Genetic decoupling of spat origin from hatchery to harvest of Mytilus galloprovincialis cultured in suspension

• Genetic traceability from hatchery to harvest has been individually achieved.
• Stock heterozygosity sank by 28% between the initial culture and the final harvest.
• Null external recruitment was observed on pre-seeded suspended ropes.
• A full outdoors harvest of hatchery-selected mussel juveniles is affordable.

Productivity of Mediterranean mussel aquaculture in suspension could be enhanced with selected hatchery strains for a variety of candidate traits. While selective breeding and reproduction are well established technologies, several key parameters required to grow selected strains outdoors still remain elusive. For instance, external recruitment dynamics on suspended devices is poorly known and therefore it is uncertain whether a harvested strain comes from the one initially seeded or has been partially replaced by wild recruits during the pre-fattening period. The incorporation of genetic technologies to the management of mussel stocks makes feasible to disentangle the genetic contribution of different seed sources, provided that an appropriate type of marker and a suitable experimental design are properly combined. Using multiplexed microsatellites we have traced back the individual origin of a batch of harvested mussels, which grew in suspension ropes that were initially seeded with full-sib juveniles. We show that mean heterozygosity decreased by 28% between months 8 and 13 in suspension, a phenomenon thought to be caused by fall-offs of the more exposed heterozygous individuals of the cohort. Bayesian tests allowed to ascertaining that 98.3% of the adult harvest came from the original hatchery full-sib family while only 1.7% mussels were recruited from the wild. Such low external recruitment on unprotected ropes allows to being confident on the feasibility of culturing selected strains, provided that both, seed density and retention timing are adequately managed.Statement of relevanceGenetic assessment of mussel recruitment in suspension.