Salmonid Field Protocols Handbook: Techniques for Assessing Status and Trends in Salmon and Trout

Hydroacoustics: Rivers

Suzanne L. Maxwell

doi: https://doi.org/10.47886/9781888569926.ch9

Hydroacoustic methods are typically used to assess abundance in migrating fish populations when other methods are not feasible (i.e., the river is too wide for weirs or too turbid for observation towers). In many instances, hydroacoustic systems may be preferable to more intrusive devices such as nets or traps. This protocol addresses the use of hydroacoustic systems in rivers from fixed, nearshore positions, although down-looking, mobile methods have also been used to assess migrating fish populations (Xie et al. 2002).

Several types of sonars have been used to assess fish populations in rivers. The simplest was the single-beam sonar. In the early 1960s a single beam, echo counting system—a Bendix counter—was developed to enumerate adult sockeye salmon Oncorhynchus nerka. The Bendix counter has been an important tool for in-season management of predominantly sockeye salmon and chum salmon O. keta in many commercial fisheries in Alaska (Dunbar 2001, 2003; McKinley 2002; Westerman and Willette 2003; Dunbar and Pfisterer 2004). Dual-beam systems first provided target strength information for individual targets, allowing quantitative estimates of fish abundances using either echo counting or echo integration procedures in the late 1960s and early 1970s (Dragesund and Olsen 1965; Craig and Forbes 1969; Forbes and Nakken 1972). Dual-beam systems have largely been used for lake surveys to assess both adult and juvenile fish (Schael et al. 1995; Vondracek and Degan 1995) but have also been used at fixed, nearshore positions to assess migrating adult salmon in rivers (Gaudet 1990; Enzenhofer et al. 1998; Pfisterer and Maxwell 2000). Later, split-beam systems (see Figure 1) were shown to provide more accurate information on fish position—and thus a more accurate measure of target strength. Split-beam systems are currently used to enumerate migrating adult salmonids in several rivers (Daum and Osborne 1998; Miller and Burwen 2002; Xie et al. 2002). Most split-beam systems are used at sites where fish are relatively spread out and fish passage estimates are relatively low (i.e., less than 2,000 fish/h). Sockeye salmon runs tend to be large and concentrated in time, and split-beam sonars were found to do a poor job of assessing sockeye salmon at passage rates higher than 2,000 fish/h (Biosonics Inc. 1999a, 1999b). Echo-counting, dual-beam, and split-beam sonar methods, along with basic sonar information, are described in Brandt (1996) and MacLennan and Simmonds (1992).

A new sonar—a dual-frequency identification sonar (DIDSON) (see Figure 2)—was recently tested in Alaska and found to work well for enumerating fish at high passage rates (i.e., more than 2,000 fish/h) (Maxwell and Gove 2004). The DIDSON is a high-frequency, multibeam sonar with a unique acoustic lens system designed to focus the beam to create high-resolution images (Belcher et al. 2001, 2002). The evaluation of the DIDSON included comparisons of sockeye salmon counts from the DIDSON, Bendix sonar, and split-beam sonars against visual observations in a clear river; range tests using an artificial target acoustically similar in size to sockeye salmon in a highly turbid river and deployment of the DIDSON on rocky river bottoms and artificial substrates to observe fish behavior at these sites (Maxwell and Gove 2004).