Optimizing sea cucumber Holothuria scabra aquaculture through dietary improvement, nursery strategies, and co-culture system assessment

The sea cucumber Holothuria scabra is a valuable tropical species that plays an important ecological role in the recycling of nutrients within coastal ecosystems. Aside from its ecological relevance, H. scabra is recognized for its high economic value and rising demand in global markets. However, overexploitation of natural populations has resulted in stock depletion, necessitating the development of sustainable aquaculture systems to ensure a reliable seed supply while minimizing pressure on wild resources. This doctoral thesis aims to enhance the sustainable aquaculture of sea cucumber H. scabra by focusing on three main stages in its culture cycle, including dietary formulation for early juveniles, nursery site management in earth pond-based hapa systems, and co-culture viability of H. scabra with abalone and macroalgae. The main objective was to develop strategies that improve survival, growth performance, and system efficiency, establishing an integrated approach from hatchery to nursery and co-culture stages. The first study focused on dietary optimization at the hatchery phase, investigating the impacts of different fermented plant and algal-based feed on the growth and survival of early juveniles H. scabra (approximately 0.1 g initial wet weight) (Chapter 2). Fermentation was implemented as a practical processing method to improve the suitability of feed sources. The result indicated a favorable acceptance of most fermented diets and revealed that feed type affected growth and survival performance. Seagrass Enhalus acoroides and macroalgae Padina australis promoted balanced growth and high survival, whereas green pond algae induced the fastest growth, but lower survival. These findings suggest that various fermented diet sources can be selectively applied based on the juvenile size and culture phase, providing a basis for developing locally sourced, cost-effective feeds for early-stage H. scabra cultivation. The second study examined the impact of varying pond environments on the nursery performance of juvenile H. scabra (approximately 1 g initial wet weight) (Chapter 3). The experiment was carried out in earthen ponds in East Lombok, Indonesia, using hapa nets placed at four sites, including a reservoir, stirred pond, non-stirred pond, and main inlet sluice. Survival rates were not significantly different across treatments. However, juveniles reared close to the main inlet sluice demonstrated the highest growth, supported by favorable water movement and natural food availability. On the other hand, stirring and fertilization treatments did not significantly enhance juvenile growth, indicating that hydrological and ecological factors, such as water circulation and food availability, play a more decisive role than physical disturbance in pond-based nursery systems. These findings underscore the importance of strategic nursery placement and water management to optimize growth during the pond-based rearing period. The third study assessed the feasibility of co-culturing juvenile sea cucumber H. scabra (approximately 2 g initial wet weight) with abalone Haliotis squamata and macroalgae Ulva lactuca in a recirculating aquaculture system with various substrate and biofilter configuration treatments (Chapter 4). The system maintained relatively high survival of sea cucumbers, but did not enhance their growth across treatments, which revealed negative growth trends. The most favorable outcome for H. scabra was observed in the treatment combining a biofilter without substrate, where temporary growth recovery suggested potential adaptation; however, performance remained limited due to insufficient nutritional input from abalone feces as the main food source. Abalone showed consistently strong survival and growth in all treatments, with substrate and macroalgae offering optimal rearing conditions, whereas the biofilter biomass of U. lactuca decreased over time, likely due to tissue degradation and stress conditions under culture environments. Overall, the co-culture system exhibited partial feasibility by maintaining abalone performance and sea cucumber survival while failing to stimulate H. scabra growth and stabilize macroalgal biomass, emphasizing the necessity for improved dietary support and system configuration to attain balanced multi-species cultivation. The three current studies collectively demonstrate that sustainable aquaculture of sea cucumber H. scabra can benefit from the integration of optimized feeding strategies, suitable nursery conditions, and multi-trophic design. Fermented diets enhance the survival and growth of early juveniles by improving feed digestibility, while appropriate nursery site selection ensures consistent performance during the transition from hatchery to grow-out. The co-culture approach expands the possibilities for H. scabra aquaculture by combining multiple species that occupy distinct ecological niches, potentially increasing system efficiency and resource utilization. Nevertheless, it also presents trade-offs that require careful balancing alongside environmental stability, biological performance, and system maintenance. This thesis contributes to the understanding of H. scabra aquaculture by identifying key factors that affect growth and survival across various culture stages. The findings provide a scientific basis for optimizing management practices and improving the sustainability of sea cucumber aquaculture systems.