Many people don’t realize that corals are animals living in partnership with tiny plants embedded in their body. These algae are too tiny to see without a microscope, but they provide corals with the colors we see when swimming around, and most important, they provide food for the coral to live.
This unique partnership allows corals to thrive in our nearshore waters, form skeletons, and eventually create the reef structures we see. The problem is that this terrific coral-algae partnership is extremely sensitive to changes in ocean temperature.
The Tropical Pacific Ocean experienced a large El Nino event last year that weakened cool ocean currents traveling from east to west along the equator. This caused parts of the central Pacific Ocean to heat up and exceed temperature limits required for corals to live and grow. The 2016 El Nino resulted in unusually high sea surface temperatures bathing the central FSM, and Kosrae in particular. In turn, these warm waters stressed the coral-algae partnership, and removed many of the tiny algae living inside the coral tissue. Because these algae provide up to 70 percent of the essential nutrition to the coral host, most corals can’t live long without them, and eventually turn white and die. This death process is called coral bleaching because corals turn bright white at first.
Recent surveys around Kosrae found that warm sea surface temperatures led to a 50-75 percent loss in living coral. This substantial decline has left many reefs on the eastern side of the island in a state of shock, with less than 15 percent living coral left in some instances. Reefs on the west side of the island fared better because coral species living there are more tolerant of the warm waters. Disturbances are a natural part of life — we get the flu, for example, and a week later we usually recover. The recovery process for coral reefs is longer, as seven-to-10 years are required for corals to re-grow based on studies from other islands.
Understanding the recovery process is key for managing the future of our reefs.
Immediately after corals turn white and die, opportunistic turf and macroalgae start to grow on the dead skeletons. These larger forms of algae are typically held in check by herbivorous fish that eat them. However, the recent bleaching event caused a rapid increase in their abundance. In response, our studies documented a near doubling of herbivorous fishes in Kosrae. The growth response of these herbivorous fishes is essential to the recovery of the reefs over the next several years, as they can keep algae in check and allow for the corals to recover.
Thus, it is important to ensure that herbivore populations remain high for the next few years. Fishermen may be happy to see the increase in fish following the coral bleaching event, but taking too many herbivores will slow down or even stop the recovery process on the reefs. Consultations will soon begin with communities in Kosrae to improve the management of their coral-reef fisheries. We hope the current bleaching event in Kosrae will be kept in mind throughout this process, so that our reefs and fishes can provide for our livelihoods for years to come.
Contributors to this story: Dr. Peter Houk, Selino Maxin, Steven Palik, and Marston Luckymis (University of Guam Marine Lab, Conservation Society of Pohnpei, Kosrae Island Resource Management Authority, and Kosrae Conservation and Safety Organization)
Read more about this in the July 7, 2017 edition of the Marshall Islands Journal.