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

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
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