Gliding the Oceans Waves - Sailors for the Sea

Gliding the Oceans Waves

By: Charles H. Greene, Ph.D. Director, Ocean Resources and Ecosystems Program, Cornell University | September 11, 2015

for Sustainable Fisheries

From its very beginning, the study of the sea has been conducted from ships. The Challenger Expedition of the mid-19th century is a classic example of men going to sea in ships for years at a time to attempt to unlock the ocean’s secrets. However, as useful as ships are for sampling beneath the ocean’s waves, they are too expensive to run and not enough of them are available to meet the full needs of the ocean science community.

During the late 20th century, satellites were added to the arsenal of tools that ocean scientists could use to expand the coverage of their data collection. However, despite the tremendous insights satellites have provided by enabling scientists to remotely sense the global ocean day after day, satellites have their limits. First, satellites gather data by the square kilometer, which makes it difficult for scientists to see what is going on at a smaller scale. Second, scientists must correlate satellite data with data collected at sea in a process known as ground truthing. Only by having both pieces of information can scientist understand the full picture. Third, satellites can only collect data at or near the sea surface because their sensors probe the ocean with electromagnetic energy, which can only see “skin deep” in the ocean.

At the beginning of the 21st century, scientists began to adopt yet another set of tools in their data-collecting arsenal—mobile robotic platforms, including autonomous underwater vehicles, drifters, floats, and gliders. The data being collected by these sea-going robots are rapidly advancing the abilities of oceanographers to understand the ocean’s circulation and biogeochemistry. They also have the potential to transform the way fisheries scientists study the dynamics of marine populations and ecosystems. A new and exciting way of gathering data by these seagoing robots is being developed with the Liquid Robotics Incorporated (LRI) Wave Glider, which can be used in large numbers to transform the scientific data gathered to help sustainably manage commercial fisheries.

What is a Wave Glider?

The LRI Wave Glider is a self-propelled robot designed for long-term deployments to collect oceanographic and other environmental data. It consists of a surface float tethered with a cable to a submersible glider. The surface float houses the brains for the robot’s communications, navigation, and power systems, while a modular payload hold houses the environmental-sensing instrumentation. The submersible glider has a series of paired wings that generate propulsive forces and a rudder to provide steering. The key innovation of the Wave Glider is its ability to harness wave energy for propulsion and solar energy to power its environmental-sensing systems. The wave glider generates propulsion with each passing wave by taking advantage of the differential motion between the surface float and submersible glider. It generates electrical power from solar panels on the deck of the surface float, which recharge a battery pack inside the Wave Glider’s hold. Two-way communication between the Wave Glider and shore is carried out by cell phone or Iridium satellite, depending on distance from the coast.

The Wave Glider’s performance and versatility at sea make it a reliable robot for collecting ocean environmental data. Its speed through water is proportional to sea state, with higher waves increasing the differential motion between the surface float and submersible glider, propelling the glider more rapidly. The Wave Glider has been found to cruise at speeds between 0.5 and 1.5 knots in light air and calm seas and at speeds greater than 1.5 knots when exposed to waves 2 feet tall or more. Over longer duration missions, speeds tend to average 1.5 knots or higher, even while weathering storms with sustained winds of 30 knots, gusts up to 80 knots, and wave heights exceeding 25 feet. In terms of endurance, the Wave Glider’s unique use of wave and solar energy for propulsion and electrical power enables it to collect data for extended periods of time.

Transforming Fisheries Science and Management

The United States possesses the world’s largest exclusive economic zone (EEZ) and enjoys the benefits of a commercial fishing industry with annual landings valued in excess of $5 billion, according to the National Marine Fisheries Service (NMFS). In addition, these fisheries support more than one million jobs yielding an additional $32 billion to the U.S. economy. Along with these valuable economic benefits, however, come the enormous responsibilities of assessing the nation’s commercial fish stocks and managing them sustainably., which is part of the job of the NMFS in the U.S.

