Dena Deck, July 20, 2006

NOAA Teacher at Sea
Dena Deck
Onboard NOAA Ship Hi’ialakai
June 26 – July 30, 2006

Mission: Ecosystem Survey
Geographical Area: Central Pacific Ocean, Hawaii
Date: July 20, 2006

Science and Technology Log

Because of their remoteness, the largely uninhabited, and dynamic ecosystems of the Northwestern Hawaiian Islands are often thought of as pristine environments. A more accurate term is “near-pristine,” because this isolated archipelago acts as a giant filtering comb in the middle of the Pacific Ocean, picking up debris that float from afar. Signs of trouble are not immediately apparent to the casual observer, but a closer look reveals ghost nests (discarded or lost fishing nets) caught on the reefs, debris of all sorts on the beaches, and plastics inside the skeleton of albatrosses. Even here, pollution has left its mark. But it is not an indelible mark, and there are devoted groups working hard to erase it from the map of the Northwestern Hawaiian Islands.

The majority of the debris that accumulates on these islands is fishing gear, and lots of it. In addition to destroying the coral upon which it settles, derelict fishing gear can also cause entanglement for the highly endangered Hawaiian monk seal, the threatened green sea turtle, fish, invertebrates, and seabirds.

The Northwestern Hawaiian Islands are in the path of the North Pacific Subtropical Convergence Zone, which stretches from Japan to the West Coast of the US. This zone is a large, shifting line that is the product of ocean currents and wind interactions where areas of the surface waters meet. The convergence of different water masses result in the aggregation of trash, being carried by each water mass and deposited along this zone. The subtropical convergence zone can actually be observed from a thousand feet in the air as a semi-continuous line of trash, earning its nickname as “East Pacific Garbage Patch.”

The group charged with the removal of these pieces from these remote atolls and islands is the Marine Debris Project, part of the Coral Reef Ecosystem Division, under the NOAA Fisheries, Pacific Islands Fisheries Science Center. The group recently completed a large-scale project over the course of the last five years (which just ended in 2006) to remove as much of the derelict fishing gear in the Northwestern Hawaiian Islands as possible. Going out on missions stretching up to four months at a time, two liveaboard mother vessels would carry eight divers each. As the seasons change, the subtropical convergence zone can be observed performing an annual dance in the North Pacific Ocean ballroom. When in the winter this line shifts south and passes the Northwestern Hawaiian Islands around January and February, the large, fractured atolls become a giant comb, trapping these floating debris from all around the North Pacific rim. During the summer, this convergence zone shifts north again.

After reaching a predetermined location, the Marine Debris Project has two methods for covering an area. In shallow reef areas, they snorkel, with the boat nearby. In deeper areas, they do “towboarding,” which involves a small board attached to the boat, which is running on average of between 1-2 knots. Inhaling deeply, and with a quick maneuver of the board, the free diver pairs can then go to the bottom, covering it in a serpentine fashion. Each diver covers a transect of 7.5 meters apiece, checking both sides of this transect while staying within visual range of the divers on either side. The distance among divers varies according to the visibility, but it’s never more than 15 cumulative meters, or approximately 45 feet, between the combined diver pair.

One of the initial efforts undertaken in 1999 at Lisianski Island and Pearl & Hermes Atoll recovered 14 tons of derelict fishing gear. Most of this gear came from trawl netting, followed by mono-filament gillnet, and maritime line. This effort also showed this gear to affect the coral reef ecosystem of the Hawaiian Archipelago (Donnohue et al., 2001). To ensure that the whole swath is observed, divers take a daily visibility measurement by placing a small piece of net underwater, and determining from how far away this net can be seen. Surveys are then conducted, with a slight overlap among each swath to ensure full coverage. When derelict debris is found, they release the board, go to the surface, and raise their hand. At this point, the towing boat turns back, obtain latitude and longitude with a GPS unit, and help the diver retrieve the fishing gear.

The Marine Debris Project, having completed their focused clean up activities on the Northwestern Hawaiian Islands, has now entered into a maintenance phase. This will help them estimate the accumulation rates at repeated zones, which will allow them to determine the frequency of future clean up efforts, and the amount of funds needed. All of this to ensure that trash does not become constant stain in an otherwise vibrant and healthy environment. Since 2002, about 200,000 lbs of net have been recovered this way each year. Many of these pieces can be retrieved by one or two divers, but occasionally a particularly large net is found. One particular net had a weight in excess of 5,000 lbs, and took all divers working together to cut it into sections and pull it out of the water. Over the years, the group has found sharks, sea turtles and monk seals trapped in these nets. One even had a portion of a whale’s spine, apparently having caught the animal in the high seas.

