Many corals are symbionts, meaning they live in harmony with another organism for the benefit of both. Zooxanthellae — a type of algae — are symbionts with corals. They live within coral polyps, and through photosynthesis provide their hosts with a large part of their energy resources — in some species more than 90 per cent!

When a coral becomes stressed by heat or light, however, the zooxanthellae will cause corals to expel them, thereby losing a majority of their food source and making survival much more difficult. This process is called coral bleaching.

At the present moment, 75 per cent of all coral reefs in the world have experienced bleaching-level heat stress, and in 30 per cent of reefs, it has reached mortality level. Corals can recover from bleaching if they have enough time to repopulate their algae colonies without recurring stress. If we do not make significant changes now, every reef in the photic zone (the surface layer of the ocean that receives sunlight) will be within bleaching threshold annually, meaning they wouldn’t have enough time to recover.

To address this, the Coral Restoration Foundation is physically “outplanting” coral babies onto reefs. The goal of the CRF is to kickstart a healthy coral population that, once established, should reproduce with each other. Currently, the majority of coral propagation is through a process called fragmentation. Fragments of genetically identical coral are grown on monofilament suspended from PVC “trees.” These trees form a coral nursery, which requires constant maintenance and monitoring.

I spent a couple of days at the nursery cleaning algal and other unwanted invertebrate growth off of the lines and the PVC. As it’s the beginning of the summer, we also pulled the trees down a couple metres to the colder water to alleviate heat and light stress. The CRF has seven different nurseries throughout the Keys, the largest being the Tavernier nursery comprised of 500 trees with different coral species and genotypes. They also have a huge experimental section where different scientific collaborators can test their methods and work in the field.

The idea behind the nursery is to make it easier for coral babies to grow to sizes that can likely succeed on a reef. Like all other ecosystems, coral reefs with higher biological and genetic diversity often do better because they can collectively resist a wider array of stressors. This is why the CRF and similar organizations try to grow many different genotypes of a species found in the wild, and outplant them together at a site, with the idea that reproduction between them will increase diversity. Over the last couple of years, they have had about 80 per cent survival, and some staghorn coral colonies they outplanted two years ago have already reproduced naturally. The extent of survival, however, varies between different species and sites. The CRF works with many collaborators to determine why this is, and how they can use scientific data to be more efficient.