The severe mortalities of oyster spat which have been affecting oyster farming in France since 2008 question the sustainability of a sector largely dependent on monoculture. The purpose of this paper is to explore the first economic impacts of these mortalities, which preliminary required an analysis of the factors determining the profitability of the oyster farming enterprises. This goal was achieved from the analysis of the oyster farming sector in the Marennes-Oleron Bay (Poitou-Charente Region), and mobilised an ad-hoc accounting study and the socio-economic background resulting from previous research work. The paper starts with a description of the institutional and regional contexts of oyster farming, followed by a presentation of the data and method used for the economic assessment, and of the main results relying on accounting, structural and economic indicators. These indicators provide the basis for exploring the links between the structure, strategies and performance of the enterprises, and for discussing preliminary findings as regards the economic impact of mortalities and the efficiency of public subsidies. All these outcomes highlight overall governance issues, in relation to the collective management of oyster farming areas and market organisation. (C) 2014 Elsevier Ltd. All rights reserved.