Acropora palmata is one of the major reef-building coral species in the Caribbean. The species has suffered drastic declines in abundance and sexual recruitment over the past decades. One method for active rehabilitation of A. palmata reefs is by assisting the production of sexual recruits under controlled lab conditions. Within this study, the effect of different aquaculture regimes and culturing periods on the survival rates of these recruits was investigated. In August 2016, coral spawn was collected on a reef nearshore New Providence, Bahamas, cross-fertilized, and reared in mobile laboratory facilities from Van Oord Dredging and Marine Contractors. Larvae were settled on pre-conditioned aragonite plugs. Sexual recruits were cultured under four different aquaculture conditions: ambient vs. high Total Alkalinity (TA) (∼2.8 mEq L⁻¹ vs. 4.8 mEq L⁻¹) and with vs. without feeding Artemia nauplii. Recruit size was monitored by tracking living tissue area and the number of polyps of a subset of recruits. Plates with recruits were outplanted to a nursery on the reef after 4, 9 and 14 weeks of aquaculture. Survival was determined during the aquaculture phase (at 4, 9 and 14 weeks after settlement), and after outplanting (at 27 and 44 weeks after settlement). During the aquaculture phase, survival was significantly lower in seawater with increased TA compared to ambient seawater conditions. The average number of polyps per recruit was significantly higher in the treatments with feeding. After outplanting to the reef, both survival and recruit size were highest in the feeding treatments. The most successful aquaculture treatment in this study was a combination of increased TA and feeding during 9 weeks of aquaculture, which resulted in a doubling of survival and recruit size at 10 months after settlement compared to ambient conditions. Ambient conditions did not enhance survivorship nor recruit size at 10 months after settlement, as compared to the other aquaculture treatments. Nevertheless, the success of ambient aquaculture conditions exceeded natural conditions, as no natural recruitment of A. palmata was observed in this study. We conclude that feeding during and ex-situ culture period enhances ex-situ growth rates and in situ recruit survival of A. palmata juveniles. No positive effects of the aquaculture treatment with only increased TA were found. Building on these results, recommendations are provided for future reef rehabilitation efforts using ex-situ rearing of sexually reproduced A. palmata recruits.

Traditionally, marine infrastructure development is regarded to have a negative impact on the ecosystem in or near which it occurs. As a consequence, a whole industry has been treated as a threat and the ingenuity of companies in this industry was forced to focus on minimizing potential negative impacts. Recent trends are to also consider potential positive spin-offs, by including nature-based components in the designs. In this context, Dutch dredging and marine contractor, Van Oord, launched its Coral Rehabilitation Initiative in 2010. A mobile laboratory, named ReefGuard, was developed, to be operated anywhere in the world for sexual reproduction and rearing corals at a large scale (Van Koningsveld et ah, 2017). ReefGuard is used to initiate 'Coral Engines': large scale nurseries with corals obtained through sexual reproduction as well as from fragmentation. These Coral Engines guarantee the long term and large-scale supply of genetically diverse corals for reef creation and rehabilitation. Having coral 'in stock' furthermore allows reefs to be repaired quickly following harmful events such as hurricanes. The involvement of local stakeholders in its setup and operation foresees in sustainable opportunities for research, education and awareness, and local employment. Since 2010 ReefGuard has been applied in five large scale field applications (Australia 2014, 2015, The Bahamas 2015, 2016, 2017) (Van Koningsveld et al, 2017; Robijns et al, 2018; Schutter et al, 2018) producing thousands of sexual recruits consistently. In 2017 a first Coral Engine was delivered with several tens of thousands newly settled coral recruits, and hundreds of one year old recruits and fragments. This paper explains the concept of the Coral Engine in more detail and shares perspectives for future implementation along marine construction projects across the world. Marine infrastructure projects thus become opportunities for coral reef rehabilitation with essential financial and logistical capacity on site, and often also a legislative requirement for compensation of harmful environmental impact.

Accelerating the recovery of marine coastal ecosystems is a global challenge that has been attempted on many systems around the world. Restoration efforts have shown varying levels of success at localized-scales, but developing techniques for large-scale application are still in their nascent stage for many systems. For seagrass meadows and marsh plants, large-scale successes have been realized by distributing seeds from moving boats or planes, respectively. Similarly for coral reefs, the harvesting, culturing and releasing of wild coral-spawn slicks to targeted reefs is anticipated to achieve cost-efficient, large-scale restoration of coral communities with low-impact technology. Yet, operational protocols for full-scale application still require development by practitioners. In this study we conducted a field trial to evaluate the actual feasibility of harvesting wild coral-spawn slicks for large-scale restoration activities, incorporating technologies used in oil spill remediation, dredging operations, and land-based aquaculture. Testing the potential for scalability to commercial vessels, our trial focused on concentrating and collecting wild coral-spawn slicks for culturing until settlement competency using an experimental 50,000 L aquaculture facility built on a tugboat. Five objectives were set and all were achieved successfully, with only one requiring further optimization. Overall, this restoration approach allows for long-distance translocation of genetically diverse coral assemblages, and may be combined with other larval conditioning techniques that are being developed to increase the resistance to stress and survival of coral recruits. Most importantly, it is fully scalable to produce billions of coral larvae for delivery to target reefs, with negligible impact to source populations.