Abstract

This study examined the occurrence of sei whale upsweep and downsweep vocalisations in 3 yrs of acoustic data (2015–2017) from the Juan Fernandez Archipelago, Chile, to determine the seasonal presence of this species in this area. Acoustic data were obtained from a hydro-acoustic station (HA03) maintained by the Comprehensive Test-Ban-Treaty Organization and analysed for target signals using automatic detectors in Raven Pro 2.0. Monthly upsweep call rates (ranging 0–1.2 calls h–1) displayed a fairly consistent seasonal pattern, with none or very few detections from the austral summer to early winter and, overall, more detections in late winter and spring, peaking in September. Monthly downsweep call rates (ranging 0–0.69 calls h–1) were higher in winter and spring. We discuss the implications of these results for understanding seasonal migration routes. Upsweeps had a duration of (mean ± SD) 1.5 ± 0.3 s with a peak frequency of (mean ± SD) 62.0 ± 8.3 Hz (n = 100), and downsweeps had a duration of 1.3 ± 0.1 s with a peak frequency of 56.3 ± 10.9 Hz (n = 100). Inter-note intervals (INIs) for upsweeps had modes at 7 s and 14 s; the dominant INIs mode for downsweeps was 4 s. This is the first multi-year passive acoustic monitoring study of sei whales in the Southeast Pacific and provides novel information on the distribution of a poorly-studied Endangered species in the southern hemisphere.