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Alaska Fisheries Sonar

Alaska Fisheries Sonar
Sonar Tools: DIDSON

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ADF&G uses several models of DIDSON transducers and lenses to meet the varying needs of its sonar projects. In rivers where biologists need particularly high-resolution images of fish far from shore, for example, DIDSON transducers are equipped with a high-resolution lens (lower left) as opposed to a standard lens (upper left and lower right). ARIS, the latest model of sonar created by SMC and which was being tested in 2011(upper right), is expected to streamline operations and improve image quality.

DIDSON Technology

DIDSON—the latest generation in sonar technology When the Alaska Department of Fish and Game adopted Dual frequency IDentification SONar (DIDSON) as a tool for detecting fish, inriver fisheries sonar advanced a giant leap forward. ADF&G biologists first encountered DIDSON at the University of Washington, where the Applied Physics Laboratory had developed it as a military tool for detecting enemy divers and underwater mines. DIDSON is now manufactured by the Sound Metrics Corporation (SMC)* and has become the sonar technology of choice for inriver fisheries sonar projects all around the world.

Video Images

Counting fish using DIDSON is similar to counting fish from a counting tower. A technician in a counting tower watches and counts fish swimming in the water below, while a technician watching DIDSON video counts images of swimming fish. But unlike a technician counting fish from a tower, a technician counting DIDSON fish images can slow down, replay and even stop the video if they are fatigued or lose their place. DIDSON can produce high-resolution video images of fish, in part because it uses high-frequency sound waves that enable it to detect the entire surface of the fish. DIDSON is the first inriver sonar to be able to do this. Using DIDSON, technicians can better distinguish between fish that are swimming side-by-side or head-to-tail and determine which direction fish are swimming. Knowing the direction in which fish are traveling is important in rivers where fish tend to mill. Scroll to the bottom of this page to see video samples.

Many little beams of sound

DIDSON emits a fan of sonar beams into the water column horizontally. When emitting 48 beams, DIDSON can be used to detect fish up to 164 feet from the transducer. When emitting 96 beams, DIDSON can be used to detect fish up to 98 feet from the transducer. The resolution of the images DIDSON produces increases with the number of beams used.

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Most sonar technologies—including Bendix and split-beam sonar—use only one beam to ensonify an area of the river. DIDSON uses a fan of multiple beams.

Operating over rough terrain

One of the advantages of DIDSON is that biologists can use it over a relatively uneven river bottom. Prior to DIDSON, biologists could not use sonar over an uneven river bottom without compromising fish detection. DIDSON software allows an observer to subtract the display of the bottom and stationary objects from the screen, leaving only the moving fish images.

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The frame at left shows an image with the bottom and stationary objects displayed while the frame at right shows the same image without them.

Measuring fish size using DIDSON images

Recently, ADF&G biologists have been field-testing DIDSON as a tool to distinguish large king salmon from sockeye salmon based on their size. Biologists cannot yet use sonar to identify fish by species, but can use DIDSON to measure fish length from images of migrating salmon. So far biologists have obtained sufficiently accurate length estimates from DIDSON images of fish swimming up to 98 feet from the transducer.

As another possible means of separating fish species using sonar, biologists are also examining differences in tail-beat frequencies observed in DIDSON video images.

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Marking tools included with the DIDSON software are used to obtain measurements directly from fish images.

Processing large quantities of data

To use DIDSON as a tool for classifying fish species based on size, biologists will need to develop a framework for processing large amounts of data quickly. The DIDSON system generates large volumes of data (more than 30 GB per day for some ADF&G projects). Reviewing data and manually measuring each individual fish is labor intensive and might not be practical for high-density fish passage rates. Consequently, ADF&G is investigating ways to automate fish size measurements.

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Examples of automatic fish sizing software from SMC (left), and Myriax, LTD (right).

DIDSON Video Examples

Seal waits for and catches a salmon:
Hooligan migration:
Seal chases salmon:
Two smaller salmon followed by a large King Salmon: