As diseased shellfish show only very seldom symptoms, any mortality event can be a disease suspicion. In theory, monitoring of mortality events in shellfish can thus provide a tool for early detection of infectious diseases in order to implement effective control measures. This consists in passive mortality event reporting by shellfish farmers to the local competent authority. Sensitivity of the detection of mortality events is a key quality indicator for this type of surveillance. Since 2008, increased mortality outbreaks occur in Pacific oysters Crassostrea gigas in France, leading to economic losses. It is important to evaluate how much the outbreaks have altered the quality indicators of the reporting system and if this modification is long-standing, to achieve an effective early detection tool. The yearly evolution of sensitivity of the passive farmer reporting was estimated during 2007-2010 in the main oyster production area in France, using a capture-recapture analysis and accounting for unequal catchability. Information on mortality events were obtained from existing annual databases of mandatory mortality reporting (capture) and from a questionnaire-based cross-sectional survey conducted in 2010 as part of larger study of oyster mortalities (recapture). Sensitivity significantly varied according to the year, from 0.55 (CI95% 0.31-1.00) to 0.76 (CI95% 0.64-0.93), 0.53 (CI95% 0.43-0.71) and 0.72 (CI95% 0.59-0.92) in 2007, 2008, 2009 and 2010, respectively. Important under-reporting was observed in 2007. Sensitivity increased concurrently with outbreaks occurrence and with implementation of financial incentive for mortality reporting. The achieved completeness was satisfactory. However, bottlenecks for reporting or solutions to facilitate reporting have to be identified to improve sensitivity of the reporting system on a long term period, whatever the epidemiological situation is.