Boosting efficiency of mussel spat collection for ecological sustainability: Identifying critical drivers and informing management

The long-term sustainability of natural and bottom-cultured mussel beds relies on the availability of spat (i.e. juvenile mussels). Traditional spat collection methods, which disrupt the donor population and its habitat, have prompted the adoption of suspended mussel spat collectors (SMCs) as an ecologically sustainable alternative. However, practical experience has demonstrated that SMC efficiency is subject to significant spatiotemporal variability, with the underlying biotic and abiotic drivers remaining unresolved. Based on 11-year SMC practices in the Dutch Wadden Sea, we first validated, through field experiments, the inference that larval abundance and settlement rate in seawater are the primary determinants of SMC efficiency. Secondly, we screened the key factors driving variation in SMC efficiency using an integrated dataset that includes both management options (i.e. user-defined factors, like SMC types) and environmental conditions (i.e. site-specific factors, such as hydrodynamics). Lastly, we developed a predictive model to explore the sensitivity of SMC efficiency to these key factors. The efficiency of SMCs was not affected by larval abundance and settlement rate, but rather regulated by management options and environmental conditions, with the key factors identified as SMC type, substrate size, mean wave height and starfish abundance. Rope-based SMCs outperform net-based SMCs in terms of harvest efficiency. Both types of SMCs exhibit consistent sensitivity to environmental conditions, with harvest efficiency higher in areas with lower mean wave height and fewer starfish. Increasing substrate size (i.e. rope length and net area) in these areas has the potential to further improve SMC efficiency. Synthesis and applications: Our study highlights the importance of environmental conditions over life cycle-related factors in regulating SMC efficiency. This offers optimistic prospects for investment in larger-scale deployment of SMCs and maximizing efficiency through site suitability assessments beforehand. The predictive model we developed can provide information for this purpose. Furthermore, strategically adjusting management options would further optimize SMC efficiency, but it is necessary to balance the associated benefits and costs. Overall, our study underscores the predictability and controllability of SMC efficiency and informs management to maximize sustainable spat supply in both mussel culture and restoration.