Growth patterns of the pearl oyster Pinctada margaritifera L. in Gazi Bay, Kenya

Culture of pearl oysters is rapidly increasing worldwide, including the western Indian Ocean. The oyster Pinctada margaritifera L., which produces the most highly valued black pearls, occurs in East Africa, and has been exploited there for the shell for many decades. The growth patterns of P. margaritifera from a natural population in the sheltered back-reef, and from oysters translocated to a tidal current-swept site, both sites within Gazi Bay, Kenya, are described. The growth rate in the natural population ranged from 31.3mm year–1 (60–65mm size-class) to 7.6mm year–1 (105–110mm sizeclass). The von Bertalanffy growth calculated with a fixed L&#8734 of 127.2mm, was 0.30 for the natural population and 0.38 for the translocated oysters. The mean growth rate during the north-east monsoon season was approximately double that for during the south-east monsoon season. The daily rate of nacre deposition ranged from 1.3µm to 5.9µm (mean 3.45µm); it declined with the size of oysters and was marginally higher at the high-energy current site. At that rate, it would take approximately two years to produce a marketable cultured half pearl with a 2.5mm layer of nacre. The results of the study are relevant to the understanding of the influence of the environment on growth, and are applicable to the optimisation of growth rate of pearl oysters in the inshore region along the east coast of Africa. Keywords: growth rate, Kenya, monsoon seasons, nacre deposition, Pinctada margaritifera, tidal currentsAfrican Journal of Marine Science 2005, 27(3): 567–57