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Simple Summary: Optimizing production strategies for clam farming is crucial to achieve economic and biological sustainability. Improving broodstock-conditioning conditions, such as the geographical origin, timing of collection, temperature, and nutrient levels, can significantly enhance outcomes. Developing Moroccan hatcheries rearing protocols adapted to local environmental factors promotes sustainable aquaculture, making clam farming a more feasible and profitable industry for the country. Effective broodstock conditioning is critical for producing high-quality larvae and spat in bivalve hatcheries. This study aimed to assess the impact of the geographic origin and season of collection on broodstock-conditioning performances to design optimal clam rearing programs. The grooved carpet shell clam (Ruditapes decussatus), widely found along Morocco's coasts and estuaries, is a key economic resource due to its high market value. However, clam production has declined over recent decades, largely due to the overexploitation of natural beds, and recruitment failures, leading to a limited wild spat availability. This study examined how the broodstock collection season (winter vs. summer) and origin (South Atlantic vs. North Mediterranean) affect broodstock performances in hatcheries. The maturity development (condition index (CI) and gonadal condition index (GCI)), histological examination, and reproductive output were evaluated. The results showed that Mediterranean clams achieved a higher maturity during winter conditioning (CI = 13.60 ± 1.02, GCI = 6.01 ± 0.90) than the Atlantic population (CI = 11.51 ± 1.50, GCI = 5.31 ± 1.14). Moreover, Mediterranean clams produced more oocytes per female (2.34 million), despite the lower spawning rate (42%), compared to the Atlantic clams (1.68 million) with a 69% spawning rate by the end of the winter conditioning. These findings highlight the importance of selecting broodstock by geographic origin to optimize shellfish hatchery production. [ABSTRACT FROM AUTHOR]

Simple Summary: The development of cost-effective protocols with optimized rearing systems and conditions is an essential requirement to assure the economic feasibility of mussel production in hatchery systems. The Mediterranean mussel Mytilus galloprovincialis plays a relevant role on the scale of bivalve aquaculture, whose expansion has deep economic and environmental consequences. The current study establishes the protocols to improve pediveliger settlement, post-larvae production and growth, through the optimization of the stocking density of larvae and post-larvae in hatchery systems. This standardized protocol for mussel rearing can enhance the production of quality seed to supply the fattening industry as well as for environmental restoration. In mussel hatchery systems, the settlement process is a crucial element influencing seed yield. The current study assayed the influence of five densities of competent pediveliger larvae on settlement success and post-larvae production. We showed an inverse relationship between density and settlement efficiency, e.g., an attachment success of 99.4% at the lowest density (35 larvae/cm2) but only 9% at the highest density (210 larvae/cm2). However, post-larvae production was higher at intermediate larvae densities (70 larvae/cm2). The reimplementation of treatments upon post-larvae density after 6 weeks post settlement showed that the lowest-density groups bore both the highest post-larvae growth rate (22.24 ± 4.60 µm/day) and the largest head batch (48% of the size distribution), as compared to the higher-post-larvae-density groups. These results highlight the importance of optimizing both pediveliger larvae density and post-larvae density, to maximize high-quality seed yield in local hatcheries. Current rearing technologies would assure a timely commercial seed production to protect natural sea rocky beds in Alboran Sea coasts. [ABSTRACT FROM AUTHOR]