Impact of season and grafter skill on nucleus retention and pearl oyster mortality rate in Pinctada margaritifera aquaculture

The mollusc Pinctada margaritifera is the top economic aquaculture species in French Polynesia (export value) and forms the basis of the black pearl industry. Mass production of this unique gem, produced by a living organism, relies on a surgical operation requiring tissue from a donor pearl oyster to be grafted, together with a nucleus made of shell, into the gonad of a receiving pearl oyster. This technique is performed by expert grafters, whose work constitutes the first step influencing pearl farm production yield. This study makes the first report of effects mediated by individual grafters and by the season of grafting on three rates scored at 45 days post operation: nucleus retention, nucleus rejection and pearl oyster mortality. These results were obtained in a very large scale grafting experiment, designed and conducted in a single culture site in the atoll of Arutua (Tuamotu Archipelago) and involving a total of 52,910 grafts performed by ten professional grafters, during two contrasting seasons: autumn (four grafting experiments) and spring (three grafting experiments). Statistical analysis using linear mixed-effect model for both univariate and multivariate analysis was performed. The results show a very highly significant season effect (p < 0.001), with a higher rate of nucleus retention in autumn than spring: 91.1 versus 79.4 %, respectively. A very highly significant season effect was also recorded for nucleus rejection rate (p < 0.001), which was greater in spring than in autumn: 12.6 versus 7.7 %. Oyster mortality rate was up to six times higher in spring than in autumn (p < 0.001). Within the two seasons, there was no significant difference between the grafting experiments for any of the three variables, adding robustness to the inter-season results. Taking into account this seasonal variation effect, grafter skill had a significant impact on the three variables (p < 0.001), making it possible to rank grafters from most to least efficient. Aside from skill differences between grafters, grafting in autumn may significantly increase graft success. Water temperature and its indirect consequences could be one of the most important environmental factors implicated in overall efficiency. These findings may help the potential development of an annual grafting schedule according to season/ lagoon temperature as a way to maximize production yield for the black-lipped pearl oyster industry in French Polynesia.