Since the 1970s, fisheries agencies around the world, including NMFS, have adopted ship-based acoustic surveys as their standard method for assessing the status of many commercially important fish stocks. While many advances in acoustic survey methods have occurred during the subsequent 40 years, one major obstruction has prevented fisheries scientists and managers from achieving even greater success. Acoustic stock assessments are conducted from manned survey vessels, and these ships are expensive to both build and operate. Therefore, even as the demand for acoustic stock assessment data has steadily increased, budgetary constraints have often limited the ability of fisheries scientists to keep up with this demand. Such budgetary constraints typically manifest themselves through the reduced availability of ship time. The high cost of building and operating a fleet of ships has resulted in fewer federally funded vessels being built and a steady decline in the numbers of operational days available for stock assessment surveys. Since it can be assumed that these budgetary constraints will continue to limit the availability of ship time into the foreseeable future, fisheries scientists must find a new way to stretch their budgets without compromising the quantity and quality of the stock assessment data being collected.

The Wave Glider Fleet

Recently, we have developed the acoustic technology that will enable NMFS scientists to use Wave Gliders in their stock assessment surveys. Wave Gliders cannot replace ships entirely because trawling and other kinds of sampling are still essential to the work of fisheries scientists, and these activities cannot be carried out with Wave Gliders. However, a fleet of Wave Gliders can supplement the use of ships and greatly improve the quality of the acoustic survey data

To illustrate the potential surveying power of a Wave Glider fleet, we look at an example from the west coast of North America. During 2012, the NMFS cooperated with the Canadian Department of Fisheries and Oceans to conduct an integrated acoustic and trawl survey of sardine and hake stocks along the entire U.S. west coast EEZ and into Canadian waters. With this stock assessment survey requiring over two months of the summer to complete, the expense of ship time alone exceeded $1 million.

For comparison, we explore the relative merits of conducting the acoustic portion of this stock assessment survey using a conventional ship-based approach versus using a fleet of Wave Gliders running the same survey lines. The ship has an operational cruising speed between 10-12 knots in calm seas, but that speed can be reduced in half when encountering rougher sea states, like those more common during seasons other than summer. For illustration purposes, we assume an average cruising speed of 7.5 knots. The cruising speed of a Wave Glider is wave-height dependent and actually increases asymptotically with increasing sea state. We assume an average cruising speed of 1.5 knots, a value consistent with many sea trials under a variety of conditions. Cruising at 7.5 knots, a ship can complete one survey line five times faster than a Wave Glider cruising at 1.5 knots, and it can complete approximately five lines in the time it would take a Wave Glider to complete just one line (Video 1).

However, the power of the Wave Glider fleet comes in numbers. With each Wave Glider in the fleet running a survey line, an acoustic stock assessment of the west coast EEZ could be completed in one week, the same time that a ship would complete approximately 12.5% of the survey. During the eight weeks that it would take a ship to complete an acoustic stock assessment survey of the entire west coast EEZ, a fleet of Wave Gliders would complete eight surveys (Video 2). With ship-time costs between $25-30 thousand per day, manned survey vessels are simply too valuable to be used for collecting only acoustic survey data, an activity often referred to by fisheries scientists as mowing the lawn. Such routine tasks should be left to unmanned robots, while ships are tasked with conducting integrated acoustic and trawling operations as well as other sampling activities that are only possible at present using ships.

Viewing the Wave Glider approach to fisheries acoustics as only an incremental improvement to the way we collect stock assessment data fails to appreciate its full potential. This approach offers an opportunity to transform fisheries science and management in a truly fundamental way. At present, ship-based assessment surveys provide fisheries scientists and managers with what can be thought of as static snapshots of fish stocks often collected at relatively infrequent intervals of a year or more. In addition, because these surveys take so long to complete, the corresponding assessments are in fact highly blurred snapshots, far from the instantaneous, or what scientists call synoptic, ideal typically assumed when the data are analyzed.

In contrast, because of much lower operational costs, a fleet of Wave Gliders would not face the same logistical constraints that compromise the stock assessment data collected by ships, making continuous monitoring of fish stocks conceivable. The resulting large volume of data would offer unprecedented potential for more sophisticated analyses and modeling, potentially entering a new era of greatly improved forecasting skill. This would enable fisheries managers to set quotas that maximize the yields to fisherman while simultaneously reducing the likelihood of overfishing. These data would also enable fisheries scientists and oceanographers to better monitor the responses of different fish stocks to climate change and ocean acidification. The global demand for seafood from a rapidly changing ocean will be staggering when the world population reaches the United Nations anticipated population of 9.6 billion people in 2050. Fisheries science and management will need the best observational and analytical tools available to help society meet this demand!

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