Assessing Efficacy of Plastic-free Alternative Ties For Coral Propagation in Reef Restoration

Abstract Coral propagation and out-planting based restoration approaches are increasingly being applied as tools to assist natural recovery and preserve resilience of coral reefs. However, many out-planting and propagation methods rely on plastic zip-ties to fasten corals to structures in coral nurseries, which is potentially problematic and unsustainable for the marine environment. Plastic-free biodegradable alternatives are becoming available but may pose unique risks by impacting coral associated bacterial communities that are integral to coral health. We therefore examined the bacterial communities of Acropora millepora coral fragments propagated in coral nurseries in two experiments on the northern Great Barrier Reef to identify whether biodegradable materials differentially impact coral associated bacterial communities. In each study coral fragments were secured to nursery frames with conventional plastic, metal and biodegradable (polyester and polycaprolactone) ties, and both tie failure and coral associated bacterial communities were characterised during a six-month period. Minimal coral mortality was observed (3.6-8 %) and all ties tested had low failure rates (0-4.2 %) with the exception of the polyester biodegradable material (29.2 % failure). No differences were observed between coral associated bacterial communities of fragments secured in the coral nursery with different tie types, and no proliferation of putatively pathogenic bacteria was recorded for fragments secured with biodegradable ties. Overall, our findings suggest that reducing reliance on conventional plastic can be achieved through transitions to biodegradable materials, without any notable impacts on coral-associated bacterial communities, but we caution the need to examine wider coral taxa of different morphologies and growth dynamics, and any new plastic-free materials prior to application.