Page 43 - Summer2020

are not capable of monitoring bottom-dwelling species, such as flatfish (e.g., flounder).
Ship Avoidance and Sonar
Estimation of fish abundance also assumes that echo sounder measurements do not affect the behavior of the species being measured. Unfortunately, hull-mounted echo sounder measurements are plagued by the prob- lem of ship avoidance. As a ship approaches, fish dive to greater depths and to the left and right of the track of the ship, thereby reducing the number of fish beneath the echo sounder. This phenomenon biases estimates of fish abundance (Scalabrin et al., 2007).
In recognition of the severity of the avoidance prob- lem, an international committee of fisheries scientists conducted a comprehensive review of the literature on ship avoidance, concluded that its main cause is acoustic noise from the engines of the ship, and recommended construction of quiet ships. Many quiet ships were built for use by fisheries biologists. Unfortunately, ship avoid- ance of the new quiet ships was just as severe as ship avoidance of the older, noisy vessels (Ona et al., 2007). A possible explanation is that fish respond to the pressure wave of approaching vessels rather than to the acoustic noise of engines.
In view of this apparently unsolvable problem, recent research has focused on removing echo sounders from ships and placing them on autonomous underwater vehicles (AUVs; Scalabrin et al., 2007) and wave glider-based sys- tems (Greene et al., 2014). These approaches will improve the quality of echo sounder measurements because they will not be affected by the ship avoidance phenomenon and will permit measurements of fish near the surface.
Another approach for eliminating the ship avoidance problem is to deploy echo sounders in an upward-looking mode on the bottom. This approach provides unbiased data of scientific interest (Kaltenberg and Benoit-Bird, 2009) but is not suitable for estimating fish abundance in large areas of commercial interest because these devices only detect fish in specific locations.
Bioacoustic Backscatter Spectroscopy
Early Impulsive Source Measurements
During the 1960s and 1970s, extensive research on swim bladder resonance was conducted in support of new, low-
frequency naval sonars. This research was motivated by need of the United States and other navies to understand the effects of echoes from fish aggregations on the detec- tion of submarines and was focused on the bioacoustics of myctophids, a group of species that reside primarily in the deep ocean during the day but that move toward the surface at night (Farquhar, 1970). The scientific measure- ments utilized impulsive devices (usually explosives) to generate broadband sound at frequencies between about 100 Hz and several kilohertz and an array of hydrophones to determine the depth of fish-reflected echoes. Mea- sured resonance frequencies were consistent with the theoretical calculations of the resonance frequencies of myctophids (Chapman et al., 1974).
Modern Directional Sonar Measurements
In recent years, impulsive devices were replaced by directional, broadband transducers (Nero et al., 1998; Stanton et al., 2010). Stanton et al. (2010) adapted a commercial, highly directional subbottom profiler with a source level (SL) of about 197 dB re 1 μPa root-mean- square (rms) to measure the frequency dependence of echoes from fish at resonance frequencies. The SL is defined as the level of sound at 1 m from the source. As a result of its high directionality, this source is unlikely to affect marine mammal behavior unless the animal is directly beneath the beam. Stanton et al. (2010) mea- sured the resonance frequency of 25-cm-long herring Clupea harengus, the dominant species at their mea- surement site, and demonstrated consistency with theoretical calculations. Because the source is towed behind a ship, this configuration is limited to the detec- tion of fish that are below the wake of the ship. The resultant measurements may be biased by changes in fish behavior in response to high-level acoustic signals, particularly at their resonance frequencies, at short dis- tances from the source.
Long-Range Sonar Measurements
Much more powerful broadband sonars, which oper- ate at low frequencies, have been employed to measure the frequency dependence of backscattered echoes from fish in the vicinity of their resonance frequen- cies at ranges of about 100 km. A major benefit of this approach is that it provides a synoptic view of the syn- chronized changes in fish behavior over areas as large as about 100 km (Makris et al., 2006). Interpretation of the resultant measurements, however, is limited by
Summer 2020 • Acoustics Today 43