Seasonal detection of three types of “pygmy” blue whale calls in the Indian Ocean

Abstract

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.

Description and seasonal detection of two potential whale calls recorded in the Indian Ocean

“…The Australian call consists of a three unit phrase with the first unit ranging from 19 to 21 Hz and with an overall duration of 43 s. The second unit consisted of 23 s duration with a 20-26 Hz frequency range. The last part is a near constant frequency 18-19 Hz tone that lasts 26-48 s and follows the second unit by 23 s (Stafford et al, 2011). Sri Lankan calls are composed of three unit phrases ranging from 20 to 108 Hz with duration of $62 min.…”

“…Knowledge of blue whale migration patterns in the IOS are not clearly understood (Samaran et al, 2013;Stafford et al, 2011). Antarctic blue whales are thought to migrate to Antarctica in the austral summer to feed and return to lower latitudes in the austral winter to breed (Samaran et al, 2013).…”

“…In particular, PAM has been used to monitor a wide range of cetacean species, from the smallest species such as the vaquita (Phocoena sinus) (e.g., Rojas-Bracho et al, 2010) to the largest-the blue whale (Balaenoptera musculus) (e.g., Stafford et al, 2011). PAM can be used in a wide range of research applications, from individual-level behavioral studies (e.g., feeding behavior; Tyack et al, 2006) to population level processes (e.g., distribution and movement patterns; Samaran et al, 2013).…”

“…Recordings from Diego Garcia have been used extensively in previous blue whale acoustic studies (Stafford et al, 2004;Stafford et al, 2011;Harris, 2012). Both unidentified acoustic signals in this study have been frequently opportunistically detected in the Diego Garcia dataset (Harris, 2012) but to date no formal description of these signals has been made.…”

“…Madagascan calls have been detected at the Crozet archipelago from January to June and in May and June further north at Diego Garcia. The differences in seasonal detections may indicate the prevalence of a population that primarily occupies the western Indian and Southern Oceans with some individuals occasionally moving northwards by the end of autumn (Stafford et al, 2011).…”