From Broodstock to Biogenic Reefs: Hatchery-Based Production of Modiolus modiolus for Biogenic Reef Restoration in Denmark

In Denmark, biogenic reef restoration with bivalves is currently relying on relaying of the abundant, short-lived blue mussel (Mytilus edulis) from spat collectors, or the endangered, disease vulnerable European flat oyster (Ostrea edulis) supplied through hatchery production. New efforts aim to add the long-lived horse mussel, Modiolus modiolus (Linnaeus, 1758), to the suite of native Danish species available for biogenic reef restoration. Like other bivalves, M. modiolus is an ecosystem engineer, with living mussels and old shells aggregating to create biogenic reefs that can be linked to healthy ecosystem functioning and high biodiversity. However, the spatial extent of M. modiolus beds in Danish waters has diminished since the early 20th century due to demersal fishing, with environmental changes contributing to further losses, leaving M. modiolus reefs endangered. To reverse the trend, biogenic reef restoration efforts aim to enhance recruitment through the relaying of hatchery-grown M. modiolus spat. However, no established hatchery protocols currently exist for M. modiolus production and past attempts described in the literature achieved only limited success. Low settlement success and survival of spat highlight gaps in our understanding of the species’ biology and underscore the need for further study to realise restoration with M. modiolus.

M. modiolus broodstock were collected from Limfjorden and studied under controlled hatchery conditions to examine feeding ecology, stressor-induced spawning methods, and early embryo development, with the aim of producing viable spat for restoration. Initial results indicate that a slow thermal-cycling method consistently induces spawning, providing opportunities to test protocols and setups related to early embryo development and larval rearing in the hatchery, which represent the first steps in producing viable M. modiolus spat for restoration purposes.

This research is part of the BioReef project, which supports the restoration of M. modiolus populations in the Danish North Sea region to improve biodiversity and ecosystem functioning. BioReef partners: Ørsted, WWF and DTU Aqua.