Dena Deck, July 12, 2006

NOAA Teacher at Sea
Dena Deck
Onboard NOAA Ship Hi’ialakai
June 26 – July 30, 2006

Mission: Ecosystem Survey
Geographical Area: Central Pacific Ocean, Hawaii
Date: July 12, 2006

A map integrating backscatter map with bathymetry, showing the seafloor in rich detail
Integrating backscatter with bathymetry, showing the seafloor in rich detail

Science and Technology Log

When soldiers from Napoleon’s army found the Rosetta Stone, it was a breakthrough discovery. Carved in ancient Egypt, it contained pieces of a message in known languages and also a language that had been dead for centuries. Without any link to other known languages, historians had been unable to decipher this language until the stone was found, which provided the necessary clues to translate it. Modern day ocean mappers are looking for their own Rosetta Stone that will allow them to link backscatter data to other ecological information.

A backscatter map, indicating substrate characteristics. Dark areas represent a harder seafloor, while lighter areas are indicative of a soft, sandy bottom.
A backscatter map, indicating substrate characteristics. Dark areas represent a harder seafloor, while lighter areas are indicative of a soft, sandy bottom.

Our ship, the NOAA ship Hi`ialakai, has a set of three sonars that, when used in conjunction, can provide accurate data about the seafloor. When emitted by a sonar, a “ping” comes back bringing two pieces of information with it: travel time and strength. The two-way travel time (the time it took from emission, bouncing off the seafloor and return back to the ship), coupled with the measured velocity of sound in the specific water location where the ship is traveling in, gives mappers a bathymetric view of the seafloor, revealing the depth of each of its points. (See “Painting the Seafloor” article.)

A second piece of data obtained from each ping is the strength of the signal. When sound hits a surface, above water or below, some of it is absorbed and the rest bounces back in what we experience as an echo. The strength of this echo depends on the hardness of the material that the sound is bouncing from. This is a very convenient fact of nature that is used when mapping to compliment the bathymetric map that provides the depth. The acoustic hardness of a substrate, or ocean bottom, affects the strength of the ping coming back to the sonar. In a real sense, the loudness of the echo changes if it is bouncing off sand or rock. Sand, being soft and full of small holes in between grains, will absorb quite a bit of sound. A more solid surface like a rock will provide a bigger echo for each ping that hits it.

A diver armed with a camera is towed from a boat, obtaining many pictures that will be used to groundtruth mapping data.
A diver armed with a camera is towed from a boat, obtaining many pictures that will be used to groundtruth mapping data.

This strength of the signal coming back is called “backscatter” and provides mappers with a second view of the seafloor. While bathymetry is a measure of the depth, backscatter gives us a clue about the nature of the seafloor being mapped. Since coral reefs, with their calcium carbonate, provide a much harder surface than a sandy sea bottom, the two will appear differently in the backscatter map. Values of intensity range from low intensity, showing up as white and representing soft, sandy bottom, to high intensity, represented as dark areas for harder substrate in the backscatter gray scale map.

When the backscatter map shows up binary data – white and black – it is easy to infer on the type of substrate being mapped. The challenge is presented with all of the gray areas in the map. Does light gray represent coarse sand? Is dark gray indicative of sand over rocks, or thousands of coral polyps? Or maybe just rock covered by sand? Every shade of gray has a value that can indicate a type of substrate.

Mapping
Mapping

Backscatter alone cannot give you these answers. With so many variables present in the mapping process, data needs to go through a “ground-truthing” process, or compared to visual observations of the sites. To do this, researchers collect video, photographs and perform actual dive observations of many of the sites that are mapped. These video and images need to be analyzed by a person. It’s a tedious process that cannot be automated – it requires having a person able to classify types of substrate from watching hour after hour of video data or many photographs. And all of these data needs to be “geo-rectified,” or coupled with GIS information to know exactly where each video segment and photograph was taken. Sometimes the payoff for “groundtruthing” backscatter is unexpected: wrecks or rich coral beds can be discovered.

