The Indian Ocean is an area in which a rich suite of cetacean fauna, including at least two subspecies of blue whale, is found; yet little information beyond stranding data and short‐term surveys for this species is available. Pygmy blue whale (Balaenoptera musculus spp.) call data are presented that provide novel information on the seasonal and geographic distribution of these animals. Acoustic data were recorded from January 2002 to December 2003 by hydrophones at three stations of the International Monitoring System, including two near the subequatorial Diego Garcia Atoll and a third southwest of Cape Leeuwin, Australia. Automated spectrogram correlation methods were used to scan for call types attributed to pygmy blue whales. Sri Lanka calls were the most common and were detected year‐round off Diego Garcia. Madagascar calls were only recorded on the northern Diego Garcia hydrophone during May and July, whereas Australia calls were only recorded at Cape Leeuwin, between December and June. Differences in geographic and seasonal patterns of these three distinct call types suggest that they may represent separate acoustic populations of pygmy blue whales and that these “acoustic populations” should be considered when assessing conservation needs of blue whales in the Indian Ocean.

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[1] Mid to low southern latitude hydrophone stations within the Indian Ocean have recorded two distinct types of low-frequency (<100 Hz) tremor that can be correlated with drifting icebergs and glacial features along the Wilkes Land coast of eastern Antarctica. The most common of these signals is a variable harmonic tremor (VHT), with spectral peaks that exhibit frequency fluctuations through time. These signals typically display a 4 to 10 Hz fundamental frequency and may have as many as ten harmonic bands. Individual VHT signal packets have durations of up to $30 min and occur throughout the year in clusters that continue for hours to days. A second, less commonly observed signal is characterized by shorter duration (25 to 90 s) pulses with a convex-upward spectrogram appearance. These cusped pulse tremors (CPT) often exhibit a near-uniform pulse spacing, with episodes continuing for minutes to hours. Tremor received levels at hydrophones near 32°S, 114°E and 7°S, 72°E reach as high as 142 and 133 dB re 1 mPa (peak to peak), respectively. Propagation likely occurs as a sea surface-reflected phase at high latitudes and a sound channel phase north of the convergence zone, with low-frequency transmission loss estimates suggesting maximum acoustic source levels of $245 dB re 1 mPa at 1 m. Source locations for a subset of the loudest VHT signals correlate with the satellite-derived locations of a large iceberg (B-15D) that migrated westward along the Wilkes Land shelf region during 2002 and early 2003. Most VHT sources, however, cannot be correlated with known iceberg locations, suggesting that these signals also may be sourced from smaller unnamed icebergs and/or associated with outlet glaciers distributed along the Wilkes Land coast. CPT signals have a more limited spatial distribution, originating from five specific regions where ice streams are observed. The harmonic nature of both signal types is consistent with the resonance of an ice layer or fluid-filled cavity within an ice mass.