Evaluation of mussels seed survival Mytilus edulis during early period of collection, development of a new tool and method

In France, today, more than 80% of farmed blue mussel seed collection is based on larvae fixation on collector ropes. Mussels are traditionally grown on bouchot, which are rows of wooden poles placed perpendicularly to the shore, stuck in the ground within the gently sloping intertidal strip. Mussels are transferred either as seed attached on collecting ropes or as juveniles placed in net sockings and then wound up around the poles or suspended to longlines, where they will fatten and grow (Garen et al., 2004). In order to optimize seed collection and to limit unwanted biofouling on ropes, mussel growers local organizations identify best timing of seed collector ropes deployment by monitoring the number and size of planktonic mussel larvae from water column. Then, number of mussel seed collected on rope per meter is estimated every two weeks and for 4-5 months to establish initial reference data of « quality collection » according origin site (Centre Régional Expérimentation et Application Aquacole –CREAA-, 2017). In Charente Maritime-Vendée, first shellfish-producing region (Agreste, 2012), monitoring of seed collection with this method over the last ten years showed that more than 50% of mussel seed collected on rope disappeared within May to July. The main deficiency of this method is that these high losses, from seeding to thinning, cannot be clearly attributed to mortalities, stalling, predation, spatial competition…? Here, we present the development of an original tool and method to assess precisely level of survival of mussel seed (1-5mm) collected on rope within the first months after spawning period. Our approach is based on little repeated pieces of rope put in semi-closed conditions witch prevent the loss of animals or empty shells. Our study allowed to assess and compare survival level of seed from three different origins, placed in three stations from Pertuis Charentais (Bay of Biscay). In this region, although adults mussels suffered heavy mortalities (>60%, Normand 2017), average mortality estimated with the present method in seed was around 10% suggesting a less sensitivity of seed and spat to the phenomenon. This work took place in 2016, in the context of massive mortality events of blue mussels in France since 2014, as a part of a specific project (MORBLEU) dedicated to identify the factors favoring the development of massive mortalities in farmed adult mussels