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How do Ocean Currents Connect Coral Reefs among Islands in the Mariana Archipelago, and How will Climate Change Affect Them?

This project began in December 2012 and was completed in August 2016

Coral ecosystems are sustained by young fish and corals born locally, and by those drifting from other reefs on ocean currents. Identifying and protecting sources of young corals and fish supports fisheries and healthy ecosystems. We tracked and predicted the directions that fish and coral larvae travel among reefs in the Mariana Archipelago. Climate change will affect ocean temperature, acidity, and current speed; we investigated how these changes will affect the transport of reef larvae.

Why We Care
Many reef organisms including corals and reef fish have a larval phase during which eggs and young are cast into ocean currents to grow and drift until they can settle on a new reef home. Some reefs are mostly self-sustaining in that offspring stay at the same reef as their parents. Other reefs are more dependent on offspring that arrive from more distant parental reefs. We investigated the exchange of reef fish and coral larvae among islands in the Mariana Archipelago. The Marianas are a string of 15 islands and shallow banks in the northwestern Pacific. Home to two U.S. jurisdictions, Guam and The Commonwealth of the Northern Mariana Islands, the Archipelago is a popular tourist destination for Asian countries and has strategic importance with a large military base.

Tourism economies depend on healthy sources of larvae to maintain the diversity of coral creatures that attract divers. Similarly, fishermen depend upon resilient populations of reef fish, clams, snails, and sea cucumbers that are maintained by larval sources originating upstream as an important part of the local economy and cultural traditions. Planning for sustainable fisheries, setting harvest limits, positioning protected areas, and ensuring resilient reef communities to support tourism, both now and in the future, are all dependent in part upon an understanding of larval sources and destinations.

What We Did
We used three types of information to understand larval connections among islands in the Marianas:

  1. Direction, speed, and seasonal patterns of the ocean currents that carry larvae were mapped using surface drifters tracked by satellite (NOAA Global Drifter Program). Over 1500 drifters have passed through the Marianas region in the last 20 years. Some of these have passed very close by or even ran aground on the Archipelago’s reefs. The paths that these drifters take probably mimic those of passive larvae floating in the ocean and were used to understand possible sources, destinations, and drift times needed to make the trip from one island reef to another.
  2. We used a computer simulation. For this, we used detailed maps of daily currents (HYCOM Hydrodynamic model) and released “virtual larvae” at the dates of known spawning. The computer model moved each larva in the direction and distance specified by the current map each day. We then summarized how many larvae went from one island to another and how long it took them to get there. Based on these simulations we determined which islands are big producers of larvae for sustaining the region’s coral reefs. Fisheries managers and conservation planners can use that information to plan fisheries regulations and marine protected areas.
  3. Climate models predict changes in the chemistry and current patterns of the oceans. Warmer water will make larvae grow faster and mature earlier. The ocean currents moving larvae from one place to another will have different speed and direction in the future. Using computer simulations that represent the future ocean, we compared patterns of larval movement predicted in today’s ocean with those likely to be seen in the next 100 years. Managers can plan fisheries and conservation activities over longer terms in the context of climate change. This component of the project was done in collaboration with Kris Karnauskas at the Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder.We completed all of this work in partnership with the local fisheries and coastal management agencies and researchers in the Marianas to be sure our study met their needs. More detailed information on our methods and partners can be found in our report and publications below.

What We Found
Results from analysis of the drifter paths indicate that the Marianas overall are relatively isolated from neighboring archipelagos. They are simply too far away for many larvae to either arrive from upstream reefs or successfully make it to downstream reefs if spawned in the Marianas. The dominant current in the southern part of the Archipelago is the North Equatorial Current. It flows rapidly westward past Guam, Rota, Tinian, and Saipan and may carry many of the larvae produced there with it on its way to the Philippine Islands. The northern part of the Marianas is characterized by more variable current directions and eddies. This may promote larval retention for these islands.

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