Using a biodegradable substrate to increase transplantation success: Effect of density and sediment on aggregation behavior of mussels

Habitat restoration through transplantation of ecosystem engineering species has become an increasingly popular conservation strategy. However, the success of these restoration efforts depends largely on the ability of transplanted organisms to establish and persist in their new environment. Ecosystem engineers typically occur in large numbers and rely on self-facilitating feedback mechanisms to overcome physical and/or biological stressors for successful establishment. These feedback mechanisms can only arise when a certain density or size threshold is reached and are driven by the interplay of facilitation and competition. To initiate the establishment of self-facilitating feedback mechanism, we used biodegradable structures known as “BioShell-SMCs”. These structures are an innovation of the nylon seed mussel collectors (SMCs) commonly used in mussel cultivation. They consist of a biodegradable net based on a compound of aliphatic polyesters, filled with empty cockle shells around a coconut fiber rope. In a mesocosm experiment, we investigated competition and facilitation processes by comparing aggregation and performance between loose seeded blue mussels (Mytilus edulis) and mussels already attached to the BioShell-SMC at two different densities (high vs. low) and two sediment compositions (mud vs shell). Our results revealed that mussels attached to the BioShell-SMC showed more pronounced clustering compared to loose mussels, particularly in low density. Mussels in high density attached to the BioShell-SMC dispersed from the SMC on both sediment compositions. Furthermore, transplanted mussels attached to the BioShell-SMC showed higher survival rates and had a better condition than loose mussels. Overall, our study emphasizes the importance of considering ecological processes such as competition and facilitation when designing and implementing restoration projects. It provides a case for optimizing transplantation success of ecosystem engineers by including temporary substrate that provide positive feedback mechanisms at establishment, effectively creating a window of opportunity.