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Application of Side Scan Sonar For the Detection of Derelict Fishing Gear
Scientists Utilize Underwater Imaging Equipment And Survey Strategies to Find and Remove Derelict Fishing Gear


Jeffrey June
Vice President

Abby Faust
Senior Surveyor
Natural Resources Consultants Inc.
Seattle, Washington
Crayton Fenn
Fenn Enterprises Inc.
Seattle, Washington
In 1999, the Northwest Straits Initiative (NWSI), a marine conservation organization in Washington state, identified the removal of derelict fishing gear as an important marine conservation issue.

The Puget Sound region, now known as part of the Salish Sea, has a history of more than 50 years of extensive commercial and recreational fishing, resulting in a legacy of lost fishing gear—including gillnets, purse-seine nets, trawl nets, monofilament fishing line, crab and shrimp traps, and various other types of equipment.

Constructed of synthetic materials such as monofilament and lost in a relatively low-energy environment, derelict fishing gear continues to entangle or entrap animals and negatively impact the marine ecosystem years and even decades after loss. Estimates suggest that more than a million marine animals were killed each year by just the derelict nets alone that had been strewn around the seabed of the Puget Sound, many since the 1960s.

In 2002, the NWSI began a program to conduct derelict fishing gear surveys in the Puget Sound and remove the equipment they located, but due to minimal funding, the survey and removal operations progressed slowly. Then, in 2009, the NWSI received $4.6 million in federal stimulus funds from the American Recovery and Reinvestment Act through NOAA. The NWSI program now operates one survey vessel, four full-time dive-recovery vessels and employs approximately 35 people.

The goal of the project is to remove at least 3,000 of the estimated 5,000 derelict nets in Puget Sound by December 2010.

A 600-kilohertz, 50-meter-range side scan sonar image showing a square recreational crab pot (upper) and a one-meter-diameter commercial pot and trailing line.

Adopting a Side Scan Sonar Method
Before the implementation of this program, recreational and commercial harvest divers had been the primary source of reports on derelict fishing gear. In order to complete its goals, the NWSI required a means of cost-effectively surveying large areas for derelict fishing gear.

In the early years of the program, the team was able to use side scan sonar to find crab pots, but finding derelict nets was a more difficult task.

Originally, drop cameras were the main method used for derelict net surveys, but these cameras have a narrow survey swath limited by the field of view, light and water clarity.

In order to more effectively survey for derelict nets, surveyor Crayton Fenn worked with Marine Sonic Technology® Ltd. (White Marsh, Virginia) to adapt their sonar equipment to the needs of the program.

The new side scan sonar system greatly expands the team's ability to discover derelict nets: A drop camera may be able to image a five-meter swath of the seafloor with enough resolution to determine the existence of derelict nets, whereas a side scan swath can be hundreds of meters wide.

Derelict crab trap surveys require a resolution high enough to image one-meter targets, which is best accomplished using a 600-kilohertz towfish operated at a 50-meter range. To characterize larger geologic features that may entangle nets, a 300-kilohertz towfish operated at a 100-meter range is the best choice.

When areas likely to contain derelict nets are encountered, the sonar range is reduced from 100 meters to 50 meters to increase the detail in the image, and the area is resurveyed. The images show scour marks on the seabed where derelict nets were recently removed, and the remaining derelict nets and seabed obstructions likely to snag nets are also evident.

With the vessel in motion at survey speed, the towfish is lowered to an altitude of approximately 10 percent of the water depth off the seabed. For example, if the sonar is operating in water 50 meters deep, the towfish should be approximately five meters above the seafloor. Once the towfish is at the appropriate altitude, the sonar operator adjusts the gain or attenuation to produce the desired image and then calculates the layback and offset of the towfish in relationship to the differential global positioning system antenna.

A 300-kilohertz, 50-meter-range side scan sonar image showing glacial debris, sand waves, remaining derelict nets and scour areas where nets were recently removed.

Challenges of the Side Scan Method
Because of the nature of the surveys, there is a high probability that the towfish could collide with or become entangled in the derelict gear or obstructions on the seabed. The NWSI team employs a Marine Sonic Technology heavy-duty, commercial-grade sonar system, which includes a 57-kilogram stainless steel towfish and clutched hydraulic winch with heavy cable. The Marine Sonic heavy towfish was chosen for its superior survivability.

