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Alaska Fisheries Sonar
Sonar Tools: DIDSON
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.
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.
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.
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.
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.