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

dB for first spectral peak and 3 dB for second spectral peak) the correlation point was deemed
correct.
This gave the presence of a component. To give the number of distinct calling animals within
each 90 s block, the number of type II components was counted (since it was not possible for a
single animal to produce more than one type II component in a 90 s block) and the number of
type II components which fell within 32.5 s of the block start added to this (since a type III
component followed around 22 s after a type II component and ran for approximately 23 s, thus a
leading type III component indicated a different caller). Type I components which fell in the last
45 s of the block also would indicate a different caller, but were ignored in this version of the
search technique as the correlation technique was not tuned for correct type I component
identification.
An example plot for a poor correlation, showing the block spectrogram, resulting correlation
vectors, and the component presence is shown on Figure 38. In this example the leading portion
of the type I component has been falsely recognised as a type III component. This was only a
problem for high SNR type I components. The type I recognition was not optimised and has
falsely recognised a second type I component.
The technique was run across 4827 records of the bluey logger and cross checked against 500
records which were manually counted for the number of callers. The resulting errors of the
number of callers manually counted minus the number of callers automatically recognised is
shown by the distribution plot of Figure 39. The correct number of callers accounted for 35% of
the sample, with the majority of the remaining blocks under-sampled. Reasons for the
discrepancies were:
• the leading portion of the type I component being mistaken for a type II component - only an
issue for the few high SNR type I components;
• very low component level or SNR - the most common error but not considered important in
the acoustic censusing technique described below, since detection was limited on received
level to those calls likely to be within the Rottnest trench region only, thus low SNR
components were rejected from counting anyway;
• occasional mistake of type II for a particular sequence of close 20 Hz 'clicking' signal
(section 3.5) - although this was rare and the algorithm was generally robust in rejecting the
clicking signals;
• propagation effects causing pronounced variability in the received level throughout a
component (ie. Figure 13) - mistakes here were worst for components with high leading and
end sections and low middle sections;
• a particular combination of vessel tones being mistaken for a component - this was rare as it
required the right mix of tones;
• propagation effects causing variability in the received level of the different spectral
components - there were some blocks where components were rejected as the harmonics
were lost due to different propagation of the different frequencies.
It is believed the correlation technique can be improved considerably by several adjustments,
particularly by differences in the normalisation methods and varying the relative intensity of
component harmonics, so that they play a greater role in the initial correlation.
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