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

Abstract
Through January-April 2000 research was carried out off the Rottnest trench to search for blue or
pygmy blue whales. A consortium of researchers carried out aerial surveys, boat surveys and
acoustical measures. Historical records led us to believe that a Western Australian population of
pygmy blue whales (Balaenopteridae musculus brevicauda, sub species of the true blue whale,
B. m. musculus) existed, while a preliminary boat survey in 1994 suggested that some of these
animals aggregated in the Rottnest trench west of Perth. This was confirmed in the 2000
observations, aerial surveys sighted up to eight blue whales/flight within the trench and in 30
days boat searching 17 blue whales were sighted. Five thousand acoustic records were made,
many of which had blue whale calling in, some having up to nine animals calling at once.
Although of a slightly different format, the calls recorded were of a similar character to those
described from other blue whale populations. A call comprised three components, the first (type
I) dominated by a 19 Hz tone for 21 s followed by a jump to a 21 Hz tone for a further 22 s. Five
to ten s later the second component (II) followed which was dominated by a long frequency
upsweep, beginning near 20 Hz and increasing to 26 Hz over 23 s. Approximately 23 s later the
third component followed, this dominated by a 20 s, 19 Hz tone. Harmonics were evident in each
component. At least one secondary source was evident in all components, consisting of a pulsed
65 Hz signal. This was especially strong in the type III component. A call thus totalled around
two minutes, repeated at intervals of 78 s. The type II component had the highest source level
and was the most obvious signal received at long range. Propagation modelling indicated that the
components could transmit extremely well along the continental shelf edge, with a bottomed
logger in 450 m of water receiving calls from in excess of 50 km and probably into the hundreds
of km. Calling showed strong day night patterns, being 2.2 times greater at night than during the
day. Although searching was concentrated in the Rottnest trench region, the distribution of
received call levels and the propagation modelling suggested calling animals were spread
relatively evenly over a large range, probably many hundreds of km along the continental shelf
edge. Limiting received calls to within the Rottnest trench indicated a maximum number of
calling animals/90 s sample/day from 1-5 with a mean at 2.2 ± 0.34, or less than 28% of the
minimum population estimate for the trench region (eight, from the aerial surveys and assuming
no corrections for sighting bias). Also extremely common in recordings were 20 Hz 'clicking'
calls of individual pulses of several hundred ms length. These occurred in bouts ranging in
length from one sampling period (ten minutes) to days, with bouts comprised of multiple
sources. The low frequency (< 100 Hz) sea noise spectra from a series of 90 s recordings made
every 10 minutes for 33.5 days was dominated by the tonal blue whale calling and 20 Hz
clicking.
Acknowledgments
This program was funded by Environment Australia through the Western Australian Museum.
Many people contributed to the success of field work, particularly Mal Perry of the Centre for
Marine Science and Technology and Micheline-Nicole Jenner and Ross Nicole of the Centre for
Whale Research.
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