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

These calls were only heard in this recording session, and not again in any of the drifting
sequences or from the bluey logger set. The gear was deployed near to a school of approximately
35 Rissos dolphins (Grampus griseus) but given the low frequency nature of the calls it was
unlikely to have originated from these animals.
3.6.4) Four second 'chirps'
At the time of finalising this report a scaled down series of field trials was being carried out
(summer 2001). A set of drifting records picked up a series of high level four second 'chirps', not
seen before. Blue whales had been sighted the previous day and were sighted several days later,
but not on the day of recording. These recordings had high flow noise as they were taken from a
drifting dinghy. An example of this call type is shown on Figure 36.
Figure 36: 'Chirps' recorded from drifting set in early 2001.
3.6.5) Dolphin signals
A large proportion of the drifting recordings contained dolphin whistles and echolocation
signals. It is probable that the bluey logger set also had dolphin signals in it, but these were not
looked for and as they were removed in the down-sampling process of the blue whale analysis,
would not have been apparent in the searching that was carried out. Several species of spinner
(Stenella spp.) and the bottlenose dolphin (Tursiops truncatus) were sighted in the boat surveys.
3.7) Blue whale call search techniques
Given the precise tonal character of the calls then the possibility of using spectrogram cross
correlation techniques, as described by Mellinger and Clark (2000) for bowhead calls, to
automate call identification seemed attractive. The 4827 records from the bluey recording set
presented an enormous and time consuming data set to search for blue whale calls. Two other
sets of the bluey logger are also available for searching for blue whale calls, 2955 records from
off Cape Leeuwin in 1998, and 5069 records from off Exmouth in 2000 (with confirmed blue
whale calling in). Automatic identification would allow the accurate placement of components
within a record, and thus allow calculation of each component level, a requirement for the
acoustic censusing technique using a single hydrophone.
Spectrogram correlation techniques were thus developed using high SNR ratio components to set
up kernels, or reference spectrogram signals containing the component structure to be searched
for, and correlating these against measured spectrograms. The standard correlation technique
multiplied the kernel against the test section then summed the result to give a single value for
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