Section 7.1: Acoustic Telemetry Overview
Douglas G. Pincock and Samuel V. Johnston
Acoustic telemetry is the use of an acoustic transmitter (hereafter referred to as tag) attached to or implanted in an aquatic animal to locate and gather information about its presence, movements, and behavior in the underwater environment. Fish are the most commonly tagged animals in acoustic telemetry studies, but other aquatic animals have been studied (Sauer et al. 1997; Niezgoda et al. 2002; Cooke et al. 2004). Early acoustic telemetry technology (Trefethen 1956) relied on relatively large tags that were only suited for use in larger fish such as adult salmon (Johnson 1960; Hallock et al.; 1970). As technology evolved, smaller tags became available (Figure 1) allowing smaller fish to be tagged (Voegeli et al. 1998; Steig 2000), often by gastric or surgical implantation (Moore et al. 1990; Adams et al. 1998). Tags and receivers evolved together to allow for greater range and reliability (Stasko and Pincock 1977; Ehrenberg and Steig 2003; Grothues 2009). Through the use of more sophisticated signal coding methods, it has become possible to simultaneously monitor much larger numbers of fish (Cole et al. 1998; Bach et al. 2003; Ransom et al. 2008; McMichael et al. 2010) and to monitor the movements of fish over large geographic areas (Hubley et al. 2008; Evans et al. 2010).
An acoustic monitoring system consists of the tags and the receivers. The fundamental components of an acoustic tag are the acoustic transducer, the battery, and the electronics, which is everything else that makes the tag function. The acoustic transducer (universally a lead zirconate titanate (PZT) cylinder as discussed later) converts electrical energy to acoustic energy that in turn propagates through the water and is detected by the receiver. The electronics facilitates and controls the conversion of the electricity from the battery to acoustic energy. The end result is a tag that emits a signal consisting of short bursts of sound waves. As sound propagates relatively well in water, tags can often be detected at significant ranges (up to 1000s of m) with appropriate signal characteristics, receiver design, and environmental conditions. Receivers can be used on a mobile platform, such as a boat, to follow or track (often referred to as mobile tracking) a tagged fish or receivers can be placed at a fixed location to detect tagged fish that move near enough to the receiver to be detected (often referred to as passive tracking). Multiple receivers may be employed to cover larger areas, and/or to position tagged fish near or within the receiver array. This Section will focus on the underlying principles, methodology, and application of acoustic telemetry.