Sheenjek River
Sonar Tools

Sonar Tools

The Alaska Department of Fish and Game has used the Sheenjek River sonar project to provide daily chum salmon estimates since 1980. ADF&G began sonar operations on the Sheenjek River using Bendix sonar, but has been using DIDSON since 2005.

Each summer sonar site technicians submerge two DIDSON units—one approximately 30 feet from the right bank and a second just off the left bank—and aim them towards the middle of the river. Between the right bank and right bank transducer technicians install a fish lead made of chain-link fence to prevent salmon from swimming behind the transducer where they cannot be detected. Due to deep water and floating debris a fish lead cannot be installed behind the left bank transducer. Instead, the left bank transducer is placed close to shore, where submerged debris and fallen clumps of riverbank block salmon from swimming behind it.

Sonar is used to detect fish at a range of 32 feet from the steeply sloping left bank and at a range of 65 feet from the gradually sloping right bank. Chum salmon migrating past the Sheenjek sonar site swim close to shore. Along the right bank, 84 percent pass within the first 32 feet of the sonar counting range and along the left bank 95 percent pass within 26 feet of the sonar counting range.

Sonar site operations begin August 8 and end September 24, and both sonar systems operate 24 hours per day, seven days a week. Other species of salmon do not migrate past the Sheenjek sonar site during the field season and all sonar-detected fish are counted as chum salmon.