Acoustic, genetic and observational evidence indicate the presence of humpback whales (Megaptera novaeangliae) from both hemispheres in Cape Verdean waters during their respective breeding seasons

A small population of humpback whales breeds around the Cape Verde Islands off West Africa. These whales exhibit a boreal seasonality, albeit two months later than that in the West Indies. Based on aseasonal observations of humpback whales and calves, Hazevoet et al. (2011) postulated that whales with an austral seasonality may also breed in Cape Verdean waters. Regions known to host breeding and calving whales from both hemispheres are rare, e.g. Costa Rica in the North Pacific. Here we present evidence consistent with this scenario in Cape Verde, using results from acoustic, molecular genetic and visual surveys. In April 2014, an SM2M+ underwater autonomous recorder (Wildlife Acoustics) was deployed to a depth of 600 m to record for 10 months, 100 km north of Sao Nicolau (N 17.6°, W 24.3°) over the abyssal plain (depth 3000 m). Analysis of 1539 hrs of recordings revealed humpback whale song during both austral and boreal spring, with absences in July and October to November. The percentage of positive detection hours conformed to a bimodal distribution, peaking in April (63%) and again in August (29%). In September 2014, 1182 km of visual survey effort was conducted from a sailing vessel throughout the archipelago, resulting in 9 encounters with a total of 12 humpback whales including 2 mother-calf pairs. Two adults were biopsy sampled, both exhibiting haplotypes (xx bp length D-loop fragments?) typical of whales sampled in the Southern Ocean. Further evidence of breeding behaviour is required to establish whether Cape Verde constitutes a breeding ground for whales with an austral seasonality. However, using three lines of evidence (acoustics, visual encounters and molecular genetics), our results build a stronger case for this scenario.