We do not have yet a backscatter “signature” for each type of substrate, or sea bottom, yet. This would be the Rosetta Stone of mapping, a development which will allow mappers to correctly identify some of the ecological characteristics of each area mapped. For instance, mappers are working towards refining their backscatter analysis to allow them to tell apart live coral from bleached ones.

The NOAA Coral Reef Conservation Program has built a pilot data set from the French Frigate Shoals, consisting of large amounts of video footage, observations, and other data. They are in the process of compiling all of this information with their backscatter maps they have for the area, and study how they relate, trying to find meaning to each gray area in these maps.

When mapping, additional and unexpected discoveries can take place. Sometimes what we think of as featureless terrains are revealed to have rich topographies. In 2004, an ocean area off the island of Oahu in Hawai`i, thought to be featureless and plain, was discovered to have sand dunes and ridges, providing important habitat to the marine fauna. Interpretation of backscatter data has improved in quality over the years, and when combined with videos and photographs, remote characterization of sea floor habitats becomes possible.

Dena Deck, July 11, 2006

NOAA Teacher at Sea
Dena Deck
Onboard NOAA Ship Hi’ialakai
June 26 – July 30, 2006

Mission: Ecosystem Survey
Geographical Area: Central Pacific Ocean, Hawaii
Date: July 2, 2006

A NOAA ship using the sonar system.
A NOAA ship using the sonar system.

Science and Technology Log

The first part in appreciating what we have is to know exactly what we have to begin with. Biologists conduct species census in both terrestrial and marine environments, and spend a great deal of time studying each species. But to gain a fuller understanding of an ecosystem, it is also necessary to know the physical characteristics of the environment that provides the foundation for these ecosystems. This is one of the main reasons why we map the seafloor.

The primary goal, as far as mapping is concerned, is to have 100% of all shallow coral reefs mapped. The group mapping the Northwestern Hawaiian Islands is a large team comprised of staff from NOAA Fisheries Coral Reef Ecosystem Division, and the University of Hawai`i. They have an exemplary set of tools at their disposal to do their work. Aboard the NOAA shipHi`ialakai, they employ two sonar systems. Used in conjunction, these sonar systems are slowly giving us a detailed account of the submerged geological features that make up the Hawaiian archipelago.

A bathymetry map showing a 15-meter drop off from several angles. Colors indicate relative depth
A bathymetry map showing a 15-meter drop off from several angles. Colors indicate relative depth

The primary objective of this mission is to produce benthic (sea bottom) habitat mapping of Kure and Pearl & Hermes Atolls. We are filling in a doughnut-shaped gap on both Kure and Pearl & Hermes Atolls, finishing a painting of the seafloor that started several expeditions ago. Coral reefs around the world are receiving increased attention because of the many threats that they face (coastal development, overfishing, climate change), and the U.S. Coral Reef Task Force has produced a number of goals and mandates relating to these ecosystems in America. Among these goals is a call for better management of these resources, and to learn more about them. Mapping all U.S. coral reefs puts Hawai`i at the center stage of this effort with its large chain of islands and atolls stretching across vast distances and volcanic islands found at every stage of geological development, from birth to eventual demise.

The research vessel Ahi operating in Kure atoll. Note the AC cabin to operate the computer equipment required for the sonar.
The research vessel operating in Kure atoll.

The two sonars that we have aboard the ship perform the same task, but each is best suited to work in different conditions. That is because they employ different frequencies which have different rates of penetration. Let’s go back to the analogy of painting a wall. If you have a large wall to paint, you can use a broad brush (or even better, a roller), to cover large areas at every stroke. But within this wall you also have edges that need to be painted more carefully. Let’s say there is a light switch placed in the middle of the wall. Using a painting roller will invariably leave white spaces in between (either that, or you end up also painting the light switch!). So for this light switch, you would use a smaller brush, allowing you to carefully get close to it, eventually covering the entire surface of the wall without painting over it. In this analogy, each light switch in the wall represents an atoll of the archipelago.When a ship maps the ocean floor, it needs to slowly cover swath areas under it. The process is very much like painting a wall with a brush. A wall cannot be painted all at once, of course. The painting is accomplished one stroke at a time, where each passing of the brush needs to slightly overlap the previous one, as to not leave any white spaces in between. When mapping the seafloor, the ship, with its sonar as a giant brush, needs to carefully cover every bit of seafloor surface, as to not leave any area between passes, or swaths, blank and unmapped.

