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Bioacoustic Attenuation Spectroscopy: A New Approach to Monitoring Fish at Sea
Orest Diachok
How does one find fishes in the sea? For millennia, catch- ing fish has depended on luck for a fisherman out for a day of recreation and both luck and knowledge of fish behavior and ecological preferences for those seeking larger catches. Still, in many ways, finding fish, especially in large quantities, was a “shot in the dark.” However, this started to change when fisheries biologists started to apply acoustics to the hunt for fishes. The various approaches that have been used and that continue to evolve now enable fishers not only to find large groups of fish more efficiently and effectively but also to enable fish- ery biologists to quantify the number of fish in areas of interest, their migration patterns, and how their numbers evolve over time as well as other aspects of their behavior.
The purpose of this article is to provide a brief historical review of acoustic approaches to fishery biology, discuss the limitations of these approaches, and describe bioacous- tic attenuation spectroscopy (BAS), a new and promising acoustic approach that has the potential to revolutionize fishery biology. The BAS approach is essentially noninva- sive, provides measures of fish abundance and the number of fish in a region, and can even estimate the number of fishes of different lengths in the ensonified region.
The most important practical application of research in fish- eries acoustics is the estimation of the abundance of species that are of commercial interest. This information is used by government agencies to set limits on commercial fishing.
Interactions Between Fish and Sound
Before reviewing the history of sonar methods to detect fish, first I review the basic physics of sound interaction with fish. The majority of species of bony fishes that occur in large numbers have a swim bladder, which is an elon- gated, air-filled chamber located in the abdominal cavity.
Swim bladders provide buoyancy and enable fish to be neutrally buoyant at their preferred depth between bouts of vigorous swimming (Helfman et al., 2009). The size of the swim bladder varies by species and is often related to the size of the species. Although the swim bladder probably evolved to provide buoyancy to the fish, it is also involved in other functions such as hearing and sound production in many species (e.g., Popper and Hawkins, 2019).
Figure 1 shows an X-ray image of a side view of the swim bladder of a pilchard sardine (Sardinops ocellatus). Because swim bladders are generally filled with air, they scatter sound in various directions and are the primary cause of backscattering and the echoes detected by fish- eries sonars. Fisheries scientists employ a concept called target strength (TS) to describe how much energy is reflected in the backscattered direction by an individual fish. TS increases with the size of the swim bladder.
As sound propagates through an aggregation of fish, each encounter with a fish within the aggregation causes some of the energy to be scattered in various directions, includ- ing sound that is backscattered, as illustrated in Figure 2. As a result, each encounter with each fish within the aggregation diminishes the energy of the sound propa- gating in the forward direction (Figure 2, red arrows).
©2020 Acoustical Society of America. All rights reserved.
 40 Acoustics Today • Summer 2020 | Volume 16, issue 2
Figure 1. X-ray image of the of the swim bladder of a pilchard sardine (Sardinops ocellatus). Anterior is to the left. Image courtesy of John Horne, University of Washington, Seattle.

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