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

Hydroacoustics: Lakes and Reservoirs

J. Christopher Taylor and Suzanne L. Maxwell

doi: https://doi.org/10.47886/9781888569926.ch10

Fisheries hydroacoustics uses transmitted sound to detect fish. Sound is transmitted as a pulse and travels quickly and efficiently through water. As the sound pulse travels through water it encounters objects that are of different density than the surrounding medium, such as fish, that reflect sound back toward the sound source. These echoes provide information on fish size, location, and abundance. The basic components of the acoustic hardware and software function to transmit the sound, receive, filter and amplify, record, and characterize the echoes. While there are many manufacturers of commercially available “fish-finders,” quantitative hydroacoustic analyses require that measurements are made with scientific-quality echo sounders that have high signal-to-noise ratios and are easily calibrated.

Over the past three decades, vertical or down-looking hydroacoustics has become increasingly important to the assessment of anadromous and land-locked salmonids (Thorne 1971, 1979; Burczynski and Johnson 1986; Mulligan and Kieser 1986; Levy et al. 1991; Yule 1992; Parkinson et al. 1994; Beauchamp et al. 1997; Wanzenbock et al. 2003), and lake and reservoir fishes (Thorne 1983; Brandt et al. 1991; Degan and Wilson 1995; Schael et al. 1995; Vondracek and Degan 1995; Cyterski et al. 2003; Taylor et al. 2005). Hydroacoustics provide a repeatable, noninvasive method of collecting high-resolution (submeter scale), continuous data along transects in three dimensions (MacLennan and Simmonds 1992). MacLennan and Simmonds (1992) as well as Brandt (1996) give a thorough introduction in the use of hydroacoustics for measuring fish abundances and distributions.

The density and distribution of lake, reservoir and lowland river fishes varies by season and time of day and is influenced by a range of abiotic, biotic and behavioral factors such as temperature, oxygen concentration, and vertical distribution of predators and prey (Lucas et al. 2002). Schools of sockeye salmon Oncorhynchus nerka occurring in lakes and reservoirs disperse in midwater at night (Johnson and Burczynski 1985; Clark and Levy 1988; Parkinson et al. 1994; Beauchamp et al. 1997). Likewise, forage fishes occur in patches, typically aggregated during the day and more dispersed at night (Appenzeller and Leggett 1992; Schael et al. 1995). Under these dispersed or disaggregated distribution patterns, densities can be acoustically estimated using vertically oriented transducers as long as the fishes are a sufficient distance from the surface to permit detection.