Target Strength of Southern Resident Killer Whales ... - BioSonics, Inc

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Code:46.2008-final-Feb9 ranges between −7 to −13 dB. Most recently, Lucifredi and Stein (2007) reported an aspect-dependent gray whale TS measurement taken at about 500-m range using a vertical line array sonar frequency centered at 23 kHz. The reported TS measurements for adult gray whales ranged from −2.9 dB at tail aspect to 12.8 dB at broadside aspect. An active sonar system was used to study killer whale hunting behavior in Norwegian waters (Simila, 1997; Nottestad and Axelson, 1999; Nottestad and Simila, 2001; Nottestad et al., 2002a, 2002b). A comprehensive and relatively well-controlled experiment was conducted to insonify a tagged killer whale using two commercial active sonar systems (the Simrad Model SP90 (20-30 kHz) and Simrad Model SH80 (110 to 120 kHz), Kongsberg Maritime AS, Horten, Norway). The goal of the study was to evaluate the feasibility of using active acoustic systems to detect killer whales and observe their behavior in the Lofoten fjords of Northern Norway (Kvadsheim et al., 2007). However, TS observations of killer whales, if obtained during this study, were not reported. Au (1996) measured the TS directivity at 67 kHz of a well-trained bottlenose dolphin in a pool like facility. He also insonified parts of the dolphin to identify the major contributors to whole body TS. Au observed relatively low TS for most portions of the dolphin's body with exception of strong echoes from the lungs. Au’s observations indicate that the air filled lung of whales, and presumably other marine mammals, is a major reflector of sound, which is similar to findings for fish that indicate that the swim bladder is the major reflector of sound in the species of fish that have one (Clay and Horne, 1994). The target strength measurements for large marine mammal at sea are very important but difficult to obtain. Usually, when these measurements are collected, they are most likely only for a specific animal and certain frequency and there are a lot of gaps that we need to fill before we can apply these measurements to practical applications. In this paper, we report the preliminary results of TS modeling we performed to extrapolate the TS measurements of bottlenose dolphin provided by Au (1996) to killer whales. We also report the results of analysis of echo data we acquired for three freely swimming killer 4
Code:46.2008-final-Feb9 whales in 2008 using a 200 kHz split-beam sonar and additional analysis we conducted to explore differences between the in-field TS measurement of killer whales and those we estimated by extrapolation from Au’s data using a 3 layer acoustic scattering model (Miller and Potter 2001). This model assumes that the lung is the major sound reflector in whales and estimates the plane wave backscattering coefficient to account for the acoustic impedance and absorption of whales flesh and blubber at different frequencies. Although it is simple and preliminary, it provides an instructive method for comparing the available measurement of various species at different frequencies and useful information for practical applications in the future. A. Measurements II. MATERIALS AND METHODS In 2008, a DT-X Echosounder (BioSonics, Inc., Seattle, Washington) with a deck unit (BioSonics, Inc.) was deployed to continuously monitor for killer whales at Lime Kiln Point on San Juan Island, Washington State. BioSonics DT-X is a 9˚ (horizontal)× 6˚ (vertical) 200 kHz digital split-beam echosounder, which has been developed and commercially available for the direct measurement of target strength (Carlson and Jackson 1980). The split-beam echosounder has a transducer, which is usually symmetrically divided into four quadrants (Ehrenberg 1974). This BioSonics DT-X echosounder has one large element whose signal is used to determine the echo amplitude, while three smaller elements are used to measure the off-axis angles. The primary goal was to demonstrate the capabilities of autonomous scientific hydroacoustics systems by using an active sonar system to study distribution, abundance, and behavior of killer whales moving past Lime Kiln Point. The first field trial of the monitoring system was carried out from the R/V Gato Verde, a catamaran sailing vessel with better stability than a monohull boat, at a depth of 14 m. The system transducer was 5
Page 1 and 2: Code:46.2008-final-Feb9 Target Stre
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Page 7 and 8: Code:46.2008-final-Feb9 each echo s
Page 9 and 10: Code:46.2008-final-Feb9 which equal
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Page 17 and 18: Code:46.2008-final-Feb9 References
Page 19 and 20: Code:46.2008-final-Feb9 Levenson, C
Page 21 and 22: Code:46.2008-final-Feb9 Similä, T.
Page 23 and 24: Code:46.2008-final-Feb9 FIGURE 2 Pl
Page 25 and 26: Code:46.2008-final-Feb9 FIGURE 4 Ta
Page 27 and 28: Code:46.2008-final-Feb9 FIGURE 5 Hi
Page 29: Code:46.2008-final-Feb9 FIGURE 7 Po

Target Strength of Southern Resident Killer Whales ... - BioSonics, Inc

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