Growth of the black-lip pearl oyster, Pinctada margaritifera, in suspended culture under hydrobiological conditions of Takapoto lagoon (French Polynesia)

Abstract

Growth of the black-lip pearl oyster, Pinctada margaritifera var. cumingi, was studied for an annual cycle, from March 1997 to April 1998, in the lagoon of Takapoto atoll (Tuamotu archipelago, French Polynesia). Growth in shell and in tissue were measured every 15 days on three successive age groups of cultivated pearl oysters. At the same time, hydrobiological parameters (temperature, salinity, oxygen concentration, suspended particulate matter), known to have influence on bivalve growth, were followed each week during culture. No seasonal trend was observed in hydrobiological parameters, except for temperature which varied between 26°C and 31°C. The potential food for pearl oysters (particulate organic matter, POM, mg l −1) was slightly concentrated, but always available, so that, in this lagoon environment, no period seemed to be unfavourable to pearl oyster growth. Effectively, growth in shell was regular and shell did not exhibit any annual ring. Nevertheless, as it is often the case for bivalves, shell growth showed a progressive decrease with the age of pearl oyster and followed a classical Von Bertalanffy model: H=160.5 (1−e −0.038 ( t−3.73) ) with H the shell height (in mm) and t the age (in months). On the other side, growth in tissue did not follow the same pattern than for shell: P. margaritifera exhibited reduced growth rate in tissue during the warm season (November–April) so that a seasonal growth model was more appropriate: W tissue=6.9/(1+e (5.58–0.208 t−0.435 sin (2 π/12 ( t−1.427) ) with W tissue, the dry tissue weight (in g) and t the age (in months). Several results concerning growth rates should be of interest for pearl farming. Firstly, the progressive decrease measured in shell growth rate implies, for pearl seeding operations, that the sooner the nucleus is implanted, the greater is the rate of nacreous deposition on this nucleus, and shorter is the time to obtain a marketable pearl for farmers. Secondly, exhaustive comparison, between growth rates obtained in our study and those obtained in other lagoons, tended to demonstrate that there is a small but significant variability in growth between lagoons of the Tuamotu archipelago. Further investigations need to be engaged in order to determine the most suitable sites for pearl farming in French Polynesia. Finally, comparison between growth of P. margaritifera var. cumingi and growth of other pearl oysters showed that P. maxima but also P. margaritifera var. erythraensis would also exhibit fast growth in Polynesian waters and then, would constitute potential candidates for further Polynesian diversification projects.