Sei whale
(Balaenoptera borealis)

In Madeira, Sei whales appear to be uncommon seasonal visitors. Confirmed records exist since September 2002, but their precise seasonal occurrence remains poorly resolved, partly because they can be difficult to distinguish from Bryde’s whales at sea (Freitas et al., 2012). Available observations suggest that they are seen mostly travelling, in small groups, rather than showing clear evidence of prolonged local use (Alves et al., 2018).

Sei whales are acoustically characterized mainly by short, stereotyped low frequency downsweeps, typically descending from ~83 Hz to ~34 Hz, over 1.2–1.4 s. These calls are produced most often as single calls, although doublets, triplets and, more rarely, quadruples also occur (Baumgartner et al., 2008; Romagosa et al., 2015). Only one Sei whale call type is used in passive acoustic analyses, reflecting how stereotyped this signal appears to be. Acoustic monitoring in the central North Atlantic has shown a bimodal pattern of Sei whale acoustic presence, with peaks in spring and autumn, consistent with migratory movements (Prieto et al., 2014; Romagosa et al., 2020).