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Alaska Fisheries Sonar

Alaska Fisheries Sonar
Sonar Tools: Split-Beam

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A split-beam sonar echogram (right) represents the echoes returned from each fish as a series of dots referred to as a fish trace. Technicians count echogram fish traces and biologists then use their counts to produce an estimate of abundance.

Split-Beam Sonar

In the 1990s the Alaska Department of Fish and Game expanded its sonar tool kit with the introduction of split-beam sonar. Split-beam sonar is technically complex to operate, but biologists can use it to collect information about a fish’s three-dimensional position and travel direction to distinguish upstream-migrating fish from downstream-moving fish and debris. Biologists can also use split-beam sonar to collect information about fish size to help separate large salmon species from small salmon species. Finally, biologists can use split-beam sonar to detect fish swimming far from shore.

When split-beam sonar is the right tool for the job

Today biologists can more easily and accurately collect information about fish position, travel direction and size using DIDSON. But split-beam sonar is still the best tool available for detecting fish at long ranges. While biologists can use DIDSON to detect fish out to about 150 feet from the transducer, they can use split-beam sonar to detect fish out to about 1,000 feet from the transducer. Three ADF&G sonar projects require long-range detection of fish and use split-beam sonar—the Kenai River king salmon sonar site and two sites on the Yukon River the Pilot Station sonar site and Eagle sonar site.

Using split-beam sonar as a tool for distinguishing fish by size

Only the Kenai River king salmon sonar site uses split-beam sonar to collect information on fish size. This information helps biologists at the site distinguish large king salmon from sockeye salmon. Biologists use split-beam sonar to provide information about fish size by analyzing what is referred to as Echo Length Standard Deviation (ELSD). See Understanding ELSD (PDF 150 kB) for more information about how biologists process and use ELSD. Starting in 2007 the Kenai River king salmon sonar project began testing DIDSON as a possible replacement for its split-beam sonar system and collected its first year of comparative split-beam and DIDSON data in 2011. DIDSON provides more accurate size information and the latest DIDSON technology may meet the site’s range requirements.

Getting into the water

To deploy a split-beam sonar transducer, sonar site technicians mount the transducer to a tripod and submerge it near shore. They then aim the transducer sonar beam offshore and perpendicular to the river’s current along a section of the river bottom with a smooth, uniform slope. They must aim the transducer beam close enough to the river bottom so that fish cannot swim below it undetected, but high enough so that contact with river bottom does not disrupt detection of fish echoes with acoustical noise.

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Numerous overlapping fish traces can be difficult to distinguish and count. Originally, fish traces were printed on paper charts (left). Today split-beam sonar data are collected and processed on computers (center and right). Technicians still need to distinguish and count individual fish traces, but several software tools have helped streamline the process.