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onar has been used to monitor salmon in Alaska streams since the 1960s, but it wasn’t until the introduction of the latest generation of sonar, DIDSON, that we could use sonar to monitor salmon in the Anchor River. Within the confines of a narrow, shallow, rocky-bottomed river like the Anchor River, sonar results in an excessive number of unwanted echoes—a condition we refer to as reverberation. Under these conditions, less advanced single-beam sonar tools (Bendix and split-beam) cannot distinguish fish echoes from background echoes, but with its multi-beam system DIDSON has largely overcome this problem. DIDSON has also overcome a challenge associated with fish behavior. In the Anchor River fish often hold or mill in the current, a behavior that easily confuses single-beam sonar. Single-beam sonar reveals little about fish swimming behavior, which can become a problem when fish don’t behave the way we expect them to. When fish hold or mill in the current instead of passing by, single-beam sonar will count some of them more than once, biasing estimates high. But DIDSON records fish passage with high-resolution, ultrasound-like fish video that allows us to easily distinguish holding or milling fish from fish actively migrating upstream.
To capture the highest-resolution images, DIDSON must be positioned relatively close to the fish. At our Anchor River site we use a single DIDSON transducer with a high-resolution range of approximately 50 feet. We use partial weirs on each bank to direct fish through the sonar beams at close range as they swim up or downstream. Sonar technicians count the upstream and downstream migrating fish while watching recorded DIDSON fish video. All upstream and downstream fish are counted as Chinook and the number of downstream fish are subtracted from the number of upstream fish. We know that some of the downstream fish are steelhead, but their numbers are small compared to the number of Chinook salmon and the bias introduced by steelhead is minimal. Many of the downstream observations are due to milling Chinook. We often see upstream migrating Chinook enter the DIDSON’s field of view and then back down before swimming back up again. As we do at nearly every sonar site in the state, at the Anchor River site we sample over time rather than run the sonar continuously. We record DIDSON video for the first 20 minutes of each hour and then expand the number of fish counted during that period to a full hour. For each day all 24 expanded hours are totaled to estimate Chinook passage.
Improved Monitoring—The Transition from Helicopter Surveys
Before we had sonar to help us monitor Anchor River Chinook, we flew helicopter surveys during peak spawning to evaluate trends in Chinook escapement. Survey conditions and surveyor differences sometimes skewed the results and year-to-year comparisons were imperfect. DIDSON/weir estimates clearly show that helicopter surveys consistently undercounted Chinook salmon. When we first field tested the sonar and weir site in 2003, for example, over 8,000 Chinook salmon were estimated at the site compared with a helicopter survey estimate of 700. In 1999, Anchor River Chinook was listed as a stock of management concern based on the helicopter surveys. However, better information provided by DIDSON and the weir have allowed the fishery to be liberalized.
Period of Sonar Operations
Sonar operations begin between May 13 and May 15, and generally continue until sometime in early or mid-June, after which the site continues as a weir site. We have used DIDSON to monitor Anchor River Chinook during the high water season every year since 2004 except in 2009. In 2009, May water levels were unusually low and we were able to install our weir at the beginning of the field season.