A recently completed bathymetry map superimposed with satellite imagery of Kure atoll. Red indicates lowest depth, and blue deepest. Satellite image has white around edge indicating the exposed reef ring.
A completed bathymetry map superimposed with satellite imagery of Kure atoll. Red indicates lowest depth, and blue deepest. White indicates exposed reef ring.

I am going to use the example of sunlight traveling through water to illustrate the way that the ship’s sonar works, both light and sound are waves.  Sunlight has many frequencies, frequencies that readily break out into all colors by raindrops or a prism. Red color has the highest frequency and, much like the 3002 kHz sonar, is the first absorbed by water. The color red is the first one to disappear underwater. Take anything cherry-colored down a few meters of water, and it will quickly loose all its brilliance, turning into a dull-looking color, an effect that is magnified with the scarcity of light at nighttime. Fish also know this very well. Many fish which are active at night tend to have a red color. Soldierfish, with their large eyes and flame-red color, are perfectly suited for the night environment.Painting over a light switch would mean running the ship into the reef! So mappers have a set of “ paint brushes” in the form of three sonars that allow them to carefully map each area. There is one low-frequency sonar (using a 300 Kilohertz (kHz) frequency) that has a long wavelength that can map between 100 meters (about 328 feet) and 4,000 meters (about 13,421 feet) – this is the ship’s big roller. There is another, high-frequency sonar (using a frequency of 3002 kHz) that can map when the seafloor is less than 100 meters (about 300 feet) from the surface – this is like a mid-sized brush. There is a third sonar, mounted on a smaller, 25-foot research vessel Ahi (which stands for Acoustic Habitat Investigator), with a sonar working at an even higher frequency, which can get really close to the reef – up to places which are 10 meters in depth (quite shallow, at 30 feet). This little boat is like the small brush used to cover areas right at the edge of what needs to be mapped.

Blue, at the other end of the visible light spectrum, has a low frequency. Its large wavelength is the last one to be absorbed by particles in the water, and penetrates deep in the ocean. If you are able to go down deep enough, say 100 meters (328 feet), all around you will look blue. I once went on a submarine ride with my sister, and when we reached 45 meters (150 feet) in depth, the entire inside of the submarine was bathed by blue light. I took her picture and, with no camera tricks, it showed how everything had acquired a sapphire hue (see picture).

When the pings return back to the ship from their very quick trip to the ocean floor, the sonar measures (or “listens”) to how long it takes for them to return, and how strong their signals are. To do this accurately, there are over 100 arrays of receivers in the sonar on the bottom of the ship, each carefully calibrated to listen carefully to each echo of a new ping. The pings coming back carry with them two bits of important information: how long they take (known as the “Two-Way Travel Time”) and the strength of the signal. The time it takes for the ping to return depends on how far it needs to travel (of course!) and how fast sound is traveling in the water.Now, how exactly does sonar work? The sonar unit emits sound, actually given the descriptive term of “ping.” This ping can be at either the low or high frequency described above. After the ping is emitted the sonar unit “listens” for it to come back. Sonar, therefore, has two essential components: the first one that emits the ping, and the second one that listens for it coming back from the seafloor. This is because when sound hits a surface, some of it is absorbed while the rest bounces back. (If you have many large walls around you, you can hear almost all of your sound coming back at you, this is the echo you hear.) The denser, and flatter the surface, the more of the sound that bounces back.

Adding a bit of complexity to this process, the speed of sound is not immutable like the speed of light. In water, it depends on the temperature of the water, its salinity, and depth (all of them affecting the density of water), so careful and constant measurements need to be taken regularly. A large array of devices, collectively known as CTD, are routinely lowered from the main ship into the water. Armed with this information, and by carefully measuring the time it took for the ping to complete its travel, we can know how far each ping had to go. If you do this many times over, you have something called “bathymetry,” a picture of the seafloor.

Putting together shallow and deep water mapping, we soon end up with a seafloor that has been completely painted, full of colors representing depths. An accurate map is essential as a base layer upon which other information can be overlaid, such as bottom cover type – coral, rocks, sand, etc. Mapping, combined with bottom characterization allows us to monitor long-term trends and changes in the marine habitat. This long-term observation is an essential tool for management of the resources. It can serve as one of the indicators for the effectiveness of the conservation efforts, allowing us to make “sound” management decisions.