Outplanting optimized: developing a more efficient coral attachment technique using Portland cement

Coral reefs are among the most valuable and vulnerable ecosystems on Earth. Their decline has spurred global interest in efforts to augment native coral populations through coral gardening. As these efforts expand, practitioners are constantly looking for new techniques to reduce costs and increase their restoration footprint. However, commonly employed coral attachment methods limit the numbers of corals that can be outplanted per day, representing a substantial bottleneck in the coral restoration process. Cement has potential as a more cost- and time-efficient coral attachment technique, but research is needed to understand its effects on coral survivorship and develop best practices for its use. Here, we use lab and field tests in a three-stage elimination format to determine the most effective cement mixture for outplanting Acropora cervicornis. We then compare this new method to two commonly used coral attachment techniques: the nail and cable tie method and two-part epoxy putty. Our tests identified the optimal cement mix to be a combination of 10 parts type I/II Portland cement to one part silica fume. This mix yielded equivalent survivorship to the other two methods, is quick and easy to use making it ideal for citizen scientists, and has roughly one-tenth of the material cost of other methods. These results support the wider use of cement for coral outplanting in order to minimize costs, maximize efficiency, and increase the effectiveness of coral restoration efforts around the world.