundance of Pi. margaritifera in Kenya was higher than reported in the Pacific Islands and similar to abundances reported in India. The overall mean (34 mm) and maximum (154 mm) sizes were lower than those reported in the Pacific Ocean, but similar to sizes in India and the Red Sea. The oysters matured before reaching 40 mm (dorsal-ventral length), principally as males. Male sex expression was dominant, and more so in samples from moorings and jetty fouling where density was exceptionally high. The high population densities, high temperature variation and high suspended matter of relatively poor quality in the nearshore shallow lagoon environment may explain the observed life history pattern observed in the oyster populations. This study is the first comprehensive report on pearl oyster populations in Kenya.