Antarctic blue whales (Balaenoptera musculus intermedia) are slowly recovering from decades of commercial whaling in the Southern Hemi-sphere that brought the species to the brink of extinction by the 1970s (Branch et al. 2004, 2007; Thomas et al. 2015). In general, these whales spend the austral summer around the Antarctic and migrate northwards in winter, although there is evidence from both whaling-era data and modern passive acoustic monitoring that some animals remain at high latitudes in winter (Hinton 1915, Širovi
c et al. 2004, Thomisch et al. 2016). The northernmost-documented wintering destinations of Antarc-tic blue whales include the west coast of Africa up to Angola (Best 1998, Figueiredo and Weir 2014), the eastern tropical Paci?c (Stafford et al. 2004), off northern New Zealand (McDonald 2006), and the Indian Ocean near the Diego Garcia atoll (Stafford et al. 2004).

Most current knowledge about the occurrence of Antarctic blue whales comes from passive acoustic data sets from hydrophones deployed from high to low latitudes (Antarctic deployments reviewed in van Opzeeland et al. 2013; for lower latitudes see Stafford et al. 2004, McDonald 2006, Samaran et al. 2013, Tsang-Hin-Sun et al. 2015, Leroy et al. 2016). This approach has proved very effective for monitoring Ant-arctic blue whales as they produce a distinct low frequency signal, known as the Z-call, easily recognizable from its Z-shape in the time-frequency domain (Fig. 1A; Ljungblad et al. 1998, Rankin et al. 2005, Širovi
c et al. 2009). Densely produced Z-calls from distant Antarctic blue whales can create a “chorus” of energy between 28 Hz and 26 Hz (Fig. 1B). Both calls and choruses have been used to identify regions and seasons of occurrence for this species, e.g.,(Širovi
c et al. 2009, Leroy et al. 2016, Thomisch et al. 2016). Here, we present the ?rst evi-dence of the surprising seasonal presence of Antarctic blue whales at the Equator in the Atlantic Ocean. 

In 2013 three hydrophones were deployed on moorings in the equato-rial Atlantic Ocean during the COLMEIA-HYDRO acoustic experiment (Maia 2013; Fig. 2). The instruments, developed by the Laboratoire Geosciences Océan (Brest, France), consisted of hydrophones connected to autonomous acquisition and storage systems (D’Eu et al. 2012). Each instrument was moored in the axis of the sound ?xing and ranging (SOFAR) channel. The acoustic data were digitized at a sample rate of 240 Hz using a 24-bit analog-to-digital conversion. The hydrophone sen-sitivity was −163.5 dB re: 1 V/Pa, including preampli?er gain. Upon recovery, the data were downloaded and examined via long-term spec-trograms. The records at the three sites clearly show a band of energy between 26 Hz and 26.5 Hz throughout the year (Fig. 3). This band indi-cates the presence of the Antarctic blue whale chorus resulting from the ?rst unit of Z-calls. The energetic bandwidth near 17–22 Hz represents a combination of Antarctic blue whales Z-calls, ?n whale 20 Hz-pulses, and possibly Atlantic blue whale calls (16–18 Hz, Mellinger and Clark 2003).