Salmon Counting · Episode 5 of 8 · 48 min
Imaging Sonar: Counting Salmon in Pitch-Black Water
When the water is too muddy, too deep or too dark to see, sound takes over. A transducer sends sound into the river, a fish's swim bladder bounces it back, and an 'acoustic camera' turns the echoes into high-resolution video in pitch-black glacial water. The episode traces the lineage from early echo-counters through split-beam sonar to DIDSON and ARIS, and the controversy when a river's king-salmon numbers are questioned after a switch to sonar.
Where it happens
- Kenai River, Alaska+1 more episodeKing-salmon sonar site whose 2013 count drew scrutiny
- Yukon River, Alaska+3 more episodesWide river where split-beam sonar is still used
Timeline
- 1980sAlaska begins deploying Bendix sonar counters.
- 2006First rigorous validation of DIDSON counts against a fish-counting fence.
- 2012ARIS first tested in Alaska.
- 2013Kenai king-salmon sonar moves to River Mile 14; the early-run count of 2,038 becomes a flashpoint.
- 2017Kenai king management shifts to large-fish escapement goals.
Still debated
When a river switches from a weir, tower or older sonar to imaging sonar, the estimate can shift, as the Kenai's 2013 early-run king count showed against weir counts on tributaries.