Coral reef ecosystems are among the most biologically diverse and complex marine ecosystems in the world. Corals provide a bio-calcified foundation that serves as habitat for multitudes of fish, invertebrates and algae, constituting a network of interacting species responsible for the overall health and functioning of the reef. Elimination or reduction of species due to global climate change, pollution, and/or unsustainable fishing practices upsets the intricate balance of biological interactions and endangers the resilience of the reef. Sadly, most coral reefs worldwide are experiencing some combination of stressors, with the result of loss of biodiversity, function, and ecosystem services that much of the world’s human population relies upon.
Restoration attempts of degraded reefs are complex and require restoring all components of a healthy reef community. As the highest mortality rates for reef organisms occur at the youngest stages, one strategy involves nurturing and protecting small corals, fishes and invertebrates through these young stages and releasing them onto the reef once they have grown into larger, hardier life stages. This is the approach we call “Holistic Reef Replenishment,” and it is currently underway on the atoll of Tetiaroa in French Polynesia.
Most tropical fish and invertebrates have a larval stage during which they spend weeks to months drifting in the open ocean. When it comes time to settle and metamorphose into juveniles, larvae smell and hear their way to the reef, and many cross the reef into the lagoon during dark nights with new moons when they are less visible to predators. However, settlement to the reef is still the most dangerous phase of the life cycle with mortality rates for larvae as high as 90%!
To collect fish and invertebrate larvae, crest nets are set up on the reef to catch larvae as they cross the reef in waves into the lagoon. The nets passively catch larvae indiscriminately of species. Because they catch larvae prior to the majority of mortality, and the larvae are carried in the waves which reduces the amount that they can avoid the net, these nets provide a good estimation of larval supply which is otherwise difficult to estimate. The larvae are collected and returned to Tetiaroa Society’s Ecostation where they are sorted by species, documented and nurtured in aquaria for weeks to months, depending on species. They are then released onto the reef. In addition to increasing the survival of larvae (many of which would have died naturally entering the lagoon to predation), raising larvae in the lab also enables researchers to classify larval fish stages, which are notoriously difficult to identify as they often look nothing like their adult selves! To date, 51 species have been collected and identified, the majority of which are surgeonfish and groupers, but also include goatfish, eels, crabs, shrimps and blennies, among others.
To compliment the fish and invertebrate replenishment of Tetiaroa’s lagoon, the Holistic Reef Replenishment project is also restoring corals on degraded reefs. Corals are raised from fragments in protected coral gardens, and then placed onto degraded reefs where hopefully they thrive. Tetiaroa suffered from a bleaching episode in April of this year, and so the corals that are re-planted are expected to help the reef recover from this disturbance. By restoring the foundation of the reef and the associated organisms that play important roles in the reef ecosystem, we are ensuring a higher level of success for the overall reef than if we were just restoring fishes, or replanting corals.
Associated species play many important roles on the reef. Crabs, fish and shrimps clean and protect host corals, and provide nutrients via their excretions. In turn, they gain a safe place to live and can eat mucus secretions from corals. Crustose coralline algae serve as a recruitment cue for coral larvae and help cement the reef together, as do other algae which are also an important food source for many organisms on the reef. Sea cucumbers clean the sediment, and worms filter out particles from the water column. Anemones provide protection for fish and shrimp and feed others with their sloppy feeding. Even crown-of-thorns sea stars, by eating corals, play an important role in clearing space for settlement of new coral recruits. There’s a whole web of life on a reef, a dynamic community creating a productive livable space in a sea of nutrient-poor water that couldn’t otherwise sustain such life. Its an interacting ecosystem that relies on all its parts to function.
This reef restoration project is being conducted in a newly created marine preserve no-take zone in the Tetiaroa lagoon, ensuring that replenished stocks are not simply fished out. Success in increasing the abundances of reef organisms in this zone will lead to a spillover effect in the lagoon adjacent to the reserve, increasing the fish stocks available to local fishermen and their families. In this way, this project represents a holistic approach to reef restoration, not only respecting the importance of all members of the reef ecosystem, but also the human component of reef ecosystem services by protecting and creating a resilient reef that can support local subsistence fishing, and support tourism by restoring the natural beauty of the reef for snorkelers and divers.
The Tetiaroa Reef Replenishment Project is a collaboration between Tetiaroa Society and the Centre de Recherches Insulaires et Observatoire de l’Environnement (CRIOBE), and is supported by Biotherm Water Lovers, Mission Blue, and Leonardo DiCaprio Foundation.
by Hannah Stewart, Chief Scientist of Tetiaroa Society