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

Changing the thickness of the Leeuwin did not dramatically change the predicted 95% detection
range (comparing Figure 26 and Figure 27). Changing the source depth only significantly
reduced the propagation from profile 7, or to the west.
Thus the bluey logger was effectively sampling all of the Rottnest trench region. Given that type
II signals in the range 95-100 dB re 1μPa were measured from the bluey logger site, then by
extrapolating the trends shown on Figure 26, it is possible that the logger was sampling perhaps
50 km or more to the north, 40 km to the west and perhaps out to 100 km or more to the south of
the site.
3.5) Transmission of blue whale calls to drifting site
Drifting sets were made using hydrophones suspended in the water column at mostly 55 m
depth. To simplify calculations a central point near the head of the trench was chosen as
representative of the location of drifting sets (31 o 58' 115 o 8', see Figure 2 for the drifting set
locations). From this site eight bathymetry paths at 45 o increments were determined (out to 50
km where possible). The rationale as used in the bluey logger calculations (previous sections)
was then used to determine the propagation of the blue whale type II component to the
hydrophone. This can be seen in Figure 29.
Figure 29: Predicted blue whale type II component level about the representative drifting site (hydrophone at 55 m
depth) for 75 m thick Leeuwin current and 40 m source depth.
By comparing Figure 24 and Figure 29 it can be seen that blue whale calls propagated
significantly better to the bottomed bluey logger than to the shallower hydrophone of the drifting
sets. Shading by the trench bathymetry can be seen at the drifting site, with propagation to the
west far better than through NE, E to SW.
The 95% probability ranges of detection for the type II component from the representative
drifting site, using two source depths and Leeuwin thicknesses are shown on Figure 30, Figure
30