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

Code:46.2008-final-Feb9
mounted on a customized aluminum frame deployed at a depth of 1 m. The coordinates for the location of
the transducer were 48° 30.920′ N and 123° 9.168′ W. The transducer was aimed horizontally at a bearing
of 238° clockwise from magnetic north. The transducer was connected to the deck unit by power and
digital signal cables. The data collected during this field trial are discussed below.
The BioSonics DT-X system could be operated in the range from 5 to 250 m at 200 kHz. The system
transmitted a rectangular pulse 0.4 ms in duration at a rate of 2 pulses/second. On May 22, 2008, three
killer whales (two adults and one juvenile) were visually observed passing by the drifting R/V Gato
Verde. The fixed-aim sonar system detected the whales and recorded their approach as shown in the
echogram presented in Figure 1. The three rectangular frames (red dashed lines) in the echogram outline
three time segments containing sequences of echoes from the visually observed whales. The killer
whales were within the beam of the sonar system for about 3 minutes.
B. Data Analysis
1) Whale Behavior
The killer whale echo sequences were processed to identify the strongest echo returns. The strongest
echo returns in each segment that appeared to be from an individual whale were used to estimate the
individual whale’s TS. The phase information from the split-beam system for each echo return, when
combined with the aiming angles of the system’s transducer, permitted the azimuth and elevation angles
to the whales relative to the acoustic axis of the transducer to be estimated. The ranges to the targets were
also provided in the system output.
While the echoes from individual whales were easily discerned in the echo sequences, the visual
observations of the whales were not sufficiently detailed to unambiguously assign a particular whale to
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