Observatoire national du cycle de vie de l’huître creuse en France. Rapport annuel ECOSCOPA 2021

Occupying a place of first order in the French coastal landscapes, oyster farming is the first contributor of the French aquaculture sector with an estimated production of 85,000 tons in recent years. Since 2008, this species has been affected by episodes of massive and recurrent mortalities triggered by the presence of viral infectious agents (Herpes virus OsHV-1 µVar type) in conjunction with environmental factors (i.e. temperature increase). This viral infection induces an alteration of the antimicrobial defenses as well as a destabilization of its microbiota. This process leads to a secondary infection by potentially pathogenic opportunistic bacteria that multiply rapidly causing irreversible tissue damage leading to the death of the animal. In parallel and although poorly informed, these mortality phenomena also affect the many wild beds present on the French coast. However, the oyster industry relies on a supply of spat that comes mainly from natural collection, now practiced in several oyster basins. However, in all these sectors, reproduction and recruitment show very variable performances from one year to the next. This crisis context (mortality episodes, recruitment failure, growth variability...) requires the implementation of relevant observation on a national scale allowing to objectively measure through different proxies the general health state of cultivated and wild oyster populations, for the different sensitive phases of their life cycle (reproduction, larval ecology, recruitment, growth, survival). The general objective of the ECOSCOPA network is to perpetuate the current observation devices and to progressively add a series of new physiological and environmental indicators, contributing to a more detailed analysis of the life cycle of this species in relation to climatic and anthropic pressures. This observatory is now part of the context of climate change and proposes to provide objective elements to better anticipate possible future crises. For the year 2021, following the COVID crisis of the previous years, part of the network had to be relaunched following the interruption of nearly two years on the monitoring of mortality and growth of standardized batches. The attached report presents results concerning only the spat age class (as opposed to a follow-up on 3 age classes in classic operation): the follow-up will be complete again in 2023. The other results are complete and concern the monitoring of environmental parameters on the eight sites, a specific experiment on survival according to bathymetry, an exhaustive analysis of the reproductive cycle and oyster recruitment (VELYGER Network) and the cytogenetic monitoring of wild spat. As far as the processing of environmental data is concerned, a particular work has been carried out, because, with now more than ten years of monitoring on some sites, it becomes possible to start observing the effect of climate change on a local scale with the temperature increase.  The main results obtained during this 2021 campaign indicate that, in terms of environmental factors, this year is in line with the general trend of global warming, but in a more moderate way than previous years. At the network sites, 2021 is in line with recent years with an average thermal anomaly +0.5°C above normal, which is mainly the result of a strong anomaly in winter: seawater temperatures were above normal, but late in the summer. In terms of phytoplankton inputs, phytoplankton concentrations were rather within normal range. The spawning and egg-laying phases took place at a "normal" period, neither early nor late, in a relatively synchronous manner allowing a supply of larvae within the normal range, with the exception of the Bourgneuf Bay site, for which the concentrations of young larvae were low. Taking into account the rather variable temperatures, which were just within the norms in the different basins, the subsequent collection was relatively heterogeneous according to the sectors, very good on certain sites, and practically null on others. Spat mortality observed for 2021 is very low for the first time (except for the sites in the Bay of Veys and Marennes-Oleron) and the national average is below 50%: this is partly linked to the absence of mortality observed on our spat oysters monitored in Marseillan (Thau lagoon).