Biological and Economic Evaluation of The Impact of Predators on Oyster Mortality (A Case study in Quiberon Bay, France).

Abstract

Pacific oysters have been cultured in several country as a result, Crassostrea gigas has become the leading species in world culture, with an estimated production of 4.6 million tons in 2006. France producing around 130 000 tons of the cupped oyster C. gigas annually and a remaining 1 500 tons of the flat oyster Ostrea edulis. Quiberon Bay has a large production of oyster, approximately 15 000 tons annually which contributes to oyster production in France. These small scale industry activities contributes to largely to France economy. Since the year of 2000 and especially 2006, the oyster mortality rates have reached quite unusual, estimated at 60% mortality in a breeding cycle ending in late 2006, according to the response of 51 companies interviewed by the CRC. This mortality brought a great impact on the France's economy. So, in this study, we observed the effects of oyster mortality in terms of biology and economics. The objectives of this study are to analyse RISCO data on oyster mortality, to determine effect of predators on oyster culture, mapping the distribution of different types of predators and to calculate the economic analysis of oyster mortality. RISCO project is conducted by Ifremer with their partners. As present in the study area, this project is held in Quiberon Bay, France and the 15 station’s of oyster culture. The observations of this research have been conducted monthly (8 surveys) in May until December 2010. Each month, there are several analyses of RISCO project which have been done, one of them is analysis oyster growth and mortality (Spat and Adult). In each station, we counted the number of predators (starfishes and gastropods) on the cage and in each bag we calculated the numbers of dead oysters then we took sampling of 30 living oysters for growth and pathology analysis. In each dead oyster, we also observed the boring oyster by oyster drill and parasitism by Polydora. The result of this research showed that there were a potential role of two species of starfishes, Asterias rubens and Marthasterias glacialis and two species of gastropods, Ocenebra erinacea and Ocenebrillus inornatus. Marthasterias glacialis indicated a great impact on mortality of adult oysters. The relation between oyster's mortality and the presence of gastropods depended on time and according to reproductive cycle. Several months show clearly a predation of oysters and others show a concentration of gastropods for spawning. Predators on oyster culture also indicated a great impact in economic value. The economic value on oyster culture model applied in RISCO project shows that BSP was higher than BAP. Based on economic value analysis on RISCO project shows that BSP was higher than BAP. Economic loss in BAP greatly happen in station 2 and 5, approximately €81.28 - €81.83. On the contrary, station 15 indicate has the higher loss in BSP approximately €54.92 or 363 oysters and for the rest, the economic value was varies between €29.67 - €40.30.