Blue whale calling in the Rottnest trench-2000, Western ... - ANP

Figure 42: Distribution of the number of calling blue whales per block from 998 manually counted 90 s blocks of the
bluey logger set.
The automatic search technique located each component within the block. This enabled the level
of the call to be determined. For the bluey logger data set, this was done by:
• selecting a section of the waveform from 2 s before to 5 s after the identified component
central point (adjusted appropriately if the selected times fell outside the actual times)
• calculating the power spectra of this section at a 0.61 Hz resolution using a Hanning window
and no overlap (5 averages for a full seven second section);
• finding the spectral peak within the frequency bands 20-26 Hz or 16-19.5 Hz for the type II
or type III component respectively;
• converting the spectral peak level over the selected frequency band to mean squared pressure
using the empirical relationship given in section 3.1.3 (type III components were assumed to
have the same relationship between spectral level and mean squared pressure as per Equation
1, and from 3.1.2 were assumed to have a 2 dB lower source level than the type II
components).
By returning the spectral level over a narrow frequency band the chance of extraneous sources
altering the component measured level was minimised. A problem with this analysis was that the
received level of many blue whale components varied throughout the component, in several
ways (ie. Figure 13). Hence measuring a small section of the component may give a slightly
different received level as compared to measuring the entire component. But this error may be
expected to be small, and when averaged over many samples of received signals, not significant.
The distribution of calculated type II component mean squared pressure values is displayed in
Figure 45 along with an overlaid curve for the received level of an equal number of theoretical
blue whale calls, spaced evenly at one km increments in a line from 280 km south to 280 km
north with a propagation loss of 15 log10 (range). Assuming the region of the Rottnest trench to
be defined as within 40 km of the bluey logger site, or ranging from Yanchep in the north to the
southern end of Garden Island in the south, then the propagation calculations of the type II
component from the bottomed bluey logger (ie. 95% probability of detection ranges Figure 26,
Figure 27 and Figure 28) suggest that those signals above approximately 110 dB re 1μPa (mean
squared pressure) were in the trench region. This assumes:
1. that most animals were located along the shelf edge, or along transects 1 (N), 4 (SE), 5 (S)
and 6 (SW) and extrapolates the trend of the 95% probability curve for transect 1 beyond the
transect length limitation;
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