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From June 30 to July 3, 2003, coral-reef-project scientists from the U.S. Geological Survey (USGS) conducted an experiment to investigate one possible cause of large variations in the amount of new reef production in adjacent areas around West Maui, Hawai'i.

A map showing the edges of three islands and the seafloor, lines show the paths that floating instruments followed.
Tracking the drifters: A representative track plot of two drifters released near West Maui, Hawai'i. The track in red on the left is from a drifter with the drogue attached directly to the surface float. That drifter was released at 10:14 p.m. local (Hawai'i standard) time June 30 and landed on Shipwreck Beach about 21 hours later, at 7:23 p.m. July 1. The track in yellow on the right is from a drifter with the drogue at 1-m depth; it was released at 10:11 p.m. July 2 and retrieved about 39 hours later, at 1:20 p.m. July 4.

Watch an animation of the drifter deployments on all 4 days of the experiment - 16.5 MB .mpg file

The goal was to determine whether the areas of poor growth are simply not getting enough new coral-larvae recruits because of local coastal-circulation effects.

The experiment studied the spawning of the reef-building coral Montipora capitata, locally known as "rice coral," because its unique reproduction cycle makes it particularly suitable for study and it is one of the primary reef-building corals in the region.

Montipora capitata in Hawai'i reproduce each summer by synchronizing their production of eggs and sperm shortly after the new moon during spring tides.

Bundles of eggs and sperm are simultaneously released into the water by millions of individual coral organisms just after sunset for approximately half an hour.

The bundles rise to the surface, break apart, and then rely on chance encounters with the eggs and sperm from other bundles for fertilization. Once fertilized, most of the eggs settle to the bottom, generally within 2 to 4 days, and attach to hard substrate to begin the creation of a new coral colony.

To track the water containing the Montipora capitata sperm and eggs as it moved during several days from its origin above a healthy reef, a system of current drifters with an integrated real-time tracking system was developed by the USGS. The system is capable of continuously tracking multiple drifters, each following individual water masses at particular depths over a wide area.

Plastic floating instruments in the ocean, one viewed above water, the other viewed from underwater.
Drifter deployed: View of a drifter from above (left) and from below (right). Photograph on right shows the underwater fin assembl,y or drogue.

The drifter's outer shell was designed to minimize its height above the water (or "sail area") and was attached either directly or by cable to a subsurface fin arrangement, called a drogue, that towed the drifter along with the water in which it floated, with little effect from surface winds. The entire drifter assembly weighed approximately 30 lb (13.5 kg) and was manageable by hand from a small boat.

To continuously monitor the track of the drifters with sufficient resolution anywhere within the Pailolo and Auau Channels, each drifter was outfitted with a global positioning system (GPS), a 900-MHz spread-spectrum data radio, and a microcontroller circuit created specifically for the project.

The GPS was WAAS capable (WAAS, or Wide Area Augmentation System, is a system of satellites and ground stations that provide GPS signal corrections and improve position accuracy), and it reported positions with an accuracy of 4 m or better.

Using the data radios, the drifters could communicate directly with the base station or, when farther offshore, by radio repeaters deployed on both Lanai and Molokai. The microcontroller removed redundant information supplied by the GPS, added an ID unique to each drifter, and controlled system timing to eliminate the possibility that two or more drifters would transmit simultaneously.

At the base station, a laptop computer was configured to receive the drifter data over the radio network and graphically display the drifters' progress. The real-time display was necessary to dispatch a recovery team in the event that a drifter landed on shore or appeared to be heading out of the study area or out of radio range.

For the display, a commercial geographic-information-system (GIS) package was modified to enable it to automatically update icons representing the drifters on the interactive map. A "trail of bread crumbs" was displayed for each drifter to facilitate tracking and forecasting the progress of the drifters. 

The drifters were successfully deployed on all four nights of the experiment and indicated that the water containing coral larvae did, indeed, reach the areas of reef off West Maui showing decline. Thus, the cause of their poor condition must be looked for elsewhere.

Additionally, the feasibility of inter-island "larval seeding" was demonstrated when three of the drifters made the channel crossing from Maui to the north Lanai reef flat within the timeframe that the larvae are still viable and in the water column.

The three drifters landed within 2 mi of one another on what is called "Shipwreck Beach." This evidence that viable larvae can drift from island to island confirms the concept that Hawaiian reef systems are an interconnected ecosystem—what happens to the reef on one island may have unanticipated effects on the reefs of neighboring islands.

The drifters and tracking system provided us with an extremely valuable new capability that enabled us to gain insight into the transport of coral larvae spawned off West Maui, Hawai'i. Building on the experience gained through this experiment, we will be able to further improve the system and provide the USGS with an even better system to track the dispersal and fate not only of larvae but also of other substances transported in the water column, such as sediment, nutrients, or contaminants.

The drifter design team consisted of Tom Reiss, responsible for the mechanical engineering, and Gerry Hatcher, responsible for the electrical and software engineering. The USGS drifter-experiment chief scientist was Mike Field. Curt Storlazzi was lead oceanographer. Joshua Logan was drifter technician.

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