Can we predict the best localities where young corals and fish are going to settle on the reef? In its ongoing effort to conserve Palau’s unique and extensive coral reefs, Palau International Coral Reef Center (PICRC) conducted a study in 2009-2011 to test whether an oceanographic computer model can predict where corals settle onto the reefs. Knowing which reefs are good source of coral “babies” and which ones mainly receives the babies from other places (sink) is important because we need to give special attention to source reefs since they provide the seeds for other reefs. Field researchers surveyed 80 randomly selected sites concentrated around Babeldaob’s main archipelago, its northern reefs, and reefs south to the Rock Islands. The study, entitled Predicting Coral Recruitment in Palau’s Complex Reef Archipelago, was published in the November 2012 issue of the scientific journal PLoS ONE (http://dx.plos.org/10.1371/journal.pone.0050998), an open access journal that is free to everyone in the world.

Previous studies on coral-reef connectivity and recruitment were focused on isolated, individual islands and reefs, and not on a system of reefs such as in Palau. The researchers concluded that when corals spawn on Palau’s reefs, resulting in drifting larvae, the larvae can end up on other reefs in the archipelago. The researchers called these reefs ‘connected’. But some reefs were more ‘connected’ than others. On some reefs, where there are abundant corals, most of the larvae stayed locally. Whereas on other reefs, which had few reefs nearby, and were more isolated, the coral larvae ended up on distant reefs, and were not “self-seeding”. Patterns of transport and settlement were examined using a computer model. “The models predicted where larvae would end up, but most remarkably the model predictions showed lots of larvae in locations where we actually recorded lots of juvenile corals”, said Dr. Yimnang Golbuu, chief researcher at PICRC and lead author of the study. “The model took years to develop”, said Dr. Eric Wolanski, one of the co-authors of the study, who has worked on oceanographic models for nearly four decades. “But this model is extremely exciting”, said Dr. Wolanski, “because in Palau we have real data from the reefs that verified the model’s results – it is no longer merely a computer game, but is now a useful tool for all stakeholders of coral reefs”. “The model can be easily modified and applied to reef fisheries “adds Dr. Wolanski.

The study also determined that Palau’s local reefs have enough coral larvae to sustain themselves, “but in times of stress and large environmental change, the ability to recruit from distant reefs may help ensure their survival”, said Dr. Golbuu. Using satellite altimetry data to reconstruct the oceanic currents in the past, the researchers suggest that recruitment of coral larvae from Yap is the reason that Palau’s reefs bounced back from the disastrous coral bleaching event that occurred during the El Niño ocean conditions of 1997-1998. Although such ‘connections’ may only occur once a decade, they are still very important. This work also reinforces the importance of PICRC's local and regional watershed work to coral reef management, as efforts must protect not only the source and sink reefs, but the water quality of corridors of transport that connect them. Reducing runoff and sedimentation helps maintain the connection among reefs, protecting recruitment processes essential to a legacy of thriving reefs for future generations.

This study contributes to the national agenda to establish a resilient Protected Area Network (PAN) for Palau’s coral reefs that will survive human impacts and climate change. The information generated will help identify potential additional sites for inclusion into the PAN so that Palau’s reef ecosystem remains robust and resilient.

Our researchers accompanied a visiting oceanographer, Eric Wolansky, to the field to look at the currents around a known grouper aggregation site. They will use the current information to develop a computer model for the site, which will be useful for testing research hypothesis.