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

Project Status: This project began in December 2012 and is Ongoing

Coral ecosystems are sustained by young fish and corals born locally, and by drifters on ocean currents from other reefs. Identifying and protecting sources of young corals and fish supports fisheries and healthy ecosystems. We are tracking and predicting 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 are investigating 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 are investigating 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 harvest from 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 vibrant 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 Are Doing

We are using 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 are 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 can be used to understand possible sources, destinations, and drift times needed to make the trip from one island reef to another.
  2. We are using a computer simulation. For this, we use detailed maps of daily currents (HYCOM Hydrodynamic model) and release “virtual larvae” at the dates of known spawning. The computer moves each larva in the direction and distance specified by the current map each day. We then summarize how many larvae went from one island to another and how long it took them to get there. Based on these simulations we can determine 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. More acidic water will make larvae less able to build their skeletons. 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 can compare patterns of larval movement predicted in today’s ocean with those likely to be seen in the next 100 years. Managers can then plan fisheries and conservation activities over longer terms in the context of climate change.

    We are doing all of this work in partnership with the local fisheries and coastal management agencies and researchers in the Marianas to be sure that our study will meet their needs. More detailed information on our methods and partners can be found in our project work plan.

    What We Found
    Preliminary 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. So far these results are based on analysis of drifter data only.

    Next Steps
    Together with local researchers, fisheries managers and conservation agency partners, we will be:
  • Identifying key species of interest to be the focus of new computer simulations. These may include endangered or threatened corals, important fisheries species, or those with a strong cultural value.
  • Teaching local scientists and managers how to set up and run future computer simulations on their own.
  • Compare patterns of larval movement in the ocean today to those predicted for the future.

Regions of Study: Pacific Ocean - Western, Guam, Pacific Islands

Primary Contacts: Matt Kendall, Matthew Poti

Research Themes: Climate Impacts ( Impacts of Changing Temperature and Hydrology, Impacts of Ocean Acidification) • Science for Coastal Ecosystem Management (Marine Spatial Planning, Protected Species, Coral)

Related NCCOS Center: CCMA

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