Another issue that can impact the quality of the side scan sonar images is thermocline or halocline differences in the water column, which affect the sonar's acoustic energy by bending or distorting the sound velocity, degrading the data and reducing the effective range of the sonar. Adjusting the depth of the sonar and reducing the range of the sonar can help solve these problems.

The side scan sometimes sonifies unintended objects in the water column like schools of fish, waves, whitecaps and propeller turbulence. This 'noise' can obscure the seafloor data and conceal the derelict fishing gear targets. Additionally, side scan sonar will not pass through gas-charged materials, such as bull kelp, eel grass, mill foil and other vegetation, which act as a bubble curtain, absorbing and reflecting the acoustic energy and blocking detection of the derelict fishing gear.

Finally, like most tools, the results obtained with side scan sonar vary dramatically depending on the skill level of the sonar operator and the boat operator's ability to stay on course.

Gear Removal Operations
Areas with historically high amounts of fishing activity were chosen as likely derelict fishing gear sites. The Washington Department of Fish and Wildlife conducts overflights to locate and count boats' fishing nets during the salmon fishing season. Twenty years' worth of this data was entered into a geographic information system and plotted over bathymetry and known seabed types. Areas with high fishing intensity and potentially entangling seabed configurations, such as rocky reefs or boulder fields, were chosen.

Once an area has been surveyed and derelict gear targets have been identified, the locations of these targets are georeferenced from the side scan image and entered as waypoints into standard navigation software, such as that of Nobeltec® Inc. (Beaverton, Oregon).

The dive recovery vessels are directed to the derelict gear target and divers using surface-supplied air and a two-way communication system are deployed to depths reaching 32 meters for gear removal.

Divers report the length and width of nets, the type of habitat affected and the species and numbers of entangled animals.

All removal work is done by hand to minimize habitat damage. Divers use serrated knives to cut derelict nets into manageable sections and float the nets or crab traps to the surface with lightweight (30 kilograms) airlift bags.

Biologists on board the recovery vessel then assess the relative age and condition of the net, count live and dead animals and collect specimens for later identification.

Crab traps are returned to owners if in serviceable condition and nets are bagged and disposed of in landfills. Information on each derelict gear item and its impacts are maintained in a database.

Thus far, side scan sonar surveys have located nearly 500 derelict nets and 2,000 derelict crab and shrimp traps. Since 2002, divers have removed 2,835 derelict nets and 1,800 derelict crab traps during 1,100 dive operation days. The derelict nets removed covered more than 5.6 square kilometers of Puget Sound seabed and weighed 200 metric tons.

Nearly 150,000 animals have been found entangled in the derelict fishing gear, including 43 marine mammals, 757 seabirds, 1,777 fish and 148,364 invertebrates.

Some preliminary research on animal catch and decomposition rates in derelict gear indicates that observed animal mortality is only a snapshot of the impact of derelict gear and represents about 14 days of actual mortality. A mortality rate model was designed that estimates the total annual mortality of the 2,835 nets removed at approximately 1.5 million animals per year, including 1,131 marine mammals, 19,891 seabirds, 50,779 fish, and 1.4 million invertebrates. Considering that nearly all of these derelict nets were 'legacy' nets lost during the 1960s through the 1980s, when nearly 3,500 gillnetters fished in Puget Sound each year, the total mortality over the life of these derelict nets is substantial.

The problem appears to be lessening, however. Fewer than 500 net fishers remain active in Puget Sound due to reduced salmon runs and Endangered Species Act restrictions. Equipped with modern navigation and depth sounder equipment, few nets are currently lost each season. Additionally, Washington state has adopted a no-fault reporting system for derelict gear so that any future gear loss can ideally be reported and managed before it is able to cause significant damage to the marine environment.

The state of Washington also plans to support a maintenance removal program in the future to ensure that the seabed remains free of such gear.

Jeffrey June is a senior scientist with Natural Resources Consultants Inc., specializing in natural resource assessments. He has been the field manager of the derelict fishing gear project for the Northwest Straits Initiative since 2002. He received his M.Sc. in fisheries from the University of Washington.

Abby Faust is a senior survey technician at Natural Resources Consultants Inc. She received a B.Sc. in geosciences from Pacific Lutheran University. Faust currently assists in survey database management and has been involved with derelict fishing gear removal in the Puget Sound.

Crayton Fenn is founder and president of Fenn Enterprises, a hydrographic survey company in Seattle, Washington. Crayton specializes in side scan sonar for discreet target identification and high-resolution imagery and has conducted marine survey operations worldwide.

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