A DIDSON-LR (Sound Metrics Corporation, Kenmore WA, www.soundmetrics.com ) was operated in parallel with visual observations to validate reliability of the instrument to enumerate fish passage in an open river. We found that even when operated at its lower frequency (0.7 MHz), the DIDSON-LR provided images of sufficient resolution to distinguish passage of fish (salmon/steelhead) at distances up to 40 m (the limit of our tests). However the level of resolution was limited, and that to distinguish between salmon/steelhead (60 to 100 cm) and large (20-50 cm) largescale suckers (also present at the counting site), the observer had to rely on both the relative size of the image and a difference in swimming pattern between species. The DIDSON-LR provided no means to distinguish between species with overlapping size ranges (Chinook salmon and steelhead). When operated at its higher frequency (1.2 MHz) the DIDSON-LR provided images of improved resolution, although the maximum distance at which it could be operated was with a 20 m Window Length and a Start Length up to 13 m.

Funding for this project was provided through a grant from the Pacific Salmon Commission - Southern Fund.

DIDSON-LR video clips (.mov videos with file size listed in parentheses)
Note: Following each description, the information on the DIDSON-LR software settings is provided in parentheses:
Frequency: HF = high (1.2 MHz), LF = low frequency (0.7 MHz)
Window settings: Start Length/Window Length (m)
Frame Rate: #1/#2 fps - #1 = frames per second (fps) at which the recording was made; #2 = fps of the playback clip

• LF & Target salmon and parr (2.2 MB)
  Dragging a tethered coho salmon back and forth across a hatchery raceway – note clouds of “sparkling” produced by reflections from numerous salmon parr which flee approach of the target fish. (LF, 20/20 m, 5/30 fps)
• LF & Target salmon (6.3 MB)
  Dragging a tethered coho salmon back and forth across a hatchery raceway (between 34 to 40 m) – note approach of 5 salmon and reaction of a cloud of parr near end of clip. (LF, 20/20 m, 5/30 fps)LF & 40 m Window Length – Three successive clips of the upstream migration of 2, 1 then 4 salmon/steelhead in three successive clips. (LF, 0.8-2.5/40 m, 4-5/20 fps)
• LF & 20 m Window Length (4.3 MB)
  Five successive clips of the upstream migration of 6, 4, 2, 1 then 2 salmon/steelhead. (LF, 16.7/20 m, 4-5/20 fps).
• HF & 20 m Window Length (3.7 MB)
  Five successive clips of the upstream migration of 2, 1, 1, 1 then 1 salmon/steelhead. (HF, 7.5/20 m, 4-5/20 fps).
• 20 m Window Length & Background Subtraction(7 MB)
  Upstream migration of 2, 1 then 5 salmon/steelhead – clip is repeated first without, then with Background Subtraction (LF, 17.5/20 m, 5/20 fps)
• Playback Speed (10.9 MB)
  Four successive repetitions of a clip showing upstream migration of 5 salmon/steelhead, with the playback speed at 1X, 2X, 4X and 8X the recording speed (with playback in Background Subtraction mode). (HF, 7.5/20 m, 6/6, 12, 24 and 48 fps)
• Lights Out (6.8 MB)
  Upstream migration of a total of 7 salmon/steelhead over the 7 minutes following immediately following Lights Out (at 23:10:10). Note “explosion” of movement (mostly upstream) of suckers following the disturbance. (LF, 17.5/20 m, 5/20 fps)
• Largescale suckers (4.4 MB)
  Upstream migration of a small then a large school of largescale suckers during early morning hours - note upstream migration of a salmon with the 2nd school. (HF, 7.5/20 m, 6.20 fps)
• Smallmouth Bass (1.3 MB)
  Apparent movement of a fish (smallmouth bass?) to catch a prey item (crawfish?) off the river bottom (note “puff” of sediment following attack).
• Beavers (4.5 MB)
  Three successive clips of the downstream passage of 1, 2 then 1 beavers – note the significant amount of reverberation signal. (LF, 17.5-18.3/20 m, 5-6/20 fps)
• Walk out to 80 meters (3.9 MB)
  Researcher walks in a zig-zag pattern downstream to 80 m, then back to DIDSON-LR; note sediment stirred up by distrurbance. (LF, 5/80m, 2/30 fps)