Alaska Department of Fish and Game sonar tools

Alaska has been a pioneer in the use of sonar to detect fish in rivers. And in the more than 40 years that ADF&G has used sonar in rivers, its tools and methods have progressed to provide increasingly more detailed information about sonar-detected fish.

Today ADF&G uses three types of sonar technology—Bendix sonar, split-beam sonar and, our latest generation of sonar technology, DIDSON. Today we rely almost entirely on DIDSON. Out of 15 ADF&G sonar sites, 11 sites use DIDSON exclusively, one site uses Bendix sonar exclusively, while three sites use a combination of split-beam and DIDSON.

 
Anchor River Map
>Watching Fish Video

ADF&G uses DIDSON to record ultrasound-like fish video. DIDSON can also be used to size fish and ADF&G biologists have been testing methods of using it to identify salmon by tail beat patterns.

 
Bendix Sonar
>Echo of the Past

Bendix sonar is the first and longest running sonar technology to count fish in Alaska rivers. ADF&F has retired Bendix at all but its Crescent River sonar site, where it counts sockeye.

Split Beam
>Going the Distance

ADF&G uses split-beam at two Yukon River sonar sites and a Kenai River king salmon site. These sites require long-distance fish detection, a task split-beam sonar excels at.

DIDSON
>From Enemy Divers to Fish

DIDSON was developed for military underwater mine and diver detection. Since ADF&G first tested DIDSON for detecting fish in 2002, it has advanced sonar fish detection considerably.

+ Counts echoes
+ Detects fish within ~30 meters of the sonar transducer
X Does not determine fish travel direction
X Cannot be used to calculate fish size
X Does not identify fish by species
+ Produces fish traces
+ Detects fish within ~300 meters of the sonar transducer
+ Determines fish travel direction
X Cannot be used to calculate fish size
X Does not identify fish by species
+ Produces high-resolution fish video
+ Detects fish within ~40-50 meters of the sonar transducer
+ Determines fish travel direction
+ Can be used to calculate fish size
X Does not identify fish by species
 

How sonar technology works
The basics of how sonar finds fish in a river are simple. First a sonar transducer submerged in the river emits a beam of sound waves into the water. When the sound waves encounter an object with a density different than water, such as a fish’s swim bladder, some of the sound waves bounce back to the transducer as echoes. The transducer detects these echoes and fisheries biologists then analyze transducer data to provide information about fish in the river.

Sound Waves
 
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Alaska Fisheries Sonar