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

callers. The aerial surveys sighted up to eight animals per flight in the Rottnest trench over the
study period (Table 5). Given that the blue whales have long down times then the aerial surveys
are unlikely to have sighted all the animals in the region. Thus the mean of the maximum
number of calling animals per day of 2.24, will represent less than 28% of the total number of
blue whales within the region (ie. 2.2/8) to give an upper bound to the proportion of blue whales
within the Rottnest trench producing the tonal signals.
To give a display of the presence of blue whales through time, the power spectral density of each
90 s sample from the bluey logger (10 minute sample separation) was determined at a 1.22 Hz
resolution using 219 averages per 90 s (Hanning window), and has been displayed with time on
Figure 47 - Figure 51. The presence of blue whales is indicated by the banding at 18-25 Hz,
while the 20 Hz clicking shows up as the intense narrow vertical stripes. It can be seen that the
two call types dominate the sea noise spectra over this period. The bluey logger ran for almost
five weeks. Although the degree of calling tapered slightly towards April, the sea noise spectra
over the 18-80 Hz band was still dominated by blue whale calling and the 20 Hz clicking at the
end of the sampling period, indicating how profuse the calling by blue whales and the 20 Hz
clicking source was, and its significant impact on ambient sea noise levels in the region.
44