Coral restoration – a systematic review of current methods, successes, failures and future directions

Coral reef ecosystems have suffered an unprecedented loss of
habitat-forming hard corals in recent decades. While marine conservation
has historically focused on passive habitat protection, demand for and
interest in active restoration has been growing in recent decades.
However, a disconnect between coral restoration practitioners, coral reef
managers and scientists has resulted in a disjointed field where it is
difficult to gain an overview of existing knowledge. To address this, we
aimed to synthesise the available knowledge in a comprehensive global
review of coral restoration methods, incorporating data from the
peer-reviewed scientific literature, complemented with grey literature and
through a survey of coral restoration practitioners. We found that coral
restoration case studies are dominated by short-term projects, with 60% of
all projects reporting less than 18 months of monitoring of the restored
sites. Similarly, most projects are relatively small in spatial scale,
with a median size of restored area of 100 m2. A diverse range of species
are represented in the dataset, with 229 different species from 72 coral
genera. Overall, coral restoration projects focused primarily on
fast-growing branching corals (59% of studies), and report survival
between 60 and 70%. To date, the relatively young field of coral
restoration has been plagued by similar ‘growing pains’ as ecological
restoration in other ecosystems. These include 1) a lack of clear and
achievable objectives, 2) a lack of appropriate and standardised
monitoring and reporting and, 3) poorly designed projects in relation to
stated objectives. Mitigating these will be crucial to successfully scale
up projects, and to retain public trust in restoration as a tool for
resilience based management. Finally, while it is clear that practitioners
have developed effective methods to successfully grow corals at small
scales, it is critical not to view restoration as a replacement for
meaningful action on climate change.