Distribution of Manila clam, Ruditapes philippinarum, into Berre Lagoon according to the environmental condition

– The Manila clam Ruditapes philippinarum (Adams & Reeve, 1850) is one of the most commercially exploited bivalve in the world (Dang et al. 2010). Since 2013, it has expanded into Berre lagoon. In 2017, the density was between 41 and 1,264 clams.m–2, higher than that of other French sites (Mahé 2017, Sanchez et al. 2014). In February 2018, the professional clam fishery opened in the Berre lagoon. Starting in July 2018, an ecological crisis occurred and the clam stock was decimated (Mayot et al. 2020). Consequently, the fishery was closed. Following this event, the impact assessment was initiated with a clam stock study in spring 2019. During this campaign, 238 stations were sampled where the number and the size of R. philippinarum were assessed. To explain the clam distribution into Berre lagoon in relation to environmental conditions, hydrological condition and grain size were analyzed. These parameters are well known to control clam expansion (Le Treut 1986) and are believed to lead the clam distribution. Preliminary results show a total abundance of 552.07 million clams. No clear correlation was found between the hydrological condition, sedimentary data and the clams’ distribution at a large scale into Berre lagoon. RuditaPes PhiliPPinaRum BERRE LAGOON FIShERY STOCK TEXTuRE hYDROLOGICAL DATA 270 M. MAhé, D. DELANGhE, R. GRISEL, J. C. POGGIALE, N. MAYOT Vie milieu, 2020, 70 (3-4) and economic officials expect such an ecological inventory from R. philippinarum for future decisions. Indeed, the objective of this study is to provide an overview of the distribution of clams in the Berre lagoon according to environmental conditions. MATERIALS AND METHODS sampling campaign: To evaluate the clam distribution and density into Berre lagoon, a field investigation has been performed between the 27th of March and the 27th of July 2019. The lagoon has been divided into 19 layers according to the depth, the grain size a priori and the hydrologic conditions (following Berthou et al. 1997). Only the first 5 meters of depth were sampled. Indeed, no clams were expected below this depth due to the ecological crisis of 2018 (Mayot et al. 2020). According to Bertignac et al. (2001), the sampling rate needs to be at least 10 stations per km2 with 2 replicates to have an adequate precision (i.e., 20 stations per km2). This strategy was created for the Arcachon Bay, where the tide plays an important role in clam biology and repartition. Into the Berre lagoon, it is not the case. In this campaign, the sampling rate was different depending on the layer specificity (depth and area) to better adjust the number of stations to the depth clam repartition. For the depth layer (between 2 and 5 meters), the sampling rate was 7 stations per km2 with 3 replicates (i.e., 21 stations per km2). For the shore layer (between 0 and 2 meters), two strategies were chosen (1) for the big shore layer, with an area superior to 1 km2, the sampling rate was 10 stations per km2 with 3 replicates (i.e., 30 stations per km2) (2) for the shore layer with an area inferior to 1 km2, the sampling rate was 20 stations per km2 with 3 replicates (i.e., 60 stations per km2). using three replicates per station allows the data to be considered as normally distributed for statistical analysis. Stations were randomly distributed inside each layer, with a distance minimal of 200 meters between them. GPS was used for reaching each station. A total of 238 stations were sampled. At each station, 3 replicates were sampled for repeatability esti-