Blue whale calling in the Rottnest trench, Western Australia, and low frequency sea noise

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 based studies 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 early 2000 observations, in 30 days boat based searching 17 pygmy blue whales were sighted. Five thousand acoustic records were made, almost all of which had blue/pygmy blue whale calling in, some having up to six animals calling at once. Although of a slightly different format, recorded call components were of a similar character to those described from other populations. Also common were impuslive 'clicking' calls which were shorter than the 12-23 s blue whale call components and of low to very low frequency (< 1 Hz to 20 Hz). The literature suggests these are produced by fin whales but none were sighted. 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 was dominated by blue whale calling.